Key words: Chemistry 8th grade. All formulas and definitions, symbols of physical quantities, units of measurement, prefixes for designating units of measurement, relationships between units, chemical formulas, basic definitions, briefly, tables, diagrams.

1. Symbols, names and units of measurement
some physical quantities used in chemistry

Physical quantity	Designation	unit of measurement
Time	t	With
Pressure	p	Pa, kPa
Amount of substance	ν	mole
Mass of matter	m	kg, g
Mass fraction	ω	Dimensionless
Molar mass	M	kg/mol, g/mol
Molar volume	V n	m 3 / mol, l / mol
Volume of matter	V	m 3, l
Volume fraction		Dimensionless
Relative atomic mass	A r	Dimensionless
	M r	Dimensionless
Relative density of gas A over gas B	D B (A)	Dimensionless
Matter density	R	kg / m 3, g / cm 3, g / ml
Avogadro constant	N A	1/mol
Temperature absolute	T	K (Kelvin)
Celsius temperature	t	°С (degree Celsius)
Thermal effect of a chemical reaction	Q	kJ/mol

2. Relations between units of physical quantities

3. Chemical formulas in grade 8

4. Basic definitions in grade 8

• Atom- the smallest chemically indivisible particle of a substance.
• Chemical element a certain type of atom.
• Molecule- the smallest particle of a substance that retains its composition and chemical properties and consists of atoms.
• Simple substances Substances whose molecules are made up of atoms of the same type.
• Complex Substances Substances whose molecules are made up of different types of atoms.
• The qualitative composition of the substance shows what atoms it consists of.
• The quantitative composition of the substance shows the number of atoms of each element in its composition.
• Chemical formula- conditional record of the qualitative and quantitative composition of a substance by means of chemical symbols and indices.
• Atomic mass unit(amu) - a unit of measurement of the mass of an atom, equal to the mass of 1/12 of a carbon atom 12 C.
• mole- the amount of a substance that contains the number of particles equal to the number of atoms in 0.012 kg of carbon 12 C.
• Avogadro constant (Na \u003d 6 * 10 23 mol -1) - the number of particles contained in one mole.
• Molar mass of a substance (M ) is the mass of a substance taken in an amount of 1 mol.
• Relative atomic mass element BUT r - the ratio of the mass of an atom of a given element m 0 to 1/12 of the mass of a carbon atom 12 C.
• Relative molecular weight substances M r - the ratio of the mass of a molecule of a given substance to 1/12 of the mass of a carbon atom 12 C. The relative molecular mass is equal to the sum of the relative atomic masses of the chemical elements that form the compound, taking into account the number of atoms of this element.
• Mass fraction chemical element ω(X) shows what part of the relative molecular weight of substance X is accounted for by this element.

ATOMIC-MOLECULAR STUDIES
1. There are substances with a molecular and non-molecular structure.
2. There are gaps between the molecules, the dimensions of which depend on the state of aggregation of the substance and temperature.
3. Molecules are in continuous motion.
4. Molecules are made up of atoms.
6. Atoms are characterized by a certain mass and size.
In physical phenomena, molecules are preserved, in chemical phenomena, as a rule, they are destroyed. Atoms in chemical phenomena rearrange, forming molecules of new substances.

THE LAW OF CONSTANT COMPOSITION OF A SUBSTANCE
Each chemically pure substance of molecular structure, regardless of the method of preparation, has a constant qualitative and quantitative composition.

VALENCE
Valency is the property of an atom of a chemical element to attach or replace a certain number of atoms of another element.

CHEMICAL REACTION
A chemical reaction is a process in which another substance is formed from one substance. Reagents are substances that enter into a chemical reaction. Reaction products are substances that are formed as a result of a reaction.
Signs of chemical reactions:
1. Release of heat (light).
2. Color change.
3. The appearance of a smell.
4. Precipitation.
5. Gas release.

>> Chemical formulas

Chemical formulas

The paragraph will help you:

> find out what a chemical formula is;
> read the formulas of substances, atoms, molecules, ions;
> correctly use the term "formula unit";
> make chemical formulas of ionic compounds;
> characterize the composition of a substance, molecule, ion by chemical formula.

Chemical formula.

Everyone has it substances there is a name. However, by name it is impossible to determine what particles the substance consists of, how many and what atoms are contained in its molecules, ions, what charges the ions have. Answers to such questions are given by a special record - a chemical formula.

A chemical formula is the designation of an atom, molecule, ion, or substance using symbols chemical elements and indexes.

The chemical formula of an atom is the symbol of the corresponding element. For example, an aluminum atom is denoted by the symbol Al, and a silicon atom by the symbol Si. Simple substances also have such formulas - the metal aluminum, the non-metal of the atomic structure silicon.

