5.1.3 Reaction Conditions & The Equilibrium Constant

• Changes in temperature change the equilibrium constants K_c and K_p
• For an endothermic reaction such as:

   An increase in temperature:

   [H₂] and [I₂] increases

   [HI] decreases

   Because [H₂] and [I₂] are increasing and [HI] is decreasing, the equilibrium constant increases

• For an exothermic reaction such as:

   An increase in temperature:

   [SO₃] decreases

   [SO₂] and [O₂] increases

   Because [SO₃] decreases and [SO₂] and [O₂] increases the equilibrium constant  decreases

Worked Example

Answer

• • Only temperature changes permanently affect the value of K_p
  • An increase in temperature shifts the reaction in favour of the products.
  • The [ products ] increases and [ reactants ] decreases, therefore, the K_p value increases.

• How the equilibrium shifts with temperature changes:

Effect on the value of the equilibrium constant

• For a reaction that is exothermic in the forward direction, increasing the temperature pushes the equilibrium from right to left
• Therefore, the value of the equilibrium constant will decrease as the ratio of [ products ] to [ reactants ] decreases
• Conversely, if the temperature is raised in an endothermic reaction, the value of the equilibrium constant will increase

• If all other conditions stay the same, the equilibrium constant K_c 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
• If all other conditions stay the same, the equilibrium constant K_c is not affected by any changes in pressure of the reactants and products
• The value of K_p 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 K_p 
• This is analogous to what happens to K_c when you change concentration in an aqueous equilibrium; a shift restores equilibrium to a new position maintaining K_c

Presence of a catalyst

• If all other conditions stay the same, the equilibrium constants K_p and K_c are 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