The rate of reaction of nitrogen with hydrogen will increase if. The rate of chemical reactions. Endothermic and exothermic reactions
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1 Reaction rate, its dependence on various factors 1. To increase the reaction rate, it is necessary to increase the pressure, add carbon monoxide (1v) cool the system, remove carbon monoxide (1v) 2. The rate of the reaction of nitrogen with hydrogen does not depend on the pressure temperature of the catalyst, the amount of the reaction product 3. The reaction rate of carbon with oxygen does not depend on the temperature of the total pressure, the degree of fineness of carbon, the amount of the reaction product 4. To reduce the reaction rate H 2 + Cl 2 \u003d 2HCl + Q, it is necessary to lower the temperature increase the pressure lower the concentration of hydrogen chloride increase the concentration of hydrogen 5. To increase the reaction rate ZN 2 + N 2 \u003d 2NH 3 + Q it is necessary to cool the system to reduce the pressure to remove ammonia to add hydrogen 6. The rate of reaction of nitrogen with hydrogen is defined as
2 7. The reaction rate of carbon monoxide with oxygen is defined as 8. Zinc (granules) and oxygen interact with the highest rate at room temperature zinc (granules) and hydrochloric acid zinc (powder) and oxygen zinc (powder) and hydrochloric acid 9. With the highest zinc and oxygen interact at room temperature hydrochloric acid and sodium carbonate solution sodium alkali and aluminum calcium oxide and water 10. The reaction rate of nitrogen with hydrogen will increase when the mixture is passed over heated iron, adding ammonia cooling the mixture, increasing the volume of the reaction vessel 11. The reaction rate of carbon monoxide (ii) with oxygen will decrease when heated, passing gases over heated platinum, adding carbon dioxide, increasing the volume of the reaction vessel 12. The reaction rate will increase when oxygen is added to copper(ii) oxide
3 nitrogen ammonia 13. The reaction rate will increase when hydrogen water nitric oxide(ii) ammonia is added 14. The reaction rate between zinc and hydrochloric acid decreases when zinc is ground when HCl is added with heating over time 15. The reaction rate between zinc and hydrochloric acid increases with grinding zinc while cooling the solution while diluting the solution over time 16. In the reaction, the decomposition rate is 0.016 mol/(l min). What is the rate of formation (in mol/(L min))? 0.008 0.016 0.032 0. In the reaction, the formation rate is 0.012 mol/(l min). What is the decomposition rate (in mol/(L min))? 0.006 0.012
4 0.024 0, The rate of an elementary reaction depends on the concentrations as follows: 19. The rate of an elementary reaction depends on the concentrations as follows: 20. Both and and interact with the highest rate at room temperature and 21. 22 reacts with the highest rate with water at room temperature. Magnesium reacts with the highest rate at room temperature with zinc water with dilute acetic acid of silver nitrate solution with copper hydrochloric acid with oxygen
5 23. The reaction rate of decomposition into simple substances increases with the addition of an increase in pressure and cooling with an increase in the volume of the reaction vessel 24. The rate of the reaction of cracking octane in the gas phase increases with cooling, an increase in pressure increases the volume of the reaction vessel decrease the pressure increase the volume of the reaction vessel 26. Which statement about catalysts is incorrect? Catalysts participate in a chemical reaction Catalysts shift the chemical equilibrium Catalysts change the reaction rate Catalysts accelerate both the forward and reverse reactions nitric acid 28. The rate of a chemical reaction is not affected by changes in the concentration of ammonia
6 pressure hydrogen concentration temperature 29. The reaction between hydrogen and fluorine bromine iodine chlorine occurs at the lowest rate 30. To increase the rate of a chemical reaction, it is necessary to increase the concentration of iron ions grind iron reduce the temperature reduce the acid concentration 31. Hydrogen reacts with the highest rate with bromine iodine fluorine chlorine 32. At room temperature, hydrogen most actively reacts with sulfur nitrogen chlorine bromine 33. The rate of reaction between iron and hydrochloric acid solution will decrease with increasing temperature, dilute the acid, increase the acid concentration, grind iron 34. To increase the rate of the hydrolysis reaction of ethyl acetate, add acetic acid, add ethanol heat the solution to increase the pressure 35. With the highest speed under normal conditions, water interacts with
7 calcium oxide iron silicon oxide (IV) aluminum 36. The reaction rate increases with increasing concentration, decreasing temperature, increasing pressure, increasing temperature 37. Increasing nitrogen concentration increases the reaction rate 38. The reaction rate of zinc with hydrochloric acid does not depend on the concentration of acid, temperature, pressure, surface area of contact reagents 39. The interaction between 40 proceeds at the lowest rate at room temperature. The rate of a chemical reaction will increase with the addition of phosphorus an increase in oxygen concentration an increase in the concentration of phosphorus oxide (V) a decrease in the volume of oxygen taken 41. An increase in the reaction rate is facilitated by:
8 addition of sulfur increase in temperature 42. The reaction between 43 proceeds at the highest rate. Reaction 44 proceeds at the highest rate at room temperature. To increase the rate of a chemical reaction, it is necessary to increase the amount of chromium increase the concentration of hydrogen ions decrease the temperature increase the hydrogen concentration iron (III) metal zinc metal nickel barium hydroxide solution 46. The rate of the chemical reaction does not depend on the concentration of hydrochloric acid the temperature of the concentration of hydrogen the degree of grinding of magnesium 47. The increase in the surface area of contact of the reagents does not affect the rate of reaction between sulfur and iron silicon and oxygen hydrogen and oxygen zinc and hydrochloric acid
9 48. With the greatest speed, sodium hydroxide interacts with metallic zinc copper (II) sulfate, nitric acid, iron (II) sulfide 49. The rate of chemical reaction depends on the amount of phosphorus taken, the temperature of the concentration of phosphorus oxide (V), the volume of oxygen taken 50. With the highest speed at Reaction 51 proceeds at room temperature. Reaction 52 proceeds at the highest rate at room temperature. An increase in the reaction rate is facilitated by: a decrease in pressure, a decrease in concentration, a cooling of the system, an increase in temperature 53. The reaction rate between zinc and a solution of hydrochloric acid will decrease if the reaction mixture is heated to dilute the acid
10 pass hydrogen chloride through the reaction mixture, use zinc powder 54. At room temperature, potassium calcium magnesium aluminum reacts with water at the highest rate 55. To increase the rate of the hydrolysis reaction of 1-bromopropane, it is necessary to add acid, lower the concentration of 1-bromopropane, increase the temperature, increase the concentration of propanol 56. Speed The reaction between magnesium and copper sulphate solution does not depend on the concentration of salt temperature of the volume of the reaction vessel, the surface area of contact of the reagents
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The rate of a chemical reaction is equal to the change in the amount of a substance per unit time in a unit of the reaction space Depending on the type of chemical reaction (homogeneous or heterogeneous), the nature of the reaction space changes. The reaction space is usually called the area in which the chemical process is localized: volume (V), area (S).
