Le Chatelier's Principle

  • At equilibrium, the forward and reverse reactions proceed at the same rate (speed of reaction.)

  • Equilibrium is disturbed when something happens to cause one of the reactions to speed up or slow down.

  • If the disturbance causes the forward reaction to speed up, the product will be produced at a faster rate so their concentrations will increase and the reaction concentration will decrease.

  • Le Chatelier’s Principle states that when a system at equilibrium is subjected to a stress, the equilibrium will shift in the direction that minimizes the effect of the stress.

  • There are three types of stresses that can disturb equilibrium:

    1. Change in concentration (adding more) - If a concentration of one of the substances increases, the rate of the reaction will increase in order to “use up” the additional substance and the opposite direction will be favored.

    2. Change in temperature - An increase in temperature will favor the reaction direction that absorbs heat.

    3. Change in pressure/volume - Changing the pressure or volume of a container enclosing an equilibrium system will only affect the reaction if gases are present. If you increase the pressure of a system at equilibrium (typically reducing the volume of the container), the reaction will favour the side with the fewest moles of gas. If both sides of the equation have the same number of moles of gas, there will be no change in equilibrium.

  • In the reaction: Fe3+ (aq) + SCN- (aq) (equilibrium arrows) FeSCN2+ (aq)

    • Stress: Adding Fe3+ will cause the reaction to shift to the right. Conversely, adding SCN- will also shift the reaction to the right, while removing either Fe3+ or SCN- will shift the reaction to the left.

    • Adding heat to the products (right) side will cause the reaction to shift to the right. Similarly, removing heat from the system will shift the reaction to the left, favoring the formation of reactants.

      • The [Fe3+] would decrease and [FeSCN2+] would increase

    • Increasing the pressure would shift the reaction to the reactant side, as systems tend to favor the side with the least amount of gas.

      • In summary, Le Chatelier's Principle demonstrates how a system at equilibrium responds to changes in concentration, temperature, or pressure by shifting the position of equilibrium to counteract the stress applied.