Halogens as Oxidising Agents
A concise revision guide to the halogens as oxidising agents: displacement reactions of chlorine, bromine and iodine with halide ions, colours in water and in a non-polar solvent, reactions with metals and with hydrogen, and the trend in oxidising power explained.
Displacement Reactions
A more reactive halogen displaces a less reactive one from a solution of its halide ions. Chlorine water added to potassium bromide solution turns it orange, because bromine is formed; added to potassium iodide it turns brown, because iodine is formed. Bromine displaces iodide ions but cannot displace chloride ions, and iodine displaces nothing.
Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ Cl₂ + 2I⁻ → 2Cl⁻ + I₂ Br₂ + 2I⁻ → 2Br⁻ + I₂
Each is a redox reaction. The halogen is reduced (0 to −1) and acts as the oxidising agent; the halide ion is oxidised (−1 to 0). The order of oxidising power is Cl₂ > Br₂ > I₂, so a halogen can only oxidise the halide ions of an element below it in the group.
Definition: A halogen higher in the group is a stronger oxidising agent, so it takes electrons from the halide ions of a halogen lower in the group.
Colours in Water and in an Organic Solvent
The product is identified by its colour. In water the colours are similar, so an organic solvent such as cyclohexane is added and the tube is shaken: the halogen dissolves in the upper organic layer and shows a distinctive colour.
| Halogen | Colour in water | Colour in cyclohexane |
|---|---|---|
| Chlorine | pale green | pale green |
| Bromine | orange (yellow when dilute) | orange-red |
| Iodine | brown | purple |
Ticks appear only where the halogen added is higher in the group than the halide in the solution; the cyclohexane layer makes the product easy to name.
Iodine dissolves poorly in water on its own; it dissolves in potassium iodide solution as the brown triiodide ion, I₃⁻, which is what is usually seen in the aqueous layer.
Exam wording: “The solution turns from colourless to brown, and the cyclohexane layer turns purple, showing that iodine has been displaced.”
Check: Predicting Displacement
Predict the observations, in water and in cyclohexane, for halogen and halide combinations not listed above, and write the ionic equations.
Explaining the Trend in Oxidising Power
The trend in oxidising power follows the trend in electronegativity. Chlorine attracts an extra electron strongly because its outer shell is close to the nucleus and only lightly shielded. Iodine’s outer shell is further away and shielded by three more inner shells, so it attracts an incoming electron much less strongly and is the weakest oxidising agent of the three.
Fluorine is stronger still, but it reacts with water, so displacement reactions are not carried out with fluorine in solution.
Exam sentence: Chlorine is a stronger oxidising agent than bromine because a chlorine atom is smaller and its outer shell is less shielded, so it attracts an electron from a bromide ion more strongly.
Reactions with Metals
The halogens oxidise metals to form ionic halides, the halogen being reduced from 0 to −1. Heated sodium burns in chlorine with a yellow flame and white smoke of sodium chloride; magnesium burns in bromine vapour to give magnesium bromide; iron wool glows in chlorine to give iron(III) chloride.
2Na + Cl₂ → 2NaCl Mg + Br₂ → MgBr₂ 2Fe + 3Cl₂ → 2FeCl₃ 2K + I₂ → 2KI
The vigour of the reaction falls down the group, so chlorine reacts violently, bromine steadily and iodine only on strong heating. With iron the choice of halogen even changes the product: chlorine gives iron(III) chloride, but iodine, the weaker oxidising agent, can only reach iron(II) iodide.
Key idea: In every reaction of a halogen with a metal the halogen is the oxidising agent and its oxidation number falls from 0 to −1.
Check: Halogens and Metals
Write the equation and describe the redox changes for a metal and halogen pair not shown above.
Reactions with Hydrogen
The halogens react with hydrogen to form the hydrogen halides, HX, and the reactivity falls sharply down the group. Fluorine reacts explosively even in the dark and cold. Chlorine reacts explosively in sunlight but only slowly in the dark. Bromine reacts on heating with a platinum catalyst. Iodine reacts very slowly on heating and the reaction is reversible, so it never goes to completion.
H₂ + F₂ → 2HF H₂ + Cl₂ → 2HCl H₂ + Br₂ → 2HBr H₂ + I₂ ⇌ 2HI
The trend has the same cause as the displacement trend: the smaller halogen attracts the hydrogen electron more strongly and forms a stronger H–X bond, so the reaction is more exothermic and faster.
Exam focus: Give the conditions with the reactivity: “chlorine and hydrogen explode in sunlight; iodine and hydrogen react slowly and reversibly on heating”.
Thermal Stability of the Hydrogen Halides
The hydrogen halides become less thermally stable down the group, because the H–X bond becomes longer and weaker as the halogen atom gets larger: H–F 562 kJ mol⁻¹, H–Cl 431, H–Br 366, H–I 299. Hydrogen fluoride and hydrogen chloride do not decompose on heating. Hydrogen bromide decomposes slightly at the temperature of a red-hot wire, and hydrogen iodide decomposes readily, giving a purple vapour of iodine:
2HI → H₂ + I₂
Exam sentence: Hydrogen iodide is less stable than hydrogen chloride because the H–I bond is longer and weaker, so less energy is needed to break it.
Check: Hydrogen Halides
Rank the hydrogen halides by stability and explain the order from bond enthalpy data you have not seen above.
Common Exam Points
State what is seen when chlorine water is added to potassium iodide solution
The solution turns brown; if cyclohexane is added and the tube shaken, the upper layer turns purple.
Explain why bromine cannot displace chloride ions
Bromine is a weaker oxidising agent than chlorine, so it cannot take electrons from chloride ions.
Identify the oxidising agent in a displacement
The halogen molecule, because its oxidation number falls from 0 to −1.
Do not say
“Chlorine is more reactive so it displaces” without explaining why; “iodine turns the cyclohexane brown”; “the halide ion is reduced”.
FAQs
Use these quick answers to check the displacement reactions and the halogens as oxidising agents.
Which halogen displaces which?
A halogen displaces the ions of any halogen below it in the group: chlorine displaces bromide and iodide, bromine displaces iodide only, iodine displaces neither.
What is oxidised and what is reduced in a displacement?
The halide ion is oxidised (−1 to 0) and the halogen molecule is reduced (0 to −1), so the halogen is the oxidising agent.
Why add cyclohexane?
The halogen formed dissolves in the organic layer and shows a clearer colour: orange-red for bromine and purple for iodine.
Why is chlorine a stronger oxidising agent than iodine?
A chlorine atom is smaller and its outer shell is less shielded, so it attracts an electron much more strongly than iodine does.
Why is hydrogen iodide less stable than hydrogen chloride?
The H–I bond is longer and weaker than the H–Cl bond, so less energy is needed to break it.
Copyright and author footprint: This OLS revision page was written for Online Learning System by Dr. Mohammed Al-Fatah. It is designed for A Level Chemistry revision and should not be copied or redistributed without permission.
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