The halogens oxidising ability

Oxidising ability of the halogens

The halogens in their chemical reactions are generally good oxidising agents. That is they are electron acceptors, they oxidise other substances and the halogens are themselves reduced when they gain electrons. Displacement reactions using halogens are a good way to demonstrate the trend in the oxidising ability of the halogens. Fluorine is the most powerful oxidising agent in group 7 and the trend in ability of the halogens to accept electrons decreases as we descend group 7.

To explain why the oxidising ability of the halogens decrease as we descend group 7 we need to consider a range of factors. The oxidiation reaction is simple enough, we can show it as:

½X2 + e → X-
where X is any halogen. The factors we need to think about which will influence how good an oxidising agent a halogen is are: So the trend is oxidising power of the halogens is a balance between all the factors listed above. Ultimately the weak F-F bond and the fact that more energy is released when it enters solution or an ionic lattice means that fluorine is the strongest oxidising of the halogens and the ability of the halogens to act as oxidising agents decreases as we descend group 7.

Halogen displacement reactions

chlorine water displacing iodide from solution Displacement reactions are a good way to show the ability of halogens to act as oxidising agents. A typical displacement reaction is shown opposite. In the first test-tube we have a solution of sodium iodide dissolved in water. On top of this is added a few centimetres of an organic solvent such as hexane or cyclohexane. Cyclohexane is a very good solvent for halogens and given the choice between dissolving in water and dissolving in cyclohexane, a halogen will always dissolve in cyclohexane before water. Cyclohexane does not dissolve in water, but like oil simply floats on top of it which is why there are two layers shown in the first test-tube in the image opposite.

Sodium iodide, being an ionic compound will dissolve in water. This solution contains sodium ions, Na+ and iodide ions, I - .

When chlorine water is added to this test-tube and shaken for around 30 seconds you can see in the image opposite that the cyclohexane layer has changed colour, it has turned a violet/purple colour.
It may be easier to understand what is happening here if we write an equation for the reaction:

sodium iodide(aq) + chlorine(aq) → sodium chloride(aq) + iodine(aq)
2NaI(aq) + Cl2(aq) → 2NaCl(aq) + I2(aq)

On the reactants side of the word equation we have two halogens present, iodine in the form of iodide in the solution and chlorine from the chlorine water. In a displacement reaction the more reactive halogen will oxidise the less reactive halide ion present in a solution or a compound, in this case chlorine will displace or oxidise the iodide ion from the sodium iodide solution. The chlorine effectively takes the place of the iodide. The iodine is kicked out of the solution and now has a choice of places to go to, either the cyclohexane solvent or the water solution, it goes into the cyclohexane solvent.

Another advantage of cyclohexane is that the halogen show up as bright clear colours when dissoloved organic solvents, iodine is purple, chlorine is green/yellow and bromine is red-brown. So the purple colour of the cyclohexane is due to the iodine dissolved in it!

Displacing bromine from a solution

In the example opposite we again have 2 halogens on the reactants side of the equation, these are bromine, in the form of bromide from the sodium bromide solution and chlorine from chlorine water. Chlorine is more reactive than bromine so will displace or oxidise it from the solution, the bromide will be kicked out of solution and will dissolve in the cyclohexane solvent, turning it red/brown.

sodium bromide(aq) + chlorine(aq) → sodium chloride(aq) + bromine(aq)
2NaBr(aq) + Cl2(aq) → 2NaCl(aq) + Br2(aq)
In this equation the chlorine has been reduced to chloride and the bromide ion has been oxidised to bromine. Chlorine is the electron acceptor or oxidising agent and the bromide is the electron donor or reducing agent.

Displacement reaction????

However be careful as you can get caught out if you are not careful! Consider the reaction shown on the below.

sodium chloride(aq) + iodine(aq) → sodium chloride(aq) + iodine(aq)
2NaCl(aq) + I2(aq) → 2NaCl(aq) + I2(aq)
Here we have iodine and chlorine as the two halogens present on the reactant side of the equation. Iodine is not able to oxidise the chloride ions present and so no reaction will happen.

Reaction of the halogens with iron(II) chloride solutions

colours of iron(II) and iron(III) chloride solutions

2Fe2+(aq) + Cl2(aq) → 2Fe3+(aq) + 2Cl-(aq)
while Iron(III) chloride solution is yellow, as shown in the image opposite. We can demonstrate the oxidising ability by reacting them with a solution of Iron(II) chloride. Here chlorine and bromine are strong enough oxidising agents to oxidise the Fe2+ ions to Fe3+ ions and in the process the chlorine and bromine are reduced to chloride and bromide ions. We can show this as:
2Fe2+(aq) + Cl2(aq) → 2Fe3+(aq) + 2Cl-(aq)
and also:
2Fe2+(aq) + Br2(aq) → 2Fe3+(aq) + 2Br-(aq)
The pale green solution containing the Fe2+ ions will change colour to a pale yellow solution containing Fe3+ when chlorine and bromine are added to it however this is not what happens with iodine. Iodine being the poorest oxidising agent in group 7 is not able to oxidise the Fe2+ ions, in fact a solution of Fe3+ will oxidise iodide ions (I-) to form iodine (I2)
2I-(aq) + 2Fe3+(aq)I2(s) + 2Fe3+(aq)
Here the soluble iodide ions (I-(aq)) are oxidised to form the almost insoluble iodine. A pale brown solution of iodine in water will form along with a solid precipitate of insoluble iodine on the bottom of the beaker or test-tube.

Key Points

Practice questions

Check your understanding - Questions on halogen displacement reactions.