Relative atomic and molecular masses. Basic concepts and laws of chemistry
In the process of developing science, chemistry faced the problem of calculating the amount of a substance for carrying out reactions and the substances obtained in their course.
Today, for such calculations of the chemical reaction between substances and mixtures, the value of the relative atomic mass entered in the periodic table is used chemical elements D. I. Mendeleev.
Chemical processes and the influence of the proportion of an element in substances on the course of a reaction
Modern science under the definition of "relative atomic mass chemical element" means how many times the mass of an atom of a given chemical element is greater than one twelfth of a carbon atom.
With the advent of the era of chemistry, the need for precise definitions the course of a chemical reaction and its results grew.
Therefore, chemists constantly tried to solve the problem of the exact masses of interacting elements in matter. One of best solutions at that time there was a binding to the lightest element. And the weight of its atom was taken as one.
The historical course of counting the substance
Initially, hydrogen was used, then oxygen. But this method of calculation turned out to be inaccurate. The reason for this was the presence of isotopes with a mass of 17 and 18 in oxygen.
Therefore, having a mixture of isotopes technically gave a number other than sixteen. Today, the relative atomic mass of an element is calculated based on the weight of the carbon atom taken as the basis, in the ratio 1/12.
Dalton laid the foundations for the relative atomic mass of an element
Only some time later, in the 19th century, Dalton proposed to calculate using the lightest chemical element - hydrogen. At lectures to his students, he demonstrated on figures carved from wood how atoms are connected. For other elements, he used data previously obtained by other scientists.
According to Lavoisier's experiments, water contains fifteen percent hydrogen and eighty-five percent oxygen. With these data, Dalton calculated that the relative atomic mass of the element that makes up water, in this case oxygen, is 5.67. The erroneousness of his calculations is due to the fact that he believed incorrectly regarding the number of hydrogen atoms in a water molecule.
In his opinion, there was one hydrogen atom per oxygen atom. Using the chemist Austin's data that ammonia contains 20 percent hydrogen and 80 percent nitrogen, he calculated what the relative atomic mass of nitrogen is. With this result, he came to an interesting conclusion. It turned out that the relative atomic mass (the ammonia formula was erroneously taken with one molecule of hydrogen and nitrogen) is four. In his calculations, the scientist relied on the periodic system of Mendeleev. From analysis, he calculated that the relative atomic mass of carbon was 4.4, instead of the previously accepted twelve.
Despite his serious blunders, it was Dalton who first created a table of some elements. It has undergone numerous changes during the lifetime of the scientist.
The isotopic component of a substance affects the relative atomic weight accuracy value
When considering the atomic masses of the elements, one can notice that the accuracy for each element is different. For example, for lithium it is four-digit, and for fluorine it is eight-digit.
The problem is that the isotopic component of each element is different and variable. For example, ordinary water contains three types of hydrogen isotope. In addition to ordinary hydrogen, they include deuterium and tritium.
The relative atomic masses of hydrogen isotopes are two and three, respectively. "Heavy" water (formed by deuterium and tritium) evaporates worse. Therefore, there are fewer isotopes of water in the vapor state than in the liquid state.
Selectivity of living organisms to different isotopes
Living organisms have a selective property in relation to carbon. Carbon with a relative atomic mass equal to twelve is used to build organic molecules. Therefore, substances of organic origin, as well as a number of minerals, such as coal and oil, contain less isotopic content than inorganic materials.
Microorganisms that process and accumulate sulfur leave behind the sulfur isotope 32. In areas where bacteria do not process, the proportion of the sulfur isotope is 34, that is, much higher. It is on the basis of the ratio of sulfur in the soil rocks that geologists come to the conclusion about the nature of the origin of the layer - whether it has a magmatic nature or a sedimentary one.
Of all the chemical elements, only one has no isotopes - fluorine. Therefore, its relative atomic mass is more accurate than other elements.
The existence of unstable substances in nature
For some elements, the relative mass is given in square brackets. As you can see, these are elements located after uranium. The fact is that they do not have stable isotopes and decay with the release of radioactive radiation. Therefore, the most stable isotope is indicated in brackets.
Over time, it turned out that it is possible to obtain a stable isotope from some of them under artificial conditions. I had to change the atomic masses of some transuranium elements in the periodic table of Mendeleev.
In the process of synthesizing new isotopes and measuring their lifetimes, it has sometimes been possible to find nuclides with half-lives millions of times longer.
Science does not stand still, new elements, laws, relationships of various processes in chemistry and nature are constantly being discovered. Therefore, in what form the chemistry and the periodic system of chemical elements of Mendeleev will turn out in the future, in a hundred years, is vague and uncertain. But I would like to believe that the works of chemists accumulated over the past centuries will serve a new, more perfect knowledge of our descendants.
DEFINITION
Iron is the twenty-sixth element of the Periodic Table. Designation - Fe from the Latin "ferrum". Located in the fourth period, VIIIB group. Refers to metals. The nuclear charge is 26.
Iron is the most common metal on the globe after aluminum: it makes up 4% (mass) of the earth's crust. Iron occurs in the form of various compounds: oxides, sulfides, silicates. Iron is found in the free state only in meteorites.
The most important iron ores include magnetic iron ore Fe 3 O 4, red iron ore Fe 2 O 3, brown iron ore 2Fe 2 O 3 × 3H 2 O and spar FeCO 3.