The reaction space of homogeneous reactions is the volume filled with reagents. Since the ratio of the amount of a substance to a unit volume is called concentration (c), the rate of a homogeneous reaction is equal to the change in the concentration of the starting substances or reaction products over time. Distinguish between average and instantaneous reaction rates.
The average reaction rate is:
where c2 and c1 are the concentrations of the initial substances at times t2 and t1.
The minus sign "-" in this expression is put when finding the speed through the change in the concentration of reagents (in this case, Dс< 0, так как со временем концентрации реагентов уменьшаются); концентрации продуктов со временем нарастают, и в этом случае используется знак плюс «+».
The reaction rate at a given moment of time or the instantaneous (true) reaction rate v is equal to:
The reaction rate in SI has the unit [mol×m-3×s-1], other units of quantity [mol×l-1×s-1], [mol×cm-3×s-1], [mol ×cm –3×min-1].
The rate of a heterogeneous chemical reaction v called, the change in the amount of the reactant (Dn) per unit time (Dt) per unit area of the phase separation (S) and is determined by the formula:
or through the derivative:
The unit of the rate of a heterogeneous reaction is mol/m2 s.
Example 1. Chlorine and hydrogen are mixed in a vessel. The mixture was heated. After 5 s, the concentration of hydrogen chloride in the vessel became equal to 0.05 mol/dm3. Determine the average rate of formation of hydrochloric acid (mol/dm3 s).
Decision. We determine the change in the concentration of hydrogen chloride in the vessel 5 s after the start of the reaction:
where c2, c1 - final and initial molar concentration of HCl.
Dc (HCl) \u003d 0.05 - 0 \u003d 0.05 mol / dm3.
Calculate the average rate of formation of hydrogen chloride, using equation (3.1):
Answer: 7 \u003d 0.01 mol / dm3 × s.
Example 2 The following reaction takes place in a vessel with a volume of 3 dm3:
C2H2 + 2H2®C2H6.
The initial mass of hydrogen is 1 g. After 2 s after the start of the reaction, the mass of hydrogen becomes 0.4 g. Determine the average rate of formation of C2H6 (mol / dm "× s).
Decision. The mass of hydrogen that entered into the reaction (mpror (H2)) is equal to the difference between the initial mass of hydrogen (mref (H2)) and the final mass of unreacted hydrogen (tk (H2)):
tpror. (H2) \u003d tis (H2) - mk (H2); tpror (H2) \u003d 1-0.4 \u003d 0.6 g.
Let's calculate the amount of hydrogen:
= 0.3 mol.
We determine the amount of C2H6 formed:
According to the equation: from 2 mol of H2, ® 1 mol of C2H6 is formed;
According to the condition: from 0.3 mol of H2, ® x mol of C2H6 is formed.
n(С2Н6) = 0.15 mol.
We calculate the concentration of the formed С2Н6:
We find the change in the concentration of C2H6:
0.05-0 = 0.05 mol/dm3. We calculate the average rate of formation of C2H6 using equation (3.1):
Answer: \u003d 0.025 mol / dm3 × s.
Factors affecting the rate of a chemical reaction . The rate of a chemical reaction is determined by the following main factors:
1) the nature of the reacting substances (activation energy);
2) the concentration of reacting substances (the law of mass action);
3) temperature (van't Hoff rule);
4) the presence of catalysts (activation energy);
5) pressure (reactions involving gases);
6) the degree of grinding (reactions occurring with the participation of solids);
7) type of radiation (visible, UV, IR, X-ray).
The dependence of the rate of a chemical reaction on concentration is expressed by the basic law of chemical kinetics - the law of mass action.
Law of acting masses . In 1865, Professor N. N. Beketov for the first time expressed a hypothesis about the quantitative relationship between the masses of the reactants and the reaction time: "... attraction is proportional to the product of the acting masses." This hypothesis was confirmed in the law of mass action, which was established in 1867 by two Norwegian chemists K. M. Guldberg and P. Waage. The modern formulation of the law of mass action is as follows: at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of reactants, taken in powers equal to the stoichiometric coefficients in the reaction equation.
For the reaction aA + bB = mM + nN, the kinetic equation of the law of mass action has the form:
, (3.5)
where is the reaction rate;
k- coefficient of proportionality, called the rate constant of a chemical reaction (at = 1 mol/dm3 k is numerically equal to ); - concentration of reagents involved in the reaction.
The rate constant of a chemical reaction does not depend on the concentration of the reactants, but is determined by the nature of the reactants and the conditions for the reactions to occur (temperature, the presence of a catalyst). For a particular reaction proceeding under given conditions, the rate constant is a constant value.
Example 3 Write the kinetic equation of the law of mass action for the reaction:
2NO (g) + C12 (g) = 2NOCl (g).
Decision. Equation (3.5) for a given chemical reaction has the following form:
.
For heterogeneous chemical reactions, the equation of the law of mass action includes the concentrations of only those substances that are in the gas or liquid phases. The concentration of a substance in the solid phase is usually constant and is included in the rate constant.
Example 4 Write the kinetic equation of the law of action of masses for reactions:
a) 4Fe(t) + 3O2(g) = 2Fe2O3(t);
b) CaCO3 (t) \u003d CaO (t) + CO2 (g).
Decision. Equation (3.5) for these reactions will have the following form:
Since calcium carbonate is a solid substance, the concentration of which does not change during the reaction, that is, in this case, the reaction rate at a certain temperature is constant.
Example 5 How many times will the rate of the reaction of oxidation of nitric oxide (II) with oxygen increase if the concentrations of the reagents are doubled?
Decision. We write the reaction equation:
2NO + O2= 2NO2.
Let us denote the initial and final concentrations of the reagents as c1(NO), cl(O2) and c2(NO), c2(O2), respectively. In the same way, we denote the initial and final reaction rates: vt, v2. Then, using equation (3.5), we obtain:
.
By condition c2(NO) = 2c1 (NO), c2(O2) = 2c1(O2).
We find v2 =k2 ×2cl(O2).
Find how many times the reaction rate will increase:
Answer: 8 times.
The effect of pressure on the rate of a chemical reaction is most significant for processes involving gases. When the pressure changes by n times, the volume decreases and the concentration increases n times, and vice versa.
Example 6 How many times will the rate of a chemical reaction between gaseous substances reacting according to the equation A + B \u003d C increase if the pressure in the system is doubled?
Decision. Using equation (3.5), we express the reaction rate before increasing the pressure:
.
The kinetic equation after increasing the pressure will have the following form:
.
With an increase in pressure by a factor of 2, the volume of the gas mixture, according to the Boyle-Mariotte law (pY = const), will also decrease by a factor of 2. Therefore, the concentration of substances will increase by 2 times.
Thus, c2(A) = 2c1(A), c2(B) = 2c1(B). Then
Determine how many times the reaction rate will increase with increasing pressure.
Chemical reactions proceed at different speeds: at a low speed - during the formation of stalactites and stalagmites, at an average speed - when cooking food, instantly - during an explosion. Reactions in aqueous solutions are very fast.
