Kp Expressions (AQA A Level Chemistry): Revision Note

K_p Expressions

• 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

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

Gaseous Equilibria

• If all the substances in the general equation above are gases:

aA (g) + bB (g) ⇌ cC (g) + dD (g)

• The equilibrium constant K_p is deduced from a gaseous reversible reaction equation

• K_p is defined as:

$K_p\mathit{=}\frac{p^{c}C\mathit{\left(}\mathit{g}\mathit{\right)}\mathit{\times }{p}^{d}D\mathit{\left(}\mathit{g}\mathit{\right)}}{p^{a}A\mathit{\left(}\mathit{g}\mathit{\right)}\mathit{\times }{p}^{b}B\mathit{\left(}\mathit{g}\mathit{\right)}}$

• Where:

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

  • p^aA (g) and p^bB (g) are the equilibrium partial pressures of A and B, kPa

  • p^cC (g) and p^dD (g) are the equilibrium partial pressures of C and D, in kPa

• Equilibrium partial pressure is calculated from  the:

  • The total number of moles

  • The mole fraction

  • The total pressure

• 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