3.8 CHAPTER 3. REACTION RATES
- The same would be true if the concentration ofthe product (NH 3 ) was changed.
If [NH 3 ] decreases, the forwardreaction is favoured andif [NH 3 ] increases, the
reverse reaction is favoured.
The effect of temperature on equilib-
rium
ESCBE
If the temperature of areaction mixture is increased, the equilibriumwill shift to de-
crease the temperature.So it will favour the reaction which will use up heat energy,
in other words the endothermic reaction. The opposite is true if the temperature is de-
creased. In this case, the reaction that produces heat energy will be favoured, in other
words, the exothermic reaction.
The reaction shown below is exothermic (shown by the negative valuefor Δ H). This
means that the forward reaction, where nitrogenand hydrogen react to form ammonia,
gives off heat. In the reverse reaction, where ammonia is broken down into hydrogen
and nitrogen gas, heat isused up and so this reaction is endothermic.
N 2 (g) + 3H 2 (g)� 2 NH 3 (g) ΔH =− 92 kJAn increase in temperature favours the reaction that is endothermic (the reverse re-
action) because it usesup energy. If the temperature is increased, then the yield of
ammonia (NH 3 ) decreases.
A decrease in temperature favours the reaction that is exothermic (the forward reaction)
because it produces energy. Therefore, if the temperature is decreased, then the yield
of NH 3 increases.
The following rules willhelp in predicting the changes that take place in equilibrium
reactions:
- If the forward reactionthat forms the product is endothermic, then anincrease
in temperature will favour this reaction and theyield of product will increase.
Lowering the temperature will decrease the product yield. - If the forward reactionthat forms the product is exothermic, then a decrease in
temperature will favourthis reaction and the product yield will increase.Increas-
ing the temperature willdecrease the product yield. - Increasing the pressurefavours the side of the equilibrium with the least number
of gas molecules. Thisis shown in the balanced symbol equation. This rule
applies in reactions withone or more gaseous reactants or products. - Decreasing the pressure favours the side of theequilibrium with the most num-
ber of gas molecules. This rule applies in reactions with one or moregaseous
reactants or products.