Determining the rate of a chemical reaction, as well as elucidating its dependence on the conditions of the process, is the task of chemical kinetics - the science of the laws governing the course of chemical reactions in time.
If chemical reactions occur in a homogeneous medium, for example, in a solution or in a gas phase, then the interaction of the reacting substances occurs in the entire volume. Such reactions are called homogeneous.
(v homog) is defined as the change in the amount of substance per unit time per unit volume:
where Δn is the change in the number of moles of one substance (most often the initial one, but it can also be the reaction product); Δt - time interval (s, min); V is the volume of gas or solution (l).
Since the ratio of the amount of substance to volume is the molar concentration C, then
Thus, the rate of a homogeneous reaction is defined as a change in the concentration of one of the substances per unit time:
if the volume of the system does not change.
If a reaction occurs between substances in different states of aggregation (for example, between a solid and a gas or liquid), or between substances that are unable to form a homogeneous medium (for example, between immiscible liquids), then it takes place only on the contact surface of substances. Such reactions are called heterogeneous.
It is defined as the change in the amount of substance per unit of time per unit of surface.
where S is the surface area of contact of substances (m 2, cm 2).
A change in the amount of a substance by which the reaction rate is determined is an external factor observed by the researcher. In fact, all processes are carried out at the micro level. Obviously, in order for some particles to react, they must first of all collide, and collide effectively: not to scatter like balls in different directions, but in such a way that the “old bonds” in the particles are destroyed or weakened and “new ones” can form. ”, and for this the particles must have sufficient energy.
The calculated data show that, for example, in gases, collisions of molecules at atmospheric pressure are in the billions per 1 second, that is, all reactions should have gone instantly. But it's not. It turns out that only a very small fraction of the molecules have the necessary energy to produce an effective collision.
The minimum excess energy that a particle (or pair of particles) must have in order for an effective collision to occur is called activation energy Ea.
Thus, on the way of all particles entering into the reaction, there is an energy barrier equal to the activation energy E a . When it is small, there are many particles that can overcome it, and the reaction rate is high. Otherwise, a "push" is required. When you bring a match to light a spirit lamp, you provide additional energy E a required for the effective collision of alcohol molecules with oxygen molecules (overcoming the barrier).
The rate of a chemical reaction depends on many factors. The main ones are: the nature and concentration of the reactants, pressure (in reactions involving gases), temperature, the action of catalysts and the surface of the reactants in the case of heterogeneous reactions.
Temperature
As the temperature rises, in most cases the rate of a chemical reaction increases significantly. In the 19th century Dutch chemist J. X. Van't Hoff formulated the rule:
An increase in temperature for every 10 ° C leads to an increase inreaction speed by 2-4 times(this value is called the temperature coefficient of the reaction).
With an increase in temperature, the average velocity of molecules, their energy, and the number of collisions increase slightly, but the proportion of "active" molecules participating in effective collisions that overcome the energy barrier of the reaction increases sharply. Mathematically, this dependence is expressed by the relation:
where v t 1 and v t 2 are the reaction rates at the final t 2 and initial t 1 temperatures, respectively, and γ is the temperature coefficient of the reaction rate, which shows how many times the reaction rate increases with each 10 ° C increase in temperature.
However, to increase the reaction rate, an increase in temperature is not always applicable, since the starting materials may begin to decompose, solvents or the substances themselves may evaporate, etc.
Endothermic and exothermic reactions
The reaction of methane with atmospheric oxygen is known to be accompanied by the release of a large amount of heat. Therefore, it is used in everyday life for cooking, heating water and heating. Natural gas supplied to homes through pipes is 98% methane. The reaction of calcium oxide (CaO) with water is also accompanied by the release of a large amount of heat.
What can these facts say? When new chemical bonds are formed in the reaction products, more energy than required to break the chemical bonds in the reactants. Excess energy is released in the form of heat and sometimes light.
CH 4 + 2O 2 \u003d CO 2 + 2H 2 O + Q (energy (light, heat));
CaO + H 2 O \u003d Ca (OH) 2 + Q (energy (heat)).
Such reactions should proceed easily (as a stone easily rolls downhill).
Reactions in which energy is released are called EXOTHERMIC(from the Latin "exo" - out).
For example, many redox reactions are exothermic. One of these beautiful reactions is an intramolecular oxidation-reduction occurring inside the same salt - ammonium dichromate (NH 4) 2 Cr 2 O 7:
(NH 4) 2 Cr 2 O 7 \u003d N 2 + Cr 2 O 3 + 4 H 2 O + Q (energy).
Another thing is the backlash. They are similar to rolling a stone uphill. It is still not possible to obtain methane from CO 2 and water, and strong heating is required to obtain quicklime CaO from calcium hydroxide Ca (OH) 2. Such a reaction occurs only with a constant influx of energy from the outside:
Ca (OH) 2 \u003d CaO + H 2 O - Q (energy (heat))
This suggests that the breaking of chemical bonds in Ca(OH) 2 requires more energy than can be released during the formation of new chemical bonds in CaO and H 2 O molecules.
Reactions in which energy is absorbed are called ENDOTHERMIC(from "endo" - inside).
Reactant concentration
A change in pressure with the participation of gaseous substances in the reaction also leads to a change in the concentration of these substances.
In order for a chemical interaction to occur between particles, they must effectively collide. The greater the concentration of reactants, the more collisions and, accordingly, the higher the reaction rate. For example, acetylene burns very quickly in pure oxygen. This develops a temperature sufficient to melt the metal. On the basis of a large amount of experimental material, in 1867 the Norwegians K. Guldenberg and P. Waage, and independently of them in 1865, the Russian scientist N. I. Beketov formulated the basic law of chemical kinetics, which establishes the dependence of the reaction rate on the concentration of reacting substances.
The rate of a chemical reaction is proportional to the product of the concentrations of the reactants, taken in powers equal to their coefficients in the reaction equation.
This law is also called the law of mass action.
For the reaction A + B \u003d D, this law will be expressed as follows:
For the reaction 2A + B = D, this law is expressed as follows:
Here C A, C B are the concentrations of substances A and B (mol / l); k 1 and k 2 - coefficients of proportionality, called the rate constants of the reaction.
The physical meaning of the reaction rate constant is not difficult to establish - it is numerically equal to the reaction rate in which the concentrations of the reactants are 1 mol / l or their product is equal to one. In this case, it is clear that the rate constant of the reaction depends only on temperature and does not depend on the concentration of substances.
Law of acting masses does not take into account the concentration of reactants in the solid state, since they react on surfaces and their concentrations are usually constant.
For example, for the combustion reaction of coal, the expression for the reaction rate should be written as follows:
i.e., the reaction rate is only proportional to the oxygen concentration.
If the reaction equation describes only the overall chemical reaction, which takes place in several stages, then the rate of such a reaction can depend in a complex way on the concentrations of the starting substances. This dependence is determined experimentally or theoretically based on the proposed reaction mechanism.
The action of catalysts
It is possible to increase the reaction rate by using special substances that change the reaction mechanism and direct it along an energetically more favorable path with a lower activation energy. They are called catalysts (from Latin katalysis - destruction).
