Equilibrium Constant (CIE AS Chemistry)

Equilibrium expression & constant

• The equilibrium expression is an expression that 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

the 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 specific and only changes if the temperature of the reaction changes

Worked example: Deducing equilibrium expressions

Answer

Partial pressure

• For reactions involving mixtures of gases, the equilibrium constant K_p is used as it is easier to measure the pressure than the concentration for gases
• The partial pressure of a gas is the pressure that the gas would have if it was in the container all by itself
• The total pressure is the sum of the partial pressure

Equation to calculate the total pressure in a mixture of gases

The diagram shows that each gas in the mixture has a partial pressure contributing towards the total pressure

Mole fraction

• The mole fraction of a gas is the ratio of moles of a particular gas to the total number of moles of gas present

Equation to calculate the mole fraction of a particular gas in a gas mixture

• To calculate the partial pressures of each gas the following relationship can be used:

Equation to calculate the partial pressure of a particular gas in a gas mixture

The sum of the mole fractions should add up to 1.00, while the sum of the partial pressures should add up to the total pressure.

Equilibrium expressions involving partial pressures

• Equilibrium expressions in terms of partial pressures are written similarly to those involving concentrations with a few differences:

The diagram shows a comparison of writing equilibrium expressions in terms of partial pressures and concentrations

Worked example: Deducing equilibrium expressions of gaseous reactions

Answer