Spontaneity of Chemical Reaction: Entropy Change in Phase Transition (For CBSE, ICSE, IAS, NET, NRA 2022)

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Entropy Change in Phase Transition

When a solid melts and produces liquid, the process occurs at the melting point of the solid.

The heat involved in the process of melting a solid is called enthalpy of fusion Therefore, the entropy of fusion is given by,

T is the melting point of the solid.

For example, ice melts at 273 K and produces water at the same temperature.

Similarly, for the equilibrium

T is the boiling point of liquid.


The enthalpy change for the transition of liquid water to steam at is . Calculate the entropy change for the process.


Third Law of Thermodynamics and Absolute Entropy

  • The entropy of a pure and perfectly crystalline solid at absolute zero temperature is zero. This is known as the Third law of thermodynamics.
  • When temperature of a substance is increased, the translational, vibrational and rotational motions become more vigorous. It leads to greater disorder and as a result the entropy of the substance increases.
  • Thus, on increasing the temperature of a substance the entropy of a substance increases. It decreases on decreasing the temperature of the substance.
  • The third law of thermodynamics helps us to calculate the absolute value of molar entropies of substances at different temperatures.

The Standard molar entropies of some substances at are given in Table

Standard Molar Entropies at

Standard Molar Entropies

Entropy Change for a Reaction

Entropy Change for a reaction can be determined by subtracting the standard entropies of reactants from those of products. Thus, for a general reaction


Calculate the entropy change for the following reaction at .

Given that the standard molar entropies of and at are and respectively.


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