Collection of basic formulas for school chemistry course

Collection of basic formulas for school chemistry course

G. P. Loginova

Elena Savinkina

E. V. Savinkina G. P. Loginova

Collection of basic formulas in chemistry

Pocket Pupil's Guide

general chemistry

The most important chemical concepts and laws

Chemical element Is a certain kind of atoms 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 - the mass of a carbon-12 atom (12 C).

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

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

Relationships between physical quantities

Atomic mass (relative) of an element B, A r (B):

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

* t and- atomic mass unit;

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

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

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

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

Molar mass of matter B, M (B), g / mol:

Where t (B)- mass B.

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

Where V M = 22.4 l / mol (a 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 hydrogen, D(gas B to H 2):

* Density of gaseous substance V by air, D(gas B by air): Mass fraction of an element NS in substance 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 of D.I. Mendeleev

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

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

Atomic nuclear charge (Z) is equal to the number of protons in the nucleus and the number of electrons in the atom:

Z = N (p +) = N (e¯).

Isotopes- atoms of one 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, 19f¯).

* Energy levels and sublevels

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

* Forms of s- and p-orbitals

Periodic Law and Periodic Table of D.I. Mendeleev

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

Period number corresponds to the number of energy levels filled with electrons, and denotes last energy level(EH).

Group number A shows and NS.

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

S-element section- the energy sublevel (EPU) is filled with electrons ns-EPU- IA– and IIА-groups, H and Not.

Section of p-elements- filled with electrons np-EPU- IIIA-VIIIA groups.

D-element section- filled with electrons (NS- 1) d-EPU - IB-VIIIB2-group.

F-element section- filled with electrons (NS-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, degradable by water: NaH, MgH 2, AlH 3.

Volatiles: 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.

Acidic: 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 quantity characterizing the ability of an atom in a molecule to acquire a negative charge.

Mechanisms of covalent bond formation

Exchange mechanism- overlapping 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.

Overlapping of orbitals during bond formation

* Hybridization type - the geometric shape of the particle - the angle between the bonds

Hybridization of the central atom orbitals- alignment of their energy and shape.

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 (e.g. water) + solute.

True solutions contain particles less than 1 nanometer in size.

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

Inhomogeneous mixtures are separated settling and filtration.

Homogeneous mixtures are separated evaporation, distillation, chromatography.

Saturated solution is or can be in equilibrium with the solute (if the solute is solid, then its excess is in the precipitate).

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

Unsaturated solution smaller,

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

Relationships between physicochemical quantities in solution

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

Where t (B)- mass B,

t (p)- the mass of the solution.

Solution weight, m (p), g:

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.

Change in the composition of the solution

Diluting the solution with water:

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

> the mass of the solution is increased by the mass of added water: m "(p) = m (p) + m (H 2 O).

Evaporation of water from solution:

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

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

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

t "(B) = t (B) + t" (B);

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

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

m "(B) = m (B) - m (sediment); m" (p) = m (p) - m (sediment).

The mass of water does not change.

Heat effect of a chemical reaction

* Enthalpy of formation of matter ΔH° (B), kJ / mol, is the enthalpy of the reaction of the formation of 1 mol of a substance from simple substances in their standard states, that is, at 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).

* Thermal effect of a chemical reaction (Hess's law)

Q = ΣQ(products) - ΣQ(reagents).

ΔН ° = ΣΔН °(products) - Σ ΔН °(reagents).

For reaction aA + bB + ... = dD + eE + ...

ΔН ° = (dΔH ° (D) + eΔH ° (E) + ...) - (aΔH ° (A) + ЬΔH ° (B) + ...),

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

Chemical reaction rate

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

For a monomolecular reaction A →…:

v = k with (A).

For the bimolecular reaction A + B →…:

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

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

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

Changing the rate of a chemical reaction

Speed ​​reaction increase:

1) chemically active reagents;

2) the rise concentration of reagents;

3) increase

4) the rise temperature;

5) catalysts. Speed ​​reaction reduce:

1) chemically inactive reagents;

2) downgrade concentration of reagents;

3) decrease surfaces of solid and liquid reagents;

4) downgrade temperature;

5) inhibitors.

* Temperature coefficient of speed(γ) is equal to the 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 the molar concentrations of reagents 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) a decrease in the concentration of products;

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

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

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

6) a 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 some substances are dissolved in water.

acids formed hydrogen cations and acid anions, for example:

HNO 3 = 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 = Na + + NO 3 ¯

KAl (SO 4) 2 = K + + Al 3+ + 2SO 4 2-

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

NaHCO 3 = 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

Dissociation degree α- 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 the solution are irreversible if the result is a precipitate, gas, weak electrolyte.

