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5.1.2 How far

Equilibrium

Types of equilibria

  • Homogenous equilibria
    • Contains equilibrium species that all have the same state or phase
    • \(K_c\) contains concentrations of all species
  • Heterogenous equilibria
    • Contains equilibrium species that all have different states or phases
    • \(K_c\) only contains concentrations of gaseous or aqueous species (concentration of solid and liquid is constant)

Equilibrium constant \(K_p\)

Mole fraction

  • \(\text{mole fraction } x(A) = \frac{\text{number of moles of A}}{\text{total number of moles in gas mixture}}\)
  • (for gases mole fraction can also be calculated by volume of gas divided by total volume)
  • Sum of mole fractions = 1

Partial pressure

  • The contribution that a gas makes to the total pressure \(P\)
  • \(\text{partial pressure } p(A) = x(A) \times P\)
  • Sum of partial pressures = total pressure

Changes that affect equilibrium

Effect of temperature change

  • When the forward reaction is exothermic
    • \(K_c\) or \(K_p\) decreases as the temperature goes up
    • \(K_c\) or \(K_p\) increases as the temperature goes down
  • When the forward reaction is endothermic
    • \(K_c\) or \(K_p\) increases as the temperature goes up
    • \(K_c\) or \(K_p\) decreases as the temperature goes down

Effect of concentration / pressure change

  • The value of \(K_c\) / \(K_p\) stays constant
  • e.g. concentration of one of the reactants increase
    • The ratio is now less than \(K_c\)
    • The system is no longer in equilibrium
    • Concentration of products increase + Concentration of reactants decrease to restore ratio
  • Total pressure changes
    • Partial pressure + ratio change
    • Exported image

Effect of catalysts on \(K\)

  • Equilibrium reached quicker
  • No change in value of \(K_c\) / \(K_p\) or equilibrium position