The catalyst acts as an experienced guide, guiding a group of tourists not through a high pass in the mountains (overcoming it requires a lot of effort and time and is not accessible to everyone), but along the detour paths known to him, along which you can overcome the mountain much easier and faster.
True, on a detour you can get not quite where the main pass leads. But sometimes that's exactly what you need! This is how catalysts, which are called selective, work. It is clear that there is no need to burn ammonia and nitrogen, but nitric oxide (II) finds use in the production of nitric acid.
Catalysts- These are substances that participate in a chemical reaction and change its speed or direction, but at the end of the reaction remain unchanged quantitatively and qualitatively.
Changing the rate of a chemical reaction or its direction with the help of a catalyst is called catalysis. Catalysts are widely used in various industries and in transport (catalytic converters that convert nitrogen oxides in car exhaust gases into harmless nitrogen).
There are two types of catalysis.
homogeneous catalysis, in which both the catalyst and the reactants are in the same state of aggregation (phase).
heterogeneous catalysis where the catalyst and reactants are in different phases. For example, the decomposition of hydrogen peroxide in the presence of a solid manganese (IV) oxide catalyst:
The catalyst itself is not consumed as a result of the reaction, but if other substances are adsorbed on its surface (they are called catalytic poisons), then the surface becomes inoperable, and catalyst regeneration is required. Therefore, before carrying out the catalytic reaction, the starting materials are thoroughly purified.
For example, in the production of sulfuric acid by the contact method, a solid catalyst is used - vanadium (V) oxide V 2 O 5:
In the production of methanol, a solid "zinc-chromium" catalyst is used (8ZnO Cr 2 O 3 x CrO 3):
Biological catalysts - enzymes - work very effectively. By chemical nature, these are proteins. Thanks to them, complex chemical reactions proceed at a high speed in living organisms at low temperatures.
Other interesting substances are known - inhibitors (from the Latin inhibere - to delay). They react with active particles at a high rate to form inactive compounds. As a result, the reaction slows down sharply and then stops. Inhibitors are often specifically added to various substances in order to prevent unwanted processes.
For example, hydrogen peroxide solutions are stabilized with inhibitors.
The nature of the reactants (their composition, structure)
Meaning activation energy is the factor through which the influence of the nature of the reacting substances on the reaction rate is affected.
If the activation energy is low (< 40 кДж/моль), то это означает, что значительная часть столкновений между частицами реагирующих веществ приводит к их взаимодействию, и скорость такой реакции очень большая. Все реакции ионного обмена протекают практически мгновенно, ибо в этих реакциях участвуют разноименно заряженные ионы, и энергия активации в данных случаях ничтожно мала.
If the activation energy is high(> 120 kJ/mol), this means that only a negligible part of the collisions between interacting particles leads to a reaction. The rate of such a reaction is therefore very slow. For example, the progress of the ammonia synthesis reaction at ordinary temperature is almost impossible to notice.
If the activation energies of chemical reactions have intermediate values (40120 kJ/mol), then the rates of such reactions will be average. Such reactions include the interaction of sodium with water or ethyl alcohol, the decolorization of bromine water with ethylene, the interaction of zinc with hydrochloric acid, etc.
Contact surface of reactants
The rate of reactions occurring on the surface of substances, i.e., heterogeneous, depends, other things being equal, on the properties of this surface. It is known that powdered chalk dissolves much faster in hydrochloric acid than an equal mass piece of chalk.
The increase in the reaction rate is primarily due to increase in the contact surface of the starting substances, as well as a number of other reasons, for example, a violation of the structure of the "correct" crystal lattice. This leads to the fact that the particles on the surface of the formed microcrystals are much more reactive than the same particles on a “smooth” surface.
In industry, for carrying out heterogeneous reactions, a “fluidized bed” is used to increase the contact surface of the reactants, the supply of starting materials and the removal of products. For example, in the production of sulfuric acid with the help of a "fluidized bed", pyrite is roasted.
Reference material for passing the test:
periodic table
Solubility table
Task number 1
They lead to a decrease in the rate of the reaction of ethylene with hydrogen.
1) lowering the temperature
3) the use of a catalyst
Answer: 14
Explanation:
1) lowering the temperature
Lowering the temperature slows down the rate of any reaction, whether exothermic or endothermic.
2) increase in ethylene concentration
Increasing the concentration of reactants always increases the rate of the reaction
3) the use of a catalyst
All hydrogenation reactions of organic compounds are catalytic; significantly accelerated in the presence of catalysts.
4) decrease in hydrogen concentration
Decreasing the concentration of the initial reagents always reduces the reaction rate
5) pressure increase in the system
Increasing the pressure when at least one of the reactants is a gas increases the rate of the reaction, since in fact, this is the same as increasing the concentration of this reagent.
Task number 2
Methanol with propionic acid.
1) temperature rise
2) pressure drop
3) lowering the temperature
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 14
Explanation:
1) temperature rise
As the temperature rises, the rate of any reaction increases (both exothermic and endothermic)
2) pressure drop
It does not affect the reaction rate in any way, tk. the initial reagents - methanol and propionic acid, are liquids, and pressure affects the rate of only those reactions in which at least one reagent is a gas
3) lowering the temperature
Lowering the temperature reduces the rate of any reaction (both exothermic and endothermic).
4) the use of strong inorganic acid as a catalyst
The interaction of alcohols with carboxylic acids (esterification reaction) is accelerated in the presence of strong mineral (inorganic) acids
5) irradiation with ultraviolet light
The esterification reaction proceeds according to the ionic mechanism, and ultraviolet light affects only some reactions proceeding according to the free radical mechanism, for example, methane chlorination.
Task number 3
Forward reaction rate
N 2 + 3H 2 ↔ 2NH 3 + Q
increases with:
1) increasing the concentration of nitrogen
2) decrease in nitrogen concentration
3) increasing the concentration of ammonia
4) a decrease in the concentration of ammonia
5) rise in temperature
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 15
Task number 4
From the proposed list of external influences, select two influences from which does not depend speed reaction
2C (tv) + CO 2 (g) → 2CO (g)
1) the degree of grinding of coal
2) temperature
3) the amount of coal
4) CO concentration
5) CO 2 concentration
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 34
Task number 5
From the proposed list of external influences, select two influences under which the reaction rate
2CaO (tv) + 3С (tv) → 2CaC 2 (tv) + CO 2 (g)
increases.
1) increasing the concentration of CO 2
2) lowering the temperature
3) pressure increase
4) rise in temperature
5) the degree of grinding CaO
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 45
Task number 6
From the proposed list of external influences, select two influences that do not provide influence on the reaction rate
HCOOCH 3 (l) + H 2 O (l) → HCOOH (l) + CH 3 OH (l).
1) change in the concentration of HCOOCH 3
2) the use of a catalyst
3) pressure increase
4) rise in temperature
5) change in HCOOH concentration
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 35
Task number 7
From the proposed list of external influences, select two influences that lead to an increase in the reaction rate
S (tv) + O 2 (g) → SO 2 (g) .