Iron is a silvery (Fig. 1) ductile metal. It lends itself well to forging, rolling and other types of machining. Mechanical properties iron is highly dependent on its purity - on the content in it of even very small amounts of other elements.
Rice. 1. Iron. Appearance.
Atomic and molecular weight of iron
Relative molecular weight of a substance(M r) is a number showing how many times the mass of a given molecule is greater than 1/12 of the mass of a carbon atom, and relative atomic mass of an element(A r) - how many times average weight atoms of a chemical element is more than 1/12 of the mass of a carbon atom.
Since iron exists in the free state in the form of monatomic Fe molecules, the values of its atomic and molecular masses are the same. They are equal to 55.847.
Allotropy and allotropic modifications of iron
Iron forms two crystalline modifications: α-iron and γ-iron. The first of them has a cubic body-centered lattice, the second - a cubic face-centered one. α-Iron is thermodynamically stable in two temperature ranges: below 912 o C and from 1394 o C to the melting point. The melting point of iron is 1539 ± 5 o C. Between 912 o C and 1394 o C, γ-iron is stable.
The temperature ranges of stability of α- and γ-iron are due to the nature of the change in the Gibbs energy of both modifications with temperature. At temperatures below 912 o C and above 1394 o C, the Gibbs energy of α-iron is less than the Gibbs energy of γ-iron, and in the range of 912 - 1394 o C - more.
Isotopes of iron
It is known that iron can occur in nature in the form of four stable isotopes 54Fe, 56Fe, 57Fe, and 57Fe. Their mass numbers are 54, 56, 57 and 58, respectively. The nucleus of an atom of the iron isotope 54 Fe contains twenty-six protons and twenty-eight neutrons, and the remaining isotopes differ from it only in the number of neutrons.
There are artificial iron isotopes with mass numbers from 45 to 72, as well as 6 isomeric states of nuclei. The most long-lived among the above isotopes is 60 Fe with a half-life of 2.6 million years.
iron ions
The electronic formula showing the distribution of iron electrons over the orbits is as follows:
1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2 .
As a result of chemical interaction, iron gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:
Fe 0 -2e → Fe 2+;
Fe 0 -3e → Fe 3+.
Molecule and atom of iron
In the free state, iron exists in the form of monatomic Fe molecules. Here are some properties that characterize the atom and molecule of iron:
iron alloys
Until the 19th century, iron alloys were mainly known for their alloys with carbon, which received the names of steel and cast iron. However, in the future, new iron-based alloys containing chromium, nickel and other elements were created. At present, iron alloys are divided into carbon steels, cast irons, alloy steels and steels with special properties.
In technology, iron alloys are usually called ferrous metals, and their production is called ferrous metallurgy.
Examples of problem solving
| Exercise | The elemental composition of the substance is as follows: the mass fraction of the iron element is 0.7241 (or 72.41%), the mass fraction of oxygen is 0.2759 (or 27.59%). Derive the chemical formula. |
| Decision | The mass fraction of the element X in the molecule of the HX composition is calculated by the following formula: ω (X) = n × Ar (X) / M (HX) × 100%. Let us denote the number of iron atoms in the molecule as "x", the number of oxygen atoms as "y". Let us find the corresponding relative atomic masses of the elements of iron and oxygen (the values of the relative atomic masses taken from the Periodic Table of D.I. Mendeleev will be rounded to integers). Ar(Fe) = 56; Ar(O) = 16. We divide the percentage of elements by the corresponding relative atomic masses. Thus, we will find the relationship between the number of atoms in the molecule of the compound: x:y= ω(Fe)/Ar(Fe) : ω(O)/Ar(O); x:y = 72.41/56: 27.59/16; x:y = 1.29: 1.84. Let's take the smallest number as one (i.e. divide all numbers by the smallest number 1.29): 1,29/1,29: 1,84/1,29; Therefore, the simplest formula for the combination of iron with oxygen is Fe 2 O 3. |
| Answer | Fe2O3 |
The physical properties of iron depend on the degree of its purity. Pure iron is a fairly ductile silvery-white metal. The density of iron is 7.87 g/cm 3 . The melting point is 1539 ° C. Unlike many other metals, iron exhibits magnetic properties.
Pure iron is fairly stable in air. In practice, iron is used containing impurities. When heated, iron is quite active against many non-metals. Consider the chemical properties of iron using the example of interaction with typical non-metals: oxygen and sulfur.
When iron is burned in oxygen, a compound of iron and oxygen is formed, which is called iron scale. The reaction is accompanied by the release of heat and light. Let's make the equation of chemical reaction:
3Fe + 2O 2 = Fe 3 O 4
When heated, iron reacts violently with sulfur to form ferrum(II) sulfide. The reaction is also accompanied by the release of heat and light. Let's make the equation of chemical reaction:
Iron is widely used in industry and everyday life. The Iron Age is an era in the development of mankind, which began at the beginning of the first millennium BC in connection with the spread of iron smelting and the manufacture of iron tools and military weapons. iron age came to replace the bronze age. Steel first appeared in India in the tenth century BC, cast iron only in the Middle Ages. Pure iron is used to make the cores of transformers and electromagnets, as well as in the production of special alloys. Most of all, iron alloys are used in practice: cast iron and steel. Cast iron is used in the production of castings and steel, steel - as structural and tool materials that are resistant to corrosion.
