Deducing Kc Expressions (Edexcel A Level Chemistry)

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:

• • 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 = 

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]