Why is I⁻ a better reducing agent than Cl⁻?

The larger ion has its outer electron further from the nucleus and more shielded, so it is lost more easily.

What is seen when solid NaCl reacts with concentrated H₂SO₄?

Steamy white fumes of HCl; no redox occurs.

Write the equation for NaCl with concentrated H₂SO₄.

NaCl + H₂SO₄ → NaHSO₄ + HCl

What are the products when HBr reduces concentrated H₂SO₄?

Br₂ (orange-brown) and SO₂: 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O.

Name the three sulfur products formed with sodium iodide and concentrated H₂SO₄.

SO₂, S (yellow solid) and H₂S (bad egg smell).

Why is silver nitrate used to identify halide ions?

Ag⁺ forms coloured, insoluble silver halide precipitates with Cl⁻, Br⁻ and I⁻.

Why is the solution acidified before adding silver nitrate?

To remove carbonate and hydroxide ions that would form precipitates and give a false positive.

Why is nitric acid used to acidify, not hydrochloric acid?

HCl would add chloride ions and give a false white precipitate.

How does solubility of the silver halides in ammonia change?

AgCl dissolves in dilute NH₃, AgBr in concentrated NH₃, AgI does not dissolve.

Give the test for ammonium ions.

Warm with NaOH(aq); ammonia turns damp red litmus paper blue.

Give the test for sulfate ions.

Add acidified barium chloride; a white precipitate of BaSO₄ forms.

In what order should carbonate, sulfate and halide be tested?

Carbonate, then sulfate, then halide.

Reactions of halide ionsAQA A-Level Chemistry: Flashcards

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Question

What is the trend in reducing ability of halide ions down Group 7?

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What is the trend in reducing ability of halide ions down Group 7?: It increases: Cl⁻ < Br⁻ < I⁻.
Why is I⁻ a better reducing agent than Cl⁻?: The larger ion has its outer electron further from the nucleus and more shielded, so it is lost more easily.
What is seen when solid NaCl reacts with concentrated H₂SO₄?: Steamy white fumes of HCl; no redox occurs.
Write the equation for NaCl with concentrated H₂SO₄.: NaCl + H₂SO₄ → NaHSO₄ + HCl
What are the products when HBr reduces concentrated H₂SO₄?: Br₂ (orange-brown) and SO₂: 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O.
Name the three sulfur products formed with sodium iodide and concentrated H₂SO₄.: SO₂, S (yellow solid) and H₂S (bad egg smell).
What are the colours of AgCl, AgBr and AgI?: White, cream and yellow.
Why is silver nitrate used to identify halide ions?: Ag⁺ forms coloured, insoluble silver halide precipitates with Cl⁻, Br⁻ and I⁻.
Why is the solution acidified before adding silver nitrate?: To remove carbonate and hydroxide ions that would form precipitates and give a false positive.
Why is nitric acid used to acidify, not hydrochloric acid?: HCl would add chloride ions and give a false white precipitate.
How does solubility of the silver halides in ammonia change?: AgCl dissolves in dilute NH₃, AgBr in concentrated NH₃, AgI does not dissolve.
Give the test for ammonium ions.: Warm with NaOH(aq); ammonia turns damp red litmus paper blue.
Give the test for sulfate ions.: Add acidified barium chloride; a white precipitate of BaSO₄ forms.
In what order should carbonate, sulfate and halide be tested?: Carbonate, then sulfate, then halide.

Exam questions on Reactions of halide ions

A technician reacts solid sodium chloride, solid sodium bromide and solid sodium iodide, in separate tests in a fume cupboard, with concentrated sulfuric acid.
Chloride ions do not reduce concentrated sulfuric acid but iodide ions do. Explain why iodide ions are the better reducing agent.2 marks
A student has a colourless aqueous solution of an unknown sodium halide. She adds dilute nitric acid followed by aqueous silver nitrate and obtains a cream precipitate.
The student adds dilute ammonia solution to the cream precipitate and then concentrated ammonia solution. State what she observes and how this confirms the halide.2 marks
A student is given a colourless aqueous solution that may contain carbonate ions, sulfate ions and chloride ions. She tests separate portions of the solution.
Describe the tests she should carry out to show that the solution contains carbonate ions and sulfate ions, and the positive result for each.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).