The Haber process makes ammonia: N2 + 3H2 ⇌ 2NH3. The reaction is reversible and the forward reaction is exothermic. The Contact process makes sulfur trioxide for sulfuric acid: 2SO2 + O2 ⇌ 2SO3, which is also exothermic in the forward direction.
A low temperature moves the position of equilibrium towards the exothermic direction, so a low temperature gives a high yield of ammonia. Raising the temperature lowers the yield. But a low temperature makes the reaction too slow, giving a slower rate of production. So a compromise temperature of about 450°C is used, with an iron catalyst. A catalyst speeds up the forward and reverse reactions equally, so it does not change the position of equilibrium.
There are four molecules of gas on the left of the Haber equation and two on the right. A high pressure therefore moves equilibrium to the right and also makes the reaction faster. About 200 atmospheres is used, because higher pressures need stronger equipment and more energy. In the Contact process the yield is already high, so only about 2 atmospheres is used, with a temperature of about 450°C and a vanadium(V) oxide catalyst.