Predict the effect of an increase in pressure and temperature on the production of ammonia in the reaction: N2(g) + 3H2(g) <-> 2NH3 (g) , where the change in enthalpy is -92.4 kJ

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An increase in temperature and an increase in pressure will have contradictory effects on the reaction.

The effect of an increase in the temperature of a system, on the equilibrium of a reaction, is dependent on whether the reaction is endothermic (positive change in enthalpy) or exothermic (negative change in enthalpy).

The forward reaction is exothermic since the change in enthalpy is negative. If we increase the temperature of the system the extra heat energy is converted into chemical energy. Thus, the position of equilibrium moves towards the endothermic reaction. In the case of our reaction an increase in temperature would shift the position of equilibrium towards the endothermic backwards reaction, reducing the yield of ammonia produced.

A change in pressure will only have an effect on equilibria involving gaseous molecules. An increase in pressure results in more molecules per area within the system. The effect of this upon the position of equilibrium is dependent on the gas molecules on either side of equilibrium. The system reacts to an increase in pressure by favouring a reduction in pressure. Since we have four gas molecules on the left and two on the right of our reaction, an increase in pressure will shift the position of equilibrium to the right, favouring the forward reaction and increasing the overall yield of ammonia produced.

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