4.25     predict the effects of changing the pressure and temperature on the equilibrium position in reversible reactions.

Le Chatelier’s principle tells us that a system at dynamic equilibrium will respond to oppose any change make to it.  We can predict what happens when we change the temperature and pressure.

e.g.       The Haber Process  (see Section 5.23)

N2(g) + 3H2(g) ⇌ 2NH3(g)         ∆H = –93 kJ/mol  (the forwards reaction is exothermic)

Pressure

·        If we increase the pressure, the system will try to reduce it.

·        There are more molecules in total on the left (4) than the right (2).

·        Turning 4 molecules into 2 (the forward reaction) reduces pressure.

·        So, if we increase pressure, equilibrium “shifts right”. This increases the yield of ammonia.

Temperature

·        If we increase the temperature, the system will try to reduce it.

·        The forward reaction is exothermic (gets hotter).

·        This means the reverse reaction must be endothermic (gets colder).

·        So, if we increase the temperature, equilibrium “shifts left”. This decreases the yield of ammonia.

You should be able to apply these arguments to other reactions in the exam such as the formation of sulphur trioxide in the manufacture of sulfuric acid, (Section 5.26).

The arguments for the sulphur trioxide equation are identical to those above. The equation is:

2SO2(g) + O2(g) ⇌ 2SO3(g)

It has more molecules on the left than the right, and is also exothermic. The pressure used is a lot lower than in the Haber Process, but that is simply because the yield at a low pressure is already quite high for this particular reaction, and the cost of increasing the pressure further is not worth the modest extra yield it would achieve.