Edexcel IAL Chemistry 11.1 Kinetics terms and rate equations

Read each exponent as the order for that reactant, add exponents for overall order and rearrange the rate equation before deriving the units of k.

Exam points

Write a rate equation from individual orders and state the overall reaction order.
Calculate k and derive its units from the rate units and overall order.
Predict a rate factor when one or more reactant concentrations are multiplied.

[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.

(a)

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 (a) — Edexcel A-Level Chemistry A2

[ 3 ]

(i)

Write the overall rate equation for Reaction 1 using your answers to (b)(i).

[ 1 ]

An answer that makes reference to the following point:

- rate $=k\left[\mathrm{H}_{2} \mathrm{O}_{2}\right]\left[I^{-}\right]$

Allow rate $=k\left[\mathrm{H}_{2} \mathrm{O}_{2}\right]\left[\left[^{-}\right]\left[\mathrm{H}^{+}\right]^{\circ}\right.$

Allow species in any order TE from (b)(i)

(ii)

Calculate a value for the rate constant of Reaction 1 using data from Mixture 1 and your answers to (b)(ii) and (b)(iv).
Include the units of the rate constant.

[ 2 ]

rearrangement of the rate equation and calculation of the rate constant

Example of calculation

$k=\operatorname{rate} \div\left(\left[\mathrm{H}_{2} \mathrm{O}_{2}\right] \times\left[\mathrm{I}^{-}\right]\right)$ $k=4.359 \times 10^{-6} \div\left(5.4 \times 10^{-2} \times 8.2 \times 10^{-3}\right)$ $=4.359 \times 10^{-6} \div 4.428 \times 10^{-4}$ $=9.844 \times 10^{-3}$

Ignore SF except 1SF

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11.1—The terms: i rate of reaction ii rate equation, rate=k[A]m[B]n where m and n are 0, 1 or 2 iii order with respect to question 1

Question (a)

Question (i)

Question (ii)