Examples of molecular and ionic reaction equations

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

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

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

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

"Complete" ionic equation: FeS + 2Н + + 2Сl¯ = Fe 2+ + 2Сl¯ + 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

The "complete" ionic equation: 3H + + 3NO 3 ¯ + ZK + + PO 4 3- = H 3 PO 4 + 3K + + 3NO 3 ¯

"Short" ionic equation: 3H + + PO 4 3- = H 3 PO 4

* Hydrogen indicator

(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 during the interaction of an acid and a base.

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

Ва 2+ + 2OH¯ + 2Н + + 2Сl¯ = Ва 2+ + 2Сl¯ + 2Н 2 O

Н + + ОН¯ = ​​Н 2 O

2. Slightly soluble base + strong acid: Cu (OH) 2 (t) + 2HCl = CuCl 2 + 2H 2 O

Сu (ОН) 2 + 2Н + + 2Сl¯ = Сu 2+ + 2Сl¯ + 2Н 2 O

Cu (OH) 2 + 2H + = 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 = 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 ≠ ;

Сl¯ + Н 2 O ≠

No hydrolysis; neutral medium, pH = 7.

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

Na 2 S = 2Na + + S 2-

Na + + H 2 O ≠

S 2- + H 2 O ↔ HS¯ + OH¯

Anion hydrolysis; alkaline medium, pH> 7.

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

End of introductory snippet.

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Key words: Chemistry grade 8. 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 substance	m	kg, g
Mass fraction	ω	Dimensionless
Molar mass	M	kg / mol, g / mol
Molar volume	V n	m 3 / mol, l / mol
Substance volume	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
Density of matter	R	kg / m 3, g / cm 3, g / ml
Avogadro's constant	N A	1 / mol
Absolute temperature	T	K (Kelvin)
Celsius temperature	t	° C (degree Celsius)
Heat effect of a chemical reaction	Q	kJ / mol

2. Relationships between units of physical quantities

3. Chemical formulas in grade 8

4. Basic definitions in grade 8

• Atom- the smallest chemically indivisible particle of matter.
• Chemical element- a certain kind of atoms.
• Molecule- the smallest particle of a substance that retains its composition and chemical properties and consists of atoms.
• Simple substances- substances whose molecules consist of atoms of the same type.
• Complex substances- substances whose molecules consist of atoms of different types.
• Qualitative composition of the substance shows which atoms it consists of.
• The quantitative composition of the substance shows the number of atoms of each element in its composition.
• Chemical formula- conditional recording 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 of 12 C.
• Moth- the amount of a substance containing a number of particles equal to the number of atoms in 0.012 kg of carbon 12 C.
• Avogadro's constant (Na = 6 * 10 23 mol -1) - the number of particles contained in one mole.
• Molar mass of matter (M ) is the mass of a substance taken in an amount of 1 mol.
• Relative atomic mass element A 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 of 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 falls on a given element.

ATOMIC-MOLECULAR TEACHING
1. There are substances with a molecular and non-molecular structure.
2. Between the molecules there are gaps, the sizes of which depend on the state of aggregation of matter 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 ones, as a rule, they are destroyed. During chemical phenomena, atoms rearrange themselves, forming molecules of new substances.

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

VALENCE
Valence 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 phenomenon as a result of which others are formed from some substances. Reagents are substances that enter into a chemical reaction. Reaction products are substances formed as a result of a reaction.
Signs of chemical reactions:
1. Release of heat (light).
2. Change in color.
3. The appearance of odor.
4. Sediment formation.
5. Gas evolution.

several basic concepts and formulas.

All substances have different mass, density and volume. A piece of metal from one element can weigh many times more than a piece of another metal of exactly the same size.

Moth(number of moles)

designation: mole, international: mol- a unit for measuring the amount of a substance. Corresponds to the amount of substance that contains NA particles (molecules, atoms, ions) Therefore, a universal value was introduced - number of moles. A common phrase in problems - “it was received ... mole of substance "

NA= 6.02 1023

NA Is Avogadro's number. Also "number by agreement". How many atoms are there in the shaft of a pencil tip? About a thousand. It is not convenient to operate with such values. Therefore, chemists and physicists all over the world have agreed - let us designate 6.02 1023 particles (atoms, molecules, ions) as 1 mole substances.

1 mol = 6.02 1023 particles

This was the first of the basic formulas for solving problems.

Molar mass of matter

Molar mass substance is the mass of one mole of substance.

Designated as Mr. It is found according to the periodic table - it is simply the sum of the atomic masses of a substance.

For example, we are given sulfuric acid - H2SO4. Let's calculate the molar mass of a substance: atomic mass H = 1, S-32, O-16.
Mr (H2SO4) = 1 2 + 32 + 16 4 = 98 g / mol.

