Changes in concentration

• If all other conditions stay the same, the equilibrium constant?Kc?is?not affected?by any changes in concentration of the reactants or products
• For example, the decomposition of hydrogen iodide:

2HI ? H₂?+ I₂

The equilibrium expression is:

Adding more HI makes the ratio of [ products ] to [ reactants ] smaller

To restore equilibrium, [H₂] and [I₂] increases and [HI] decreases

Equilibrium is restored when the ratio is 6.25 x 10^-3?again

Changes in pressure

• A change in pressure?only?changes the?position?of the equilibrium?(see Le Chatelier’s principle)
• If all other conditions stay the same, the equilibrium constant?Kc?is?not affected?by any changes in pressure of the reactants and products

Changes in temperature

• Changes in temperature?change?the equilibrium constant?Kc
• For an endothermic reaction such as:

An increase in temperature:

[H₂] and [I₂]?increases

[HI]?decreasesBecause [H₂] and [I₂] are?increasing?and [HI] is?decreasing, the equilibrium constant?Kc?increases

• For an exothermic reaction such as:

An increase in temperature:

[SO₃]?decreases

[SO₂] and [O₂]?increasesBecause [SO₃]?decreases?and [SO₂] and [O₂]?increases?the equilibrium constant?Kc?decreases

Presence of a catalyst

• If all other conditions stay the same, the equilibrium constant?Kc?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

Worked example: Factors affecting?Kc

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.

Worked example: Factors which increase?Kp?value

Answer

Only temperature changes permanently affect the value of?Kp

An increase in temperature shifts the reaction in favour of the products.

The [ products ] increases and [ reactants ] decreases, therefore, the?Kp?value increases.