Deducing Kp Expressions (Edexcel A Level Chemistry): Revision Note

Kp Expressions

• We have seen previously that equilibrium reactions can be quantified by reference to an equilibrium expression and equilibrium constant

• 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

• K_c is defined as follows:

K_c = [C]c[D]d[A]a[B]b

• Where:

  • [A] and [B] = the equilibrium reactant concentrations (mol dm^-3)

  • [C] and [D] = the equilibrium product concentrations (mol dm^-3)

  • a, b, c and d = the number of moles of the respective reactants and products

• Solids are ignored in equilibrium expressions

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

Homogeneous reactions

• In the generic example above, if all the substances are gases, we can show the equation with that state symbol

aA (g) + bB (g) ⇌ cC (g) + dD (g)

• We can write a different equilibrium expression in terms of the partial pressure of the gases

• This equilibrium constant is called K_p and is defined as follows

K_p = pcC (g)eqm×pdD (g)eqmpaA (g)eqm×pbB (g)eqm

• Where:

  • p = the equilibrium partial pressure of each gas in atm

  • a, b, c and d = the coefficients in the balanced equation.

Heterogeneous reactions

• For heterogenous reactions, 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