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Thermal effects

The chemical reactions take place through the breaking of

chemical bonds and the forming of new ones. Therefore,

chemical reactions are accompanied by important thermal

effects (heat release or absorption).

Exothermic reactions = reactions that take place with heat

release.

Endothermic reactions = reactions that take place with

heat absorption.

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Chemical reactions are represented using chemical

equations.

Reactants = substances initially involved in a chemical

reaction. They are written in the left term of the equation.

Reaction products = substances formed in a chemical

reaction. They are written in the right term of the equation

Because in a chemical reaction, the nature of atoms of

the substances is not changed, the chemical equations are

equalized so that the number of atoms of a certain element

from the left term is equal to the one from the right term.

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Let’s consider the chemical reaction between hydrogen and

chlorine, when hydrochloric acid is formed:

H2 + Cl2 = 2HCl

For the hydrochloric acid we chose the coefficient 2 so that

the number of chlorine atoms, as well as the number of

hydrogen atoms is not modified.

The primary signification of this chemical reaction is that a

hydrogen molecule interacts with a chlorine molecule in

order to form two molecules of hydrochloric acid.

During this transformation, the covalent bonds: H – H

and Cl – Cl are broken, and a new bond is formed: H – Cl.

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The chemical equations have the same properties as

mathematical equations. Thus, the equation can be

multiplied with Avogadro’s number, and we obtain:

NA H2 + NA Cl2 = 2 NA HCl

The second signification of the chemical equation is:

that 1 mole of hydrogen reacts with 1 mole of chlorine to

obtain 2 moles of hydrochloric acid.

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In some situations, in order not to create confusion, chemical

formulas of the reactants and the reaction products are

followed by the symbol of the aggregation state written

between brackets:

2Na (s) + 2H2O (l) = 2NaOH (aq) + H2 (g)

The next symbols are used: s – solid, l – liquid, g –

gas, aq – aqueous solution.

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Classification of chemical reactions

It is very difficult to choose unique and well defined

criteria for the chemical reactions classification. One criterion

can be the way the reactants interact in order to form the

reaction products. Based on these criteria, we can

distinguish:

combination reactions (synthesis),

decomposition reactions,

single displacement reactions,

double displacement reactions.

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N2 + 3H2 = 2NH3

Fe + S = FeS

Ca + Cl2 = CaCl2

SO3 + H2O = H2SO4

a) Combination reactions (synthesis) are reactions in

which two substances interact to form a single compound.

There are many examples for this:

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CaCO3 = CaO + CO2

4NH4NO3 = 3N2 + N2O4 + 8H2O

Fe2(SO4)3 = Fe2O3 + 3SO3

b) Decomposition reactions are transformations in which

from one substance, two or more substances are formed:

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Fe + CuSO4 = Cu + FeSO4

Mg + 2H2O = Mg(OH)2 + H2

Zn + 2HCl = ZnCl2 + H2

Cl2 + 2KI = 2KCl + I2

c) Single displacement or substitution reactions are

transformations in which one element or one group of

elements from a combination is replaced with another

element or group of elements:

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Pb(NO3)2 + 2KI = PbI2 + 2KNO3

AgNO3 + KCl = AgCl + KNO3

H2SO4 + BaCl2 = BaSO4 + 2HCl

CaCl2 + K2CO3 = CaCO3 + 2KCl

d) Double displacement or coupling substitution is

transformation in which two elements or groups of elements

are exchanged between two chemical combinations:

A special case of double substitution reactions is the reaction

between acids and bases:

H2SO4 + 2NaOH = Na2SO4 + 2H2O

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Based on the nature of the reactants or products there are:

- combustion reactions

- hydrolysis reaction

- precipitation and complexation reactions

a) Combustion reactions: oxygen reacts with a carbon

compound containing hydrogen and/or other element like O,

S, N. Example: the combustion of hydrocarbons (toluene,

methane, acetylene), alcohols (methanol) or sulfur

compounds (thiophene)

C6H5-CH3 + 9O2 = 7CO2 + 4H2O

CH4 + 2O2 = CO2 +2H2O

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2C2H2 + 5O2 = 4CO2 + 2H2O

2CH3OH + 3O2 = 2CO2 + 2H2O

C4H4S + 6O2 = 4CO2 + 2H2O + SO2

The burning of carbon can also be considered a

combustion reaction: C + O2 = CO2

b) Hydrolysis reaction: the reactant is water; this reactions

are frequent in inorganic chemistry as well as in organic

chemistry:

Al2(SO4)3 + 6H2O = 2Al(OH)3 + 3H2SO4

R-CN + H2O = R-CONH2

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c) The precipitation and complexation reactions: the

classification criteria is the nature of the reaction products:

Pb(NO3)2 + K2SO4 = PbSO4 + 2KNO3

CoCl3 + 6NH3 = [Co(NH3)6]Cl3

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Stoichiometric calculation

Stoichiometric calculation is based on the law of

conservation of mass:

In a chemical reaction, the mass of the reactants is

equal to the mass of the reaction products.

Let us consider the reaction between metallic sodium

and water that occurs according to the chemical equation:

2Na + 2H2O = 2NaOH + H2

Atomic masses: Na – 23, H – 1, O – 16.

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In a vessel filled with sufficiently enough water we

introduce 0.23 g sodium. Calculate the quantity (mass) of

water that has reacted, as well as the quantities (masses) of

sodium hydroxide and hydrogen that have resulted.

The quantity of water that has reacted with sodium:

2·23g Na………………………………2·18g H2O

0.23g Na………………………………x g H2O

________________________________________

20.23 2 18x 0.18gH O

2 23

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Similarly, we calculate the mass of the resulted NaOH:

2·23g Na………………………………2·40g NaOH

0.23g Na………………………………x g NaOH

_________________________________________

NaOH0.23 2 40

x 0.4g2 23

The resulted hydrogen mass:

2·23g Na………………………………2g H2

0.23g Na………………………………x g H2

_____________________________________

2

0.23 2x 0.01gH

2 23

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We can calculate directly the volume of H2 that results

from the reaction in normal conditions of temperature and

pressure:

2·23g Na………………………………22.4L H2 (cn)

0.23g Na………………………………x L H2 (cn)

___________________________________________

2(cn)0.23 22.4

x 0.12LH2 23