Under the influence of atmospheric oxygen and moisture, iron alloys turn into rust. The rust product can be described by the chemical formula Fe 2 O 3 · xH 2 O. One sixth of the cast iron smelted dies from rust, so the issue of corrosion control is very relevant. Corrosion protection methods are very diverse. The most important of them are: protection of the metal surface with a coating, creation of alloys with anticorrosive properties, electrochemical means, change in the composition of the medium. Protective coatings are divided into two groups: metallic (coating of iron with zinc, chromium, nickel, cobalt, copper) and non-metallic (varnishes, paints, plastics, rubber, cement). With the introduction of special additives into the composition of the alloys, stainless steel is obtained.
Iron. The prevalence of iron in nature
Iron. Distribution of iron in nature. Biological role gland
The second important chemical element after oxygen, the properties of which will be studied, is Ferum. Iron is a metallic element that forms a simple substance - iron. Iron is a member of the eighth group of the secondary subgroup of the Periodic Table. According to the group number, the maximum valence of iron should be eight, however, in compounds, Ferum more often exhibits valences of two and three, as well as known compounds with iron valence of six. The relative atomic mass of iron is fifty-six.
In terms of its abundance in the composition of the earth's crust, Ferum occupies the second place among metallic elements after aluminum. Mass fraction of iron in earth's crust is almost five percent. In the native state, iron is very rare, usually only in the form of meteorites. It was in this form that our ancestors were able to get to know iron for the first time and appreciate it as a very good material for making tools. It is believed that iron is the main constituent of the nucleus the globe. Ferum is more often found in nature as part of ores. The most important of them are: magnetic iron ore (magnetite) Fe 3 O 4, red iron ore (hematite) Fe 2 O 3, brown iron ore (limonite) Fe 2 O 3 nH 2 O, iron pyrite (pyrite) FeS 2, spar iron ore ( siderite) FeCO3, goethite FeO (OH). In the waters of many mineral springs contains Fe (HCO 3) 2 and some other iron salts.
Iron is a vital element. In the human body, as well as in animals, ferrum is present in all tissues, but its largest part (about three grams) is concentrated in the blood globules. Iron atoms occupy a central position in hemoglobin molecules; hemoglobin owes them its color and ability to attach and split off oxygen. Iron is involved in the process of transporting oxygen from the lungs to the tissues of the body. daily requirement body in Ferum is 15-20 mg. Its total amount enters the human body with plant foods and meat. With blood loss, the need for Ferum exceeds the amount that a person receives from food. Iron deficiency in the body can lead to a condition characterized by a decrease in the number of red blood cells and hemoglobin in the blood. Medical preparations iron should only be taken as directed by a doctor.
Chemical properties of oxygen. Connection reactions
Chemical properties of oxygen. Connection reactions. The concept of oxides, oxidation and combustion. Conditions for the onset and cessation of combustion
Oxygen reacts vigorously with many substances when heated. If you put red-hot charcoal C into a vessel with oxygen, it becomes white-hot and burns. Let's make the equation of chemical reaction:
C + ONaHCO 2 = CONaHCO 2
Sulfur S burns in oxygen with a bright blue flame to form a gaseous substance - sulfur dioxide. Let's make the equation of chemical reaction:
S + ONaHCO 2 = SONaHCO 2
Phosphorus P burns in oxygen with a bright flame to form thick white smoke, which consists of solid particles of phosphorus (V) oxide. Let's make the equation of chemical reaction:
4P + 5ONaHCO 2 = 2PNaHCO 2 ONaHCO 5
The equations for the reactions of the interaction of oxygen with coal, sulfur and phosphorus are united by the fact that one substance is formed from two starting substances in each case. Such reactions, as a result of which only one substance (product) is formed from several initial substances (reagents), are called communication reactions.
The products of the interaction of oxygen with the considered substances (coal, sulfur, phosphorus) are oxides. Oxides are complex substances containing two elements, one of which is oxygen. Almost all chemical elements form oxides, with the exception of some inert elements: helium, neon, argon, krypton and xenon. There are some chemical elements that do not combine directly with oxygen, such as Aurum.
Chemical reactions of interaction of substances with oxygen are called oxidation reactions. The concept of "oxidation" is more general than the concept of "combustion". Combustion is a chemical reaction in which the oxidation of substances occurs accompanied by the release of heat and light. For combustion to occur, the following conditions are necessary: close contact of air with a combustible substance and heating to the ignition temperature. For various substances, the ignition temperature is different meanings. For example, the ignition temperature of wood dust is 610 ° C, sulfur - 450 ° C, white phosphorus 45 - 60 ° C. In order to prevent the occurrence of combustion, it is necessary to excite at least one of the indicated conditions. That is, it is necessary to remove the combustible substance, cool it below the ignition temperature, block the access of oxygen. Combustion processes accompany us in everyday life, therefore, each person must know the conditions for the onset and cessation of combustion, and also observe necessary rules handling of flammable substances.
