Inorganic Chemistry
7 marks · EasyThis question is about methods for showing the percentage of oxygen in air and spectator ions in displacement reactions.
Describe one method for measuring the percentage of oxygen in air, including the apparatus used and the main steps.
A student adds iron filings to copper(II) sulfate solution. Write the full word equation for this reaction and explain what is observed.
Write the full ionic equation for the reaction and identify the spectator ions.
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(a) Using copper: 100 cm³ of air is passed over excess heated copper filings in a silica tube between two gas syringes. The copper turns black as it reacts with oxygen to form copper(II) oxide. When the volume of gas stops decreasing, measure the remaining volume to calculate oxygen percentage. · must show mentions copper or iron filings with appropriate apparatus, describes reaction with oxygen, states measurement of volume decrease
(b) Iron + copper(II) sulfate → iron(II) sulfate + copper. Observation: iron filings become coated with brown copper, and blue colour fades. · must show correct word equation, mentions brown copper deposit and fading blue solution
(c) Full ionic equation: Fe(s) + Cu²⁺(aq) + SO₄²⁻(aq) → Fe²⁺(aq) + SO₄²⁻(aq) + Cu(s). Spectator ion: SO₄²⁻. · must show correct full ionic equation, SO₄²⁻ identified as spectator ion
Common mistakes: Omitting apparatus details; Not subtracting spectator ions when writing ionic equations
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Mentions use of copper or iron filings in suitable apparatus
M2 (a) · 1 mark · Describes a reaction with oxygen (e.g., copper turns black or iron rusts)
A1 (a) · 1 mark · Mentions measurement of volume decrease enables percentage calculation
B1 (b) · 1 mark · Correct word equation given
B2 (b) · 1 mark · Correct observations stated
B3 (c) · 1 mark · Correct full ionic equation written
B4 (c) · 1 mark · Correct identification of spectator ion
Worked solution
a) One method: Pass 100 cm³ air over excess heated copper filings in a glass tube between gas syringes. As copper reacts with oxygen to form black copper(II) oxide, the volume of gas decreases. Measure final volume when no further change occurs and calculate percentage oxygen by difference.
b) Word equation: iron + copper(II) sulfate → iron(II) sulfate + copper. Observation: Brown copper appears on iron, blue solution fades.
c) Full ionic equation: Fe(s) + Cu²⁺(aq) + SO₄²⁻(aq) → Fe²⁺(aq) + SO₄²⁻(aq) + Cu(s). The sulfate ion, SO₄²⁻, is the spectator ion (unchanged on both sides).