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IB Chemistry HL 2.2.10 Hl Reaction Order Questions

Use real HL exam questions to determine individual and overall reaction orders from initial-rate data and concentration–time or rate–concentration graphs.

Syllabus
First assessment 2025
Course
Chemistry HL
Level
HL

Exam points

  • Deduce the order with respect to a reactant from initial-rate data or concentration–time evidence, including zero-, first- and second-order behaviour.
  • Identify or calculate the overall reaction order by summing the experimentally supported individual reactant orders.
  • Analyse and interpret rate–concentration or integrated-rate graphs to evaluate the reaction-order model and its quantitative consequences.

Learn this topic

Study 2.2.10 (HL)—Reaction order conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

IB Chemistry HL 2.2.10 Hl Reaction Order Questions question 1

[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.

Na2 S2O3(aq)+2HCl(aq)→2NaCl(aq)+SO2( g)+S( s)+H2O(1)\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 IB Chemistry HL 2.2.10 Hl Reaction Order Questions question 1 — IB Chemistry HL
Figure for Question IB Chemistry HL 2.2.10 Hl Reaction Order Questions question 1 — IB Chemistry HL

Initially they measured out 10.0 cm310.0 \mathrm{~cm}^{3} of 0.500moldm−30.500 \mathrm{moldm}^{-3} hydrochloric acid and then added 40.0 cm340.0 \mathrm{~cm}^{3} of 0.0200 moldmm−30.0200 \mathrm{~mol} \mathrm{dm} \mathrm{m}^{-3} aqueous sodium thiosulfate. The mark on the paper was obscured 47 seconds after the solutions were mixed.

Question (a)

(a)

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

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Question (i)

(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 ]

Question (b)

(b)

One proposed mechanism for this reaction is:

S2O32−(aq)+H+(aq)⇌HS2O3−(aq) Fast HS2O3−(aq)+H+(aq)→SO2( g)+S( s)+H2O(l) Slow \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}
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Question (i)

(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
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