1) increase in the concentration of sulfur dioxide
2) temperature increase
3) decrease in oxygen concentration
4) lowering the temperature
5) increase in oxygen concentration
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 25
Task number 8
From the proposed list of external influences, select two influences that do not affect on the reaction rate
Na 2 SO 3 (solution) + 3HCl (solution) → 2NaCl (solution) + SO 2 + H 2 O.
1) change in the concentration of hydrochloric acid
2) pressure change
3) temperature change
4) change in the concentration of sodium sulfite
5) change in the concentration of sodium chloride
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 25
Task number 9
From the proposed list of substances, select two pairs each, the reaction between which proceeds at the highest rate at room temperature.
1) zinc and sulfur
2) solutions of sodium carbonate and potassium chloride
3) potassium and dilute sulfuric acid
4) magnesium and hydrochloric acid
5) copper and oxygen
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 34
Task number 10
From the proposed list of external influences, select two influences that lead to an increase in the reaction rate
CH 4 (g) + 2O 2 (g) → CO 2 (g) + H 2 O (g).
1) increase in oxygen concentration
2) lowering the temperature
3) increase in carbon dioxide concentration
4) increase in methane concentration
5) pressure reduction
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 14
Task number 11
From the proposed list of external influences, select two influences that lead to an increase in the reaction rate
2AgNO 3 (tv) → 2Ag (tv) + O 2 (g) + 2NO 2 (g).
1) lowering the pressure in the system
2) pressure increase in the system
3) temperature increase
4) the degree of grinding of silver
5) the degree of grinding of silver nitrate
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 35
Task number 12
From the proposed list of substances, select two pairs, the reaction between which proceeds at the lowest rate at room temperature.
1) copper sulfate (solution) and sodium hydroxide (solution)
2) sodium and water
3) magnesium and water
4) oxygen and zinc
5) sulfuric acid (solution) and potassium carbonate (solution)
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 34
Task number 15
From the proposed list of external influences, select two influences that lead to an increase in the reaction rate
Fe (tv) + 2H + → Fe 2+ + H 2 (g).
1) an increase in the concentration of iron ions
2) metal iron grinding
3) adding a few pieces of iron
4) increase in acid concentration
5) temperature decrease
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 24
Task number 16
From the proposed list of substances, select two pairs, the reaction rate between which does not depend from an increase in the surface area of contact of the reagents.
1) sulfur and iron
2) silicon and oxygen
3) hydrogen and oxygen
4) sulfur dioxide and oxygen
5) zinc and hydrochloric acid
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 34
Task number 17
From the proposed list of external influences, select two influences that lead to an increase in the rate of the reaction of nitrogen with hydrogen.
1) temperature increase
2) use of an inhibitor
3) the use of a catalyst
4) decrease in ammonia concentration
5) decrease in hydrogen concentration
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 13
Task number 18
From the proposed list of external influences, select two influences that do not lead to a change in the reaction rate
CH 3 COOC 2 H 5 + OH - → CH 3 COO - + C 2 H 5 OH.
1) temperature change
2) change in alcohol concentration
3) change in alkali concentration
4) change in salt concentration
5) change in ether concentration
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 24
Task #19
From the proposed list of external influences, select two influences in which the rate of the ester hydrolysis reaction will increase significantly.
1) temperature increase
2) adding alkali
3) decrease in alcohol concentration
4) decrease in the concentration of ether
5) pressure increase
Write down in the "ANSWER" field the numbers of the selected types of reactions.
Answer: 12
Task number 20
From the proposed list of external influences, select two influences that lead to a change in the reaction rate between copper and nitric acid.
Chemical reactions proceed at different rates. Some of them completely end in small fractions of a second, others in minutes, hours, days. In addition, the same reaction can proceed rapidly under certain conditions, for example, at elevated temperatures, and slowly under others, for example, upon cooling; in this case, the difference in the rate of the same reaction can be very large.
When considering the rate of a reaction, it is necessary to distinguish between reactions occurring in homogeneous system and reactions taking place in heterogeneous system.
A phase is a part of a system separated from its other parts by an interface .
A homogeneous system is called a system consisting of one phase (if the reaction proceeds in a homogeneous system, then it takes place in the entire volume of this system):
H 2 + Cl 2 \u003d 2HCl
Heterogeneous - a system consisting of several phases (if a reaction occurs between substances that form a heterogeneous system, then it can only take place on the interface of the phases that form the system):
Fe + 2HCl \u003d FeCl 2 + H 2
The reaction takes place only on the surface of the metal, because only here both reactants come into contact with each other. In this regard, the rate of a homogeneous reaction and the rate of a heterogeneous reaction are determined differently.
Any gaseous system, for example, a mixture of nitrogen and oxygen, can serve as an example of a homogeneous system. Another example of a homogeneous system is a solution of several substances in one solvent, for example, a solution of sodium chloride, magnesium sulfate, nitrogen and oxygen in water. Examples of heterogeneous systems include the following systems: water with ice, saturated solution with sediment, coal and sulfur in air. In the latter case, the system consists of three phases: two solid and one gas.
The rate of a homogeneous reaction is the ratio of the change in the molar concentration of reactants or reaction products to a unit of time:
V=∆C⁄∆t=∆n⁄(V∙∆t)
n is the amount of substance.
The rate of a heterogeneous reaction is the change in the amount of a substance entering into a reaction or formed during a reaction per unit time per unit area of the phase surface:
V=∆n⁄(S∙∆t)
The most important factors that affect the rate of a reaction are:
1. the nature of the reactants;
2. their concentration;
3. temperature;
4. the presence of catalysts in the system;
5. the rate of some heterogeneous reactions also depends on the intensity of movement of the liquid or gas near the surface on which the reaction occurs, the area of contact.
Let's start with the simplest and most important:
Dependence of the reaction rate on the concentrations of reactants.
A necessary condition for a chemical interaction to occur between the particles of the initial substances is their collision with each other. That is, the particles must come close to each other so that the atoms of one of them would experience the action of electric fields created by the atoms of the other. Therefore, the reaction rate is proportional to the number of collisions that the molecules of the reactants undergo.
The number of collisions, in turn, is the greater, the higher the concentration of each of the starting substances or the greater the product of the concentrations of the reacting substances. So the reaction rate is:
is proportional to the product of the concentration of substance A and the concentration of substance B. Denoting the concentrations of substances A and B, respectively, by [A] and [B], we can write^
v =k∙[A]∙ [V]
k - coefficient of proportionality - the rate constant of this reaction (determined experimentally).
The resulting relation expresses the law mass action for a chemical reaction that occurs when two particles collide: at constant temperature, the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants. (K. Guldberg and P. Waage in 1867 G).
It is logical to assume that if 3 particles participate in the reaction (The probability of a simultaneous collision of more than three particles is extremely small, equations containing more than 3 particles are chain reactions, each of which occurs separately and has its own speed), then the law of mass action is written accordingly:
v \u003d k ∙ [A] 2 ∙ [V]
v \u003d k ∙ [A] ∙ [B] ∙ [N]
As can be seen, in this case, the concentration of each of the reactants is included in the reaction rate expression to a degree equal to the corresponding coefficient in the reaction equation.
