Inorganic Chemistry
7 marks · EasyThis question is about determining the amount of oxygen in air and key ideas about oxidation and reduction.
Describe how copper metal can be used to determine the percentage of oxygen in air, including the main observations and calculation.
Explain why copper must be in excess in this experiment.
Define what is meant by 'oxidising agent' and 'reducing agent' in a chemical reaction.
State which substance is the oxidising agent when magnesium reacts with copper(II) oxide, and briefly justify your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Air is passed over heated copper in a tube, causing copper to react with oxygen to form black copper(II) oxide. The volume of air decreases as oxygen reacts. By measuring the drop in gas volume, the percentage of oxygen in air can be calculated.
(b) So that all the oxygen can react and be removed, giving an accurate measurement.
(c) An oxidising agent accepts electrons (or gives oxygen to another substance) and is reduced itself. A reducing agent donates electrons (or removes oxygen from another substance) and is oxidised itself.
(d) Copper(II) oxide. It gives oxygen to magnesium and is reduced.
Common mistakes: Omitting that the copper must be in excess; Not linking the definition of agents to electron or oxygen transfer; Failing to identify CuO as oxidising agent
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Describes air passed over heated copper and change to powder
A2 (a) · 1 mark · Observes drop in gas volume due to oxygen reacting
A3 (a) · 1 mark · Calculates oxygen % from initial and final volumes
B1 (b) · 1 mark · Explains need for copper in excess
C1 (c) · 1 mark · Defines oxidising and reducing agents in terms of electrons or oxygen
C2 (c) · 1 mark · States they are reduced/oxidised accordingly
D1 (d) · 1 mark · Identifies copper(II) oxide as oxidising agent and explains
Worked solution
a) Air is passed back and forth over heated copper, which turns from pink-brown to black as copper reacts with oxygen to form copper(II) oxide. The gas volume decreases as oxygen is removed. The percentage is calculated from the drop in volume (e.g., (initial − final)/initial × 100%).
b) Copper must be in excess to ensure all oxygen is removed, ensuring an accurate calculation.
c) An oxidising agent accepts electrons (or provides oxygen); it is reduced in the process. A reducing agent donates electrons (or removes oxygen); it is oxidised.
d) Copper(II) oxide is the oxidising agent, as it provides oxygen to magnesium and is itself reduced.