Inorganic Chemistry
8 marks · NormalThe following experiment investigates the reactions of alkali metals and halogens, and how their reactivity relates to redox processes.
A student leaves a freshly cut piece of sodium exposed to air and measures how long it takes to lose its shine. Another student does the same for potassium. Explain, using observations and knowledge of the alkali metals’ reactions with air, which metal loses its shine faster and why.
The following table shows the time taken (in seconds) for lithium, sodium, and potassium to lose their shine when exposed to air:
| Metal | Time to lose shine / s |
|---|---|
| Lithium | 70 |
| Sodium | 20 |
| Potassium | 4 |
When potassium reacts with bromine, a halide salt is formed. Write a balanced symbol equation for this reaction, including state symbols.
A displacement reaction occurs when chlorine water is added to a solution of potassium bromide. Use a labelled electron transfer diagram or a written explanation to describe how this reaction can be viewed as a redox reaction.
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) Potassium loses its shine faster than sodium because it reacts more rapidly with oxygen in the air. Potassium tarnishes almost instantly, while sodium tarnishes less quickly. This is because potassium is more reactive than sodium. · must show Potassium loses shine faster, Potassium more reactive than sodium, Reference to reaction with air or oxygen
(b) Potassium · must show Potassium
(c) 2K(s) + Br2(l) → 2KBr(s) · must show Balanced equation, Correct state symbols
(d) Chlorine is reduced and bromide ions are oxidised. Chlorine gains electrons to become chloride ions, while bromide ions lose electrons to become bromine molecules. This is a redox reaction. The electron transfer can be shown as: Cl2(aq) + 2Br–(aq) → 2Cl–(aq) + Br2(aq) 2Br–(aq) → Br2(aq) + 2e– (oxidation) Cl2(aq) + 2e– → 2Cl–(aq) (reduction) · must show Chlorine reduced, bromide oxidised, Loss and gain of electrons, Half equations or clear written description
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states potassium loses shine faster than sodium (with or without quantitative support).
B2 (a) · 1 mark · Gives correct explanation based on greater reactivity of potassium due to its position in Group 1.
B3 (b) · 1 mark · States potassium is the metal that loses shine fastest (from table data).
B4 (c) · 1 mark · Balanced formula with correct reactants and products.
B5 (c) · 1 mark · Correct state symbols for all substances.
M1 (d) · 1 mark · Identifies that chlorine is reduced and bromide ions are oxidised (or electrons transferred from Br– to Cl2).
M2 (d) · 1 mark · Describes/equates loss of electrons from Br– as oxidation, gain by Cl2 as reduction.
A1 (d) · 1 mark · Writes the relevant half-equations or gives an accurate electron transfer diagram/description.
Worked solution
a) Potassium loses its shine faster than sodium because potassium reacts more rapidly with oxygen in the air. When both are freshly cut, potassium tarnishes almost instantly, while sodium takes longer. This is because potassium is further down Group 1 and is more reactive than sodium.
b) According to the table, potassium loses its shine the fastest (in 4 seconds).
c) The balanced equation is:
d) In this reaction, chlorine is reduced (gains electrons) and bromide ions are oxidised (lose electrons). The ionic equation is:
Half equations: Br–(aq) → Br2(aq) + 2e– (oxidation: loss of electrons) Cl2(aq) + 2e– → 2Cl–(aq) (reduction: gain of electrons) This shows electron transfer – chlorine is reduced, bromide is oxidised – so the reaction is a redox reaction.