Predict what changes the equilibrium constant
Distinguish a change in equilibrium position from a change in Kc or Kp.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Equilibrium constants apply at a fixed temperature; include only gaseous or aqueous species in the relevant expressions. For acid–base work, distinguish strength from concentration. Written practice is self-checked
Before you begin
- Balance equations, interpret reversible arrows and use concentration in mol dm⁻³.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
At a fixed temperature, what does adding reactant do to K?
What you will learn
- State which condition changes the equilibrium constant
- Distinguish a composition shift from a change in K
- Explain why a catalyst does not change K
At fixed temperature, K stays the same
For a particular balanced reaction, equilibrium constant Kc or Kp has a fixed value at a fixed temperature. Changing concentrations or pressure can shift the equilibrium composition, but the system returns to the same K value at that temperature.
Only a temperature change changes the value of the equilibrium constant for the reaction. The new equilibrium composition changes accordingly.
A catalyst changes rates, not equilibrium composition
A catalyst lowers the activation energy for both forward and reverse pathways, so equilibrium is reached faster. It does not favour one direction at equilibrium and does not change K.
Use Le Chatelier's principle for the position shift and the equilibrium-constant rule for K itself; they answer different questions.
Pause and explain
Which change alters the value of an equilibrium constant?
Worked example
Which changes alter K for a reaction: adding reactant, increasing pressure, adding a catalyst, or increasing temperature?
Show the worked solution
- Adding reactant can shift the equilibrium position.
- Pressure can shift a gaseous equilibrium; a catalyst changes rates.
- Only a temperature change changes K.
Answer Only increasing temperature changes the equilibrium constant.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
An equilibrium is disturbed by adding product at constant temperature. State what changes and what remains the same.
Give me a hint
Apply Le Chatelier to composition, then use the fixed-temperature K rule.
Compare my reasoning
- Adding product shifts position toward reactants.
- Concentrations change as the system re-equilibrates.
- K has the same value at the unchanged temperature.
The equilibrium shifts toward reactants; composition changes, but K remains unchanged at constant temperature.
Look for these in your work
- Position shift correct
- Composition change stated
- K invariant stated
A catalyst is added to a mixture at equilibrium. Predict the effect on rate, equilibrium position and K.
Give me a hint
Compare forward and reverse rates rather than just the initial reaction.
Compare my reasoning
- A catalyst increases both forward and reverse rates.
- It does not favour a direction, so equilibrium position is unchanged.
- K is unchanged at the same temperature.
Equilibrium is approached faster if disturbed, but the position and K are unchanged by adding a catalyst.
Look for these in your work
- Both rates increased
- No position shift
- K unchanged
A gaseous equilibrium is compressed at constant temperature and its composition shifts. Explain why the equilibrium constant is still unchanged.
Give me a hint
Separate the effect on concentrations from the fixed-temperature K rule.
Compare my reasoning
- Compression changes partial pressures and can shift position.
- The system re-establishes equilibrium.
- At the same temperature, K retains its value for the balanced reaction.
Compression can change the equilibrium composition, but K remains unchanged because temperature is constant.
Look for these in your work
- Pressure effect on position recognised
- New equilibrium allowed
- K's temperature dependence stated
Common mistakes
- Saying concentration changes K
- Saying pressure changes Kp at fixed temperature
- Claiming a catalyst increases K
- Confusing new equilibrium composition with a new K value
What can you explain now?
At a fixed temperature, what does adding reactant do to K?
Compare with the explanation
It does not change K
It may shift composition, but K remains fixed at that temperature.
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Make it stick
Which changes affect composition, and which one changes the value of K?
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