Example

Consider the following two reactions –

It is observed that there is difference in the value of ΔH if water is obtained in gaseous or liquid state. ΔH value in second case is higher because heat is evolved when steam condenses. Hence, physical state always affects the heat of reaction.

Allotropic forms of the elements

Heat energy is also involved when one allotropic form of an element is converted into another. Thus, the value of ΔH depends on the allotropic form used in the reaction.

Example:

The  value of ΔH is different when carbon in the form of diamond or graphite is used.

The difference between two values is equal to the heat absorbed when 12 g of diamond is converted into 12 g of graphite

Reaction carried out at constant pressure or constant volume

When a chemical reaction  occurs at constant volume, the heat change is called the internal energy. However, most of the reactions are carried out at constant pressure and the enthalpy change is termed as the energy of reaction at constant pressure.

The relation between ΔH (Enthalpy change) and ΔE (Internal energy change) is given as follows:

ΔE + Δn_gRT = ΔH

Dn_g =             (Total number of moles of products) – (total number of moles of  reactants).

R = Gas Constant

T = Temperature (in Kelvin)

The difference between ΔH and ΔE value is negligible when solids and liquids are involved in a chemical change. But, in reactions which involve gases, the difference in two values is considerable.