The oxygen cycle in nature
The oxygen cycle in nature. The use of oxygen, its biological role
Approximately a quarter of the atoms of all living matter is accounted for by oxygen. Insofar as total oxygen atoms in nature invariably, with the removal of oxygen from the air due to respiration and other processes, it must be replenished. The most important source of oxygen in inanimate nature is carbon dioxide and water. Oxygen enters the atmosphere mainly as a result of the process of photosynthesis, which involves this-o-two. An important source of oxygen is the Earth's atmosphere. Part of the oxygen is formed in upper parts atmosphere due to the dissociation of water under the action of solar radiation. Part of the oxygen is released by green plants in the process of photosynthesis with ash-two-o and this is-in-two. In turn, atmospheric it-o-two is formed as a result of the reactions of combustion and respiration of animals. Atmospheric o-two is spent on the formation of ozone in the upper atmosphere, oxidative processes weathering rocks, in the process of animal respiration and in combustion reactions. The transformation of t-two into tse-two leads to the release of energy, respectively, energy must be spent on the transformation of this-two into o-two. This energy is the Sun. Thus, life on Earth depends on cyclic chemical processes made possible by solar energy.
The use of oxygen is due to its chemical properties. Oxygen is widely used as an oxidizing agent. It is used for welding and cutting metals, in the chemical industry - to obtain various compounds and intensify some production processes. In space technology, oxygen is used to burn hydrogen and other fuels, in aviation - when flying on high altitudes, in surgery - to support patients with shortness of breath.
The biological role of oxygen is due to its ability to support respiration. A person, when breathing for one minute, consumes on average 0.5 dm3 of oxygen, during the day - 720 dm3, and during the year - 262.8 m3 of oxygen.
1. The reaction of thermal decomposition of potassium permanganate. Let's make the equation of chemical reaction:
The substance potassium-manganese-o-four is widely distributed in everyday life under the name "potassium permanganate". The oxygen that has formed is shown by a smoldering torch, which flashes brightly at the opening of the gas outlet tube of the device in which the reaction is carried out, or when introduced into a vessel with oxygen.
2. Decomposition reaction of hydrogen peroxide in the presence of manganese (IV) oxide. Let's make the equation of chemical reaction:
Hydrogen peroxide is also well known from everyday life. It can be used to treat scratches and minor wounds (an ash-two-o-two wt three percent solution should be in every first aid kit). Many chemical reactions accelerated in the presence of certain substances. In this case, the hydrogen peroxide decomposition reaction is accelerated by manganese-o-two, but manganese-o-two itself is not consumed and is not part of the reaction products. Manganese-o-two is a catalyst.
Catalysts are substances that speed up chemical reactions, but are themselves not consumed. Catalysts are not only widely used in the chemical industry, but also play an important role in human life. Natural catalysts, which are called enzymes, are involved in the regulation of biochemical processes.
Oxygen, as noted earlier, is slightly heavier than air. Therefore, it can be collected by forcing air into a vessel placed with the hole up.
They restored it with charcoal in a furnace (see), arranged in a pit; they pumped it into the furnace with bellows, the product - kritsa was separated from the slag by blows and various products were forged from it. As the blowing methods improved and the height of the hearth increased, the process increased and part of it became carburized, i.e. cast iron was obtained; this relatively fragile product was considered a waste product. Hence the name pig iron, pig iron - English pig iron. Later, it was noticed that when loading not iron, but cast iron into the furnace, low-carbon iron bloom is also obtained, and such a two-stage process (see Chrychny redistribution) turned out to be more profitable than raw blown. In the 12th-13th centuries. the screaming method was already widespread. In the 14th century cast iron began to be smelted not only as a semi-finished product for further processing, but also as a material for casting various products. The reconstruction of the hearth into a mine (“domnitsa”), and then into a blast furnace, also dates back to the same time. In the middle of the 18th century in Europe, the crucible process of obtaining steel began to be used, which was known in Syria back in early period Middle Ages, but later it was forgotten. With this method, steel was obtained by melting metal mixtures in small (crucibles) from a highly refractory mass. In the last quarter of the 18th century the puddling process of redistribution of cast iron into a flame reflective hearth began to develop (see Puddling). Industrial revolution of the 18th - early 19th centuries, invention of the steam engine, construction railways, large bridges and a steam fleet caused a huge need for and his. However, all existing methods of production could not meet the needs of the market. Mass production of steel began only in the middle of the 19th century, when the Bessemer, Thomas, and open-hearth processes were developed. In the 20th century the electric steelmaking process arose and became widespread, giving high-quality steel.
distribution in nature. In terms of content in the lithosphere (4.65% by weight), it ranks second among (on the first). It vigorously migrates in the earth's crust, forming about 300 (, etc.). accepts Active participation in magmatic, hydrothermal and supergene processes associated with the formation various types its deposits (see Iron). - Earth's depths, it accumulates in the early stages of magma, in ultrabasic (9.85%) and basic (8.56%) (in granites it is only 2.7%). B accumulates in many marine and continental sediments, forming sedimentary sediments.
The following are physical properties relating mainly to those with a total impurity content of less than 0.01% by mass:
A kind of interaction with Concentrated HNO 3 (density 1.45 g / cm 3) passivates due to the appearance of a protective oxide film on its surface; more dilute HNO 3 dissolves with the formation of Fe 2+ or Fe 3+ , recovering to MH 3 or N 2 O and N 2 .
Receipt and application. The pure is obtained in relatively small amounts of water of it or it. A method is being developed to directly obtain from. Gradually increases the production of sufficiently pure by its direct from ore concentrates, or coal at relatively low levels.
