Changing Conditions - General
Effects of temperature
- How the equilibrium shifts with temperature changes:
Effect on the value of Kc
- For a reaction that is exothermic in the forward direction, increasing the temperature pushes the equilibrium from right to left
- Therefore, the value of Kc will decrease as the ratio of [ products ] to [ reactants ] decreases
- Conversely, if the temperature is raised in an endothermic reaction, the value of Kc will increase
Effect on the value of Kp
- For a reaction that is exothermic in the forward direction, increasing the temperature pushes the equilibrium from right to left
- Therefore, the value of Kp will decrease as the ratio of [ products ] to [ reactants ] decreases
- Conversely, if the temperature is raised in an endothermic reaction, the value of Kp will increase
Effects of pressure
- Changes in pressure only affect reactions where the reactants or products are gases
- How the equilibrium shifts with pressure changes:
Effect on the value of Kc
- The value of Kc is not affected by any changes in pressure as this does not involve gases.
Effect on the value of Kp
- The value of Kp is not affected by any changes in pressure.
- Changes in pressure cause a shift in the position of equilibrium to a new position which restores the value of Kp
- This is analogous to what happens to Kc when you change concentration in an aqueous equilibrium; a shift restores equilibrium to a new position maintaining Kc
Presence of a catalyst
- If all other conditions stay the same, the equilibrium constant Kp is not affected by the presence of a catalyst
- A catalyst speeds up both the forward and reverse reactions at the same rate so the ratio of [ products ] to [ reactants ] remains unchanged
- Catalysts only cause a reaction to reach equilibrium faster
- Catalysts therefore have no effect on the position of the equilibrium once this is reached
Worked example
Factors affecting Kc
An equilibrium is established in the reaction
AB (aq) + CD (aq) ⇌ AC (aq) + BD (aq) ΔH = +180 kJ mol-1
Which factors would affect the value of Kc in this equilibrium?
Answer
-
- Only a change in temperature will affect the value of Kc and any other changes in conditions would result in the position of the equilibrium moving in such way to oppose this change.
- Adding a catalyst will increase the rate of reaction meaning the state of equilibrium will be reached faster but will have no effect on the position of the equilibrium and therefore Kc is unchanged.