Chemical formula molecule of a simple substance contains the symbol of the corresponding element and a subscript - a small number written below and to the right. The index indicates the number of atoms in the molecule.

An oxygen molecule consists of two oxygen atoms. Its chemical formula is O 2 . This formula is read by pronouncing first the symbol of the element, then the index: “o-two”. The formula O 2 denotes not only the molecule, but also the substance oxygen itself.

The O 2 molecule is called diatomic. Of these molecules (their general formula is E 2), the simple substances of Hydrogen, Nitrogen, Fluor, Chlorine, Bromine, Iodine are composed.

Ozone contains three-atom molecules, white phosphorus - four-atom, and sulfur - eight-atom. (Write the chemical formulas of these molecules.)

H 2
O2
N 2
Cl2
Br2
I 2

In the formula of a molecule of a complex substance, the symbols of the elements whose atoms are contained in it, as well as the indices, are written. The carbon dioxide molecule consists of three atoms: one carbon atom and two oxygen atoms. Its chemical formula is CO 2 (read "tse-o-two"). Remember: if there is one atom of any element in the molecule, then the corresponding index, i.e. I, is not written in the chemical formula. The formula of the carbon dioxide molecule is also the formula of the substance itself.

In the formula of an ion, its charge is additionally recorded. To do this, use the superscript. In it, a number indicates the amount of charge (they do not write a unit), and then a sign (plus or minus). For example, a Sodium ion with a charge of +1 has the formula Na + (read "sodium plus"), a Chlorine ion with a charge - I - SG - ("chlorine minus"), a hydroxide ion with a charge - I - OH - (" o-ash-minus"), a carbonate ion with a charge of -2 - CO 2- 3 ("tse-o-three-two-minus").

Na + , Cl -
simple ions

OH -, CO 2- 3
complex ions

In the formulas of ionic compounds, they first write down, without indicating charges, positively charged ions, and then - negatively charged (Table 2). If the formula is correct, then the sum of the charges of all ions in it is equal to zero.

table 2
Formulas of some ionic compounds

In some chemical formulas, a group of atoms or a complex ion is written in brackets. As an example, take the formula for slaked lime Ca (OH) 2. This is an ionic compound. In it, for every Ca 2+ ion, there are two OH - ions. The compound formula reads " calcium-o-ash-twice", but not "calcium-o-ash-two".

Sometimes in chemical formulas, instead of the symbols of the elements, "foreign" letters are written, as well as index letters. Such formulas are often called general. Examples of formulas of this type: ECI n , E n O m , Fe x O y. First
the formula denotes a group of compounds of elements with Chlorine, the second - a group of compounds of elements with Oxygen, and the third is used if the chemical formula of the Ferrum compound with Oxygen unknown and
it should be installed.

If you need to designate two separate neon atoms, two oxygen molecules, two carbon dioxide molecules or two sodium ions, use the notation 2Ne, 20 2, 2CO 2 , 2Na +. The number in front of the chemical formula is called the coefficient. The coefficient I, like the index I, is not written.

formula unit.

What does 2NaCl mean? There are no NaCl molecules; table salt is an ionic compound that consists of Na + and Cl - ions. A pair of these ions is called the formula unit of matter (it is highlighted in Fig. 44, a). Thus, the notation 2NaCl represents two formula units of table salt, i.e., two pairs of Na + and C l- ions.

The term "formula unit" is used for complex substances not only of ionic, but also of atomic structure. For example, the formula unit for quartz SiO 2 is the combination of one silicon atom and two oxygen atoms (Fig. 44, b).

Rice. 44. formula units in compounds of ionic (a) atomic structure (b)

A formula unit is the smallest "brick" of a substance, its smallest repeating fragment. This fragment can be an atom (in simple matter), molecule(in simple or complex matter),
a collection of atoms or ions (in a complex substance).

An exercise. Compose the chemical formula of a compound that contains Li + i SO 2- 4 ions. Name the formula unit of this substance.

Solution

In an ionic compound, the sum of the charges of all ions is zero. This is possible provided that there are two Li + ions for each SO 2- 4 ion. Hence the formula of the compound is Li 2 SO 4.

The formula unit of a substance is three ions: two Li + ions and one SO 2- 4 ion.

Qualitative and quantitative composition of the substance.

A chemical formula contains information about the composition of a particle or substance. Characterizing the qualitative composition, they name the elements that form a particle or substance, and characterizing the quantitative composition, indicate:

The number of atoms of each element in a molecule or complex ion;
the ratio of atoms of different elements or ions in a substance.

