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.
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.