Download Chemical reactions

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.3
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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a) Combination reactions (synthesis) are reactions in
which two substances interact to form a single compound.
There are many examples for this:
N2 + 3H2 = 2NH3
Fe + S = FeS
Ca + Cl2 = CaCl2
SO3 + H2O = H2SO4
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b) Decomposition reactions are transformations in which
from one substance, two or more substances are formed:
CaCO3 = CaO + CO2
4NH4NO3 = 3N2 + N2O4 + 8H2O
Fe2(SO4)3 = Fe2O3 + 3SO3
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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:
Fe + CuSO4 = Cu + FeSO4
Mg + 2H2O = Mg(OH)2 + H2
Zn + 2HCl = ZnCl2 + H2
Cl2 + 2KI = 2KCl + I2
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d) Double displacement or coupling substitution is
transformation in which two elements or groups of elements
are exchanged between two chemical combinations:
Pb(NO3)2 + 2KI = PbI2 + 2KNO3
AgNO3 + KCl = AgCl + KNO3
H2SO4 + BaCl2 = BaSO4 + 2HCl
CaCl2 + K2CO3 = CaCO3 + 2KCl
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
________________________________________
x
0.23  2  18
 0.18gH2O
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
_________________________________________
x
0.23  2  40
 0.4gNaOH
2  23
The resulted hydrogen mass:
2·23g Na………………………………2g H2
0.23g Na………………………………x g H2
_____________________________________
x
0.23  2
 0.01gH2
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)
___________________________________________
0.23  22.4
x
 0.12LH2 (cn)
2  23
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