The most important modern technology. In its pure form, due to its low value, it is practically not used, although in everyday life steel or cast iron products are often called “iron”. The bulk is used in the form of very different in composition and properties. It accounts for approximately 95% of all metal products. Rich (over 2% by weight) - cast iron, smelted in blast-furnace from enriched iron (see Blast-furnace production). Steel of various grades (content less than 2% by mass) is smelted from cast iron in open-hearth and electric and converters by (burning out) excess, removal of harmful impurities (mainly S, P, O) and addition of alloying elements (see Martenovskaya, Converter). High alloy steels (with great content, and other elements) are smelted in electric arc and induction. For the production of steels and especially important purposes, new processes are used - vacuum, electroslag remelting, plasma and electron beam melting, etc. Methods are being developed for steel smelting in continuously operating units that provide high quality and process automation.
On the basis, materials are created that can withstand the effects of high and low, and high, aggressive environments, large alternating voltages, nuclear radiation, etc. Production and it is constantly growing. In 1971, 89.3 million tons of pig iron and 121 million tons of steel were smelted in the USSR.
L. A. Shvartsman, L. V. Vanyukova.
It has been used as an artistic material since antiquity in Egypt (for the head from the tomb of Tutankhamun near Thebes, mid-14th century BC, Ashmolean Museum, Oxford), Mesopotamia (daggers found near Carchemish, 500 BC, British Museum , London)
One of the main characteristics of any chemical element is its relative atomic mass.
(An atomic mass unit is 1/12 of the mass of a carbon atom, the mass of which is assumed to be 12 amu and is1,66 10 24 G.
Comparing the masses of atoms of elements with one amu, find the numerical values of the relative atomic mass (Ar).
The relative atomic mass of an element shows how many times the mass of its atom is greater than 1/12 of the mass of a carbon atom.
For example, for oxygen Ar (O) = 15.9994, and for hydrogen Ar (H) = 1.0079.
For molecules of simple and complex substances, determine relative molecular weight, which is numerically equal to the sum of the atomic masses of all the atoms that make up the molecule. For example, the molecular weight of water is H2O
Mg (H2O) = 2 1.0079 + 1 15.9994 = 18.0153.
Avogadro's Law
In chemistry, along with units of mass and volume, a unit of quantity of a substance, called a mole, is used.
!MOLE (v) - a unit of measurement of the amount of a substance containing as many structural units (molecules, atoms, ions) as there are atoms in 0.012 kg (12 g) of the carbon isotope "C''.
This means that 1 mole of any substance contains the same number of structural units, equal to 6,02 10 23 . This value is called constant Avogadro(notation NBUT, dimension 1/mol).
The Italian scientist Amadeo Avogadro put forward a hypothesis in 1811, which was later confirmed by experimental data and later received the name Avogadro's law. He drew attention to the fact that all gases are equally compressed (Boyle-Mariotte law) and have the same coefficients of thermal expansion (Gay-Lussac law). In this regard, he suggested that:
equal volumes of different gases under the same conditions contain the same number of molecules.
Under the same conditions (usually they talk about normal conditions: the absolute pressure is 1013 millibars and the temperature is 0 ° C), the distance between the molecules of all gases is the same, and the volume of the molecules is negligible. Given all of the above, we can make an assumption:
!if equal volumes of gases under the same conditions contain the same number of molecules, then the masses containing the same number of molecules must have the same volumes.
In other words,
Under the same conditions, 1 mole of any gas occupies the same volume. Under normal conditions, 1 mole of any gas occupies a volume v, equal to 22.4 liters. This volume is calledmolar volume of gas (dimension l/mol or m³ /mol).
The exact value of the molar volume of gas under normal conditions (pressure 1013 millibars and temperature 0 ° C) is 22.4135 ± 0.0006 l/mol. Under standard conditions (t=+15° C, pressure = 1013 mbar) 1 mole of gas occupies a volume of 23.6451 liters, and att\u003d + 20 ° C and a pressure of 1013 mbar, 1 mol occupies a volume of about 24.2 liters.
In numerical terms, the molar mass coincides with the masses of atoms and molecules (in amu) and with the relative atomic and molecular masses.
Therefore, 1 mole of any substance has such a mass in grams, which is numerically equal to the molecular weight of this substance, expressed in atomic mass units.
For example, M(O2) = 16 a. e.m. 2 \u003d 32 a.m. e. m., thus, 1 mol of oxygen corresponds to 32 g. The densities of gases measured under the same conditions are related as their molar masses. Since molecular substances (liquids, vapors, gases) are the main object of practical problems during the transportation of liquefied gases on gas carriers, the main sought quantities will be the molar mass M(g/mol), amount of substance v in moles and mass t substances in grams or kilograms.
Knowing the chemical formula of a particular gas, it is possible to solve some practical problems that arise during the transportation of liquefied gases.
Example 1. There are 22 tons of liquefied ethylene in the deck tank (WITH2 H4 ). It is necessary to determine whether there is enough cargo on board to purge three cargo tanks of 5000 m 3 each, if, after purge, the temperature of the tanks is 0 ° C and the pressure is 1013 millibars.
1. Determine the molecular weight of ethylene:
M \u003d 2 12.011 + 4 1.0079 \u003d 28.054 g / mol.