An exercise. Describe the composition of methane CH 4 (molecular compound) and soda ash Na 2 CO 3 (ionic compound)

Solution

Methane is formed by the elements Carbon and Hydrogen (this is a qualitative composition). The methane molecule contains one carbon atom and four hydrogen atoms; their ratio in the molecule and in the substance

N(C): N(H) = 1:4 (quantitative composition).

(The letter N denotes the number of particles - atoms, molecules, ions.

Soda ash is formed by three elements - Sodium, Carbon and Oxygen. It contains positively charged Na + ions, since Sodium is a metallic element, and negatively charged CO -2 3 ions (qualitative composition).

The ratio of atoms of elements and ions in a substance is as follows:

conclusions

A chemical formula is a record of an atom, molecule, ion, substance using the symbols of chemical elements and indices. The number of atoms of each element is indicated in the formula with a subscript, and the charge of the ion is indicated with a superscript.

Formula unit - a particle or a collection of particles of a substance, represented by its chemical formula.

The chemical formula reflects the qualitative and quantitative composition of a particle or substance.

?
66. What information about a substance or particle does a chemical formula contain?

67. What is the difference between a coefficient and a subscript in chemical records? Complete your answer with examples. What is superscript used for?

68. Read the formulas: P 4 , KHCO 3 , AI 2 (SO 4) 3 , Fe(OH) 2 NO 3 , Ag + , NH + 4 , CIO - 4 .

69. What do the entries mean: 3H 2 0, 2H, 2H 2, N 2, Li, 4Cu, Zn 2+, 50 2-, NO - 3, ZCa (0H) 2, 2CaC0 3?

70. Write down chemical formulas that read like this: es-o-three; boron-two-o-three; ash-en-o-two; chrome-o-ash-thrice; sodium-ash-es-o-four; en-ash-four-twice-es; barium-two-plus; pe-o-four-three-minus.

71. Make a chemical formula of a molecule that contains: a) one Nitrogen atom and three Hydrogen atoms; b) four hydrogen atoms, two phosphorus atoms and seven oxygen atoms.

72. What is the formula unit: a) for soda ash Na 2 CO 3; b) for the ionic compound Li 3 N; c) for the compound B 2 O 3, which has an atomic structure?

73. Make formulas for all substances that can contain only such ions: K + , Mg2 + , F - , SO -2 4 , OH - .

74. Describe the qualitative and quantitative composition:

a) molecular substances - chlorine Cl 2, hydrogen peroxide (hydrogen peroxide) H 2 O 2, glucose C 6 H 12 O 6;
b) ionic substance - sodium sulfate Na 2 SO 4;
c) H 3 O +, HPO 2- 4 ions.

Popel P. P., Kriklya L. S., Chemistry: Pdruch. for 7 cells. zahalnosvit. navch. zakl. - K .: Exhibition Center "Academy", 2008. - 136 p.: il.

Lesson content lesson summary and support frame lesson presentation interactive technologies accelerating teaching methods Practice quizzes, testing online tasks and exercises homework workshops and trainings questions for class discussions Illustrations video and audio materials photos, pictures graphics, tables, schemes comics, parables, sayings, crossword puzzles, anecdotes, jokes, quotes Add-ons abstracts cheat sheets chips for inquisitive articles (MAN) literature main and additional glossary of terms Improving textbooks and lessons correcting errors in the textbook replacing obsolete knowledge with new ones Only for teachers calendar plans training programs methodological recommendations

Check information. It is necessary to check the accuracy of the facts and the reliability of the information presented in this article. There is a discussion on the talk page on the topic: Doubts about terminology. Chemical formula ... Wikipedia

A chemical formula is a reflection of information about the composition and structure of substances using chemical signs, numbers and separating brackets. Currently, the following types of chemical formulas are distinguished: The simplest formula. Can be obtained by experienced ... ... Wikipedia

A chemical formula is a reflection of information about the composition and structure of substances using chemical signs, numbers and separating brackets. Currently, the following types of chemical formulas are distinguished: The simplest formula. Can be obtained by experienced ... ... Wikipedia

A chemical formula is a reflection of information about the composition and structure of substances using chemical signs, numbers and separating brackets. Currently, the following types of chemical formulas are distinguished: The simplest formula. Can be obtained by experienced ... ... Wikipedia

A chemical formula is a reflection of information about the composition and structure of substances using chemical signs, numbers and separating brackets. Currently, the following types of chemical formulas are distinguished: The simplest formula. Can be obtained by experienced ... ... Wikipedia

Main article: Inorganic compounds List of inorganic compounds by elements an informational list of inorganic compounds, presented in alphabetical order (by formula) for each substance, hydrogen acids of the elements (with their ... ... Wikipedia