The value of the rate constant k depends on the nature of the reactants, on the temperature and on the presence of catalysts, but does not depend on the concentrations of the substances.
In homogeneous reactions:
v =k∙3∙
In a heterogeneous reaction, the reaction rate equation includes the concentration only gaseous matter :
2Na (solid) + H 2 (gas) → 2NaH (solid)
In a state of equilibrium, when the rate of the forward reaction is equal to the rate of the reverse reaction, the relation is fulfilled:
aA + bB+… = zZ+dD+…
K=([A] a ∙ [B] b ...) ⁄ ([D] d ∙ [Z] z …)
To express the state of equilibrium in reactions between gaseous substances, their partial pressures are often used:
N 2 (gas) + 3H 2 (gas) → 2NH 3 (gas)
It is interesting:
Dependence of the equilibrium constant on temperature and pressure. As mentioned in an article about thermodynamics, the equilibrium constant is related to the Gibbs energy by the equation:
Or
It can be seen from this equation that the equilibrium constant is very sensitive to an increase/decrease in temperature, and almost insensitive to a change in pressure. The dependence of the equilibrium constant on the entropy and enthalpy factors shows its dependence on the nature of the reagents.
Dependence of the equilibrium constant on the nature of the reagents.
This dependence can be demonstrated by a simple experiment:
Zn + 2HCl \u003d ZnCl 2 + H 2
Sn + 2HCl \u003d SnCl 2 + H 2
Hydrogen is released more intensively in the 1st reaction, since Zn is a more active metal than Sn.
Zn + H 2 SO 4 \u003d ZnSO 4 + H 2
Zn + 2CH 3 COOH \u003d Zn (CH 3 COO) 2 + H 2
Hydrogen is released more intensively in the 1st reaction, since H 2 SO 4 is a stronger acid than CH 3 COOH.
Conclusion: the more active the substance, the more actively it reacts. In the case of acids, activity is their strength (the ability to donate a proton), in the case of metals, a place in the voltage series.
The dependence of the rate of heterogeneous reactions on the intensity of movement of a liquid or gas near the surface on which the reaction occurs, the contact area.
This dependence is also demonstrated experimentally. Here the dependence on the contact area will be shown; the dependence on the velocity of the gas or liquid at the interface is subject to logic.
4Al (solid) +3O 2 →2Al 2 O 3
4Al (crushed) + 3O 2 → 2Al 2 O 3
Al (crushed) reacts more intensively with oxygen (a column of flame, if you want to repeat, throw a little silver into the fire, but very carefully, observing all safety measures) than Al (solid), it does not even light up.
Conclusion: the degree of grinding affects the reaction rate: the finer the substance, the greater the contact area of the reactants, the higher the rate of heterogeneous reactions.
The dependence of the reaction rate on temperature.
The molecular-kinetic theory of gases and liquids makes it possible to calculate the number of collisions between the molecules of certain substances under certain conditions. If we use the results of such calculations, it turns out that the number of collisions between the molecules of substances under normal conditions is so large that all reactions should proceed almost instantly. However, in reality, not all reactions end quickly. This contradiction can be explained if we assume that not every collision of the molecules of the reacting substances leads to the formation of a reaction product. In order for a reaction to occur, i.e., for the formation of new molecules, it is first necessary to break or weaken the bonds between the atoms in the molecules of the starting substances. It takes a certain amount of energy to do this. If the colliding molecules do not have this energy, then the collision will be inefficient - it will not lead to the formation of a new molecule. If the kinetic energy of the colliding molecules is sufficient to weaken or break the bonds, then the collision can lead to the rearrangement of atoms and the formation of a molecule of a new substance.
The energy that molecules must have in order for their collision to lead to the formation of a new substance is called the activation energy of this reaction.
As the temperature rises, the number of active molecules increases. It follows that the rate of a chemical reaction must also increase with increasing temperature.
This dependence is expressed by the van't Hoff rule: With an increase in temperature for every 10 ℃ the reaction rate increases by 2-4 times:
V 2 is the final reaction rate; V 1 is the initial reaction rate; γ (∆t ℃)⁄10 is a temperature coefficient showing how many times the speed will increase when the temperature rises by 10℃ (coefficient degree).
It is interesting:
As mentioned above, in order for collisions of molecules to be useful, they must have an activation energy. The activation energy of different reactions is different. Its value is the factor through which the influence of the nature of the reacting substances on the reaction rate is affected. For some reactions, the activation energy is small, for others, on the contrary, it is large.
If the activation energy is very low (less than 40 kJ/mol), then this means that a significant part of the collisions between the particles of the reactants leads to the reaction. The speed of such a reaction is great. If the activation energy of the reaction is very high (more than 120 kJ/mol), then this means that only a very small part of the collisions of the interacting particles leads to the occurrence of a chemical reaction. The rate of such a reaction is very slow. If the activation energy of the reaction is not very small and not very large (40-120 kJ / mol), then such a reaction will not proceed very quickly and not very slowly. The rate of such a reaction can be measured.
Reactions that require a noticeable activation energy for their course begin with a break or weakening of bonds between atoms in the molecules of the starting substances. In this case, the substances pass into an unstable intermediate state, characterized by a large amount of energy. This state is called an activated complex. It is for its formation that the activation energy is needed. The unstable activated complex exists for a very short time. It decomposes to form reaction products. In the simplest case, an activated complex is a configuration of atoms in which old bonds are weakened. Consider the reaction:
Where at the beginning are the initial reagents, then the activated complex, then the reaction products.
This energy required for the transition of substances into an activated complex is called the Gibbs energy of activation. It is related to the entropy and enthalpy of activation by the equation:
The energy required to transfer substances to the state of an activated complex is called the enthalpy of activation. H≠ But just as important is the entropy of activation, it depends on the number and orientation of molecules at the moment of collision.
There are favorable orientations ("a") and unfavorable ones ("b" and "c").
The energy levels in the reacting system are shown in the diagram below. It can be seen from it that only those molecules that have the necessary Gibbs energy of activation enter into the interaction; the highest point is the state when the molecules are so close together and their structures are distorted that the formation of reaction products is possible:
Thus, the Gibbs energy of activation is an energy barrier that separates reactants from products. Spent on the activation of molecules then released as heat.
Dependence on the presence of a catalyst in the system.Catalysis.
Substances that are not consumed as a result of the reaction, but affect its rate, are called catalysts.
The phenomenon of changing the rate of a reaction under the action of such substances is called catalysis. Reactions that take place under the action of catalysts are called catalytic.
In most cases, the effect of a catalyst is explained by the fact that it reduces the activation energy of the reaction. In the presence of a catalyst, the reaction proceeds through different intermediate stages than without it, and these stages are energetically more accessible. In other words, in the presence of a catalyst, other activated complexes arise, and their formation requires less energy than the formation of activated complexes that arise without a catalyst. Thus, the activation energy of the reaction is lowered; some molecules, whose energy was insufficient for active collisions, now turn out to be active.
