Summer 2026 Predicted
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,1 This question is about equilibrium involving iron(III) ions and thiocyanate ions.
Iron(III) chloride solution is pale yellow. Potassium thiocyanate solution is
colourless.
When these solutions are mixed, an equilibrium is established:
Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)
pale yellow colourless deep red
(a) (i) A student adds a few drops of concentrated iron(III) chloride solution to
the equilibrium mixture.
Explain the effect on the appearance of the solution.
(3)
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, (ii) Which is the expression for the equilibrium constant, Kc, for this reaction?
(1)
A Kc = [FeSCN²⁺] / [Fe³⁺][SCN⁻]
B Kc = [Fe³⁺][SCN⁻] / [FeSCN²⁺]
C Kc = [FeSCN²⁺]² / [Fe³⁺]²[SCN⁻]²
D Kc = [Fe³⁺] / [FeSCN²⁺][SCN⁻]
(b) A test tube contains an equilibrium mixture of the three ions at room
temperature.
The test tube is placed in a beaker of ice-cold water.
The mixture becomes lighter in colour/less red.
(i) State what can be deduced about the sign of ΔH for the forward reaction from
this observation. Justify your answer.
(2)
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for more papers: tyrionpapers.com
for more papers: tyrionpapers.com
,1 This question is about equilibrium involving iron(III) ions and thiocyanate ions.
Iron(III) chloride solution is pale yellow. Potassium thiocyanate solution is
colourless.
When these solutions are mixed, an equilibrium is established:
Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)
pale yellow colourless deep red
(a) (i) A student adds a few drops of concentrated iron(III) chloride solution to
the equilibrium mixture.
Explain the effect on the appearance of the solution.
(3)
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, (ii) Which is the expression for the equilibrium constant, Kc, for this reaction?
(1)
A Kc = [FeSCN²⁺] / [Fe³⁺][SCN⁻]
B Kc = [Fe³⁺][SCN⁻] / [FeSCN²⁺]
C Kc = [FeSCN²⁺]² / [Fe³⁺]²[SCN⁻]²
D Kc = [Fe³⁺] / [FeSCN²⁺][SCN⁻]
(b) A test tube contains an equilibrium mixture of the three ions at room
temperature.
The test tube is placed in a beaker of ice-cold water.
The mixture becomes lighter in colour/less red.
(i) State what can be deduced about the sign of ΔH for the forward reaction from
this observation. Justify your answer.
(2)
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