The second necessary formula for solving problems is

mass formula:

That is, to find the mass of a substance, you need to know the number of moles (n), and we find the molar mass from the Periodic Table.

The law of conservation of mass - the mass of substances that have entered into a chemical reaction is always equal to the mass of the substances formed.

If we know the mass (masses) of the substances that have entered the reaction, we can find the mass (masses) of the products of this reaction. And vice versa.

The third formula for solving chemistry problems is

volume of matter:

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Where does the number 22.4 come from? From Avogadro's law:

equal volumes of different gases, taken at the same temperature and pressure, contain the same number of molecules.

According to Avogadro's law, 1 mole of ideal gas under normal conditions (n.v.) has the same volume Vm= 22.413 996 (39) l

That is, if we are given normal conditions in the problem, then, knowing the number of moles (n), we can find the volume of the substance.

So, basic formulas for solving problems in chemistry

Avogadro's numberNA

6.02 1023 particles

Amount of substance n (mol)

n = V \ 22.4 (l \ mol)

Mass of substance m (g)

Volume of substance V(l)

V = n 22.4 (l \ mol)

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These are formulas. Often, to solve problems, you first need to write the reaction equation and (necessarily!) Arrange the coefficients - their ratio determines the ratio of moles in the process.

Quantity and its dimension	Ratio
Atomic mass of element X (relative)
Element ordinal	Z = N(e –) = N(R +)
Mass fraction of element E in substance X, in fractions of a unit, in%)
The amount of substance X, mol
The amount of gas substance, mol	V m= 22.4 L / mol (n.u.) Well. - R= 101 325 Pa, T= 273 K
Molar mass of substance X, g / mol, kg / mol
Mass of substance X, g, kg	m(X) = n(X)  M(X)
Molar volume of gas, l / mol, m 3 / mol	V m= 22.4 l / mol at standard conditions
Gas volume, m 3	V = V m × n
Product yield
Density of substance X, g / l, g / ml, kg / m 3
Density of gaseous substance X in terms of hydrogen
Density of gaseous substance X in air	M(air) = 29 g / mol
Unified gas law
Mendeleev-Clapeyron equation	PV = nRT, R= 8.314 J / mol × K
Volume fraction of a gaseous substance in a mixture of gases, in fractions of a unit or in%
Molar mass of gas mixture
Molar fraction of substance (X) in the mixture
Heat quantity, J, kJ	Q = n(X)  Q(X)
Heat effect of reaction	Q = -H
Heat of formation of substance X, J / mol, kJ / mol
Chemical reaction rate (mol / lsec)
Mass action law (for a simple reaction)	a A + v B = with C + d D u = k with a(A)  with v(B)
Van't Hoff's rule
Solubility of substance (X) (g / 100 g solvent)
Mass fraction of substance X in the mixture A + X, in fractions of a unit, in%
Solution weight, g, kg	m(rr) = m(X) + m(H 2 O) m(rr) = V(rr)   (rr)
Mass fraction of a solute in a solution, in fractions of a unit, in%
Density of the solution
Solution volume, cm 3, l, m 3
Molar concentration, mol / l
The degree of dissociation of the electrolyte (X), in fractions of a unit or%
Ionic product of water	K(H 2 O) =
Hydrogen exponent	pH = –lg

Main:

Kuznetsova N.E. and etc... Chemistry. 8 cells-10 cells .. - M .: Ventana-Graf, 2005-2007.

Kuznetsova N.E., Litvinova T.N., Levkin A.N. Chemistry. 11 class in 2 parts, 2005-2007.

Egorov A.S. Chemistry. New textbook for university preparation. Rostov n / a: Phoenix, 2004. - 640 p.

Egorov A.S. Chemistry: a modern course to prepare for the exam. Rostov n / a: Phoenix, 2011. (2012) - 699 p.

Egorov A.S. Self-study guide for solving chemical problems. - Rostov-on-Don: Phoenix, 2000.- 352 p.

Chemistry / tutorial-tutor for university applicants. Rostov-n / D, Phoenix, 2005– 536 p.

Khomchenko G.P., Khomchenko I.G.... Chemistry tasks for university applicants. M .: Higher school. 2007. – 302s.

Additional:

Vrublevsky A.I... Educational and training materials for preparation for centralized testing in chemistry / A.I. Vrublevsky –Mn .: LLC “Unipress”, 2004.– 368 p.

Vrublevsky A.I... 1000 problems in chemistry with chains of transformations and control tests for schoolchildren and applicants. - Minsk: OOO "Unipress", 2003. - 400 p.

Egorov A.S... All types of computational problems in chemistry to prepare for the exam. –Rostov n / a: Phoenix, 2003. – 320s.

