Inorganic Chemistry
8 marks · NormalThis question is about trends and reactions involving Group 1 alkali metals and Group 7 halogens.
Potassium reacts vigorously with water to produce potassium hydroxide and hydrogen gas. Write a balanced chemical equation for this reaction.
Describe one difference you would observe in the reaction of potassium with water compared to that of lithium with water.
Explain, using ideas about atomic structure, why potassium is more reactive than sodium.
Chlorine water is added to a solution of potassium iodide.
Write an ionic equation for the reaction that occurs.
State one visible observation when chlorine water is added to potassium iodide solution and explain why this occurs, referring to relative reactivity.
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(a) 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
(b) Potassium reacts more vigorously; eg, produces a lilac flame or explodes, while lithium does not.
(c) Potassium's outer electron is further from the nucleus, so it is less strongly attracted and lost more easily, making it more reactive than sodium.
(d(i)) Cl2(aq) + 2I^-(aq) → 2Cl^-(aq) + I2(aq)
(d(ii)) The solution turns brown because iodine is formed; chlorine is more reactive than iodine and displaces iodide ions.
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Marking points
A1 (a) · 1 mark · Correct balanced equation for potassium and water.
B1 (b) · 1 mark · One valid observation difference explained (e.g., potassium flame, explosion, faster reaction).
M1 (c) · 1 mark · Correctly refers to atomic structure (electron shells, distance from nucleus).
A2 (c) · 1 mark · Correct explanation relating weaker attraction to increased reactivity.
A3 (d(i)) · 2 marks · Correct ionic equation, mark both species and balancing.
B2 (d(ii)) · 1 mark · Correct visible observation (solution turns brown).
M2 (d(ii)) · 1 mark · Explanation refers to reactivity: chlorine displaces iodide due to being more reactive.
Worked solution
a) Potassium and water equation:
b) Potassium reacts more vigorously: it may produce a lilac flame or explode, and the reaction is faster than with lithium.
c) Potassium's outer electron is in a shell further from the nucleus than sodium's. This means the outer electron is less strongly attracted, so it is lost more easily, making potassium more reactive than sodium.
d)(i) The ionic equation for the reaction is:
d)(ii) The solution turns brown because iodine is formed. Chlorine is more reactive than iodine and so displaces iodide ions from solution.