This article or section needs revision. Please improve the article in accordance with the rules for writing articles ... Wikipedia

A chemical equation (chemical reaction equation) is a conditional record of a chemical reaction using chemical formulas, numerical coefficients and mathematical symbols. The chemical reaction equation gives qualitative and quantitative ... ... Wikipedia

Chemical software are computer programs used in the field of chemistry. Contents 1 Chemical editors 2 Platforms 3 Literature ... Wikipedia

Books

• Japanese-English-Russian Dictionary of Industrial Equipment Installation. About 8,000 terms, Popova I.S. The dictionary is intended for a wide range of users, and primarily for translators and technical specialists involved in the supply and implementation of industrial equipment from Japan or ...
• Brief Dictionary of Biochemical Terms, Kunizhev S.M.. The dictionary is intended for students of chemical and biological specialties of universities studying the course of general biochemistry, ecology and the basics of biotechnology, and can also be used in ...

Collection of basic formulas for a school course in chemistry

Collection of basic formulas for a school course in chemistry

G. P. Loginova

Elena Savinkina

E. V. Savinkina G. P. Loginova

Collection of basic formulas in chemistry

Student pocket guide

general chemistry

The most important chemical concepts and laws

Chemical element A certain type of atom with the same nuclear charge.

Relative atomic mass(A r) shows how many times the mass of an atom of a given chemical element is greater than the mass of a carbon-12 atom (12 C).

Chemical substance- a collection of any chemical particles.

chemical particles

formula unit- a conditional particle, the composition of which corresponds to the given chemical formula, for example:

Ar - substance argon (consists of Ar atoms),

H 2 O - water substance (consists of H 2 O molecules),

KNO 3 - substance potassium nitrate (consists of K + cations and NO 3 ¯ anions).

Relations between physical quantities

Atomic mass (relative) of an element B, Ar(B):

Where *t(atom B) is the mass of an atom of element B;

*t and is the atomic mass unit;

*t and = 1/12 t(atom 12 C) \u003d 1.6610 24 g.

Amount of substance B, n(B), mol:

Where N(B) is the number of particles B;

N A is the Avogadro constant (NA = 6.0210 23 mol -1).

Molar mass of a substance V, M(V), g/mol:

Where t(B)- weight B.

Molar volume of gas AT, V M , l/mol:

Where V M = 22.4 l/mol (consequence of Avogadro's law), under normal conditions (n.o. - atmospheric pressure p = 101 325 Pa (1 atm); thermodynamic temperature T = 273.15 K or Celsius temperature t = 0°C).

B for hydrogen, D(gas B to H 2):

* Density of a gaseous substance AT by air, D(gas B by air): Mass fraction of the element E in matter B, w(E):

Where x is the number of atoms E in the formula of substance B

The structure of the atom and the Periodic Law D.I. Mendeleev

Mass number (A) - the total number of protons and neutrons in the atomic nucleus:

A = N(p 0) + N(p +).

The charge of the nucleus of an atom (Z) equals the number of protons in the nucleus and the number of electrons in the atom:

Z = N(p+) = N(e¯).

isotopes- atoms of the same element, differing in the number of neutrons in the nucleus, for example: potassium-39: 39 K (19 p + , 20n 0 , 19e¯); potassium-40: 40 K (19 p+, 21n 0 , 19e¯).

*Energy levels and sublevels

*Atomic Orbital(AO) characterizes the region of space in which the probability of an electron having a certain energy to stay is the greatest.

*Shapes of s- and p-orbitals

Periodic Law and Periodic System D.I. Mendeleev

The properties of elements and their compounds are periodically repeated with increasing serial number, which is equal to the charge of the nucleus of the element's atom.

Period number corresponds the number of energy levels filled with electrons, and means last energy level(EU).

Group number A shows and etc.

Group number B shows number of valence electrons ns and (n – 1)d.

s-element section- the energy sublevel (EPL) is filled with electrons ns-epu- IA- and IIA-groups, H and He.

p-elements section- filled with electrons np-epu– IIIA-VIIIA-groups.

d-element section- filled with electrons (P- 1) d-EPU - IB-VIIIB2-groups.

f-element section- filled with electrons (P-2) f-EPU - lanthanides and actinides.

Changes in the composition and properties of hydrogen compounds of elements of the 3rd period of the Periodic system

Non-volatile, decomposed by water: NaH, MgH 2 , AlH 3 .

Volatile: SiH 4 , PH 3 , H 2 S, HCl.

Changes in the composition and properties of higher oxides and hydroxides of elements of the 3rd period of the Periodic system

Basic: Na 2 O - NaOH, MgO - Mg (OH) 2.

Amphoteric: Al 2 O 3 - Al (OH) 3.

