Enthalpy and Internal Energy

Enthalpy is the amount of heat absorbed during a chemical reaction, while Internal energy is the sum of kinetic energy and potential energy. Enthalpy is represented by “H”, while internal energy is represented by “E”. More differences between Enthalpy and internal energy are given below.

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What is Enthalpy?

The total heat contents of a system are called the Enthalpy of the system and are equal to the sum of internal energy and pressure-volume work.

H = E + W

                                                           H= E + PV                                           [ W = PV]

The Enthalpy of a system cannot be measured in a given state. However, the change in Enthalpy can be measured in a given state as follows:

ΔH = ΔE + Δ(PV)  …………. (i)

Enthalpy is a state function and Enthalpy change is measured in Joules.

See Also: Entropy

What is Internal Energy?

The sum of the kinetic energy, as well as the potential energies of all the particles present in the system, is called the internal energy of the system. Internal energy is represented by “E”.

Change in internal energy (ΔE) is a state function. It is not possible to measure the absolute value of internal energy. However, the change in internal energy can be determined.

The kinetic energies of the particles are due to:

  1. Translational Motion
  2. Vibrational Motion
  3. Rotational Motion

Potential energy accounts for all types of attractive forces present in the system. These Attractive forces include all types of bonds and Van der Waal’s forces present between particles.

Comparison: Enthalpy Vs Internal energy





Internal Energy

The sum of internal energy and the product of pressure and volume of the system is called Enthalpy. It is the sum of the Kinetic energy and Potential Energy of the system.
It is the function so only change in enthalpy can be measured. It is also a state function so change in it can be measured.
Its formula is: H = E + PV


Its formula is: E =  + PE
Its Units are Kcal mol-1 or KJ mol-1


Its Unit is Joules Or calories.

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