Chemical Thermodynamics: Exothermic and Endothermic Reaction (For CBSE, ICSE, IAS, NET, NRA 2022)

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Exothermic Reaction

  • Exothermic reactions are those reactions which proceed with the evolution of heat.
  • For example, When a fuel like cooking gas or coal is burnt in air, heat is evolved, Adding dilute hydrochloric acid in a test tube containing a few pieces of granulated zinc.

Endothermic Reaction

  • Endothermic reactions are those reactions which proceed with the absorption of heat from the surroundings.
  • For example, Adding a small amount of solid ammonium chloride in a test tube half-filled with water.

Thermo Chemical Equations

  • The chemical equations which will specify heat energy changes and states of the reactants and products. These are called the thermochemical equations.
  • Gaseous, Liquid and Solid states are represented by putting symbols (g) , (l) , and (s) alongside the chemical formulae, respectively.
  • The amount of heat evolved or absorbed by a symbol H.
  • The amount of heat evolved or absorbed is written after the equation followed by semicolon.
  • H is negative for exothermic reactions and it is positive for endothermic reactions.

For example, to represent burning of methane in oxygen, we write

H is , so it is exothermic reaction.

An endothermic reaction is written as

First Law of Thermodynamics

  • According to this law , Energy can neither be created nor destroyed. The total energy of the universe or an isolated system is constant.
  • Mathematically, It is stated as

U = change in internal energy, q = heat absorbed by the system, and w = work done on the system.

Internal Energy (U)

  • The internal energy may be defined as the sum of the energies of all the atoms, molecules or ions contained in the system.
  • Every system has a definite amount of energy. This amount is different for different substances.
  • It is not possible to measure the absolute values of internal energy . So, we calculate the change in internal energy.

If the internal energy of the system in the initial state is and in the final state is then change in internal energy is independent of the path taken from the initial state to final state. we can write as

The internal energy of the system can be changed either by allowing heat to flow into the system or out of the system; and by work done on the system or by the system.

Work of Expansion

Let us assume that pressure p is constant and the volume of the system changes from to . The work done by a system is given as

Here, w is because the work is done by the system.

From first law of thermodynamic

Substituting w in this equation

If the process is carried ot at constant volume, i.e..

v in subscript denotes that volume is constant.

Enthalpy (H)

Enthalpy is denoted by the symbol H and is given by

Enthalpy change, is given by

If the change is carried out at constant pressure, then . Then equation (2) is written as

(at Constant Pressure)


(at Constant Pressure) or

Relation between and

Let, be the total volume of the gaseous reactants, the total volume of the gaseous products, the number of moles of gaseous reactants, and the number of moles of gaseous products, at constant pressure and temperature.

Using ideal gas law, we can write

Subtracting equation (1) from equation (2) , we get

At constant pressure

For solids and liquids is very small. We can neglect the term , hence is nearly equal to .

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