Acid: SiO 2 - H 4 SiO 4, P 2 O 5 - H 3 PO 4, SO 3 - H 2 SO 4, Cl 2 O 7 - HClO 4.

chemical bond

Electronegativity(χ) is a value that characterizes the ability of an atom in a molecule to acquire a negative charge.

Mechanisms for the formation of a covalent bond

exchange mechanism- the overlap of two orbitals of neighboring atoms, each of which had one electron.

Donor-acceptor mechanism- overlapping of the free orbital of one atom with the orbital of another atom, which has a pair of electrons.

Orbital overlap during bond formation

*Type of hybridization - geometric shape of the particle - angle between bonds

Hybridization of orbitals of the central atom– alignment of their energy and form.

sp– linear – 180°

sp 2– triangular – 120°

sp 3– tetrahedral – 109.5°

sp 3 d– trigonal-bipyramidal – 90°; 120°

sp 3 d 2– octahedral – 90°

Mixtures and solutions

Solution- a homogeneous system consisting of two or more substances, the content of which can be changed within certain limits.

Solution: solvent (eg water) + solute.

True Solutions contain particles smaller than 1 nanometer.

Colloidal solutions contain particles 1-100 nanometers in size.

Mechanical mixtures(suspensions) contain particles larger than 100 nanometers.

Suspension=> solid + liquid

Emulsion=> liquid + liquid

Foam, fog=> gas + liquid

Heterogeneous mixtures are separated settling and filtering.

Homogeneous mixtures are separated evaporation, distillation, chromatography.

saturated solution is or can be in equilibrium with the solute (if the solute is a solid, then its excess is in the sediment).

Solubility is the content of a solute in a saturated solution at a given temperature.

unsaturated solution less,

Supersaturated solution contains a solute more, than its solubility at a given temperature.

Relationships between physicochemical quantities in solution

Mass fraction of solute AT, w(B); fraction of a unit or %:

Where t(B)- mass B,

t(p) is the mass of the solution.

The mass of the solution m(p), r:

m(p) = m(B) + m(H 2 O) = V(p) ρ(p),

where F(p) is the volume of the solution;

ρ(p) is the density of the solution.

Solution volume, V(p), l:

molar concentration, s(B), mol/l:

Where n(B) is the amount of substance B;

M(B) is the molar mass of substance B.

Changing the composition of the solution

Diluting the solution with water:

> t "(B)= t(B);

> the mass of the solution increases by the mass of the added water: m "(p) \u003d m (p) + m (H 2 O).

Evaporation of water from solution:

> the mass of the solute does not change: t "(B) \u003d t (B).

> the mass of the solution is reduced by the mass of evaporated water: m "(p) \u003d m (p) - m (H 2 O).

Merging two solutions: the masses of the solutions, as well as the masses of the solute, add up:

t "(B) \u003d t (B) + t" (B);

t"(p) = t(p) + t"(p).

Drop of crystals: the mass of the solute and the mass of the solution are reduced by the mass of the precipitated crystals:

m "(B) \u003d m (B) - m (draft); m" (p) \u003d m (p) - m (draft).

The mass of water does not change.

Thermal effect of a chemical reaction

*Enthalpy of formation of matter ΔH° (B), kJ / mol, is the enthalpy of the reaction of formation of 1 mol of a substance from simple substances in their standard states, that is, at a constant pressure (1 atm for each gas in the system or at a total pressure of 1 atm in the absence of gaseous participants in the reaction) and constant temperature (usually 298 K , or 25°C).

*Heat effect of a chemical reaction (Hess' law)

Q = ΣQ(products) - ΣQ(reagents).

ΔН° = ΣΔН°(products) – Σ ΔH°(reagents).

For reaction aA + bB +… = dD + eE +…

ΔH° = (dΔH°(D) + eΔH°(E) +…) – (aΔH°(A) + bΔH°(B) +…),

where a, b, d, e are the stoichiometric quantities of substances corresponding to the coefficients in the reaction equation.

The rate of a chemical reaction

If during the time τ in the volume V amount of reactant or product changed by Δ n, speed reaction:

For a monomolecular reaction А → …:

v=k c(A).

For a bimolecular reaction A + B → ...:

v=k c(A) c(B).

For the trimolecular reaction A + B + C → ...:

v=k c(A) c(B) c(C).

Change in the rate of a chemical reaction

Speed ​​reaction increase:

1) chemically active reagents;

2) promotion reagent concentrations;

3) increase

4) promotion temperature;

5) catalysts. Speed ​​reaction reduce:

1) chemically inactive reagents;

2) downgrade reagent concentrations;

3) decrease surfaces of solid and liquid reagents;

4) downgrade temperature;

5) inhibitors.

