Heat Content or Enthalpy
When the change of state of a system is brought about at constant pressure, there will be a change in volume. The heat transferred such a process is known as heat content or enthalpy and is denoted by H.
It may be defined as:
H = E + PV
Where E is an internal energy. P and V are the pressure and volume of the system respectively.
Heat change at constant pressure may be expressed as:
Where, = Heat of reaction at constant pressure
= Change in internal energy
= No. of moles of gaseous reactants
R = Gas constant
T = Absolute temperature
If are the heat of reaction at the temperatures respectively, then
Where = Molar heat capacity of products — molar heat capacity of reactants at constant pressure. Similarly, if are the change in internal
Where is the difference in the heat capacities of the products and reactants at constant volume.
These relations of are known as Kirchhoff’s equations.
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