Part 2: Disproportionation, chlorine uses and halide tests

Track chlorine into two oxidation states and distinguish halide ions using precipitation and ammonia solubility.

One element is oxidised and reduced

In water, chlorine establishes Cl₂ + H₂O ⇌ HCl + HClO. Chlorine changes from 0 to −1 in HCl and to +1 in HClO. Aqueous HCl provides H⁺ and Cl⁻; the solution is acidic and HClO has bleaching/disinfecting action.

With cold, dilute NaOH, chlorine makes chloride and chlorate(I). Bromine and iodine can undergo analogous disproportionation under appropriate supplied conditions. Identify the oxidation state of the same element in both products. Hot concentrated alkali gives different products; do not use that equation when the question specifies cold dilute conditions.

Weigh the benefit and risk

Controlled chlorination kills harmful microorganisms in drinking water and pools, reducing waterborne disease. Chlorine itself is toxic, and reactions with organic matter can form potentially harmful chlorinated by-products. Treatment therefore involves controlled dosing and monitoring. Explain both the public-health benefit and the specific risk rather than simply calling chlorine “dangerous”.

Precipitate the silver halide

Acidify a sample with dilute nitric acid to remove carbonate interference, then add aqueous silver nitrate. Do not use HCl, which adds the very chloride ions being tested. The precipitate is the new insoluble solid, not merely a coloured solution.

Add dilute ammonia to the precipitate; if it remains, test its behaviour with concentrated ammonia under the prescribed laboratory controls. State that the silver halide solid dissolves, not that the original halide ion dissolves. The complex-ion equations are not needed for this AS outcome.

Worked disproportionation: check atoms, charge and oxidation states

Remove Na⁺ from the cold dilute alkali equation to obtain Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O. There are two chlorine atoms, two oxygen atoms, two hydrogen atoms and total charge −2 on each side. In ClO⁻, oxygen contributes −2 and the ion totals −1, so chlorine must be +1.

Of the two chlorine atoms initially at 0, one ends at −1 and one at +1. The gain and loss of one electron balance. Chlorine has undergone both reduction and oxidation: that is disproportionation. Writing oxidation numbers without linking them to the correct product species leaves the explanation incomplete.

For a supplied analogous bromine equation, replace Cl by Br throughout, then check the products actually stated. Do not assume every halogen/alkali reaction under every temperature gives the same product oxidation states.

A complete halide identification, including the inference

Suppose nitric-acid-treated sample X gives a pale precipitate with AgNO₃. Colour alone makes chloride versus bromide uncertain. If the solid remains in dilute ammonia but dissolves in concentrated ammonia, the combined evidence supports AgBr and therefore Br⁻ in X. It does not show that elemental bromine was originally present.

Use enough nitric acid to remove carbonate interference before adding AgNO₃; record effervescence separately if it occurs. Keep the sample, reagents and droppers uncontaminated. Chloride from hydrochloric acid, tap water or a reused dropper can generate AgCl unrelated to the unknown.

A silver-halide test detects ions already present in solution. A covalent C–Br bond in a haloalkane must first undergo hydrolysis before it supplies Br⁻; the two practical questions are related but have different starting species.

Match the scope of an evaluation question

If asked for one disadvantage, give one specific risk such as toxicity of chlorine or formation of harmful chlorinated organic by-products. If asked to evaluate treatment, connect that risk to the benefit of destroying pathogens and the role of controlled dosing. A benefit alone does not answer a disadvantages-only question.

For chlorine with water, HCl and HClO have different roles: HCl contributes acidity while HClO provides disinfecting and bleaching action. Do not call the solution a simple mixture of chloride ions and oxygen or confuse it with the NaOH reaction.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources