How to find out the qualitative and quantitative composition of a substance. Qualitative and quantitative composition of organic substances. The simplest and molecular formulas Describe the qualitative and quantitative composition of substances ch4

Let's consider the qualitative and quantitative composition of substances. Let us determine its features for compounds of organic and inorganic origin.

What does the qualitative composition of a substance show?

It demonstrates the types of atoms that are present in the molecule being analyzed. For example, water is formed by hydrogen and oxygen.

The molecule includes sodium and oxygen atoms. Sulfuric acid contains hydrogen, oxygen, and sulfur.

What does the quantitative composition show?

It demonstrates the quantitative content of each element within a complex substance.

For example, water contains two hydrogen atoms and one oxygen atom. Sulfuric acid consists of two hydrogens, one sulfur atom, four oxygens.

It contains three hydrogen atoms, one phosphorus, and four oxygen atoms.

The qualitative and quantitative composition of substances also exists in organic matter. For example, methane contains one carbon and four hydrogens.

Methods for determining the composition of a substance

The qualitative and quantitative composition of substances can be determined chemically. For example, when a molecule of a complex compound decomposes, several molecules with a simpler composition are formed. So, when heating calcium carbonate, consisting of calcium, carbon, four oxygen atoms, you can get two and carbon.

And the resulting chemical decomposition compounds can have different qualitative and quantitative composition of substances.

Simple and complex connections can be of molecular as well as non-molecular composition.

The first group is in different states of aggregation. For example, sugar is a solid, water is a liquid, and oxygen is a gas.

Compounds of non-molecular structure are found in solid form under standard conditions. These include salts. When heated, they melt and change from a solid to a liquid state.

Examples of composition determination

“Describe the qualitative and quantitative composition of the following substances: sulfur oxide (4), sulfur oxide (6).” This task is typical in school course inorganic chemistry. In order to cope with it, you first need to create formulas for the proposed compounds, using valences or oxidation states.

Both proposed oxides contain the same chemical elements, therefore, their qualitative composition is the same. They include sulfur and oxygen atoms. But in quantitative terms, the results will differ.

The first compound contains two oxygen atoms, and the second has six.

Let's do it next task: “Describe the qualitative and quantitative composition of H2S substances.”

A hydrogen sulfide molecule consists of a sulfur atom and two hydrogens. The qualitative and quantitative composition of the H2S substance allows us to predict it chemical properties. Since the composition contains a hydrogen cation, hydrogen sulfide can exhibit oxidizing properties. For example, similar characteristics manifest themselves in interaction with an active metal.

Information about the qualitative and quantitative composition of a substance is also relevant for organic compounds. For example, knowing the quantitative content of components in a hydrocarbon molecule, you can determine whether it belongs to a certain class of substances.

Such information allows one to predict the chemical and physical characteristics of the analyzed hydrocarbon and identify its specific properties.

For example, knowing that the composition contains four carbon atoms and ten hydrogens, we can conclude that this substance belongs to the class of saturated (saturated) hydrocarbons with the general formula SpH2n+2. All representatives of this homologous series are characterized by radical mechanism, as well as oxidation by atmospheric oxygen.

Conclusion

Any inorganic and organic substance has a certain quantitative and qualitative composition. Information is necessary to establish the physical and chemical properties of the analyzed inorganic compound, and for organic substances, the composition allows one to establish class membership and identify characteristic and specific chemical properties.

During the lesson, you will learn about the qualitative and quantitative compositions of organic substances, what the simplest, molecular, structural formula is.

One simple formula can correspond to many molecular formulas.

A formula that shows the order of connection of atoms in a molecule is called a structural formula.

Hexene and cyclohexane have the same molecular formulas C 6 H 12, but they are two different substances with different physical and chemical properties. See table. 1.

Table 1. Difference in properties of hexene and cyclohexane

To characterize an organic substance, it is necessary to know not only the composition of the molecule, but also the order of arrangement of atoms in the molecule - the structure of the molecule.

The structure of substances is reflected by structural (graphical) formulas, in which covalent bonds between atoms are indicated by dashes - valence strokes.

IN organic compounds carbon forms four bonds, hydrogen forms one, oxygen forms two, and nitrogen forms three.

Valence. Number of covalent nonpolar or polar bonds, which an element can form are called valence

A bond formed by one pair of electrons is called simple or single communication

A bond formed by two pairs of electrons is called double connection, it is denoted by two dashes, like the “equal” sign. Three electron pairs form triple connection, which is indicated by three dashes. See table. 2.

