Edexcel A-Level Chemistry A2 11.5 Deduce the Order 0 1 Or 2 With Respect to a Substance in a Rate Equation Questions

Practise Edexcel IAL Chemistry 11.5 by comparing concentration and rate factors, reading graph shapes, using proportional volumes and identifying first-order half-lives.

Exam points

Deduce order by comparing concentration and initial-rate factors between controlled runs.
Recognise zero-, first- and second-order shapes on rate–concentration graphs.
Use proportional preparation volumes when absolute concentration is unnecessary for order.
deduce first-order behaviour from a concentration–time graph using constant half-life

[Maximum number: 3]

A group of students investigated the kinetics of a 'clock' reaction.
The reaction investigated was that between hydrogen peroxide and iodide ions in the presence of acid.

Reaction $1 \quad \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{I}^{-}(\mathrm{aq}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{I}_{2}(\mathrm{aq})$

In this 'clock' reaction, a fixed volume of aqueous sodium thiosulfate, $\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}$ , and a small amount of starch were added to the reaction mixture.

The added thiosulfate ions react with the iodine produced in Reaction 1.

Reaction $22 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}(\mathrm{aq})+\mathrm{I}_{2}(\mathrm{aq}) \rightarrow \mathrm{S}_{4} \mathrm{O}_{6}^{2-}(\mathrm{aq})+2 \mathrm{I}^{-}(\mathrm{aq})$

When all the thiosulfate ions have reacted, the presence of iodine is detected by the formation of a starch-iodine complex. The students recorded the time taken for this complex to form.

Four reaction mixtures, with different initial concentrations of hydrogen peroxide, hydrogen ions and iodide ions, were prepared.

Each mixture had the same volume and contained the same amount of sodium thiosulfate and starch.

Table for Question Edexcel A-Level Chemistry A2 11.5 Deduce the Order 0 1 Or 2 With Respect to a Substance in a Rate Equation Questions question 1 — Edexcel A-Level Chemistry A2

Use the results in the table to deduce the order of Reaction 1 with respect to hydrogen peroxide, hydrogen ions and iodide ions. Justify each answer by referring to relevant data from the table.

Hydrogen peroxide

Hydrogen ions
lodide ions

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Edexcel A-Level Chemistry A2 11.5 Deduce the Order 0 1 Or 2 With Respect to a Substance in a Rate Equation Questions question 1