*Temperature coefficient of speed(γ) is equal to a number that shows how many times the reaction rate increases when the temperature rises by ten degrees:

Chemical equilibrium

*Law of mass action for chemical equilibrium: in a state of equilibrium, the ratio of the product of molar concentrations of products in powers equal to

Their stoichiometric coefficients, to the product of molar concentrations of reactants in powers equal to their stoichiometric coefficients, at a constant temperature is a constant value (concentration equilibrium constant).

In a state of chemical equilibrium for a reversible reaction:

aA + bB + … ↔ dD + fF + …

K c = [D] d [F] f …/ [A] a [B] b …

*Shift of chemical equilibrium towards the formation of products

1) Increasing the concentration of reagents;

2) decrease in the concentration of products;

3) increase in temperature (for an endothermic reaction);

4) decrease in temperature (for an exothermic reaction);

5) increase in pressure (for a reaction proceeding with a decrease in volume);

6) decrease in pressure (for a reaction proceeding with an increase in volume).

Exchange reactions in solution

Electrolytic dissociation- the process of formation of ions (cations and anions) when certain substances are dissolved in water.

acids formed hydrogen cations and acid anions, for example:

HNO 3 \u003d H + + NO 3 ¯

With electrolytic dissociation grounds formed metal cations and hydroxide ions, for example:

NaOH = Na + + OH¯

With electrolytic dissociation salts(medium, double, mixed) are formed metal cations and acid anions, for example:

NaNO 3 \u003d Na + + NO 3 ¯

KAl (SO 4) 2 \u003d K + + Al 3+ + 2SO 4 2-

With electrolytic dissociation acid salts formed metal cations and acid hydroanions, for example:

NaHCO 3 \u003d Na + + HCO 3 ‾

Some strong acids

HBr, HCl, HClO 4 , H 2 Cr 2 O 7 , HI, HMnO 4 , H 2 SO 4 , H 2 SeO 4 , HNO 3 , H 2 CrO 4

Some strong foundations

RbOH, CsOH, KOH, NaOH, LiOH, Ba(OH) 2 , Sr(OH) 2 , Ca(OH) 2

Degree of dissociation α is the ratio of the number of dissociated particles to the number of initial particles.

At constant volume:

Classification of substances according to the degree of dissociation

Berthollet's rule

Exchange reactions in solution proceed irreversibly if a precipitate, gas, or weak electrolyte is formed as a result.

Examples of molecular and ionic reaction equations

1. Molecular equation: CuCl 2 + 2NaOH = Cu(OH) 2 ↓ + 2NaCl

The "complete" ionic equation: Cu 2+ + 2Cl¯ + 2Na + + 2OH¯ = Cu(OH) 2 ↓ + 2Na + + 2Cl¯

"Short" ionic equation: Сu 2+ + 2OH¯ \u003d Cu (OH) 2 ↓

2. Molecular equation: FeS (T) + 2HCl = FeCl 2 + H 2 S

"Full" ionic equation: FeS + 2H + + 2Cl¯ = Fe 2+ + 2Cl¯ + H 2 S

"Short" ionic equation: FeS (T) + 2H + = Fe 2+ + H 2 S

3. Molecular equation: 3HNO 3 + K 3 PO 4 = H 3 PO 4 + 3KNO 3

"Full" ionic equation: 3H + + 3NO 3 ¯ + ZK + + PO 4 3- \u003d H 3 RO 4 + 3K + + 3NO 3 ¯

"Short" ionic equation: 3H + + PO 4 3- \u003d H 3 PO 4

*Hydrogen index

(pH) pH = – lg = 14 + lg

*PH range for dilute aqueous solutions

pH 7 (neutral medium)

Examples of exchange reactions

Neutralization reaction- an exchange reaction that occurs when an acid and a base interact.

1. Alkali + strong acid: Ba (OH) 2 + 2HCl \u003d BaCl 2 + 2H 2 O

Ba 2+ + 2OH¯ + 2H + + 2Cl¯ = Ba 2+ + 2Cl¯ + 2H 2 O

H + + OH¯ \u003d H 2 O

2. Slightly soluble base + strong acid: Сu (OH) 2 (t) + 2НCl = СuСl 2 + 2Н 2 O

Cu (OH) 2 + 2H + + 2Cl¯ \u003d Cu 2+ + 2Cl¯ + 2H 2 O

Cu (OH) 2 + 2H + \u003d Cu 2+ + 2H 2 O

*Hydrolysis- an exchange reaction between a substance and water without changing the oxidation states of atoms.

