Edexcel A-Level Chemistry AS Paper 1, November 2020: Question 5

12 marks · Medium difficulty · Open Response

Assess various properties, reactions, and trends of halogens and halide ions including displacement, redox reactions with sulfuric acid, preparation of potassium iodate(V), testing for iodide, and boiling point trends.

Practise this question

Question

Question text

5 This question is about the reactions of the halogens and halide ions.

(a) (i) When chlorine gas is bubbled through an aqueous solution of

potassium iodide, the reaction involves

(1)

A oxidation only

B reduction only

C redox

D disproportionation

(ii) Cyclohexane was added to the resulting solution from (a)(i). The mixture was

shaken and then allowed to stand for a few minutes. Two layers were formed.

[ Density: aqueous layer solution = 1.10 g cm–3, cyclohexane layer = 0.78 g cm–3]

The colour of the lower layer was

(1)

A pale yellow

B purple

C red

D pale green

(b) Concentrated sulfuric acid was added to a small quantity of solid potassium iodide

in a test tube.

(i) In this exothermic reaction, which of the following mixtures of gases would be

produced?

(1)

A hydrogen iodide and sulfur dioxide only

B hydrogen iodide and hydrogen sulfide only

C hydrogen iodide, sulfur dioxide and hydrogen sulfide

D hydrogen iodide, hydrogen sulfide and iodine

(ii) Hydrogen iodide is a gas which reacts in a similar way to hydrogen chloride.

State the observation when the hydrogen iodide gas is passed over the

mouth of an open bottle of concentrated ammonia solution.

Write an equation, including state symbols, for the reaction.

(3)

Observation …

Equation

of potassium hydroxide.

The equation for the reaction is

3I2 + 6KOH → KIO3 + 5KI + 3H2O

Potassium iodate(V) can be separated from the other reaction product using their

differing solubilities in water.

Solubility in water at 25°C

/ mol dm–3

KI 8.92

KIO3 0.43

(i) Outline a procedure that you could use to obtain a sample of dry, solid

potassium iodate(V) from the reaction mixture.

(3)

(ii) Describe how you would show that iodide ions are present in an aqueous

solution of potassium iodide.

(2) 11

… *P62306A01124*

(d) Fluorine is an element in Group 7.

Group 7 includes the elements chlorine, bromine and iodine.

Some information about the melting and boiling temperatures of Group 7

elements is shown in the table.

Element Melting temperature / K Boiling temperature / K

chlorine 172 238

bromine 266 332

iodine 387 457

Which is the expected boiling temperature of fluorine, in kelvin, K?

(1)

A 4

B 85

C 575

D 610

(Total for Question 5 = 12 marks)

Mark scheme

Show the mark scheme

How to answer it

Reactions of Halogens and Halide Ions

What this question tests

This exam question evaluates your understanding of Group 7 chemistry trends, halogen displacement reactions, solvent extraction colour identification, reducing abilities of halide ions with concentrated sulfuric acid, testing for halide ions using silver nitrate, crystallisation purification techniques, and predicting physical properties down a group.

Question 5(a)

Halogen Displacement & Solvent Extraction

✅ Correct Answers

(a)(i): C (redox)
(a)(ii): A (pale yellow)

💡 Key Knowledge

Chlorine is a stronger oxidising agent than iodine; it displaces iodide ions ( 2I⁻ + Cl₂ → I₂ + 2Cl⁻ ).
Iodine ( I₂ ) is formed in the aqueous solution. When extracted into cyclohexane (upper layer), iodine gives a purple colour, but the remaining aqueous layer appears pale yellow.

❌ Common Errors

Selecting option B (purple) for (a)(ii) by forgetting that the question specifically asks for the colour of the lower (aqueous) layer rather than the upper organic layer.

Mark allocation: 1 mark for (a)(i), 1 mark for (a)(ii).

Question 5(b)

Sulfuric Acid Reactions & Ammonia Test

✅ Correct Answers

(b)(i): D (hydrogen iodide, hydrogen sulfide and iodine)
(b)(ii) Observation: White smoke
(b)(ii) Equation: HI(g) + NH₃(g) → NH₄I(s) (Allow NH₄I(s) or NH₄⁺I⁻(s) )

🧠 Exam Technique

For part (b)(ii), make sure you list both observation components explicitly: state the colour ( white ) and the state ( smoke or solid fumes). For the equation, include correct state symbols ( (g) and (s) ).

❌ Common Errors

Forgetting that iodide is a strong reducing agent capable of reducing sulfur in H₂SO₄ all the way down to H₂S , leading to incorrect combinations in multiple choice (b)(i).
Omitting state symbols in chemical equations, which costs marks.

Mark allocation: 1 mark for (b)(i); 3 marks for (b)(ii) (1 for white, 1 for smoke, 1 for balanced equation with state symbols).

Question 5(c)

Purification & Halide Ion Identification

✅ Correct Answers

(c)(i) Procedure: 1. Cool the reaction mixture to crystallise the product.
2. Filter off the solid KIO₃ .
3. Dry the resulting solid (e.g., leave to dry, warm oven, or press between filter papers).
(c)(ii) Test: Add aqueous silver nitrate ( AgNO₃ ) followed by dilute/nitric acid ( HNO₃ ).
(c)(ii) Result: A yellow precipitate (or solid/crystals).

💡 Key Knowledge

Solubility data shows that KIO₃ has a much lower solubility than KI at 25 °C ( 0.43 mol dm⁻³ vs 8.92 mol dm⁻³ ), making separation via cooling and filtration effective.

❌ Common Errors

Adding hydrochloric acid instead of nitric acid during the silver nitrate test (hydrochloric acid introduces chloride ions, giving a false positive precipitate).
Failing to mention acidification entirely in the halide test.

Mark allocation: 3 marks for (c)(i); 2 marks for (c)(ii).

Question 5(d)

Group 7 Trends & Boiling Temperatures

✅ Correct Answers

(d): B (85 K)

📐 Step-by-Step Logic

Step 1: Examine the trend in boiling temperatures moving down Group 7 (chlorine: 238 K, bromine: 332 K, iodine: 457 K). Boiling temperature increases down the group due to stronger London forces as the number of electrons increases.
Step 2: Extrapolate the trend upwards for fluorine since it sits above chlorine. Its boiling point must be significantly lower than chlorine's 238 K.
Step 3: Reject extremes like 4 K (too low, absolute zero proximity) or values matching melting temperatures of heavier elements (575 K / 610 K). Therefore, 85 K is the only logical extrapolation.

Mark allocation: 1 mark for (d). Total for question: 12 marks.

Topics

Inorganic Chemistry · Physical Chemistry · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 3: Redox I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.