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the common-ion effect.

Common-ion effect

In solutions the change in equilibrium position can come about due to the common-ion effect. The common-ion effect is where one substance releases ions (upon dissociating or dissolving) which are already present in the equilibrium reaction.

If solid sodium chloride is added to an aqueous solution and dissolves, the following dissociation occurs:

NaCl(s)NaCl(s) →→ Na+(aq)+Cl−(aq)Na+(aq)+Cl−(aq)

If that solution contains the following equilibrium:

HCl(l)+H2O(l)HCl(l)+H2O(l) ⇌⇌ Cl−(aq)+H3O+(aq)Cl−(aq)+H3O+(aq)

The added Cl−Cl− ion (common-ion) interferes with the equilibrium by raising the concentration of the Cl−Cl− ion. According to Le Chatelier’s principle the reverse reaction speeds up as it tries to reduce the effect of the added Cl−Cl−. As a result the equilibrium position shifts to the left.

The effect of temperature on equilibrium (ESCNQ)

If the temperature of a reaction mixture is changed, the equilibrium will shift to minimise that change.

  • If the temperature is increased the equilibrium will shift to favour the reaction which will reduce the temperatureThe endothermic reaction is favoured.
  • If the temperature is decreased the equilibrium will shift to favour the reaction which will increase the temperatureThe exothermic reaction is favoured.

For example the forward reaction shown below is exothermic (shown by the negative value for ΔHΔH). This means that the forward reaction, where nitrogen and hydrogen react to form ammonia, gives off heat, increasing the temperature (the forward reaction is exothermic). In the reverse reaction, where ammonia decomposes into hydrogen and nitrogen gas, heat is taken in by the reaction, cooling the vessel (the reverse reaction is endothermic).

Remember that heat is released during an exothermic reaction. It is a product of the reaction.

N2(g)+3H2(g)⇋2NH3(g)ΔH=−92N2(g)+3H2(g)⇋2NH3(g)ΔH=−92 kJkJ

  • An increase in temperature:
    • Favours the endothermic reaction because it takes in energy (cools the container).
    • The reverse reaction is endothermic, so the reverse reaction is favoured.
    • The yield of ammonia (NH3)(NH3) will decrease.
  • decrease in temperature:
    • Favours the exothermic reaction because it releases energy (warms the container).
    • The forward reaction is exothermic, so the forward reaction is favoured.
    • The yield of NH3NH3 will increase.