Kp Expressions (AQA A Level Chemistry)

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

Gaseous Equilibria

• This section covers how we manage gases in calculating the equilibrium constant and how an equilibrium yield is affected by the partial pressures of reactants and products

• Many industrial process involve reactions between gases so this application has important consequences for controlling reaction conditions

• 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

• 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