Effect of temperature on reaction rate — AQA A-Level Chemistry
Test yourself on Effect of temperature on reaction rate with AQA A-Level practice questions.
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Effect of temperature on reaction rate explained
The rate of reaction is the change in the concentration of a reactant or product per unit time, with units mol dm⁻³ s⁻¹.
Read the full explanation
On a concentration against time graph the rate at any moment is the gradient of the tangent at that point, steepest at the start when the reactants are most concentrated and falling to zero when the reaction stops. Because concentration is often awkward to follow directly, experiments track something proportional to it: the volume of gas released, the loss of mass, the intensity of a colour, or the time taken for a fixed visible change. In that last case the rate is proportional to 1/t, so a plot of 1/t against temperature or concentration behaves like a rate plot even though it is not in absolute units.
The qualitative effect of temperature changes on the rate of reaction.
Raising the temperature makes a reaction faster, and usually much faster: as a rough guide a 10 K rise roughly doubles the rate of many reactions near room temperature. A plot of rate against temperature is not a straight line but a curve that steepens, an exponential rise. Two things change when a gas or solution is heated. The particles move faster, so collisions happen more often, and their energy per collision is greater. The second effect dominates by far. The collision frequency rises only slightly, roughly with the square root of the absolute temperature, while the fraction of particles with energy greater than or equal to Eₐ rises steeply, and it is this fraction that multiplies up the number of successful collisions.
Students should be able to: use the Maxwell–Boltzmann distribution to explain why a small temperature increase can lead to a large increase in rate.
The puzzle is that a 10 K rise, which changes the mean molecular energy by only a few per cent, can double the rate. The distribution explains it. Warming the gas shifts the whole curve to the right and flattens it, but the important change is out in the tail. Because the tail falls away steeply, a small rightward shift moves a disproportionately large slice of molecules past Eₐ: the shaded area beyond the barrier might double while the mean energy rose by three per cent. Since the rate depends on the number of molecules with energy greater than or equal to Eₐ, not on the mean, the rate roughly follows that shaded area. Collision frequency also rises slightly, but it is the change in the area beyond Eₐ that dominates.
Required practical 3 Investigation of how the rate of a reaction changes with temperature.
Required practical 3 follows one reaction at several temperatures. The usual choice is sodium thiosulfate with dilute hydrochloric acid, which slowly clouds with sulfur; the mixture is placed over a printed cross and the time for the cross to disappear is recorded. Since the same amount of sulfur forms each time, the rate is proportional to 1/t. Everything except temperature must be held constant: the concentrations, the total volume, the depth of liquid, the same cross, and ideally the same observer, because judging the moment the cross vanishes is subjective. The solutions are brought to temperature separately in a water bath before mixing, and the actual temperature of the mixture is measured rather than assumed. A plot of 1/t against temperature curves upwards.
Your focus
- Define rate of reaction with reference to concentration and time and state its units.
- Determine the rate at a stated time by drawing a tangent to a concentration against time graph and calculating its gradient.
- Select an appropriate measurable quantity to follow a given reaction and justify why it tracks concentration.
Show all 16 objectives
- Explain why 1/t is a valid measure of rate when the time for a fixed visible change is recorded.
- Describe the shape of a graph of rate against temperature and state what the shape shows about how rate changes.
- Explain a temperature induced rate increase using both collision frequency and the proportion of collisions exceeding Eₐ, and say which dominates.
- Predict the effect on rate of a stated temperature rise for a reaction and justify the prediction.
- Identify why an answer referring only to more frequent collisions is incomplete for a temperature change.
- Explain, using the shape of the energy tail, why a small rise in temperature produces a large rise in rate.
- Sketch two labelled distribution curves and use the areas beyond Eₐ to justify a stated change in rate.
- Compare the contribution of increased collision frequency with that of the increased proportion of energetic molecules.
- Criticise an answer that explains a temperature effect using collision frequency alone, and correct it.
- Describe a method for measuring how the rate of a named reaction changes with temperature, using a fixed visible endpoint.
- Identify the control variables in the experiment and explain the effect of failing to control each one.
- Plot 1/t against temperature, describe the shape obtained and relate it to the Maxwell–Boltzmann distribution.
