O-LevelChemistryRates of Reaction5070/21 · Oct/Nov 2014 · Q910 Marks

Bromate(V) ions, BrO3¯, react with bromide ions, Br¯, in acidic solution to form bromine. BrO3¯(aq) + 5Br¯(aq) + 6H⁺(aq) → 3Br2(aq) + 3H2O(l)

(a)(i)Explain why the acidity of the reaction mixture decreases as the reaction proceeds.[2]
(a)(ii)State the colour of aqueous bromine.[1]
(a)(iii)Explain, using the kinetic particle theory, why increasing the temperature increases the rate of this reaction.[2]
(b)Bromine oxidises aqueous iodide ions to iodine. Write the equation for this reaction.[1]
(c)Aqueous potassium iodide can be used to test for oxidising agents. Describe and explain the colour change when excess aqueous potassium iodide is added to aqueous acidified potassium manganate(VII), KMnO4.[2]
(d)Describe how aqueous bromine is used to test for an unsaturated hydrocarbon.[1]
(e)Draw a 'dot-and-cross' diagram for a bromine molecule. Show only the outer electrons.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[2] (a)(i) acidity caused by H⁺ ions (1) H⁺ ions consumed in the reaction/H⁺ ions used up in the reaction (1)
[1] (a)(ii) orange/reddish-brown (1)
[2] (a)(iii) ions or particles have more energy/move faster (1) more particles or ions have energy above the activation energy/more energetic collisions/more effective collisions/more successful collisions / more fruitful collisions (1)
[1] (b) Br2 + 2I¯ → I2 + 2Br¯ (1)
[2] (c) purple solution goes brown (1) iodide oxidised to iodine/iodine is brown (1)
[1] (d) aqueous bromine decolourised (1)
[1] (e) correct dot and cross diagram for bromine molecule (1)

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