Enthalpy: Definition and Derivation

You already know that heat absorbed or released by a body is related to its internal energy. The heat absorbed is equal to the change in the internal energy. This is given by ∆U = qV. But this equation holds good only at a constant volume. However, when heat is absorbed or released by a body, there is a change in its volume. The alternate solution is to carry out the reaction at a constant pressure – in flasks and test tubes. Hence, there is a need for another state function which defines the change in internal energy at a constant pressure. The concept of enthalpy comes into picture here.

enthalpy

Enthalpy

At a constant pressure, the equation for change in internal energy, ∆U = q + w can be written as

∆U = qP – p∆V

Where qP represents the heat absorbed by the system at a constant pressure and – p∆V is the expansion work done due to the heat absorbed by the system.

We can write the above equation in terms of initial and final states of the system as:

UF – UI = qP –p(VF – VI)

Or qP = (UF + pVF) – (UI + pVI)

Enthalpy H can be given by H = U + PV. Substituting it in the above equation, we get:

qP = HF – HI = ∆H

Hence, change in enthalpy ∆H = qP, which is the heat absorbed by the system at a constant pressure.

At constant pressure, we can also write,

∆H = ∆U + p∆V

Some pointers to be kept in mind:

  • In exothermic reactions, heat from the system is lost to the surrounding. For such reactions, ∆H is negative.
  • In endothermic reactions, heat is absorbed by the system from the surroundings. For such reactions, ∆H is positive.

To learn more about enthalpy, the differences between enthalpy and energy, and to watch video lectures on these topics, download Byju’s The Learning App.


Practise This Question

The bond enthalpies of H2,X2 an HX are in the ratio of 2 : 1 : 2. If enthalpy of formation of HX is 50 kJ mol1, the bond enthalpy of X2 is