- Suggest an improvement that removes the subjective judgement of the endpoint and explain why it improves the data.
Effect of temperature on reaction rate exam tips
Marking Points
- one mark for defining rate as the change in concentration of a reactant or product per unit time
- one mark for the units mol dm⁻³ s⁻¹
- one mark for identifying the rate at a given moment as the gradient of the tangent to a concentration against time curve
- one mark for recognising 1/t as a measure of rate when a fixed extent of reaction is timed
- one mark for stating that rate increases as temperature increases
- one mark for describing the increase as exponential, or as rising by an ever greater factor
- one mark for stating that many more particles have energy greater than or equal to the activation energy
- one mark for identifying the increase in the energy of the particles, rather than the increase in collision frequency, as the dominant effect
- one mark for both curves drawn correctly on axes labelled number of molecules and energy, with Eₐ marked on the energy axis
- one mark for the higher temperature curve having a lower peak shifted to the right and crossing the first curve once
- one mark for stating that at the higher temperature many more molecules have energy greater than or equal to Eₐ
- one mark for concluding that the frequency of successful collisions, and therefore the rate, increases greatly
- one mark for noting that the increase in collision frequency alone is too small to account for the change
- one mark for stating that the time for a fixed visible change is measured and that rate is proportional to 1/t
- one mark for naming the control variables, including concentration, total volume and depth of solution
- one mark for bringing the solutions to temperature in a water bath and measuring the temperature of the mixture
- one mark for describing the shape of the 1/t against temperature plot as an upward curve rather than a straight line
Examiner Tips
- 💡Draw tangents with a ruler over a wide span so the gradient can be read accurately.
- 💡State the units every time you quote a rate; they are frequently a mark in their own right.
- 💡If the experiment times a disappearing cross or a colour change, say explicitly that rate is proportional to 1/t.
- 💡Always pair the two ideas: particles have more energy, so a greater proportion of collisions exceed Eₐ.
- 💡Say many more particles exceed Eₐ; the word many is what conveys the exponential effect.
- 💡When describing a curve, name the shape, then explain it; description and explanation carry separate marks.
- 💡Shade the two areas beyond Eₐ on your sketch and refer to them by name in the explanation.
- 💡Use the word proportion or fraction of molecules beyond Eₐ, then convert that to frequency of successful collisions.
- 💡A three mark version of this question usually wants: curve shifts right, many more molecules exceed Eₐ, so more frequent successful collisions.
- 💡Name the specific control variables from this experiment rather than saying keep everything else the same.
- 💡When asked for an improvement, target the subjective endpoint, for example by using a light sensor and data logger.
- 💡Mention that sulfur dioxide is released, so the work is done in a well ventilated room or fume cupboard.
Common Mistakes
- defining rate as how fast a reaction goes, with no reference to concentration or time
- quoting the units as mol dm⁻³ or s⁻¹ alone
- calculating a gradient from two widely spaced points on a curve instead of drawing a tangent
- treating the time taken as the rate, so a longer time is reported as a faster reaction
- assuming the rate is constant because the graph looks nearly straight over a short interval
- explaining the increase only as more frequent collisions, which the mark scheme refuses on its own
- writing more successful collisions with no reference to energy or the activation energy, which is also refused alone
- describing the rate against temperature graph as a straight line of positive gradient
- saying the activation energy falls when the temperature rises
- claiming the particles gain activation energy rather than gaining kinetic energy
- attributing the whole rate increase to more frequent collisions, which the mark scheme explicitly refuses
- saying the mean energy rises above Eₐ, rather than that more molecules lie beyond Eₐ
- stating that the activation energy is lowered by heating
- drawing the hotter curve with a higher peak, so the area under it is no longer constant
- writing more successful collisions without linking it to the area beyond Eₐ on the curve
- plotting time against temperature and describing the falling curve as showing that rate decreases
- heating the mixed solutions, so reaction occurs before timing starts
- changing the total volume when varying a concentration, which alters the other concentration too
- quoting the water bath setting as the reaction temperature instead of the measured value
- treating the subjective endpoint as a systematic error when a different observer judges each run, where the inconsistency between observers makes it a random error - it is systematic only when one observer judges every run the same way.