Equilibrium Constants, Kc & Kp (Cambridge (CIE) A Level Chemistry): Revision Note

Equilibrium Constant: Concentrations

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

• K_c is defined as:

$K_{\mathrm{c}}=\frac{{\left[\mathrm{C}\right]}^{\mathrm{c}}{\left[\mathrm{D}\right]}^{\mathrm{d}}}{{\left[\mathrm{A}\right]}^{\mathrm{a}}{\left[\mathrm{B}\right]}^{\mathrm{b}}}$

• Where:

  • [A] and [B] are the equilibrium concentrations of A and B, in mol dm^-3 

  • [C] and [D] are the equilibrium concentrations of C and D, in mol dm^-3

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

• Solids are ignored in equilibrium expressions

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

Mole Fraction & Partial Pressure

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:

P_total = P_A + P_B + P_C + .......

• P_total = total pressure

  • P_A, P_B, P_C = partial pressures

 How partial pressures contribute to 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

$\mathbf{Mole}\mathbf{fraction}\mathbf{=}\frac{\mathbf{number}\mathbf{of}\mathbf{moles}\mathbf{of}\mathbf{a}\mathbf{particular}\mathbf{gas}}{\mathbf{total}\mathbf{number}\mathbf{of}\mathbf{moles}\mathbf{of}\mathbf{all}\mathbf{the}\mathbf{gases}\mathbf{in}\mathbf{the}\mathbf{mixture}}$

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

$\mathbf{Partial}\mathbf{pressure}\mathbf{=}\mathbf{mole}\mathbf{fraction}\mathbf{\times }\mathbf{total}\mathbf{pressure}$

• 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 Constant: Partial Pressures

Equilibrium expressions involving partial pressures

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

Comparing K_p and K_c expressions