Table 2. Examples of organic substances with different bonds

In practice it is usually used abbreviated structural formulas, in which the bonds of carbon, oxygen and other atoms with hydrogen are not indicated:

Rice. 1. Volumetric model of an ethanol molecule

Structural formulas convey the order in which atoms are connected to each other, but do not convey the arrangement of atoms in space. Structural formulas are a two-dimensional drawing, but molecules are three-dimensional, i.e. are volumetric, this is shown in the example of ethanol in Fig. 1.

The lesson covered the issue of qualitative and quantitative compositions of organic substances, what the simplest, molecular, structural formula is.

References

1. Rudzitis G.E. Chemistry. Basics general chemistry. 10th grade: textbook for educational institutions: basic level/ G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Chemistry. 10th grade. Profile level: textbook for general education institutions/ V.V. Eremin, N.E. Kuzmenko, V.V. Lunin et al. - M.: Bustard, 2008. - 463 p.

3. Chemistry. 11th grade. Profile level: academic. for general education institutions/ V.V. Eremin, N.E. Kuzmenko, V.V. Lunin et al. - M.: Bustard, 2010. - 462 p.

4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering universities. - 4th ed. - M.: RIA " New wave": Publisher Umerenkov, 2012. - 278 p.

Homework

1. Nos. 6-7 (p. 11) Rudzitis G.E. Chemistry. Fundamentals of general chemistry. 10th grade: textbook for general education institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. -M.: Education, 2012.

2. Why do organic substances, the composition of which is reflected by the same molecular formula, have different chemical and physical properties?

3. What does the simplest formula show?

Mass fractions are usually expressed as percentages:

ω%(O) = 100% – ω%(H) = 100% – 11.1% = 88.9%.

Questions for control

1. What particles are usually formed by combining atoms?

2. How can you express the composition of any molecule?

3. What are the subscripts in chemical formulas?

4. What do chemical formulas show?

5. How is the law of constancy of composition formulated?

6. What is a molecule?

7. What is the mass of the molecule?

8. What is relative molecular weight?

9. What is it equal to mass fraction of this element in this substance?

1. Describe the qualitative and quantitative composition of the following molecules:

active substances: methane CH4, soda Na2 CO3, glucose C6 H12 O6, chlorine Cl2, aluminum sulfate Al2 (SO4)3.

2. The phosgene molecule consists of one carbon atom, one oxygen atom and two chlorine atoms. The urea molecule consists of one carbon atom, one oxygen atom and two NH atomic groups 2. Write the formulas for phosgene and urea.

3. Count it up total number atoms in the following molecules: (NH 4 )3 PO4 , Ca(H2 PO4 )2 , 2 SO4 .

4. Calculate the relative molecular weights of the substances indicated in exercise 1.

5. What are the mass fractions of elements in the following substances: NH 3, N2 O, NO2, NaNO3, KNO3, NH4 NO3? Which of these substances has the largest mass fraction of nitrogen and which has the smallest?

§ 1.5. Simple and complex substances. Allotropy.

Chemical compounds and mixtures

All substances are divided into simple and complex.

Simple substances are substances that consist of atoms of one element.

In some simple substances, atoms of one element

connect with each other and form molecules. Such simple substances have molecular structure . These include

are: hydrogen H2, oxygen O2, nitrogen N2, fluorine F2, chlorine Cl2, bromine Br2, iodine I2. All these substances consist of diatomic

molecules (Please note that the names of simple substances

match the names of the elements!)

Other simple substances have atomic structure, that is, they consist of atoms between which there are certain bonds (we will consider their nature in the section “Chemical bonds and structure of matter”). Examples of such simple substances are all metals (iron Fe, copper Cu, sodium Na, etc.) and some non-metals (carbon C, silicon Si, etc.). Not only the names, but also the formulas of these simple substances coincide with the symbols of the elements.

There is also a group of simple substances called noble gases. These include: helium He,

neon Ne, argon Ar, krypton Kr, xenon Xe, radon Rn. These simple substances consist of atoms not chemically bonded to each other.

Each element forms at least one simple substance. Some elements can form more than one,

but two or more simple substances. This phenomenon is called allotropy.

Allotropy is the phenomenon of the formation of several simple substances by one element.

Different simple substances that are formed by the same chemical element are called allotropic

modifications (modifications).

Allotropic modifications may differ from each other composition of molecules. For example, the element oxygen forms

two simple substances. One of them consists of diatomic O2 molecules and has the same name as the element - oxygen. Another simple substance consists of triatomic O3 molecules and has its own name - ozone:

Oxygen O2 and ozone O3 have different physical and chemical properties.

Allotropes can be solids that have different structure of the crystal

tallow An example is allotropic modifications carbon C - diamond and graphite.

The number of known simple substances (approximately 400) is significantly greater than the number chemical elements, since many elements can form two or more allotropic modifications.

Complex substances are substances that consist of atoms of different elements.

