8.2. Effect of temperature on reaction rates and the concept of activation energy
A subsection of Chemistry, 9701, through 8. Reaction kinetics
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A large excess of 2-bromo-2-methylpropane is added to 0.0010 mol of NaOH, which contains a few drops of phenolphthalein indicator. A stopwatch is started as soon as the substances are mixed. The time taken for the pink colour to disappear is recorded. The experiment is repeated at different temperatures, keeping all concentrations and volumes of reagents constant. temperature / °C time taken for pink colour to disappear / s Explain what is meant by the term rate of reaction. The graph shows the energy distribution of molecules in a sample of 2‑bromo‑2‑methylpropane at 25 °C. Ea represents the activation energy for the reaction. proportion of molecules with a given energy molecular energy Ea Label the graph to show the proportion of 2‑bromo‑2‑methylpropane molecules which have sufficient energy to react. Use the same axes to sketch the distribution of energies of molecules in a sample of 2‑bromo‑2‑methylpropane at 50 °C. State the effect of an increase in temperature on Ea for this reaction. Draw the mechanism to show the reaction of 2‑bromo‑2‑methylpropane with OH–. Show the intermediate formed in this reaction. Include all charges, partial charges, lone pairs and curly arrows as appropriate. H3C CH3 CH3 Br C  Name the mechanism for this reaction. The original experiment is repeated at 25 °C with 2‑chloro‑2‑methylpropane instead of 2‑bromo‑2‑methylpropane. All other variables remain constant. Predict the effect of using 2‑chloro‑2‑methylpropane compared to 2‑bromo‑2‑methylpropane on the time taken for the pink colour to disappear. Explain your answer. 
9701_s21_qp_22
THEORY
2021
Paper 2, Variant 2
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