Deducing Kc Expressions (Edexcel AS Chemistry): Revision Note

Deducing Kc Expressions

Equilibrium expression & constant

• The equilibrium constant expression is an expression that links the equilibrium constant, K, 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 corresponding equilibrium constant expression is written as:

$\mathit{K}\mathbf{=}\frac{{\left[C\right]}^{\mathbf{c}}{\left[D\right]}^{\mathbf{d}}}{{\left[A\right]}^{\mathbf{a}}{\left[B\right]}^{\mathbf{b}}}$

• Where:

  • [A] and [B] = equilibrium reactant concentrations (mol dm^-3)

  • [C] and [D] = equilibrium product concentrations (mol dm^-3)

  • a, b, c and d = number of moles of corresponding reactants and products

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

Homogeneous systems and K_c 

• A homogeneous system is where all of the reactants and products are in the same physical state, e.g.

CH₃COOH (l) + C₂H₅OH (l) ⇌ CH₃COOC₂H₅ (l) + H₂O (l)

• For this reaction, all of the reactants and products are in the same, liquid state / phase and will, therefore, all feature in the K_c expression

  • K_c = $\frac{\left[{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_5\right]\left[{\mathrm{H}}_2\mathrm{O}\right]}{\left[{\mathrm{CH}}_3\mathrm{COOH}\right]\left[{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\right]}$

Heterogeneous systems and K_c 

• A heterogeneous system is where not all of the reactants and products are in the same physical state, e.g.

CaCO₃ (s) ⇌ CaO (s) + CO₂ (g)

• Solids are ignored in equilibrium expressions

  • This leads to a K_c expression of K_c = [CO₂]