The Halogens
7 marks · HardThis question is about halogens and redox processes in their reactions.
Describe the appearance and physical state of chlorine and iodine at room temperature.
Bromine water is added to a solution of potassium iodide. The following reaction occurs:
In terms of oxidation and reduction, explain which species is reduced and which is oxidised in the reaction.
State the colour change that is observed during this reaction. Explain the reason for this change.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Chlorine: green gas; Iodine: dark grey solid · must show Chlorine = green gas, Iodine = dark grey solid
(b(i)) Iodide ions are oxidised to iodine (lose electrons), bromine is reduced to bromide (gains electrons). · must show Iodide ions oxidised (loss of electrons, forms I2), Bromine is reduced (gains electrons, forms Br-)
(b(ii)) The solution changes from orange-brown (bromine) to dark brown (due to iodine formed). The colour change occurs because bromine displaces iodine from iodide ions. Iodine is less soluble and forms a brown solution. · must show Initial colour = orange-brown (bromine water), Final colour = brown (iodine in solution), Explains that bromine displaces iodine from iodide ions, and the iodine gives the brown colour
Common mistakes: Mixing up which species is oxidised and which is reduced; Not mentioning both initial and final colours; Giving the symbol instead of the colour
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States chlorine: green gas
B2 (a) · 1 mark · States iodine: dark grey solid
M1 (b(i)) · 1 mark · Identifies that iodide ions are oxidised (loses electrons to form I2)
M2 (b(i)) · 1 mark · Identifies that bromine is reduced (gains electrons to form Br-)
B3 (b(ii)) · 1 mark · Initial colour correct (orange-brown from bromine water)
B4 (b(ii)) · 1 mark · Final colour correct (brown, due to iodine in solution)
B5 (b(ii)) · 1 mark · Explains colour change due to formation of iodine from iodide ions displaced by bromine
Worked solution
a) Chlorine is a green gas at room temperature; iodine is a dark grey solid.
b)(i) In the reaction, iodide ions (I^-) are oxidised (they lose electrons) to form iodine (I2). Bromine (Br2) is reduced (it gains electrons) to form bromide ions (Br^-).
b)(ii) The colour changes from orange-brown (bromine water) to dark brown (iodine in solution). This occurs because bromine displaces iodine from the iodide ions, and the liberated iodine in solution gives a dark brown colour.