Percentage yield; why yields are less than 100 percent; atom economy; and how both are used to judge how efficient and sustainable a reaction is.

What this dot point is asking

AQA wants you to calculate percentage yield, explain why yields are below 100 percent, calculate atom economy, and use both measures to discuss how efficient and sustainable an industrial process is. Yield and atom economy answer two different questions: yield asks "how much of the possible product did we actually get?", while atom economy asks "how much of the reactant mass ended up as useful product rather than waste?". A good industrial process needs both to be high.

Percentage yield

The theoretical mass is the maximum that could form, found from the balanced equation and reacting masses. The actual mass is what is really obtained in the laboratory or factory. Because the theoretical mass is a reacting-mass calculation, every percentage-yield question is really a two-stage problem: first work out the maximum mass from moles, then divide the actual mass by it.

Why yields are below 100 percent

These reasons are about losing or failing to form product; they do not change the atom economy, which is a fixed property of the chosen reaction equation.

Atom economy

Atom economy measures how much of the mass of the reactants ends up as the useful product rather than waste. Because the total mass of products equals the total mass of reactants (conservation of mass), you can use the sum of the product formula masses as the denominator. A reaction that makes only the desired product (such as a simple addition reaction) has an atom economy of 100 percent.

Judging efficiency and sustainability

A reaction with a high atom economy wastes fewer raw materials and produces less waste, so it is more sustainable and often more profitable. Industry chooses processes that balance a high yield with a high atom economy, while also considering whether by-products can be sold, the energy needed, and the safety and cost of the reactants. A by-product that has a use (for example sold to another industry) effectively raises the usefulness of the process even if the headline atom economy for one product is low.

Try this

Q1. A reaction has a theoretical yield of $50$ g but produces $40$ g. Calculate the percentage yield. [2 marks]

• Cue. $(40 / 50) \times 100 = 80\%$.

Q2. Give one reason why the percentage yield of a reaction may be less than 100 percent. [1 mark]

• Cue. Product is lost in transfer (or the reaction is incomplete or reversible, or there are side reactions).

Q3. For the addition reaction $C_2H_4 + Br_2 \rightarrow C_2H_4Br_2$, state the atom economy and explain your answer. [2 marks]

• Cue. 100 percent, because there is only one product, so all the reactant atoms end up in the useful product.