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:

\Delta H = \Delta E + P \Delta V \\[3mm] = \Delta E + \Delta n RT


Where, \Delta H = Heat of reaction at constant pressure

\Delta E = Change in internal energy

\Delta n = No. of moles of gaseous reactants

R = Gas constant

T = Absolute temperature

If \Delta H_1 \text{and} \Delta H_2 are the heat of reaction at the temperaturesT_1 and T_2 respectively, then

\Delta C_p = \dfrac{\Delta H_2- \Delta H_1}{T_2- T_1}


Where \Delta C_P=  Molar heat capacity of products — molar heat capacity of reactants at constant pressure. Similarly, if \Delta E_1 and \Delta E_2 are the change in internal

\Delta C_v = \dfrac{\Delta E_2- \Delta E_1}{T_2- T_1}

Where \Delta C_c is the difference in the heat capacities of the products and reactants at constant volume.

These relations of \Delta C_P \text{and are} \Delta C_V are known as Kirchhoff’s equations.

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