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Edexcel A-Level Chemistry A2 11.10 Calculations and Graphical Methods to Find the Activation Energy for a Reaction From Experimental Data the Arrhenius Questions

Convert temperature to kelvin, calculate the required reciprocal or logarithmic terms, plot the stated linear form and use the signed gradient with R before converting to kJ mol⁻¹.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
A2

Exam points

  • Transform temperature and rate data into the variables specified by the Arrhenius form.
  • Plot a labelled best-fit Arrhenius graph and calculate its signed gradient with units.
  • Use gradient = −Eₐ/R to calculate positive activation energy in kJ mol⁻¹.

Edexcel A-Level Chemistry A2 11.10 Calculations and Graphical Methods to Find the Activation Energy for a Reaction From Experimental Data the Arrhenius Questions question 1

[Maximum number: 7]

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 1H2O2(aq)+2H+(aq)+2I−(aq)→2H2O(l)+I2(aq)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, Na2 S2O3\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 S2O32−(aq)+I2(aq)→S4O62−(aq)+2I−(aq)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.

Question (a)

(a)

The activation energy for Reaction 1 may be found by repeating the experiment at different temperatures.
Each student carried out an experiment at a different temperature.
One of the students misread the thermometer in their experiment.

Table for Question (a) — Edexcel A-Level Chemistry A2

The activation energy, EaE_{\mathrm{a}}, for a reaction may be found by plotting a graph of In rate against 1 / T.
The gradient of the resulting line of best fit can be used in the Arrhenius equation to determine a value for EaE_{\mathrm{a}}, in kJmol−1\mathrm{kJ} \mathrm{mol}^{-1}.

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Question (i)

(i)

Determine the value for EaE_{\mathrm{a}} for Reaction 1 by plotting a graph using the axes provided.
You should take into account the error made by one of the students.

 In rate =−EaR×1T+ constant R=8.31JK−1 mol−1\text { In rate }=-\frac{E_{\mathrm{a}}}{R} \times \frac{1}{T}+\text { constant } \quad R=8.31 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}
Figure for Question (i) — Edexcel A-Level Chemistry A2

EaE_{a}kJmol−1\mathrm{kJ} \mathrm{mol}^{-1}

[ 5 ]

Question (ii)

(ii)

The students all used thermometers capable of reading to the same precision. Use your graph to deduce the temperature that the student who made the error should have read on their thermometer.

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