Equilibrium Constant Kp for Homogeneous Systems (Oxford AQA International A Level (IAL) Chemistry): Revision Note

K_p Expressions

• The equilibrium expression links the equilibrium constant, K_c, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account

• So, for a given reaction:

aA + bB ⇌ cC + dD

Gaseous Equilibria

• If all the substances in the general equation above are gases:

aA (g) + bB (g) ⇌ cC (g) + dD (g)

• The equilibrium constant K_p is deduced from a gaseous reversible reaction equation

• K_p is defined as:

• Where:

  • a, b, c and d are the respective number of moles of each reactant and product

  • p^aA (g) and p^bB (g) are the equilibrium partial pressures of A and B, kPa

  • p^cC (g) and p^dD (g) are the equilibrium partial pressures of C and D, in kPa

• Equilibrium partial pressure is calculated from  the:

  • The total number of moles

  • The mole fraction

  • The total pressure

• Solids and liquids are ignored in K_p equilibrium expressions

• The K_p of a reaction is constant and only changes if the temperature of the reaction changes

K_p, Equilibrium & Condition Changes

• Le Chatelier's Principle can be applied to gaseous equilibria in the same way it is applied to aqueous systems

  • It states that if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this change

• It can be used to predict the qualitative effects of changes in temperature and pressure on:

  • The position of equilibrium

  • The value of K_p

How temperature affects K_p

• For a reaction that is exothermic in the forward direction:

  • Increasing the temperature pushes the equilibrium to the left

  • This means that there are more reactants and less products

  • The ratio of products to reactants decreases

  • Therefore, the value of K_p decreases

• For a reaction that is endothermic in the forward direction:

  • Increasing the temperature pushes the equilibrium to the right

  • This means that there are less reactants and more products

  • The ratio of products to reactants increases

  • Therefore, the value of K_p increases

How pressure affects K_p

• 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 similar to K_c when you change concentration in an aqueous equilibrium; a shift restores equilibrium to a new position maintaining K_c

How a catalyst affects K_p

• If all other conditions stay the same, K_p 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