Distinguish between homogeneous and heterogeneous catalysis.
In the case of homogeneous catalysis, the catalyst and reactants form one phase (gas or solution).
In the case of heterogeneous catalysis, the catalyst is present in the system as an independent phase. In heterogeneous catalysis, the reaction proceeds on the surface of the catalyst; therefore, the activity of the catalyst depends on the size and properties of its surface. In order to have a large (“developed”) surface, the catalyst must have a porous structure or be in a highly crushed (highly dispersed) state. In practical application, the catalyst is usually applied to a carrier having a porous structure (pumice, asbestos, etc.).
Catalysts are widely used in the chemical industry. Under the influence of catalysts, reactions can be accelerated millions of times or more. In some cases, under the action of catalysts, such reactions can be excited that practically do not proceed without them under given conditions.
It is interesting:
As already mentioned, a change in the reaction rate in the presence of a catalyst occurs due to a decrease in the activation energy of its individual stages. Let's look at this in more detail:
(A…B)-activated complex.
Let this reaction have a high activation energy and proceed at a very low rate. Let there be substance K (catalyst), which easily interacts with A and forming AK :
(A…K)-activated complex.
AK easily interacts with B to form AB:
AK+B=(AK…B)=AB+K
(AK…B)-activated complex.
AK+B=(AK…B)=AB+K
Summing these equations, we get:
All of the above is shown in the graph:
It is interesting:
Sometimes the role of catalysts is played by free radicals, due to which the reaction proceeds according to a chain mechanism (explanation below). For example reaction:
But if water vapor is introduced into the system, then free radicals are formed. ∙OH and H∙.
∙OH+CO=CO 2 +H∙
H∙+O 2 =∙OH+∙O
CO+∙O=CO2
Thus, the reaction proceeds much faster.
Chain reactions. Chain reactions proceed with the participation of active centers - atoms, ions or radicals (fragments of molecules) that have unpaired electrons and, as a result, exhibit very high reactivity.
During the acts of interaction of active centers with molecules of the initial substances, molecules of the reaction product are formed, as well as new active particles - new active centers capable of an act of interaction. Thus, active centers serve as creators of chains of successive transformations of substances.
An example of a chain reaction is the synthesis of hydrogen chloride:
H2(gas)+ Cl2(gas)=2HCl
This reaction is caused by the action of light. Absorption of a quantum of radiant energy λυ chlorine molecule leads to its excitation. If the vibrational energy exceeds the binding energy between atoms, then the molecule breaks up:
Cl 2 +λυ=2Cl∙
The resulting chlorine atoms easily react with hydrogen molecules:
Cl∙+H 2 =HCl+H∙
The hydrogen atom, in turn, easily reacts with the chlorine molecule:
H∙+Cl 2 =HCl+Cl∙
This sequence of processes continues on. In other words, one absorbed light quantum leads to the formation of many HCI molecules. The chain can end when the particles collide with the walls of the vessel, as well as when two active particles and one inactive one collide, as a result of which the active particles combine into a molecule, and the released energy is carried away by the inactive particle. In such cases, the circuit breaks:
Cl∙+Cl∙=Cl 2
Cl∙+Cl∙+Z=Cl 2 +Z∙
Where Z is the third particle.
This is the mechanism of a chain reaction to a straight chain reaction: with each elementary interaction, one active center forms, in addition to the molecule of the reaction product, one new active center.
Branched chain reactions include, for example, the reaction of the formation of water from simple substances. The following mechanism of this reaction was experimentally established and confirmed by calculations:
H 2 +O 2 \u003d 2 ∙OH
∙OH+H 2 = H 2 O+H∙
H ∙ + O 2 \u003d ∙ OH + O ∙ ∙
O ∙ ∙ +H 2 =∙OH+H∙
Such important chemical reactions as combustion, explosions, hydrocarbon oxidation processes (obtaining alcohols, aldehydes, ketones, organic acids) and polymerization reactions proceed through the chain mechanism. Therefore, the theory of chain reactions serves as the scientific basis for a number of important branches of engineering and chemical technology.
Chain processes also include nuclear chain reactions occurring, for example, in nuclear reactors or during the explosion of an atomic bomb. Here, the role of an active particle is played by a neutron, whose penetration into the nucleus of an atom can lead to its decay, accompanied by the release of high energy and the formation of new free neutrons that continue the chain of nuclear transformations.
It is interesting:
Reaction rate in heterogeneous systems. Heterogeneous reactions are of great importance in technology.
Considering heterogeneous reactions, it is easy to see that they are closely related to the processes of matter transfer. Indeed, in order for a reaction, for example, the combustion of coal to proceed, it is necessary that the carbon dioxide formed during this reaction be constantly removed from the surface of the coal, and new quantities of oxygen would approach it. Both processes (withdrawal CO2 from the surface of the coal and supply O2 to it) are carried out by convection (displacement of a mass of gas or liquid) and diffusion.
Thus, in the course of a heterogeneous reaction, at least three stages can be distinguished:
1. Supply of the reactant to the surface;
2. Chemical reaction on the surface;
3. Removal of the reaction product from the surface.
In the steady state of the reaction, all three stages of it proceed at equal rates. Moreover, in many cases, the activation energy of the reaction is low, and the second stage (the actual chemical reaction) could proceed very quickly if the supply of the reactant to the surface and the removal of the product from it would also occur quickly enough. Therefore, the rate of such reactions is determined by the rate of substance transfer. It can be expected that with an increase in convection, their speed will increase. Experience confirms this assumption. So, the reaction of burning coal:
C + O 2 \u003d CO 2
the chemical stage of which requires a small activation energy, proceeds the faster, the more intensively oxygen (or air) is supplied to the coal.
However, not in all cases the rate of a heterogeneous reaction is determined by the rate of substance transfer. The determining stage of reactions, the activation energy of which is high, is the second stage - the actual chemical reaction. Naturally, the rate of such reactions will not increase with increased stirring. For example, the oxidation reaction of iron with oxygen from moist air does not accelerate with an increase in the air supply to the metal surface, since here the activation energy of the chemical stage of the process is quite high.
The step that determines the rate of the reaction is called the rate-limiting step. In the first example, the rate-limiting step is the transfer of matter, in the second, the actual chemical reaction.
irreversible and reversible reactions. chemical balance. Shift in chemical equilibrium. Le Chatelier's principle.
All chemical reactions can be divided into two groups: irreversible and reversible reactions. Irreversible reactions proceed to the end - until one of the reactants is completely consumed. Reversible reactions do not proceed to the end: in a reversible reaction, none of the reactants is completely consumed. This difference is due to the fact that an irreversible reaction can only proceed in one direction. A reversible reaction can proceed both in the forward and reverse directions.