Examples complex substances: HCI, H 2 O, NaCl, CO 2,

H2 SO4, Cu(NO3)2, C6 H12 O6, etc.

Complex substances are often called chemical compounds. IN chemical compounds the properties of the simple substances from which these compounds are formed are not preserved

are. The properties of a complex substance differ from the properties of the simple substances from which it is formed.

For example, sodium chloride NaCl can be formed from simple substances - sodium metal Na And chlorine gas Cl 2. The physical and chemical properties of NaCI differ from the properties of Na and Cl 2.

IN In nature, as a rule, non-pure substances are found,

and mixtures of substances. IN practical activities we also

We usually use mixtures of substances. Any mixture consists of

two or more substances called compounds

components of the mixture.

For example, air is a mixture of several gaseous substances: oxygen O 2 (21% by volume), nitrogen N 2 (78%), carbon dioxide CO 2, etc. Mixtures are dis-

solutions of many substances, alloys of some metals, etc. Mixtures of substances can be homogeneous (uniform) and he-

terogenic (heterogeneous).

Homogeneous mixtures are mixtures in which there is no interface between the components.

Mixtures of gases (in particular, air) and liquid solutions (for example, a solution of sugar in water) are homogeneous.

Heterogeneous mixtures are mixtures in which the components are separated by an interface.

TO heterogeneous includemixtures solids (sand +

Chalk powder), mixtures of liquids insoluble in each other (water + oil), mixtures of liquids and solids insoluble in them (water + chalk).

Liquid solutions, which are the most important representatives of homogeneous systems, we will study in detail in our course.

The most important differences between mixtures and chemical compounds:

1. In mixtures, the properties of individual substances (components)

are saved.

2. The composition of mixtures is not constant.

Questions for control

1. What two types are all substances divided into?

2. What are simple substances?

3. What simple substances have a molecular structure (names and formulas)?

4. What simple substances have an atomic structure? Give examples.

5. What simple substances are made up of atoms that are not bonded to each other?

6. What is allotropy?

7. What are allotropic modifications called?

8. Why is the number of prime substances more number chemical elements?

9. What are complex substances?

10. Are the properties of simple substances preserved when a complex substance is formed from them?

11. What are homogeneous mixtures? Give examples.

12. What are heterogeneous mixtures? Give examples.

13. How do mixtures differ from chemical compounds?

Tasks for independent work

1. Write the formulas of the following known to you: a) simple substances (5 examples); b) complex substances (5 examples).

2. Divide the substances whose formulas are given below into simple and complex: NH 3, Zn, Br2, HI, C2 H5 OH, K, CO, F2, C10 H22.

3. The element phosphorus forms three simple substances that differ, in particular, in color: white, red and black phosphorus. What are these simple substances in relation to each other?

§ 1.6. Valence of elements. Graphic formulas of substances

Let's consider the chemical formulas of some compounds

As can be seen from these examples, the atoms of the elements chlorine, oxygen, nitrogen, carbon not any, but only a certain number of hydrogen atoms are added (1, 2, 3, 4 atoms, respectively).

Between atoms in chemical compounds there are chemical bonds. Let us write formulas in which each chi-

a mic connection is indicated by a dash:

Such formulas are called graphic.

Graphic formulas of substances - these are formulas that show the order of connection of atoms in molecules and the number of bonds that each atom forms.

Number chemical bonds, which forms one atom of a given element in a given molecule, is called the valency of the element.

Valency is usually indicated by Roman numerals: I, II, III, IV, V, VI, VII, VIII.

In all the molecules under consideration, each hydrogen atom forms one bond: therefore, the valence of hydrogen is equal to one (I).

Chlorine atom in HCl molecule forms one bond, its valency in this molecule is equal to I. The oxygen atom in the H2 O molecule forms two bonds, its valence is equal to II. Valence

nitrogen in NH3 is III, and the valency of carbon in CH4 is IV. Some items have constant valence.

Elements with constant valency are elements that in all connections exhibit the same valence

Elements with constant valency I are: hydrogen H, fluorine F , alkali metals: lithium Li, sodium Na,

potassium K, rubidium Rb, cesium Cs.

The atoms of these monovalent elements always form

only one chemical bond.

Elements with constant valency II:

oxygen O, magnesium Mg, calcium Ca, strontium Sr, barium Ba, zinc Zn.

The element with constant valency III is aluminum Al.

Most items have variable valence.

Variable valency elements are elements that are different connections may have different valency values*.

Consequently, the atoms of these elements in different compounds can form different numbers of chemical bonds (Table 4).

* We will consider the physical meaning of valence, the reasons for the existence of elements with constant and variable valence after studying the theory of atomic structure.

