Methanol is made by reacting carbon monoxide with hydrogen in a closed vessel.
CO+2H2⇌CH3OH\mathrm{CO + 2H_2 \rightleftharpoons CH_3OH}CO+2H2⇌CH3OH
The atom economy for making methanol by this reaction is 100%.
In a trial, no substances other than methanol are formed and none of the methanol is lost when it is collected. The percentage yield of methanol is 75%.
Which statement explains why the percentage yield is below 100%?
Some carbon monoxide and hydrogen remain unreacted when the reaction reaches equilibrium.
The unreacted carbon monoxide and hydrogen lower the atom economy of the reaction to 75%.
The theoretical mass of methanol is 75% of the mass predicted by the equation.
Some of the mass of the reactants is changed into heat energy during the reaction.
104 exam-style questions on AQA GCSE Chemistry 3.3 Yield and atom economy of chemical reactions, covering 3.3.1a Percentage yield, 3.3.1b Theoretical mass of a product, 3.3.2a Atom economy, and 3.3.2b Choosing a reaction pathway. Each one has a worked solution and a mark scheme showing where the marks go.