Temperature's Effect on Reaction Rate Revision Notes | A Level AQA Chemistry

1. Why Temperature Is the Most Powerful Rate Variable

You have probably noticed that food cooks faster at higher temperatures, or that a cold pack slows a chemical reaction to a crawl. Temperature is not merely one factor among many — it is arguably the most dramatic lever you can pull on a reaction rate. A rise of just 10°C can roughly double the rate of many reactions, and the reason why goes far deeper than “particles move faster.”

To understand this properly, you need two things: a precise definition of what rate actually means, and a model of how molecular energies are distributed in a sample. Once those two ideas are in place, the temperature effect becomes not just memorable but inevitable.

2. Rate of Reaction — The Exam-Standard Definition

Notice that rate is always expressed per unit time. A reaction that produces 0.5 mol dm⁻³ of product in 1 second is ten times faster than one that produces the same amount in 10 seconds. Examiners frequently award a mark specifically for including the time component in a definition, so make sure it is always there.

3. The Qualitative Effect of Temperature on Rate

Increasing the temperature of a reaction mixture increases the rate of reaction. Decreasing the temperature decreases the rate. This is the qualitative picture, and it holds for virtually all reactions you will encounter at A Level.

As temperature increases, the fraction of molecules with energy equal to or greater than the activation energy increases disproportionately, which is why rate rises much faster than temperature alone might suggest.

The key insight is that not all collisions lead to reaction. Only those collisions where the combined kinetic energy of the colliding particles meets or exceeds the activation energy $E_a$ are successful. Temperature changes the proportion of molecules with sufficient energy, and this effect is disproportionately large.

4. The Maxwell–Boltzmann Distribution — The Heart of the Explanation

Picture a sample of gas containing billions of molecules. At any given moment, those molecules are not all moving at the same speed — they are constantly colliding with each other and exchanging energy. The result is a spread of energies, and the Maxwell–Boltzmann distribution is the graph that describes this spread.

The x-axis shows molecular energy; the y-axis shows the number of molecules (or the probability of finding a molecule) at each energy. The curve starts at the origin, rises to a peak at the most probable energy, then tails off asymptotically towards the right — meaning there is always a small number of molecules with very high energies, no matter what the temperature.

Three features of this graph are critical for exam answers. First, the peak of the higher-temperature curve shifts to the right (higher energy) and is lower in height. Second, the total area under each curve is the same, because the total number of molecules has not changed. Third, the area under the curve to the right of $E_a$ — representing molecules with enough energy to react — is significantly larger at the higher temperature.

That third point is the crux of everything. Even a modest temperature rise shifts the tail of the distribution noticeably to the right. Because the activation energy sits in the tail of the curve, a small rightward shift can double or even triple the area beyond $E_a$. This is why a 10°C rise can double the rate: the fraction of molecules exceeding $E_a$ increases dramatically, even though the average energy has only risen slightly.

5. Connecting the Distribution to Rate — The Full Explanation

Now bring the two ideas together. Rate depends on the frequency of successful collisions. A successful collision requires both sufficient energy ($\geq E_a$) and correct orientation. Temperature affects the energy criterion directly: as temperature rises, the Maxwell–Boltzmann distribution shifts so that a larger proportion of molecules possess energy $\geq E_a$.

Because the activation energy sits in the high-energy tail of the distribution, even a small temperature increase causes a large proportional increase in the number of molecules that can react. This is the reason the effect of temperature on rate is so pronounced compared with, say, a small change in concentration.

The Arrhenius equation, $k = Ae^{-E_a/RT}$, captures this relationship mathematically: the rate constant $k$ increases exponentially with temperature. You are not required to use this equation quantitatively at this stage, but it confirms that the relationship between temperature and rate is exponential, not linear — which is exactly what the Maxwell–Boltzmann tail predicts.

The reaction profile above shows $E_a$ as the energy barrier that colliding particles must overcome. On the Maxwell–Boltzmann distribution, $E_a$ is the vertical line on the energy axis: only molecules in the shaded area to the right of that line can react. Raising the temperature increases the shaded area substantially.

6. Investigating How Rate Changes with Temperature — Required Practical

The specification requires you to be able to describe and evaluate an investigation into how reaction rate changes with temperature. The most common method at A Level uses the reaction between sodium thiosulfate and hydrochloric acid, where a precipitate of sulfur forms and obscures a cross drawn beneath the flask. The time taken for the cross to disappear is a measure of the rate.

One important evaluation point: the cross-disappearance method is subjective — different observers may judge the endpoint differently. Using the same observer throughout, or replacing the visual method with a colorimeter to measure light transmission, reduces this source of error. A colorimeter gives a continuous, objective measurement and allows you to track rate throughout the reaction rather than at a single endpoint.

7. Exam Technique — Writing High-Mark Answers

Temperature questions appear in almost every AQA kinetics paper, and the mark schemes are very specific about the language required. The most common reason students lose marks is stopping the explanation too early — for example, writing “particles have more energy so react faster” without linking this to the Maxwell–Boltzmann distribution and the proportion of molecules exceeding $E_a$.

A full ‘explain’ answer for a temperature question needs three linked steps: (1) state that molecules have a range of energies described by the Maxwell–Boltzmann distribution; (2) state that only molecules with energy $\geq E_a$ can react; (3) state that at higher temperature, a greater proportion of molecules have energy $\geq E_a$, so the rate of successful collisions increases. Each of these steps typically carries a separate mark.

Worked example: Writing a Full Maxwell–Boltzmann Explanation for a 4-Mark Question

Explain, with reference to the Maxwell–Boltzmann distribution, why increasing the temperature from 25°C to 35°C causes a large increase in the rate of a reaction. [4 marks]

1. Molecules in the reaction mixture have a range of kinetic energies, described by the Maxwell–Boltzmann distribution. — [B1] — Naming the distribution and stating that molecules have a range of energies earns the first mark.
2. Only molecules with energy equal to or greater than the activation energy $E_a$ can react when they collide. — [B1] — Linking $E_a$ to the condition for a successful collision earns the second mark. Do not say Eₐ decreases.
3. At 35°C, the Maxwell–Boltzmann curve shifts to the right: the peak is lower and broader, and the high-energy tail extends further. — [B1] — Describing the shape change correctly (lower peak, shifted right, same area) earns the third mark.
4. The area under the curve to the right of $E_a$ is significantly larger at 35°C. A much greater proportion of molecules now possess energy $\geq E_a$, so the frequency of successful collisions increases substantially, causing a large increase in rate. — [B1] — Explicitly stating that the proportion exceeding Eₐ increases significantly, and linking this to rate, earns the fourth mark. The word ‘proportion’ is key.

Answer: Four marking points: (1) molecules have a range of energies (Maxwell–Boltzmann distribution); (2) only molecules with energy ≥ Eₐ react; (3) at higher temperature the distribution shifts right with a lower, broader peak; (4) a greater proportion of molecules have energy ≥ Eₐ, so the rate of successful collisions increases significantly.

Bringing everything together: rate of reaction measures how quickly concentrations change over time. Temperature increases rate because it shifts the Maxwell–Boltzmann distribution to higher energies, dramatically increasing the proportion of molecules with energy $\geq E_a$. Even a small temperature rise produces a large rate increase because the activation energy sits in the tail of the distribution, where small shifts in the curve correspond to large changes in the area beyond $E_a$. This understanding underpins every kinetics question you will face at A Level.