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:

Equilibrium expression linking the equilibrium concentration of reactants and products at equilibrium

• 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

Equilibrium expression linking the partial pressures of reactants and products at equilibrium

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

Answer 1

Answer 2

K_p = pCO₂(g) = kPa