DP Chemistry (last assessment 2024)

Question 18M.2.hl.TZ2.4

Date	May 2018	Marks available	[Maximum mark: 4]	Reference code	18M.2.hl.TZ2.4
Level	hl	Paper	2	Time zone	TZ2
Command term	Calculate, Determine, Formulate	Question number	4	Adapted from	N/A

[Maximum mark: 4]

18M.2.hl.TZ2.4

(a)

In acidic solution, bromate ions, BrO₃⁻(aq), oxidize iodide ions, I⁻(aq).

BrO₃⁻(aq) + 6H⁺(aq) + 6e⁻ $\rightleftharpoons$ Br⁻(aq) + 3H₂O(l)

2I⁻(aq) $\rightleftharpoons$ I₂(s) + 2e⁻

Formulate the equation for the redox reaction.

[1]

Markscheme

BrO₃^–(aq) + 6H⁺(aq) + 6I^–(aq) $\rightleftharpoons$ Br^–(aq) + 3I₂(s) + 3H₂O(l)

Accept → for $\rightleftharpoons$ .

[1 mark]

(b)

The change in the free energy for the reaction under standard conditions, ΔG^Θ, is −514 kJ at 298 K.

Determine the value of E^Θ, in V, for the reaction using sections 1 and 2 of the data booklet.

[2]

Markscheme

n = 6

«ΔG^Θ = –nFE^Θ»

« ${E^\Theta } = - \frac{{\Delta {G^\Theta }}}{{{\text{n}}F}} = \frac{{514 \times {{10}^3}{\text{ J mo}}{{\text{l}}^{ - 1}}}}{{6 \times 9.65 \times {{10}^4}{\text{ C mo}}{{\text{l}}^{ - 1}}}} =$ » 0.888 «V»