You will learn in this chapter about:
* Exothermic and Endothermic reaction.
* Heat of Reactions.
* Measurement of heat of reactions.
* Heat of Neutralization.
The chemical reactions during which material changes are
accompanied with change in heat energy are called thermo
chemical reactions. The branch of chemistry which deals
with the study of heat changes in chemical reactions is
called thermo chemistry. There are two types of thermo
chemical reactions i.e. exothermic and endothermic
10.1 EXOTHERMIC AND ENDOTHERMIC REACTIONS
10.1.1 Exothermic Reaction:
It is Greek word and composed of EXO means out of or to
evolve + THERME means heat. An exothermic reaction is
the chemical change during which heat is given out or
released. The change of heat is represented by AH and it
is shown by negative, sign.
The combustion of coal in air is the example of
exothermic reaction. 393.7 Kilo joules of heat energy is
released when 1 mole of coal is burnt in 1 mole of O2
to produce 1 mole of CO2.
C(s) + O2 (g)
CO2 (g) ΔH = - 393.7 K.J/mol
Burning of methane in presence of oxygen is another
example of exothermic reaction. When 1 mole of methane
is burnt in 2 moles of O2 then 1 mole of CO2
and 2 moles of water are formed. During these reaction
890 Kilo joules per mole of heat energy is released.
CH4 (g) + 2O2 (g)
CO2 (g) + 2H2O (1) ΔH = -890 K.J/mol.
The formation of water from hydrogen and oxygen is also
example of exothermic reaction 286 Kilo joules per mole
of heat energy is released, when 1 mole of H2
reacts with ½ moles of O2 to form 1 mole of H2O.
H2 (g) + O2 (g)
H2O (1) ΔH = -286 K.J/mol.
Generally in exothermic reactions, heat flows from the
system to surroundings and container becomes hot, this
is only possible, when total energy of the reactants is
greater than total energy of products. The difference in
the energies is the heat supplied by the system to
Using Exothermic Reactions to Warm
In modern army, food rations can be warmed without
benefit of stove or campfire. The pouch that contains
the food is attached to flameless radiation heater. The
heater contains chemicals that react with water to
produce heat. When the pouch is placed in a bag and
water added, temperature of the food reaches to 60°C in
about 15 minutes.
Mg(s) + 2H2O(1)
Mg (OH) 2(s) + H2 (g) ΔH=
The reaction of (Mg) with water is slow, because of the
formation of film oxide (MgO). The reaction of Mg with
water is highly accelerated in the presence of iron (Fe)
and ordinary salt (NaCl). Thus, the flameless radiation
heater contains a mixture of Mg, Fe and NaCl.
10.1.2 Endothermic Reactions:
It is Greek word and composed of ENDO mean into or to
absorb and THERME means heat. Endothermic reaction is
the chemical change during which heat is absorbed or
taken in. The change of heat energy is represented by Δ
H and sign of A H is positive (+Δ H= Absorbs heat).
During the endothermic reaction heat is absorbed from
the surroundings, it means heat flows from surroundings
to the system and container becomes cold. This is
because the total energy of the products is greater than
the total energy of reactants. Here, the difference in
the energies is the heat supplied to the system by the
The decomposition of water into hydrogen and oxygen is
example of endothermic reaction. During decomposition of
1 mole of water to 1 mole of hydrogen and half mole of
oxygen 286 Kilo joules per mole of heat energy is
------à H2 (g) +1 O2
(g) ΔH = +286 KJ/mol
1 mole of nitric oxide (NO) is formed by combination of
½ mole of N2 and ½ mole of O2
.This is the example of endothermic reaction and heat
absorbed is about 90.25 Kilo joules per mole.
½ N2 (g) +½ O2 (g)
------à NO (g) ΔH = +
10.2 HEAT CONTENTS OF REACTION
Every substance possesses a characteristic internal
energy and internal energy depends upon the structure
and physical state of that substance. The energy
possessed by a substance is called heat contents of that
substance. During a chemical reaction, the reactants are
converted into products and heat energy is either
absorbed or evolved. This is because the heat contents
of these respective substances are different. The heat
evolved or absorbed at constant pressure is called as
enthalpy of the reaction.
The heat content (enthalpy) of a substance is
represented by "H" and the change in heat content during
a chemical reaction is then represented by ΔH. Here
Greek letter Δ (delta) signifying the change in the
property. It means the change in heat content during
chemical reaction is the difference between the heat
content of products and reactants of that reaction.
It is difficult to measure the enthalpy of a reaction,
but we can measure the change in enthalpy which is
denoted by (ΔH). It is obtained by subtracting the
enthalpy of reactant (HJ from enthalpy of products (H2).
ΔH = (H2
Change of Enthalpy Heat content of product Heat
content of reactions
If enthalpy of products is greater than the enthalpy of
reactants, then the sign of A H will be positive and
over all reaction is endothermic and heat is absorbed.
½ N2 (g) +½ O2 (g)
NO (g) ΔH = + 90.25 KJ/mol
C(s) + H2O (1)
CO (g) + H2 (g) ΔH = + 118 KJ/mol.
------à H2 (g) + ½ O2
(g) ΔH = + 286 KJ/mol.
If enthalpy of product is smaller than the reactants,
then the sign of ΔH will be negative and overall
reaction is exothermic and heat is evolved. The examples
of exothermic reactions are following:
H2O (1) +½ O2 (g)
H2O (l) ΔH = - 286 KJ/mol.
S (g) + O2 (g)
ΔH = -296.8 KJ/mol.
CH4 (g) +O2 (g)
CO2 (g) +2H2O (1) ΔH = -890.4
N2 (g) + 3H2 (g)
2NH3 (g) ΔH = -92.0 KJ/mol.