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ChemistryEdu Logo Chemical Equilibrium | Le Chatelier's Principle#

Le Chatelier's Principle#

Le Chatelier's Principle states that if any change is imposed on equilibrium state, then equilibrium will shift itself in that direction in which the effect of that change is nullified.

Effect of change in concentration of reactants and products#

  • Let us consider a reaction: A + B ⇌ C + D
  • If concentration of reactants is increased, then to achieve equilibrium again, the concentration of reactants needs to de decreased. For this to happen, the reaction will shift in forward direction.
  • If concentration of products is increased, then to achieve equilibrium again, the concentration of products needs to be decreased. For this to happen, reaction will shift in backward direction.
  • If concentration of reactants is decreased, then to achieve equilibrium again, the concentration of reactants needs to be increased. For this to happen, reaction will shift in backward direction.
  • If concentration of products is decreased, then to achieve equilibrium again, the concentration of products needs to be increased. For this to happen, reaction will shift in forward direction.

Effect of change in temperature#

  • Exothermic reactions are favoured at low temperature and endothermic reactions are favoured at high temperature.
  • For exothermic reaction, if temperature is increased, then reaction will shift in backward direction and if temperature is decreased, then reaction will shift in forward direction.
  • For endothermic reaction, if temperature is increased, then reaction will shift in forward direction and if temperature is decreased, then reaction will shift in backward direction.

Effect of change in pressure#

  • Let us consider a reaction: N2(g) + 3H2(g) ⇌ 2NH3(g)
\[K_p = {(p_{NH_3})^2 \over p_{N_2} \times (p_{H_2})^3}\]
  • Also, we know that partial pressure of a gas depends on total pressure and mole fraction of gas. Partial pressure = Mole fraction * Total pressure
\[K_p = {P^2 \times (Χ_{NH_3})^2 \over {P \times Χ_{N_2} \times P^3 \times (Χ_{H_2})^3}}\]
\[K_p = {{1 \over P^2} \times {(Χ_{NH_3})^2 \over {Χ_{N_2} \times (Χ_{H_2})^3}}}\]

  • In this reaction, if total pressure P is increased, then to maintain equilibrium, the moles of NH3 should increase. Hence, reaction will move in forward direction.

  • Similarly, if total pressure P is decreased, then to maintain equilibrium, the moles of NH3 should decrease and that of reactants should increase. Hence, reaction will move in backward direction.

Conclusion

  • On increasing pressure, the reaction will move in that direction in which total number of gaseous moles is less and on decreasing pressure, the reaction will move in that direction in which total number of gaseous moles is more.

  • The reaction for which Δng = 0 has no effect of pressure.

Effect of addition of inert gas#

  • If inert gas is added at constant volume, then there will be no effect on equilibrium state. This is because at constant volume if inert gas is added, then total pressure increases but mole fraction of individual gases decreases, so partial pressure of individual gases will remain same.
  • If inert gas is added at constant pressure, then mole fraction of individual gases will decrease. This decreases the individual partial pressure of each gas. However, the direction of reaction will depend on Δng.

Let us consider an example where Δng > 0:

\[A_{(g)} ⇌ B_{(g)} + C_{(g)}\]
\[K_p = {p_B \times p_C \over p_A}\]
\[K_p = {P \times Χ_B \times P \times Χ_C \over P \times Χ_A}\]
\[K_p = {P {(n_B/N) \times (n_C/N) \over (n_A/N)}}\]
\[K_p ∝ {1 \over N},\ where\ N = total\ moles\ of\ gases\]

Since no. of gaseous moles are increased and Qp will become less than Kp, the reaction will shift in forward direction to achieve equilibrium.

Conclusion

If inert gas is added at constant pressure:

  • Δng > 0 : Forward Shift

  • Δng < 0 : Backward Shift

  • Δng = 0 : No effect on equilibrium

Effect of catalyst#

  • Catalyst is a substance which can alter the rate of a reaction.
  • Catalyst has no effect on equilibrium state but it changes the time taken to achieve equilibrium state.
  • Positive catalyst decreases the time taken to achieve equilibrium while negative catalyst increases the time taken to achieve equilibrium.