Consider two examples:
1) The interaction between zinc and concentrated nitric acid proceeds:
Zn + 4HNO 3 → Zn (NO 3) 2 + NO 2 + 2H 2 O
With a sufficient amount of nitric acid, the reaction will end only when all the zinc has dissolved. In addition, if you try to carry out this reaction in the opposite direction - to pass nitrogen dioxide through a solution of zinc nitrate, then metallic zinc and nitric acid will not work - this reaction cannot proceed in the opposite direction. Thus, the interaction of zinc with nitric acid is an irreversible reaction.
2) The synthesis of ammonia proceeds according to the equation:
3H 2 +N 2 ↔2NH 3
If one mol of nitrogen is mixed with three mols of hydrogen, conditions favorable for the reaction to occur in the system, and after a sufficient time the gas mixture is analyzed, the analysis results will show that not only the reaction product (ammonia) will be present in the system, but also the initial substances (nitrogen and hydrogen). If now, under the same conditions, not a nitrogen-hydrogen mixture, but ammonia, is placed as the starting substance, then it will be possible to find that part of the ammonia decomposes into nitrogen and hydrogen, and the final ratio between the amounts of all three substances will be the same as in that case when starting from a mixture of nitrogen and hydrogen. Thus, the synthesis of ammonia is a reversible reaction.
In the equations of reversible reactions, arrows can be used instead of the equal sign; they symbolize the flow of the reaction in both forward and reverse directions.
In reversible reactions, reaction products simultaneously appear, and their concentration increases, but as a result, the reverse reaction begins to occur, and its rate gradually increases. When the rates of the forward and reverse reactions become the same, chemical equilibrium. So, in the last example, an equilibrium is established between nitrogen, hydrogen and ammonia.
Chemical equilibrium is called dynamic equilibrium. This emphasizes that at equilibrium, both forward and reverse reactions occur, but their rates are the same, as a result of which changes in the system are not noticeable.
A quantitative characteristic of chemical equilibrium is a quantity called the constant of chemical equilibrium. Let's take a look at the reaction as an example:
The system is in equilibrium:
Hence:
The equilibrium constant of this reaction.
At a constant temperature, the equilibrium constant of a reversible reaction is a constant value showing the ratio between the concentrations of the reaction products (numerator) and starting materials (denominator), which is established at equilibrium.
The equilibrium constant equation shows that under equilibrium conditions, the concentrations of all substances participating in the reaction are interconnected. A change in the concentration of any of these substances entails a change in the concentrations of all other substances; as a result, new concentrations are established, but the ratio between them again corresponds to the equilibrium constant.
To express the equilibrium constant of heterogeneous reactions, as well as the expression of the law of mass action, the concentrations of only those substances that are in the gas phase are included. For example, for a reaction:
the equilibrium constant has the form:
The value of the equilibrium constant depends on the nature of the reactants and on the temperature. It does not depend on the presence of catalysts. As already mentioned, the equilibrium constant is equal to the ratio of the rate constants of the forward and reverse reactions. Since the catalyst changes the activation energy of both the forward and reverse reactions by the same amount, it does not affect the ratio of their rate constants. Therefore, the catalyst does not affect the value of the equilibrium constant and, therefore, can neither increase nor decrease the yield of the reaction. It can only speed up or slow down the onset of equilibrium. This can be seen on the chart:
Shift in chemical equilibrium. Le Chatelier's principle. If the system is in a state of equilibrium, then it will remain in it as long as the external conditions remain constant. If the conditions change, then the system will go out of balance - the rates of the direct and reverse processes will change unequally - the reaction will proceed. Of greatest importance are cases of imbalance due to changes in the concentration of any of the substances involved in the equilibrium, pressure or temperature.
Le Chatelier's principle:
If any impact is exerted on a system in equilibrium, then as a result of the processes occurring in it, the equilibrium will shift in such a direction that the impact will decrease.
Indeed, when one of the substances ( is affected by an increase/decrease in the concentration of only a gaseous substance) involved in the reaction, the equilibrium shifts towards the consumption of this substance. When the pressure rises, it shifts so that the pressure in the system decreases; as the temperature rises, the equilibrium shifts towards an endothermic reaction - the temperature in the system drops (more on that below).
Le Chatelier's principle applies not only to chemical, but also to various physico-chemical equilibria. Equilibrium shift when changing the conditions of such processes as boiling, crystallization, dissolution occurs in accordance with the Le Chatelier principle.
1. An imbalance due to a change in the concentration of any of the substances involved in the reaction.
Let hydrogen, hydrogen iodide and iodine vapor be in equilibrium with each other at a certain temperature and pressure. Let us introduce an additional amount of hydrogen into the system. According to the law of mass action, an increase in hydrogen concentration will entail an increase in the rate of the forward reaction - the synthesis of HI, while the rate of the reverse reaction will not change. In the forward direction, the reaction will now proceed faster than in the reverse. As a result, the concentrations of hydrogen and iodine vapor will decrease, which will entail a slowdown in the forward reaction, and the concentration of HI will increase, which will cause an acceleration of the reverse reaction. After some time, the rates of the forward and reverse reactions will again become equal - a new equilibrium will be established. However, the HI concentration will now be higher than it was before the addition of H 2 , and the H 2 concentration will be lower.
The process of changing concentrations caused by imbalance is called displacement or equilibrium shift.
If in this case there is an increase in the concentrations of substances on the right side of the equation, then they say that the equilibrium shifts to the right, i.e., in the direction of the flow of the direct reaction; with a reverse change in concentrations, they speak of a shift of equilibrium to the left - in the direction of the reverse reaction. In this example, the equilibrium has shifted to the right. At the same time, the substance (H 2), the increase in the concentration of which caused an imbalance, entered into a reaction - its concentration decreased.
Thus, with an increase in the concentration of any of the substances participating in the equilibrium, the equilibrium shifts towards the consumption of this substance; when the concentration of any of the substances decreases, the equilibrium shifts towards the formation of this substance.
2. An imbalance due to a change in pressure (by reducing or increasing the volume of the system).
When gases are involved in the reaction, the equilibrium can be disturbed by a change in the volume of the system. With an increase in pressure by compressing the system, the equilibrium shifts towards a decrease in the volume of gases, i.e., towards a decrease in pressure; with a decrease in pressure, the equilibrium shifts towards an increase in volume, i.e., towards an increase in pressure:
3H 2 +N 2 ↔2NH 3
With increasing pressure, the reaction will shift towards the formation of ammonia; when the pressure decreases, towards the reagents.
3. Disequilibrium due to temperature change.
The equilibrium of the vast majority of chemical reactions shifts with temperature. The factor that determines the direction of the equilibrium shift is the sign of the thermal effect of the reaction. It can be shown that when the temperature rises, the equilibrium shifts in the direction of the endothermic reaction, and when it decreases, it shifts in the direction of the exothermic reaction:
This means that with an increase in temperature, the yield of hydrogen iodine will increase, with a decrease, the equilibrium will shift towards the reactants.
Physical methods for stimulating chemical transformations.
The reactivity of substances is influenced by: light, ionizing radiation, pressure, mechanical action, radiolysis, photolysis, laser photochemistry, etc. Their essence is to create in various ways superequilibrium concentrations of excited or charged particles and radicals, the reactions of which with other particles lead to certain chemical transformations.