2. We calculate the density of ethylene vapor under normal conditions:
ρ \u003d M / V \u003d 28.054: 22.4 \u003d 1.232 g / l.
3. Find the volume of cargo vapor under normal conditions:
22∙10 6: 1.252= 27544 m 3 .
The total volume of cargo tanks is 15,000 m 3 . Consequently, there is enough cargo on board to purge all cargo tanks with ethylene vapor.
Example 2. It is necessary to determine how much propane (WITH3 H8 ) will be required to purge cargo tanks with a total capacity of 8000 m 3 if the temperature of the tanks is +15 ° C, and the propane vapor pressure in the tank after the end of the purge will not exceed 1013 millibars.
1. Determine the molar mass of propane With3 H8
M = 3 12,011 + 8 1,0079 = 44.1 g/mol.
2. Determine the density of propane vapor after purging the tanks:
ρ \u003d M: v \u003d 44.1: 23.641 \u003d 1.865 kg / m 3.
3. Knowing the vapor density and volume, we determine the total amount of propane required to purge the tank:
m \u003d ρ v \u003d 1.865 8000 \u003d 14920 kg ≈ 15 tons.
Atomic-molecular doctrine defines the atom as the smallest chemically indivisible particle. And if it is a particle, then it must have a mass, which is very small. Modern methods studies allow us to determine this value with great accuracy.
Example: m(H) = 1.674 10 -27 kg
m(O) = 2.667 10 -26 kg Absolute masses
m (C) = 1.993 10 -26 kg
The presented values are very inconvenient for calculations. Therefore, in chemistry, not absolute, but relative atomic masses are often used. Relative atomic mass (Ar) is the ratio of the absolute mass of an atom to 1/12 of the mass of a carbon atom. Using a formula, this can be written as 
1/12m(c) is the comparison value and is called 1 amu.
1a.u.m. \u003d 1/12 1.993 10 -26 kg \u003d 1.661 10-27 kg
Let's calculate Ar for some elements.
Ar(O) = = = 15.99 ~ 16
Ar(H) = = = 1.0079 ~ 1
Comparing the relative atomic masses of oxygen and hydrogen with the absolute ones, the advantages of Ar are clearly seen. The Ar values are much simpler. They are more convenient to use in calculations. The finished values of Ar are given in the periodic table. Using the Ar elements, one can compare their masses.
This calculation shows that the zinc atom weighs 2.1 times more than the phosphorus atom.
Relative molecular weight (Mr) is equal to the sum of the relative atomic masses of its constituent atoms (dimensionless). Calculate the relative molecular weight of water. You know that a water molecule contains two hydrogen atoms and one oxygen atom. Then its relative molecular mass will be equal to the sum of the products of the relative atomic mass of each chemical element and the number of its atoms in a water molecule:
calculate the relative molecular weights of the substances.
Mr(Cu2O)= 143,0914
Mr(Na3PO4)= 163,9407
Mr(AlCl3)= 133,3405
Mr(Ba3N2)= 439,9944
Mr (KNO 3)= 101,1032
Mr(Fe(OH)2)= 89,8597
Mr (Mg (NO 3) 2) \u003d 148,3148
Mr (Al 2 (SO 4) 3) \u003d 342,1509
The amount of substance (n) is a physical quantity that characterizes the number of structural units of the same type contained in a substance. Structural units are any particles that make up a substance (atoms, molecules, ions, electrons, or any other particles).
The unit for measuring the amount of a substance (n) is the mole. mole- the amount of a substance containing as many structural elementary units (molecules, atoms, ions, electrons, etc.) as there are atoms in 0.012 kg (12 g) \u003d 1 mol of the carbon isotope 12 C.
The number of N A atoms in 0.012 kg (12 g) of carbon, or 1 mole, can be easily determined as follows:
The value of N A is called the Avogadro constant.
When describing chemical reactions, the amount of a substance is a more convenient quantity than the mass, since the molecules interact regardless of their mass in quantities that are multiples of whole numbers.
For example, the hydrogen combustion reaction (2H2 + O2 → 2H2O) requires twice large quantity hydrogen substances than oxygen. The ratio between the amounts of reactants is directly reflected by the coefficients in the equations.
Example: in 1 mole of calcium chloride \u003d contains 6.022 × 10 23 molecules (formula units) - CaCl 2.
1 mole (1 M) iron = 6 . 10 23 Fe atoms
1 mol (1 M) chloride ion Cl - = 6 . 10 23 ions Cl - .
1 mol (1 M) electrons e - = 6 . 10 23 electrons e - .
To calculate the amount of a substance based on its mass, the concept of molar mass is used:
Molar mass (M) is the mass of one mole of a substance (kg / mol, g/mol). The relative molecular weight and the molar mass of a substance are numerically the same, but have different dimensions, for example, for water, M r = 18 (the relative atomic and molecular masses are dimensionless), M = 18 g/mol. The amount of a substance and the molar mass are related by a simple relationship:
The basic stoichiometric laws, which were formulated at the turn of the 17th and 18th centuries, played an important role in the formation of chemical atomistics.
