Group 17

Syllabus points

• Describe the trends in volatility and colour of the halogens.
• Describe and explain the relative reactivity of the halogens as oxidising agents, including displacement reactions.
• Describe the reactions of halide ions with silver nitrate and with concentrated sulfuric acid.
• Describe and explain the reaction of chlorine with cold and with hot aqueous sodium hydroxide.
• Explain the use of chlorine in water treatment, and the balance of risks and benefits.

The elements

Volatility decreases down the group, so the melting and boiling points rise. Each atom has more electrons, so the van der Waals forces between molecules are stronger and more energy is needed to separate them. Note that the covalent bond inside the molecule is not being broken here; only the forces between molecules are.

The colours deepen down the group, which is why the organic-solvent test works.

Reactivity as oxidising agents

The halogens are oxidising agents: each gains one electron to form a 1- ion.

Oxidising power decreases down the group. Going down, the atomic radius increases and there is more shielding, so the incoming electron is attracted less strongly and the halogen accepts it less readily.

So chlorine is a stronger oxidising agent than bromine, which is stronger than iodine.

Displacement reactions

A more reactive halogen displaces a less reactive one from its halide:

Cl₂ + 2KBr → 2KCl + Br₂

The solution turns from colourless to orange, because bromine has been released. Chlorine has been reduced and the bromide oxidised.

Cl₂ + 2KI → 2KCl + I₂ turns the solution brown.

Br₂ + 2KI → 2KBr + I₂ also gives brown.

Iodine displaces neither, because it is the weakest oxidising agent of the three. Adding a little organic solvent and shaking makes the results clearer: bromine gives orange in the solvent layer, iodine gives violet.

Reactions of halide ions

With silver nitrate

This is the standard test. Acidify with dilute nitric acid, then add silver nitrate solution:

Ag⁺ + Cl⁻ → AgCl

The ammonia step is what makes the test reliable, because white, cream and pale yellow are hard to tell apart by eye. Solubility in ammonia falls down the group, so the sequence separates them cleanly.

Nitric acid is added first to remove carbonate and hydroxide ions, which would give their own precipitates.

With concentrated sulfuric acid

This one distinguishes the halides by how strongly they reduce, and the products change down the group:

• Chloride gives steamy HCl fumes only. Chloride is too weak a reducing agent to reduce sulfuric acid, so this is simply an acid displacement.
• Bromide gives HBr, and then some red-brown Br₂ vapour and SO₂, because bromide is a strong enough reducing agent to reduce the sulfur from +6 to +4.
• Iodide gives HI, then purple I₂ vapour, and reduces the sulfur further still, to H₂S with its bad-egg smell, and to yellow sulfur.

The trend to state is that reducing power increases down the group, which is the mirror image of the oxidising trend of the elements. The larger the ion, the more easily it gives up its electron.

Chlorine with sodium hydroxide

Cold and dilute sodium hydroxide:

Cl₂ + 2NaOH → NaCl + NaClO + H₂O

Chlorine goes from 0 to -1 in NaCl and to +1 in NaClO, so this is disproportionation. The product, sodium chlorate(I), is the active ingredient of household bleach.

Hot and concentrated sodium hydroxide:

3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O

Again disproportionation, but now to -1 and +5. Sodium chlorate(V) is used as a weedkiller.

The pair is a favourite question, and the temperature is the only thing that distinguishes them.

Chlorine in water treatment

Chlorine reacts with water in a further disproportionation:

Cl₂ + H₂O ⇌ HCl + HClO

Chloric(I) acid, HClO, is the active species. It kills bacteria, and adding chlorine to drinking water has prevented enormous numbers of deaths from cholera and typhoid.

There are real risks alongside that benefit. Chlorine is toxic, and it can react with organic matter in water to form chlorinated hydrocarbons, some of which are suspected carcinogens. The syllabus expects the judgement as well as the facts: the benefits of disinfection are generally considered to outweigh the risks, and that is a decision about weighing harms rather than a purely chemical conclusion.

Common mistakes

• Saying the covalent bonds break when iodine sublimes. Only van der Waals forces between molecules are overcome.
• Saying oxidising power increases down the group. It decreases; reducing power of the halide ions increases.
• Skipping the ammonia step in the silver nitrate test, when the precipitate colours are hard to distinguish.
• Forgetting to acidify with nitric acid before adding silver nitrate.
• Mixing up the two sodium hydroxide reactions. Cold and dilute gives chlorate(I); hot and concentrated gives chlorate(V).
• Saying chlorine itself kills the bacteria in water treatment. The active species is HClO.