Why doesn't adding a solid change the equilibrium constant
Adding a solid does not change the equilibrium constant because the activity of a pure solid is defined as 1 and is omitted from the K expression, so its amount does not appear in the calculation.
Chemistry · Chemical equilibrium
Why solids are omitted from K
In the law of mass action the equilibrium constant uses activities. For a pure solid (or pure liquid) the activity is defined as 1, because its concentration or pressure does not change as the reaction proceeds.
Implications of adding more solid:
- The reaction quotient Q remains unchanged
- The concentrations of aqueous or gaseous species stay the same
- The calculated K value is unaffected
Writing the equilibrium expression for a heterogeneous system:
- 1Write the balanced equation including the solid
- 2Omit the solid from the K expression
- 3Include only species whose amounts change (aq, g)
Comparison of expressions
| Species | Included in K? |
|---|---|
| CaCO₃(s) | No |
| Ca²⁺(aq) | Yes |
| CO₃²⁻(aq) | Yes |
Example: CaCO₃(s) ⇌ Ca²⁺(aq) + CO₃²⁻(aq). If [Ca²⁺]=0.010 M and [CO₃²⁻]=0.010 M, K = (0.010)(0.010)=1.0×10⁻⁴. Adding extra CaCO₃(s) does not change those concentrations, so K stays 1.0×10⁻⁴.
Check yourself
What does the activity of a pure solid equal in the equilibrium expression?
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