In the previous unit, we have learnt the concept of energy, enthalpy and also the enthalpy
changes which take place during physical and chemical processes. In this unit, we shall mainly focus on the standard state conditions and standard enthalpy changes of some important chemical processes along with other aspects related to them.
17.1 ENTHALPY OF A REACTION OR HEAT OF A REACTION
The enthalpy of a reaction is defined as the enthalpy change accompanying the chemical
reaction when the molar quantities of reactants and products are the same as indicated in
the chemical equation. It is also known as heat of reaction. It is represented by L\H.
FACTORS ON WHICH ENTHALPY OF A REACTIQN (A~HJ DEPENDS
The enthalpy of a reaction depends on the following factors:
1. Physical state of reactants and products. As pointed out earlier that change of physical state also involve heat changes. Thus, the value of 6H does depend upon the physical state of the reactants and products.
2. Quantities of reactants and products. The amount of heat evolved or absorbed depends upon the amounts of reactants and products. For example, combustion of 12 g(1 mol) of carbon produces 393.5 kJ of heat, while combustion of 1.2 g of carbon produces 39.35 kJ of heat.
3. Allotropic modifications. For different allotropic forms of the substances different amount of heat is evolved or absorbed.
C (graphite ) + O2 (g) à CO2 (g); ΔH = – 393.5 kj
C (diamond) + O2 (g) à CO2 ; ΔH = – 395.4 KJ.
4. Temperature. The value of ΔH also depends upon the temperature at which reactants and products are considered. For example, for a reaction
H2 (g) + CI2(g) à 2HCI
The variation of ΔH with temperature is given by Kirchhoff’s equation as[ΔH2 – H1] = ( T2 – T1) [ΣCP (P) – ΣCP ( R) ]
5. Conditions of constant pressure or constant volume. The heat of reaction also depends on the conditions of constant volume or constant pressure at a given temperature.