Displacement reactions involving solutions of salts
5 marks · HardA piece of magnesium is placed in a solution of nickel(II) chloride. Magnesium displaces nickel from the solution.
Identify the spectator ion present in this reaction.
Explain what is meant by a spectator ion in a displacement reaction.
State, in terms of electron transfer, which ion acts as the oxidising agent in this reaction.
State, with a reason, which atom or ion acts as the reducing agent.
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.
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(a) Cl^- (chloride ion)
(b) A spectator ion is an ion that does not undergo any chemical change during the reaction; it remains in solution and is unchanged both before and after the reaction.
(c) Ni^{2+} (nickel(II) ion) acts as the oxidising agent because it gains electrons from magnesium.
(d) Magnesium (Mg) acts as the reducing agent because it loses electrons to reduce nickel(II) ions.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly identifies chloride ion (Cl^-) as the spectator ion.
B2 (b) · 1 mark · Defines a spectator ion as unchanged during the reaction.
B3 (b) · 1 mark · States that the spectator ion remains in solution or does not participate in the reaction.
B4 (c) · 1 mark · Identifies Ni^{2+} as the oxidising agent and links to electron gain from Mg.
B5 (d) · 1 mark · Identifies Mg as reducing agent and links to electron loss reducing nickel(II) ions.
Worked solution
a) The reaction is:
In ionic form: Mg(s) + Ni^{2+}(aq) + 2Cl^-(aq) \rightarrow Mg^{2+}(aq) + 2Cl^-(aq) + Ni(s)
Chloride ion (Cl^-) appears unchanged on both sides, so it is the spectator ion.
b) A spectator ion is an ion that does not change during a chemical reaction; it stays in solution and is not involved in the actual redox process.
c) Ni^{2+} is the oxidising agent because it gains electrons from magnesium to become Ni(s).
d) Magnesium acts as the reducing agent because it donates electrons to reduce the nickel(II) ions.