Yield

Yield is the amount of product formed in a reaction. In a reversible industrial reaction it is decided by the position of equilibrium, which depends on temperature and pressure but not on a catalyst. Malay: hasil · Chinese: 产率.

Because the industrial reactions in this chapter are reversible and their forward reactions are exothermic, temperature affects the yield in a way students must reason about carefully. Lowering the temperature shifts the equilibrium toward the products and raises the yield of an exothermic reaction, while raising the temperature lowers it. Pressure matters when the number of gas molecules changes across the equation: a higher pressure shifts the equilibrium toward the side with fewer gas molecules and so raises the yield there. A catalyst changes neither of these, because it speeds the forward and backward reactions equally.

Example. In the Haber process, N₂ + 3H₂ ⇌ 2NH₃, four gas molecules become two, so a high pressure raises the yield of ammonia. In the Contact process the yield of sulfur trioxide is already high at low pressure, so a very high pressure is not worth its cost.

Confusion to avoid. Do not confuse yield with rate. Yield is how much product is obtained; rate is how fast it forms. A catalyst raises the rate but leaves the yield unchanged, and a high temperature can raise the rate while lowering the yield of an exothermic reaction.

A neat way to keep this straight in an answer is to ask two separate questions of every change: does it move the equilibrium, and does it change the speed. Temperature and pressure can do both, a catalyst does only the second, and stating this cleanly is what earns the mark rather than a vague comment that a change “helps the reaction”.

Being able to separate what changes the yield from what changes the rate is the central skill this chapter tests, and it comes up repeatedly in SPM Chemistry.