When a process undergoes at constant pressure, the heat observed/ released is equal to the change in Enthalpy. It is sometimes referred as heat content. Enthalpy is the sum of the Internal energy and the product of the Pressure and Volume. Mathematically,

Enthalpy (H) = U + PV


U = Internal Energy of the system,

P = Pressure of the system,

V = Volume of the system.

The quantity U + PV is known as the enthalpy of the system and is denoted by H. It represents the total energy stored in the system. Internal energy and enthalpy are also an extensive property as well as a state function. The absolute enthalpy can’t be measured, however the change in enthalpy can be written as,

ΔH = ΔU + Δ (PV)

At constant temperature, for the process heat flow (q) is equal to the change in enthalpy. Mathematically,

ΔH = q

The energy transferred between a system and the surroundings under constant pressure. We can’t measure absolute enthalpy of a system but we can measure the change in enthalpy for a process. If the initial and the final states have enthalpies Hᵢ and Hf respectively then the change in enthalpy for the process can be written as,

ΔH = Hf – Hᵢ

How to find the Change in Enthalpy and Internal Energy?

Problem: For a particular reaction, the system absorbs 7kJ of heat and does 1.5 kJ of work on its surroundings. What is the change in Internal energy and changes in Enthalpy of the system?

Solution: Given,

Heat Absorbed (q) = + 7 kJ

Work done on the surroundings (W) = – 1.5 kJ

Change in Internal Energy (ΔU) =?

Change in Enthalpy (ΔH) =?

Using First law of thermodynamics:

ΔU = q + W

ΔU = 7 kJ – 1.5 kJ = 5.5 kJ

ΔH = qp = Heat supplied at constant pressure = + 7 kJ

Therefore, Heat change in Internal Energy is 5.5 kJ and change in Enthalpy is 7 kJ.