1. LAW OF CONSERVATION OF MASS (M.V. Lomonosov, 1748).
The sum of the masses of the reaction products is equal to the sum of the masses of the starting substances. As an addition to this law, the law of conservation of the mass of an element (1789, A.L. Lavoisier) can serve - the mass of a chemical element does not change as a result of the reaction. These laws are of decisive importance for modern chemistry, since they allow simulating chemical reactions with equations and performing quantitative calculations on their basis.
2. THE LAW OF CONSTANT COMPOSITION (J. Proust, 1799-1804).
An individual chemical substance of a molecular structure has a constant qualitative and quantitative composition, regardless of how it was obtained. Compounds that obey the law of constant composition are called daltonides. Daltonides are all currently known organic compounds (about 30 million) and some (about 100 thousand) inorganic substances. Substances with a non-molecular structure (bertolides) do not obey this law and may have a variable composition depending on the method of sample preparation. These include the majority (about 500 thousand) of inorganic substances.
3. LAW OF EQUIVALENTS (I. Richter, J. Dalton, 1792-1804).
Each complex substance, regardless of the method of its preparation, has a constant qualitative and quantitative composition. Hence, chemical substances interact with each other in strictly defined (equivalent) ratios. The masses of reactants are directly proportional to their equivalent masses..
where E A and E B are the equivalent masses of the reactants.
4. LAW OF AVOGADRO (A. Avogadro, 1811).
Equal volumes of different gases measured under the same conditions (pressure, temperature) contain the same number of molecules. It follows from the law that:
Ø Under normal conditions (n.s., T \u003d 273 K, p \u003d 101.325 kPa), one mole of any gas occupies the same volume - molar volume(V m), equal to 22.4 l / mol.
Ø Mass ratio equal volumes different gases measured under the same conditions ( relative density of gas to gas), is equal to the ratio of their molecular (molar) masses .
Most often, the relative density is determined by hydrogen or air. Respectively,
,
where 29 is the average, more precisely weighted average, molecular weight of air.
Ø The volumes of the reacting gases are related to each other and to the volumes of the gaseous reaction products as simple integers(the Gay-Lussac law of volumetric relations).
Task
How many grams of gaseous chlorine should be spent and how many grams of liquid phosphorus (III) chloride will be obtained if 1.45 grams of phosphorus is used in the reaction?
P 4 (tv.) + Cl 2 (g.) \u003d PCl 3 (l.)
Solution: 1. It is necessary to make sure that the equation is in equilibrium, i.e. it is necessary to put down stoichiometric coefficients: P 4 (solid) + 6Cl 2 (g.) = 4PCl 3 (l.). For 1 mole P 4 I can spend 6 moles Cl 2 to get 4 moles PCl 3
2. We have the mass of P 4 in the reaction, therefore, we can find out how many moles of phosphorus are used. According to T.M. we find out the atomic mass of phosphorus ~ 31, this says that 1 mole of phosphorus will have a mass of 31 g (molar mass), and the atomic mass of P 4 will be 124 g. Let's find how many moles are in 1.45 g of phosphorus:
1.45 g - x mol x \u003d 0.0117 mol
124 g - 1 mol
3. Now we find out how many moles of chlorine need to be taken to use 0.0117 moles of phosphorus. According to the equilibrium reaction, we see that 6 moles of chlorine must be taken for 1 mole of phosphorus, therefore, chlorine must be taken 6 times more. We believe:
0.0117 x 6 = 0.07 moles of chlorine.
0.07 moles x 70.906 g (in 1 mole of Cl 2) = 4.963 g of Cl 2
5. Now let's find how many grams of liquid phosphorus (III) chloride to get. You can use two different solutions:
5.1. The law of conservation of mass 1.45 g P 4 (tv.) + 4.963 g. Cl 2 (g.) \u003d 6.413 PCl 3 (w.)
5.2. And you can use the method as we found the mass of the necessary phosphorus.
Examples:
Condition
Determine the mass fraction of water of crystallization in barium chloride dihydrate BaCl2 2H2O
Decision
The molar mass of BaCl2 2H2O is:
M (BaCl2 2H2O) \u003d 137+ 2 35.5 + 2 18 \u003d 244 g / mol
From the formula BaCl2 2H2O it follows that 1 mol of barium chloride dihydrate contains 2 mol of H2O.
We determine the mass of water contained in BaCl2 2H2O: m (H2O) \u003d 2 18 \u003d 36 g.
Find the mass fraction of water of crystallization in barium chloride dihydrate
BaCl2 2H2O. ω(H2O) = m(H2O)/ m(BaCl2 2H2O) = 36/244 = 0.1475 = 14.75%.
Example on my own
1. The chemical compound contains, by weight, 17.56% sodium, 39.69% chromium, and 42.75% oxygen. Determine the simplest compound formula. (Na 2 Cr 2 O 7).