Egorov A.S., Aminova G.Kh... Sample tasks and exercises to prepare for the chemistry exam. - Rostov n / a: Phoenix, 2005.- 448 p.

Unified State Exam 2007. Chemistry. Educational and training materials for the preparation of students / FIPI - M .: Intellect-Center, 2007. - 272 p.

Unified State Exam-2011. Chemistry. Training kit, ed. A.A. Kaverina. - M .: National Education, 2011.

The only real options for assignments to prepare for the unified state exam. Unified State Exam. 2007. Chemistry / V.Yu. Mishina, E.N. Strelnikov. M .: Federal Testing Center, 2007. – 151s.

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List of sites for chemistry:

1. Alhimik. http:// www. alhimik. ru

2. Chemistry for everyone. Electronic reference book for a complete course of chemistry.

http:// www. informika. ru/ text/ database/ chemy/ START. html

3. School chemistry - a reference book. http:// www. schoolchemistry. by. ru

4. Chemistry tutor. http: // www. chemistry.nm.ru

Internet resources

Alhimik. http:// www. alhimik. ru

Chemistry for everyone. Electronic reference book for a complete course of chemistry.

http:// www. informika. ru/ text/ database/ chemy/ START. html

School chemistry - a reference book. http:// www. schoolchemistry. by. ru

http://www.classchem.narod.ru

Chemistry tutor. http: // www. chemistry.nm.ru

http://www.alleng.ru/edu/chem.htm- educational resources of the Internet in chemistry

http://schoolchemistry.by.ru/- school chemistry. On this site there is an opportunity to pass On-line testing on various topics, as well as demo versions of the Unified State Exam

Chemistry and Life – XXI Century: Popular Science Journal. http:// www. hij. ru

>> Chemical formulas

Chemical formulas

The material in the paragraph will help you:

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

Chemical formula.

Everyone has it substances there is a name. However, by the 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.

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

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

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

The O 2 molecule is called diatomic. Simple substances of Hydrogen, Nitrogen, Fluor, Chlorine, Bromine, and Iodine are composed of such molecules (their general formula is E 2).

Ozone contains triatomic molecules, white phosphorus has tetraatomic molecules, and sulfur has octatomic molecules. (Write the chemical formulas of these molecules.)

H 2
O 2
N 2
Cl 2
Br 2
I 2

In the formula of the molecule of a complex substance, the symbols of the elements, the atoms of which are contained in it, are written, as well as the indices. The carbon dioxide molecule consists of three atoms: one carbon atom and two oxygen. Its chemical formula is CO 2 (read "tse-o-two"). Remember: if there is one atom of some element in a molecule, then the corresponding index, that is, 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. For this, a superscript is used. In it, the number indicates the amount of charge (the unit is not written), and then the 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 - СГ - ("chlorine-minus"), hydroxide ion with a charge - I - OH - (" o-ash-minus "), 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, 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 parentheses. Let's take the formula for slaked lime Ca (OH) 2 as an example. It is an ionic compound. In it, for each 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 symbols of elements, "extraneous" letters are written, as well as letters-indices. Such formulas are often referred to as general formulas. Examples of formulas of this type: ECI n, E n O m, Fe x O y. The 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 Ferrum's compound with Oxygen unknown and
it should be installed.

If you need to designate two separate atoms of Neon, two molecules of oxygen, two molecules of carbon dioxide 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. Coefficient I, like index I, is not written.

Formula unit.

And what does the 2NaCl record mean? NaCl molecules do not exist; table salt is an ionic compound that consists of Na + and Cl - ions. A pair of these ions is called the formula unit of the substance (it is highlighted in Fig. 44, a). Thus, the record 2NaCl represents two formula units of sodium chloride, i.e., two pairs of ions Na + and Cl-.

The term "formula unit" is used for complex substances of not only ionic, but also atomic structure. For example, the formula unit for quartz SiO 2 is the combination of one Silicium 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 a simple substance), molecule(in a simple or complex substance),
a set of atoms or ions (in a complex substance).

The exercise. Draw up the chemical formula of a compound that contains ions Li + i SO 2- 4. 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 call 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.

The 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 symbols of chemical elements and indices. The number of atoms of each element is indicated in the formula using a subscript, and the charge of an ion is indicated by a superscript.

Formula unit - a particle or 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 the 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 records mean: 3H 2 0, 2H, 2H 2, N 2, Li, 4Cu, Zn 2+, 50 2-, NO - 3, ЗСа (0Н) 2, 2СаС0 3?

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

71. Make the chemical formula of the molecule, which 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 of all substances, which may 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) ions H 3 O +, HPO 2- 4.

Popel P. P., Kriklya L.S., Khimia: Pidruch. for 7 cl. zagalnoosvit. navch. prl. - К .: EC "Academy", 2008. - 136 p .: il.

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