A student investigates the reactions of halogens with halide solutions. The tubes are shown in Fig. 11.1 and the results are recorded in Table 11.1. Fig. 11...
Question 1 Report
A student investigates the reactions of halogens with halide solutions. The tubes are shown in Fig. 11.1 and the results are recorded in Table 11.1.
Fig. 11.1
| tube | halide solution | halogen added | observation |
|---|---|---|---|
| A | potassium chloride | bromine water | no change |
| B | potassium bromide | chlorine water | |
| C | potassium iodide | chlorine water | |
| D | potassium iodide | bromine water |
(a) Complete Table 11.1 by describing the observation (colour change) in tubes B, C and D. [3]
(b) Name the type of reaction being investigated. [1]
(c) State which halogen is shown to be the most reactive by these results. [1]
(d) Explain how the results show that iodine is the least reactive of the three halogens. [2]
(e) The student shakes each tube with an organic solvent to see the colours more clearly. Suggest why the colours are easier to see in the solvent. [1]
(f) State one safety precaution the student should take during this experiment. [1]
(g) Write the word equation for the reaction in tube B. [1]
(h) Bromine water is added to potassium chloride solution (tube A). Predict the observation and explain your answer. [2]
Answer Details
This is a halogen displacement investigation. A halogen displaces any halogen below it in the reactivity order Cl > Br > I, and the new halogen formed gives a colour change.
(a) The completed observations are [3] (one mark each):
| tube | halide solution | halogen added | observation |
|---|---|---|---|
| A | potassium chloride | bromine water | no change |
| B | potassium bromide | chlorine water | colourless turns orange / brown (bromine formed) |
| C | potassium iodide | chlorine water | colourless turns dark brown (iodine formed) |
| D | potassium iodide | bromine water | colourless turns dark brown (iodine formed) |
(b) The reaction type is displacement [1].
(c) Chlorine displaces both bromine and iodine, so chlorine is the most reactive halogen shown [1].
(d) Iodine does not displace chlorine or bromine from their solutions [1], yet both chlorine and bromine displace iodine from potassium iodide, which shows iodine is the least reactive of the three [1].
(e) Shaking with an organic solvent concentrates the halogen into the small solvent layer, so the colour is more concentrated and clearer to see [1].
(f) A safety precaution: work in a fume cupboard (also acceptable: wear eye protection, or wear gloves) [1], because chlorine and bromine are toxic and irritating.
(g) Word equation for tube B: chlorine + potassium bromide → potassium chloride + bromine [1].
(h) Tube A shows no change / no reaction [1], because bromine is less reactive than chlorine and so cannot displace chloride from potassium chloride [1].
Exam tip: a colour change means a reaction happened (more reactive halogen added); "no change" means the added halogen is less reactive than the one already in the salt.
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