2. The elemental composition of the substance is as follows: the mass fraction of the iron element is 0.7241 (or 72.41%), the mass fraction of oxygen is 0.2759 (or 27.59%). Derive the chemical formula. (Fe 3 O 4)
Example (parsing) . Set the molecular formula of a substance if the mass fraction of carbon in it is 26.67%, hydrogen - 2.22%, oxygen - 71.11%. The relative molecular weight of this substance is 90.
| Solution 1. To solve the problem, we use the formulas: w = ; n = ; x: y: z = n(C) : n(H) : n(O). 2. We find the chemical quantities of the elements that make up the substance, assuming that m (C x H y O z) \u003d 100 g. m (C) \u003d w (C) m (C x H y O z) \u003d 0, 2667 100 g = 26.67 g m(H) = w(H) m(C x H y O z) = 0.0222 100 g = 2.22 g m(O) = w(O ) m(C x H y O z) = 0.7111 100 g = 71.11 g. n(C) = = = 2.22 mol.; n(H) = = = 2.22 mol.; n(O) = = = 4.44 mol. 3. Determine the empirical formula of the substance: n (C) : n (H) : n (O) \u003d 2.22 mol: 2.22 mol: 4.44 mol. x: y: z \u003d 1: 1: 2. The empirical formula of the substance is CHO 2. 4. We establish the true molecular formula of the substance: M r (CHO 2) \u003d A r (C) + A r (H) + 2A r (O) \u003d 12 + 1 + 2 16 \u003d 45; M r (CHO 2): M r (C x H y O z) = 45: 90 = 1: 2. The true molecular formula of the substance is C 2 H 2 O 4. Answer: molecular formula of the substance C 2 H 2 O 4 . Problem. Find the chemical formula of a substance that contains 9 wt. including aluminum and 8 wt. hours of oxygen. Solution: Find the ratio of the number of atoms: Answer: Chemical formula given substance: . Relative density of gas X by gas Y - D by Y (X). Often in tasks they are asked to determine the formula of a substance (gas) depending on Relative density D is a value that shows how many times gas X is heavier than gas Y. It is calculated as the ratio of the molar masses of gases X and Y: D according to Y (X) \u003d M (X) / M (Y) Often, relative densities of gases are used for calculations by hydrogen and by air. Relative density of gas X for hydrogen: D for H2 = M (gas X) / M (H2) = M (gas X) / 2 Air is a mixture of gases, so only the average molar mass can be calculated for it. Its value is taken as 29 g/mol (based on the approximate average composition). Therefore: D by air. \u003d M (gas X) / 29 Example: Determine the formula of a substance if it contains 84.21% C and 15.79% H and has a relative density in air of 3.93. Let the mass of the substance be 100 g. Then the mass C will be 84.21 g, and the mass H will be 15.79 g. 1. Find the amount of substance of each atom: ν(C) = m / M = 84.21 / 12 = 7 .0175 mol, ν(H) = 15.79 / 1 = 15.79 mol. 2. We determine the molar ratio of C and H atoms: C: H \u003d 7.0175: 15.79 (we divide both numbers by a smaller one) \u003d 1: 2.25 (we will multiply by 1, 2.3.4, etc. until 0 or 9 appears after the decimal point. In this problem, you need to multiply by 4) \u003d 4: 9. Thus, the simplest formula is C 4 H 9. 3. Based on the relative density, we calculate the molar mass: M = D (air) 29 = 114 g / mol. Molar mass corresponding to the simplest formula C 4 H 9 - 57 g / mol, this is 2 times less true molar mass. So the true formula is C 8 H 18. | |
1. Fill in the gaps in the sentences.
Absolute atomic mass shows the mass of one twelfth part 1/12 of the mass of one molecule of the carbon isotope 12 6 C is measured in the following units: g, gc, mg, t.
Relative atomic mass shows how many times the mass of a given substance of an element is greater than the mass of a hydrogen atom; does not have a unit of measure.
2. Write down using the notation oku = rounded to an integer value:
a) relative atomic mass of oxygen - 16:
b) relative atomic mass of sodium - 23;
c) relative atomic mass of copper - 64 .
3. Names of chemical elements are given: mercury, phosphorus, hydrogen, sulfur, carbon, oxygen, potassium, nitrogen. In the empty cells, enter the symbols of the elements in such a way that a series is obtained, in which the relative atomic mass increases.
4. Underline the correct statements.
a) The mass of ten oxygen atoms is equal to the mass of two bromine atoms;
b) The mass of five carbon atoms is greater than the mass of three sulfur atoms;
c) The mass of seven oxygen atoms is less than the mass of five magnesium atoms.
5. Complete the diagram.
6. Calculate the relative molecular masses of substances according to their formulas:
a) M r (N 2) \u003d 2 * 14 \u003d 28
b) M r (CH 4) = 12+4*1=16
c) M r (CaCO 3) = 40+12+3*16=100
d) M r (NH 4 Cl) \u003d 12 + 41 + 35.5 \u003d 53.5
e) M r (H 3 PO 4) = 3*1+31+16*4=98
7. Before you is a pyramid, the "building stones" of which are formulas chemical compounds. Find a path from the top of the pyramid to its base so that the sum of the relative molecular masses of the compounds is minimal. When choosing each next "stone", you need to take into account that you can choose only the one that is directly adjacent to the previous one.
In response, write down the formulas of the substances of the winning path.
Answer: C 2 H 6 - H 2 CO 3 - SO 2 - Na 2 S
8. Citric acid is found not only in lemons, but also in unripe apples, currants, cherries, etc. citric acid used in cooking household(for example, to remove rust stains from fabric). The molecule of this substance consists of 6 carbon atoms, 8 hydrogen atoms, 7 oxygen atoms.
C 6 H 8 O 7
Mark the correct statement:
a) the relative molecular weight of this substance is 185;
b) the relative molecular weight of this substance is 29;
c) the relative molecular weight of this substance is 192.