Reaction kinetics

• Concentration changes how often; temperature changes what fraction; a catalyst moves the line, not the curve.
• Concentration changes how often; temperature changes what fraction; a catalyst moves the line, not the curve. And: the minimum energy required for a collision to be effective.

• 8.1.1 I can explain — explain and use the terms: rate of reaction, frequency of collisions, effective collisions and non-effective collisions
• 8.1.2 I can explain — explain qualitatively, in terms of frequency of effective collisions, the effect of concentration and pressure changes on the rate of a reaction
• 8.1.3 I can use — use experimental data to calculate the rate of a reaction
• 8.2.1 I can define — define activation energy, E_a, as the minimum energy required for a collision to be effective
• 8.2.2 I can sketch — sketch and use the Boltzmann distribution to explain the significance of activation energy
• 8.2.3 I can explain — explain qualitatively, in terms both of the Boltzmann distribution and of frequency of effective collisions, the effect of temperature change on the rate of a reaction
• 8.3.1 I can explain — explain and use the terms catalyst and catalysis: (a) to explain that, in the presence of a catalyst, a reaction has a different mechanism, i.e. one of lower activation energy (b) to explain this catalytic effect in terms of the Boltzmann distribution (c) to construct and interpret a reaction pathway diagram for a reaction in the presence and absence of an effective catalyst

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “A higher concentration gives the particles more energy, so more collisions are successful.” Correct Concentration changes how often particles collide. The fraction of collisions with at least Ea is set by the temperature, so it is unchanged.
• “Heating speeds a reaction up because the particles collide more often.” Correct True but minor: about 2% more collisions for a 10 K rise near room temperature. The main reason is the much larger proportion of collisions with energy at or above Ea, shown on the Boltzmann distribution. Give both, the distribution first.
• “At the higher temperature the Boltzmann curve is taller.” Correct Its peak is lower and further right. The area under the curve is the number of molecules, which has not changed.
• “A catalyst lowers the activation energy of the reaction.” Correct A catalyst provides an alternative mechanism with a lower activation energy. On the Boltzmann distribution the Ea line moves left; the curve does not move.
• “Activation energy is the energy needed to start a reaction.” Correct The syllabus definition: the minimum energy required for a collision to be effective.
• “The rate at 90 s is the concentration change from 60 s to 120 s divided by 60 s.” Correct That is an average rate, the gradient of a chord. The rate at a moment is the gradient of the tangent at that moment.
• “Increasing the concentration gives the particles more energy, so more collisions are successful.” Repair Concentration changes how often particles collide. The fraction with at least Ea is set by the temperature and is unchanged.
• “A higher temperature speeds the reaction because the particles collide more often.” Repair True but minor, about 2% for 10 K. The main effect is the far larger proportion of collisions with energy at or above Ea, shown on the Boltzmann distribution. Give both, the distribution first.
• “At a higher temperature the Boltzmann curve is taller.” Repair The peak is lower and further right; the area is the same because the number of molecules is the same.
• A Boltzmann curve drawn starting part-way up the vertical axis, with its tail meeting the energy axis. Repair It starts at the origin, because no molecule has zero energy, and the tail approaches the axis without touching it, because there is no upper limit to a molecule’s energy.
• “A catalyst lowers the activation energy of the reaction.” Repair It provides an alternative mechanism with a lower activation energy. The original route still exists, with its original Ea.
• “A catalyst shifts the Boltzmann curve to the left.” Repair The curve depends only on the temperature. A catalyst moves the Ea line to the left.
• “A catalyst makes the reaction more exothermic and increases the yield.” Repair ΔH, the two levels on the pathway diagram and the position of equilibrium are all unchanged. Only the rate changes, in both directions equally.
• “Activation energy is the energy needed to start a reaction.” Repair The minimum energy required for a collision to be effective: the syllabus’s words.
• “Raising the temperature lowers the activation energy.” Repair Ea belongs to the reaction and its route. Temperature changes how many molecules have at least Ea, not Ea itself.
• “Rate at 120 s = concentration at 120 s ÷ 120 s.” Repair The rate at a time is the gradient of the tangent there. A concentration change over an interval divided by the interval is an average rate, the gradient of a chord.
• “Increasing the pressure speeds up any reaction.” Repair Only reactions involving gases. Pressure does not change the concentration of a solution.
• “Human error was the largest source of error.” Repair Never acceptable. Name it (the judgement of when the cross disappears) and classify it: random.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. Topic 26 (reaction kinetics at A Level) builds directly on this chapter: re-answer retrieval questions 6 and 11 when you start it. In topic 7, re-explain why a catalyst leaves the position of equilibrium unchanged; in topics 12, 14 and 18, name the catalyst in each process and say whether it is homogeneous or heterogeneous. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 8 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A multiple-choice item on this topic can turn on which quantity a change affects: collision frequency, the fraction of collisions that is effective, the Boltzmann curve or the Ea line. A structured question asks you to define activation energy in the syllabus’s words, sketch a Boltzmann distribution or a pathway diagram, and explain a change in rate in terms of the frequency of effective collisions.
• The calculations are a rate from the gradient of a tangent (or an average rate from a chord), with its unit; a gas volume converted to moles with Vm = 24.0 dm³ mol⁻¹ from the Data section; 1/t from a timed observation; and Ea of the reverse reaction from a pathway diagram. No activation energy or rate is in the Data section: every such value is supplied by the question.
• Topic 8 is the theory behind the syllabus’s named rate experiment: sodium thiosulfate and acid, timing how long the sulfur takes to hide a cross. A planning or evaluation question asks for the variables and how each is held, the results table with 1/t, the graph, and the largest error, which the syllabus itself classifies as random.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.