Equilibrium Constant & Temperature (Edexcel International A Level (IAL) Chemistry): Revision Note

Changing Conditions - Temperature

• Changes in temperature change the equilibrium constants K_c and K_p

• For an endothermic reaction such as:

2HI (g) ⇌ H₂ (g) + I₂ (g)

K_c = $\frac{\left[{\mathrm{H}}_2\right]\left[{\mathrm{I}}_2\right]}{{\left[\mathrm{HI}\right]}^2}$

• With 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:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

K_c = $\frac{{\left[{\mathrm{SO}}_3\right]}^2}{{\left[{\mathrm{SO}}_2\right]}^2\left[{\mathrm{O}}_2\right]}$

• With an increase in temperature:

  • [SO₃] decreases

  • [SO₂] and [O₂] increases

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

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