1. Irreversible hydrolysis of binary compounds:

Mg 3 N 2 + 6H 2 O \u003d 3Mg (OH) 2 + 2NH 3

2. Reversible hydrolysis of salts:

A) salt is formed strong base cation and strong acid anion:

NaCl = Na + + Сl¯

Na + + H 2 O ≠ ;

Cl¯ + H 2 O ≠

Hydrolysis is absent; the medium is neutral, pH = 7.

B) Salt is formed strong base cation and weak acid anion:

Na 2 S \u003d 2Na + + S 2-

Na + + H 2 O ≠

S 2- + H 2 O ↔ HS¯ + OH¯

Anion hydrolysis; alkaline environment, pH>7.

B) Salt is formed a cation of a weak or sparingly soluble base and an anion of a strong acid:

End of introductory segment.

Text provided by LitRes LLC.

You can safely pay for the book with a Visa, MasterCard, Maestro bank card, from a mobile phone account, from a payment terminal, in an MTS or Svyaznoy salon, via PayPal, WebMoney, Yandex.Money, QIWI Wallet, bonus cards or in another way convenient for you.

Modern symbols of chemical elements were introduced into science in 1813 by J. Berzelius. At his suggestion, the elements are denoted by the initial letters of their Latin names. For example, oxygen (Oxygenium) is denoted by the letter O, sulfur (Sulfur) - by the letter S, hydrogen (Hydrogenium) - by the letter H. In cases where the names of the elements begin with the same letter, one of the following is added to the first letter. So, carbon (Carboneum) has the symbol C, calcium (Calcium) - Ca, copper (Cuprum) - Cu.

Chemical symbols are not only abbreviated names of elements: they also express their certain quantities (or masses), i.e. each symbol denotes either one atom of an element, or one mole of its atoms, or the mass of an element equal to (or proportional to) the molar mass of that element. For example, C means either one carbon atom, or one mole of carbon atoms, or 12 mass units (usually 12 g) of carbon.

Formulas of chemicals

The formulas of substances also indicate not only the composition of the substance, but also its quantity and mass. Each formula represents either one molecule of a substance, or one mole of a substance, or the mass of a substance equal to (or proportional to) its molar mass. For example, H 2 O denotes either one molecule of water, or one mole of water, or 18 mass units (usually (18 g) of water.

Simple substances are also denoted by formulas showing how many atoms a molecule of a simple substance consists of: for example, the formula for hydrogen is H 2. If the atomic composition of a molecule of a simple substance is not exactly known or the substance consists of molecules containing a different number of atoms, and also if it has not a molecular, but an atomic or metallic structure, a simple substance is denoted by the element symbol. For example, a simple substance phosphorus is denoted by the formula P, since, depending on the conditions, phosphorus can consist of molecules with a different number of atoms or have a polymeric structure.

Formulas in chemistry for solving problems

The formula of the substance is established based on the results of the analysis. For example, according to the analysis, glucose contains 40% (wt.) carbon, 6.72% (wt.) hydrogen and 53.28% (wt.) oxygen. Therefore, the masses of carbon, hydrogen and oxygen are related to each other as 40:6.72:53.28. Let's designate the required glucose formula as C x H y O z , where x, y and z are the numbers of carbon, hydrogen and oxygen atoms in the molecule. The atomic masses of these elements are respectively equal to 12.01; 1.01 and 16.00 amu Therefore, the glucose molecule contains 12.01x a.m.u. carbon, 1.01u a.m.u. hydrogen and 16.00za.u.m. oxygen. The ratio of these masses is 12.01x: 1.01y: 16.00z. But we have already found this ratio, based on the data of glucose analysis. Consequently:

12.01x: 1.01y: 16.00z = 40:6.72:53.28.

According to proportion properties:

x: y: z = 40/12.01:6.72/1.01:53.28/16.00

or x: y: z = 3.33: 6.65: 3.33 = 1: 2: 1.

Therefore, in a glucose molecule, there are two hydrogen atoms and one oxygen atom per carbon atom. This condition is satisfied by the formulas CH 2 O, C 2 H 4 O 2, C 3 H 6 O 3, etc. The first of these formulas, CH 2 O-, is called the simplest or empirical formula; it corresponds to a molecular weight of 30.02. In order to find out the true or molecular formula, it is necessary to know the molecular weight of a given substance. When heated, glucose is destroyed without turning into a gas. But its molecular weight can be determined by other methods: it is equal to 180. From a comparison of this molecular weight with the molecular weight corresponding to the simplest formula, it is clear that the formula C 6 H 12 O 6 corresponds to glucose.

