A thermochemical equation may be defined as a chemical equation which indicates the amount of heat evolved or absorbed during the chemical reaction in addition to the quantities of reactants or products. It may be noted that in thermochemical equations the fractional coefficients are also commonly used contrary to our usual practice for balancing the chemical equations. A thermochemical equation can be written in two ways:

(a) Heat effect can be written as one of the term along with the products.

For example

2SO_{2}(g) + O_{2} à 2SO_{3}(g) + 694.6 kj

(b) Heat evolved or absorbed can be expressed in terms of Δ.H.

For example,

2SO_{2}(g) +O_{2}(g) –7 2SO_{3}(g); 6.H = – 694.6 kJ.

It is very important to mention the physical states of various reactants and products while writing thermochemical equations because change of physical state is also accompanied by the enthalpy changes. For example: When 1 mol of hydrogen gas reacts with f mol of oxygen gas to produce 1 mol of liquid water, 286 kJ of heat is produced.

When 1 mol of hydrogen gas reacts with 1/2 mol of oxygen gas to produce 1 mol of liquid water, 286 kJ of heat is produced

H_{2}(g) + 1/2 O_{2}(g) à H_{2}O(g) ; ΔH = – 249 KJ

On the other hand, if 1 mol of water vapours is produced instead of 1 mol of liquid water, the value of 6.H will be different.

H_{2}(g) + 1/2 O_{2}(g) à H_{2}O(g) ; ΔH = – 249 KJ

**Some Important Conventions about Thermochemical Equations**

1. The coefficients of various substances of chemical equation represent the number of their respective moles. In thermodynamic interpretation of an equation, we never interpret the coefficients as number of molecules. Hence, it is acceptable to write coefficients in fractions wherever necessary.

2. The value of 6.H in a thermochemical equation corresponds to the enthalpy change taking place when a specified number of moles of various reactants and products (as indicated by the coefficients of the various substances in the chemical equation), are involved in the reaction.

3. In case of the coefficients in the chemical equation are multiplied or divided by some integer, the 6.H value must also be multiplied or divided by the same integer. For example:

H_{2}(g) + 1/2 O_{2}(g) à H_{2}O(l) ; ΔH = – 286 KJ