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the concentration of the product is decreased

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.