Table 4

The most typical valence values ​​of some elements

Elements	The most characteristic
	valency
	II, III, IV, VI, VII

To determine the valency of such elements in any given compound, you can use the valency rule

ribbon.

According to this rule, in most binary compounds of type A m B n, the product of the valence of element A (x) by the number of its atoms (t) is equal to the product of the valence of element

ta B (y) by the number of its atoms (n):

x · t = y · n * .

Let us determine, for example, the valence of phosphorus in the following compounds:

x I	x" II
PH3	P2 O5
Valence of hydrogen	Oxygen valence
is constant and equal to I	is constant and equal to II
x 1 = 1 3	x" 2 = 2 5
x = 3	x" = 5
PH3	P2 O5
Phosphorus in PH3 is	Phosphorus in P2 O5 is
trivalent	pentavalent
element	element

* The valence rule does not apply to binary compounds, in which atoms of the same element are directly bonded to each other. For example, the valency rule does not obey the first

hydrogen oxide H2 O2, since in its molecule there is a bond between oxygen atoms: H-O-O-H.

Using the valency rule, you can make up formulas binary compounds, i.e. determine the indices in these formulas.

Let's create, for example, the formula for the compound aluminum with oxygen. Al and O have constant valency values, co-

responsible III and II:

The least common multiple (LCD) of the numbers 3 and 2 is 6. Divide the LCM by the valence of Al:

6: 3 = 2 and for valence O: 6: 2 = 3

These numbers are equal to the indices of the corresponding symbols

elements in the compound formula:

Al2 O3

Let's look at two more examples.

Write down formulas for compounds that consist of:

note that in most binary compounds

In general, atoms of the same element do not combine directly with each other.

Let's write graphic formulas all the connections we looked at in this paragraph:

Compare the number of dashes for each element with its valence, which is indicated in the text of the paragraph.

Questions for control

1. What is the valence of an element?

2. What numbers usually indicate valency?

3. What are constant valency elements?

4. Which elements have constant valence?

5. What are elements with variable valency? Indicate the most typical valency values ​​for chlorine, sulfur, carbon, phosphorus, and iron.

6. How is the valency rule formulated?

7. What are the names of the formulas that show the order of connection of atoms in molecules and the valence of each element?

Tasks for independent work

1. Determine the valence of elements in the following compounds: AsH 3, CuO, N 2 O 3, CaBr 2, AlI 3, SF 6, K 2 S, SiO 2, Mg 3 N 2.

Write graphic formulas for these substances.

2. Define Indexes m and n in the following formulas:

Hm Sen, Pm Cln, Pbm On, Om Fn, Fem Sn Write graphic formulas for these substances.

3. Make molecular and graphic formulas for compounds of chromium with oxygen in which chromium exhibits valence II, III and VI.

4. Write down formulas for compounds that consist of:

a) manganese (II) and oxygen; b) manganese (IV) and oxygen; c) manganese (VI) and oxygen; d) chlorine (VII) and oxygen; e) barium and oxygen. Write graphic formulas for these substances.

§ 1.7. Mol. Molar mass

The mass of a substance is expressed in kg, g or other units

The unit of quantity of a substance is the mole.

Most substances are made up of molecules or atoms.

A mole is the amount of a substance that contains as many molecules (atoms) of this substance as there are atoms in 12 g (0.012 kg) of carbon C.

Let's determine the number of C atoms in 12 g of carbon. To do this, divide 0.012 kg by absolute mass carbon atom m a (C) (see § 1.3):

0.012 kg/19.93 10–27 kg ≈ 6.02 1023.

From the definition of the concept “mole” it follows that this number

equal to the number of molecules (atoms) in one mole of any substance. It is called Avogadro's number and is denoted by the symbol

ox N A:

(Note that Avogadro's number is a very large number!)

If a substance consists of molecules, then 1 mole is 6.02 x 1023 molecules of this substance.

For example: 1 mole of hydrogen H2 is 6.02 · 1023 molecules of H2; 1 mole of H2O water is 6.02 x 1023 H2O molecules;

1 mole of glucose C6 H12 O6 is 6.02 1023

molecules C6 H12 O6.

If a substance consists of atoms, then 1 mole is 6.02 x 1023 atoms of this substance.

For example: 1 mole of iron Fe is 6.02 1023 Fe atoms;

1 mole of sulfur S is 6.02 1023 atoms of S. Therefore:

1 mole of any substance contains the Avogadro number of particles that make up this substance, i.e. approximately 6.02 × 1023 molecules or atoms.

The amount of substance (i.e. the number of moles) is denoted by Latin letter p (or the Greek letter v). Any given number of molecules (atoms) is denoted by the letter N.

The amount of substance n is equal to the ratio of a given number of molecules (atoms) N to the number of molecules (atoms) in 1 mole NA.