Question

Explain why pure liquids and solids can be ignored while writing the equilibrium constant expression?

Answer 1

Explanation:

While writing the equilibrium constant expression, the concentrations of pure solids and pure liquids are omitted because their concentrations remain constant throughout the reaction.

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Reason:

• The concentration (or density) of a pure solid or pure liquid does not change with the progress of the reaction.
• Their activity is taken as unity (1).
• Since equilibrium constant expressions involve only variable concentrations, constant terms are absorbed into the value of the equilibrium constant.

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Illustration:

For the reaction:
CaCO₃(s) ⇌ CaO(s) + CO₂(g)

The equilibrium constant expression is:

K_c = [CO₂]

The solids CaCO₃ and CaO are not included in the expression.

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Conclusion:

Thus, pure solids and pure liquids are ignored in equilibrium constant expressions because their concentrations remain constant and do not affect the position of equilibrium.