Thermochemistry is the study of heat changes in chemical reactions. Exothermic reactions evolve heat while endothermic reactions absorb heat. The enthalpy of a reaction is the quantity of heat absorbed or evolved. Standard enthalpy of reaction is measured under standard conditions. Hess's law states that the enthalpy change of a reaction is the same regardless of the reaction pathway. Thermochemistry is applied in pyrometallurgy which involves processes like calcination, roasting, smelting, and refining to extract metals from ores.

2. M. Ali Mohsin MME-16-04

3. Thermochemistry
 The study of heat changes
accompanying a chemical reaction
is known as thermochemistry.

4. Heat of a reaction
In a chemical reaction, the energy in the form of heat will either be evolved or
absorbed and this is called Heat of Reaction.
Heat is denoted by the symbol q.
• The sign of q is +ve if heat is Absorbed by the system.
• The sign of q is -ve if heat is Evolved by the system.

5. Exothermic Reactions
 In all chemical changes energy, when heat is evolved by the reaction is
called Exothermic Reaction.
C + O2 CO2 ∆H = -393.7 kj/mol
Endothermic Reactions
 In all chemical changes energy, when heat is absorbed by the reaction is
called Endothermic Reaction.
N2 + O2 2NO ∆H = +180.51 kj/mol

6. Heat of Reaction
Exothermicity
“out of” a system
Dq < 0
Endothermicity
“into” a system
Dq > 0
Energy
System
Surroundings
Energy
System
Surroundings

7. Spontaneous Reaction
 A process which takes place on its own without any outside assistance and
moves from a non-equilibrium state towards an equilibrium state is termed
as Spontaneous Process or Natural Process.
 Water flows from Higher level to lower level.
 Neutralization of a strong acid with a strong base.
NaOH +HCl NaCl +H2O

8. Non Spontaneous Reaction
A process which doesn’t take place on its own and take place with
external assistance is termed as Non Spontaneous Reaction.
 Pumping of water uphill.
 Transfer of heat from cold interior part of the refrigerator to the hot
surroundings.
 When nitrogen reacts with oxygen, nitric oxide is formed. This
reaction takes place by the absorbance of heat.
N2 + O2 2NO

9. Enthalpy
 To take account of increase in internal energy and accompanying work done by
the gas, there is another property of the system is called Enthalpy or Heat
Content.
 It is represented by H.
H = E + PV
 Enthalpy is a State Function.
 It is measured in Joules.
 It is not possible, to measure the enthalpy of a system in a given state. however
change in enthalpy can be measured.

10. Enthalpy of a Reaction
The standard enthalpy of reaction is enthalpy of change which occurs when the certain
number of moles of reactants as indicated by the balanced chemical equation, react
together completely to give the products under standard condition, i.e 25◦C (298K) and one
atmospheric pressure.
2H2 + O2 2H2O ∆H = -285.8 KJ/mol
Enthalpy of Formation
The standard enthalpy of formation of a compound is the amount of heat absorbed or
evolved when one mole of the compound is formed from its elements.
The enthalpy of formation, for MgO is -692kJ/mol
Mg + 1/2O2 MgO ∆Hf=-692kj/mol

11. Enthalpy of Atomization
The amount of heat absorbed when one mole of gaseous atoms are formed
from the elements under standard conditions.
For example, the standard enthalpy of atomization of hydrogen under
standard conditions is given:
½ H2 H ∆Hat=218 kj/mol
Enthalpy of Combustion
The amount of heat evolved when one mole of a substance is completely burnt in
excess of oxygen under standard conditions.
For example, standard enthalpy of combustion of ethanol is ∆Hc = -1368 kj/mol.
C2H5OH + 3O2 2CO2 +3H2O ∆Hc = -1368 kj/mol

12. Enthalpy of Neutralization
 The amount of heat evolved when one mole of hydrogen ions from an acid, react with one mole of
hydroxide ions from a base react to form one mole of water, is called enthalpy of neutralization.
 For example, the enthalpy of neutralization of sodium hydroxide by hydrochloric acid is -57.4
Kj/mol.
HCl H+ (aq) + Cl-
NaOH Na+ + OH- (aq) ∆Hn = -57.4 Kj/mol
Enthalpy of Solution
The amount of heat absorbed or evolved when one ole of a substance is dissolved in so much
solvent that further dilution results in no detectable heat change.
For example , the enthalpy of solution of ammonium chloride is +16.2kj/mol and sodium carbonate
is -25.0kj/mol.

13. Measurement of Enthalpy of a Reaction
1. Glass Calorimeter
 An ordinary glass calorimeter is used to find the value of ∆H for various
purposes. It consists of:
(a) Insulated container
(b) Thermometer
(c) Stirrer
Working
 Reactants are placed in calorimeter.
 When the reaction proceeds, the heat energy evolved or absorbed will
either warm or cool the system.
 The temperature of the system is recorded before and after the chemical
reaction.
 Knowing the change in temperature, mass of reactants & specific heat
of the reaction
Q = m × s × ∆T

14. 2. Bomb Calorimeter
A bomb calorimeter is used for the accurate determination of the enthalpy of
combustion for food, fuel and other compounds.
It consists of a strong cylindrical steel vessel usually lined with enamel to prevent
corrosion.
 WORKING
 A known mass is placed in a platinum crucible inside the Bomb.
 The lid is screwed & oxygen is provided to maintain the
 Pressure around about 20atm.
 The initial temperature is measured
 The enthalpy of combustion can be calculated from this formula
q = c x ∆T

15. Hess's Law Of Constant Heat Summation
“If a chemical change takes place by several different routes, the overall
enthalpy change is the same, regardless of the route by which the
chemical change occurs (provided the initial and final conditions are the
same).”
 Single Step Process
2NaOH + CO2 Na2CO3+H2O ∆H=-89.08kJ
 Two Step Process
NaOH+CO2 NaHCO3 ∆H1=-48.06kJ
NaHCO3+NaOH Na2CO3+H2O ∆H2=-41.02kJ
According to Hess’s law:
∆H=∆H1+∆H2
-89.08=-48.06-41.02
-89.08=-89.08

16. Application of Thermochemistry
Pyrometallurgy:
 Pyrometallurgy is a branch of Extractive Metallurgy. It consists of the
thermal treatment of minerals and metallurgical ores and concentrates to
bring about physical and chemical transformations in the materials to enable
recovery of valuable metals
Pyrometallurgical processes are generally grouped into one or more of the
following categories:
1. Calcination,
2. Roasting,
3. Smelting,
4. Refining.

17. 1. Calcination:
 The process of calcining - heating a substance to a high temperature, but
below its melting point, to bring about thermal decomposition.
2. Roasting:
 Roasting is a process of heating of sulphide ore to a high temperature in
presence of air.
 This process is similar like the reaction happening with Blast Furnace.

18. 3. Smelting:
 Smelting involves thermal reactions in which at least one product is a molten
phase.
 Metal oxides can then be smelted by heating with coke or charcoal (forms of
carbon), a reducing agent that liberates the oxygen as carbon dioxide leaving
a refined mineral.
4. Refining:
 Refining is the removal of impurities from materials by a thermal process.
 This covers a wide range of processes, involving different kinds of furnace or
other plant.