CAIE A Level Chemistry 9701 · Group 17
In an industrial process, hydrogen gas is passed over heated iodine to form hydrogen iodide. The reaction is reversible and reaches equilibrium:
(a) Describe the trend in reactivity of the halogens as oxidising agents down the group.
(b) Explain why the reaction between hydrogen and iodine is less vigorous than the reaction between hydrogen and chlorine.
(c) Describe and explain the relative thermal stabilities of hydrogen chloride, hydrogen bromide and hydrogen iodide.
5 marks, so the mark scheme is looking for about 5 separate creditworthy points. Making the same point twice in different words earns it once.
Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.
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