Thus, a chemical formula is an image of the composition of a substance using the symbols of chemical elements, numerical indices, and some other signs. There are the following types of formulas:

— protozoa , which is obtained empirically by determining the ratio of chemical elements in a molecule and using the values ​​of their relative atomic masses (see the example above);

— molecular , which can be obtained by knowing the simplest formula of a substance and its molecular weight (see the example above);

— rational , displaying groups of atoms characteristic of classes of chemical elements (R-OH - alcohols, R - COOH - carboxylic acids, R - NH 2 - primary amines, etc.);

— structural (graphic) , showing the mutual arrangement of atoms in a molecule (it can be two-dimensional (in a plane) or three-dimensional (in space));

— electronic, which displays the distribution of electrons in orbits (written only for chemical elements, not for molecules).

Let's take a closer look at the example of an ethanol molecule:

1. the simplest formula of ethanol is C 2 H 6 O;
2. the molecular formula of ethanol is C 2 H 6 O;
3. the rational formula of ethanol is C 2 H 5 OH;

Examples of problem solving

EXAMPLE 1

Exercise	With the complete combustion of oxygen-containing organic matter weighing 13.8 g, 26.4 g of carbon dioxide and 16.2 g of water were obtained. Find the molecular formula of a substance if its relative hydrogen vapor density is 23.
Solution	Let's draw up a scheme for the combustion reaction of an organic compound, denoting the number of carbon, hydrogen and oxygen atoms as "x", "y" and "z", respectively: C x H y O z + O z →CO 2 + H 2 O. Let us determine the masses of the elements that make up this substance. The values ​​of relative atomic masses taken from the Periodic Table of D.I. Mendeleev, rounded up to integers: Ar(C) = 12 a.m.u., Ar(H) = 1 a.m.u., Ar(O) = 16 a.m.u. m(C) = n(C)×M(C) = n(CO 2)×M(C) = ×M(C); m(H) = n(H)×M(H) = 2×n(H 2 O)×M(H) = ×M(H); Calculate the molar masses of carbon dioxide and water. As is known, the molar mass of a molecule is equal to the sum of the relative atomic masses of the atoms that make up the molecule (M = Mr): M(CO 2) \u003d Ar (C) + 2 × Ar (O) \u003d 12+ 2 × 16 \u003d 12 + 32 \u003d 44 g / mol; M(H 2 O) \u003d 2 × Ar (H) + Ar (O) \u003d 2 × 1 + 16 \u003d 2 + 16 \u003d 18 g / mol. m(C)=×12=7.2 g; m(H) \u003d 2 × 16.2 / 18 × 1 \u003d 1.8 g. m(O) \u003d m (C x H y O z) - m (C) - m (H) \u003d 13.8 - 7.2 - 1.8 \u003d 4.8 g. Let's define the chemical formula of the compound: x:y:z = m(C)/Ar(C) : m(H)/Ar(H) : m(O)/Ar(O); x:y:z = 7.2/12:1.8/1:4.8/16; x:y:z = 0.6: 1.8: 0.3 = 2: 6: 1. This means the simplest formula of the compound is C 2 H 6 O and the molar mass is 46 g / mol. The value of the molar mass of an organic substance can be determined using its hydrogen density: M substance = M(H 2) × D(H 2) ; M substance \u003d 2 × 23 \u003d 46 g / mol. M substance / M(C 2 H 6 O) = 46 / 46 = 1. So the formula of an organic compound will look like C 2 H 6 O.
Answer	C2H6O

EXAMPLE 2

Exercise	The mass fraction of phosphorus in one of its oxides is 56.4%. The oxide vapor density in air is 7.59. Set the molecular formula of oxide.
Solution	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%. Calculate the mass fraction of oxygen in the compound: ω (O) \u003d 100% - ω (P) \u003d 100% - 56.4% \u003d 43.6%. Let us denote the number of moles of elements that make up the compound as "x" (phosphorus), "y" (oxygen). Then, the molar ratio will look like this (the values ​​​​of relative atomic masses taken from the Periodic Table of D.I. Mendeleev will be rounded to whole numbers): x:y = ω(P)/Ar(P) : ω(O)/Ar(O); x:y = 56.4/31: 43.6/16; x:y = 1.82: 2.725 = 1: 1.5 = 2: 3. This means that the simplest formula for the combination of phosphorus with oxygen will have the form P 2 O 3 and a molar mass of 94 g / mol. The value of the molar mass of an organic substance can be determined using its density in air: M substance = M air × D air; M substance \u003d 29 × 7.59 \u003d 220 g / mol. To find the true formula of an organic compound, we find the ratio of the obtained molar masses: M substance / M(P 2 O 3) = 220 / 94 = 2. This means that the indices of phosphorus and oxygen atoms should be 2 times higher, i.e. the formula of the substance will look like P 4 O 6.
Answer	P 4 O 6