A student adds a few drops of chlorine water to a test-tube of aqueous potassium bromide, as shown in Fig. 2.1. Fig. 2.1 (a) State the colour of the aqueous...
Question 1 Report
A student adds a few drops of chlorine water to a test-tube of aqueous potassium bromide, as shown in Fig. 2.1.
Fig. 2.1
(a) State the colour of the aqueous potassium bromide before the chlorine water is added. [1]
(b) Describe the colour change seen in the test-tube after the chlorine water is added. [1]
(c) Name the type of reaction taking place. [1]
(d) Name the halogen that is formed in the solution. [1]
(e) Explain why chlorine is able to react in this way with potassium bromide. [1]
(f) Complete the word equation for the reaction:
chlorine + potassium bromide → .............. + .............. [2]
Answer Details
This is a halogen displacement reaction. A more reactive halogen pushes a less reactive halogen out of its salt. Chlorine is above bromine in Group VII, so chlorine is more reactive and displaces bromine from potassium bromide.
(a) Aqueous potassium bromide is colourless before the chlorine water is added [1].
(b) After adding chlorine water the solution turns orange / brown (yellow-brown) [1], the colour of bromine forming in the solution.
(c) The reaction type is displacement [1].
(d) The halogen formed is bromine [1], which gives the orange-brown colour.
(e) Chlorine can react this way because chlorine is more reactive than bromine, so it displaces bromine from the bromide [1]. Reactivity falls down Group VII, and chlorine sits above bromine.
(f) Completed word equation: chlorine + potassium bromide → potassium chloride [1] + bromine [1]. The chlorine takes the place of the bromine, forming potassium chloride, and the freed bromine appears as the coloured product.
Exam tip: to predict a displacement, remember the order of reactivity Cl > Br > I; a halogen can only displace one below it in the group.
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