Enthalpy and entropy concept

Enthalpy, H, is a thermodynamic property of a system. It is defined as the sum of internal energy E of a system and the product of the pressure and volume of the system.

H=E PV

The PV term represents the mechanical work done on or by the system.

Since we are usually more interested in changes than in absolute values, we can write

ΔH=ΔE PΔV

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I.e., the change in enthalpy is the sum of the change in the internal energy and the work done.

Example:

What is the change in enthalpy in a reaction if it releases 100 kJ of energy and the PΔV work is 10 kJ?

Solution:

ΔH=ΔE PΔV = -100 kJ 10 kJ = -90 kJ

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Entropy, S, is an another thermodynamic property, which we can consider as a measure of the disorder or randomness of a system.

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An ordered system has low entropy. A disordered system has highentropy.

For example, in the solid state, molecules are strongly attracted to each other (less disorder). In the gaseous state, molecules are not strongly attracted (more disorder). Because of that, entropy is greater in a gas.

ΔS equals the heat transferred between the system and its surroundings divided byT.

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