Group 17: Question 2

Worked solution

Part (a): Predicting the observations

The key idea is the trend in oxidising power down Group 17: it decreases from chlorine to bromine to iodine. A halogen can only displace (oxidise) a halide ion that lies below it in the group, it cannot displace its own ion, and it cannot displace an ion from a halogen above it.

• Tube 1 (sodium chloride): chlorine is above bromine in the group, so bromine is not a strong enough oxidising agent to displace chloride ions. No reaction occurs. The only colour seen is the pale orange of the bromine water itself, unchanged.
• Tube 2 (sodium bromide): bromine cannot oxidise its own ion. There is nothing for it to displace. No reaction occurs, and again only the orange colour of the added bromine water is visible.
• Tube 3 (sodium iodide): iodine lies below bromine, so bromine (the stronger oxidising agent) readily oxidises iodide ions to iodine. The orange bromine water is used up as it reacts, and the solution turns a red-brown/dark brown colour as iodine is formed in solution.

Part (b): Ionic equation for tube 3

Bromine oxidises iodide ions to iodine, while itself being reduced to bromide ions:

$\text{Br}_2\text{(aq)} + 2\text{I}^-\text{(aq)} \rightarrow 2\text{Br}^-\text{(aq)} + \text{I}_2\text{(aq)}$

Two iodide ions are needed to balance the two electrons released when one $\text{Br}_2$ molecule is reduced to two $\text{Br}^-$ ions.

Part (c): Why chlorine water gives a different result in tube 2

Down Group 17, atomic radius increases from chlorine to bromine to iodine. A smaller atom, like chlorine, has its outer shell closer to the nucleus and experiences less shielding from a similar effective nuclear charge, so it attracts an incoming electron more strongly than a larger atom does. This makes chlorine a stronger oxidising agent than bromine.

Because of this, chlorine water is able to oxidise bromide ions. Something bromine water cannot do to itself. Using chlorine water instead of bromine water in tube 2 would therefore produce a reaction:

$\text{Cl}_2\text{(aq)} + 2\text{Br}^-\text{(aq)} \rightarrow 2\text{Cl}^-\text{(aq)} + \text{Br}_2\text{(aq)}$

The solution would turn (or deepen to) orange as bromine is formed. A clear colour change that was not seen when bromine water alone was added to bromide ions in the original experiment.

Final answers

• (a) Tube 1: no reaction (pale orange bromine water colour only). Tube 2: no reaction (pale orange bromine water colour only). Tube 3: reaction occurs. Bromine water is decolourised and the solution turns red-brown/dark brown as iodine forms.
• (b) $\text{Br}_2\text{(aq)} + 2\text{I}^-\text{(aq)} \rightarrow 2\text{Br}^-\text{(aq)} + \text{I}_2\text{(aq)}$
• (c) Chlorine’s smaller atomic radius gives it stronger attraction for an incoming electron, making it a stronger oxidising agent than bromine; chlorine water would displace bromine from bromide ions (turning tube 2 orange), unlike bromine water, which cannot react with its own ion.