IB Chemistry HL 2.2.10 Reaction order Question Bank

Practise IB Chemistry HL 2.2.10 by applying reaction order to exam-style questions.

[Maximum number: 4]

A group of students investigated the rate of the reaction between aqueous sodium thiosulfate and hydrochloric acid according to the equation below.

\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow 2 \mathrm{NaCl}(\mathrm{aq})+\mathrm{SO}_{2}(\mathrm{~g})+\mathrm{S}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}(1)

The two reagents were rapidly mixed together in a beaker and placed over a mark on a piece of paper. The time taken for the precipitate of sulfur to obscure the mark when viewed through the reaction mixture was recorded.

Figure for Question 2.2.10 (HL)—Reaction order question 1 — IB Chemistry HL

Initially they measured out $10.0 \mathrm{~cm}^{3}$ of $0.500 \mathrm{moldm}^{-3}$ hydrochloric acid and then added $40.0 \mathrm{~cm}^{3}$ of $0.0200 \mathrm{~mol} \mathrm{dm} \mathrm{m}^{-3}$ aqueous sodium thiosulfate. The mark on the paper was obscured 47 seconds after the solutions were mixed.

(a)

The teacher asked the students to measure the effect of halving the concentration of sodium thiosulfate on the rate of reaction.

[ 1 ]

(i)

If the reaction were first order with respect to the thiosulfate ion, predict the time it would take for the mark on the paper to be obscured when the concentration of sodium thiosulfate solution is halved.

[ 1 ]

$94(\mathrm{~s}) / 1 \min 34 \mathrm{~s}$ ;

(b)

One proposed mechanism for this reaction is:

\begin{array}{ll} \mathrm{S}_{2} \mathrm{O}_{3}^{2-}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq}) \rightleftharpoons \mathrm{HS}_{2} \mathrm{O}_{3}^{-}(\mathrm{aq}) & \text { Fast } \\ \mathrm{HS}_{2} \mathrm{O}_{3}^{-}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{S}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) & \text { Slow } \end{array}

[ 3 ]

(i)

The results of an experiment investigating the effect of the concentration of hydrochloric acid on the rate, while keeping the concentration of thiosulfate at the original value, are given in the table below.

Table for Question (i) — IB Chemistry HL

On the axes provided, draw an appropriate graph to investigate the order of the reaction with respect to hydrochloric acid.

Figure for Question (i) — IB Chemistry HL

[ 3 ]

Figure for Answer to Question (i) — IB Chemistry HL

correct scale and units on y-axis;
Accept other suitable scales (such as 1 / t ) and units (such as $m s^{-1}$ ).
Axes do not have to show origin/start at zero.
correct calculation of rate in $s^{-1}$ ;

$[\mathrm{HCl}] / \mathrm{mol} \mathrm{dm}^{-3}$

0.02

0.04

0.06

0.08

Time / s

89.1

72.8

62.4

54.2

Rate $/ s^{-1}$

0.0112

0.0137

0.0161

0.0185

If graph correct, assume this has been done on calculator and not written down.
correct plotting of points that the student decides to use and a connecting line;

Award final mark if 3 or more points are correct, irrespective of what is plotted on y-axis.
If line goes through the correct values at given concentrations of HCl, assume that points are marked there.

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2.2.10 (HL)—Reaction order question 1

Question (a)

Question (i)

Question (b)

Question (i)