Effect of Temperature on Rate

This lesson covers how changing the temperature affects the rate of reaction using collision theory, as required by the Edexcel GCSE Combined Science specification (1SC0). Temperature is arguably the most important factor — it affects both the frequency and the energy of collisions.

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The Key Idea

When the temperature is increased:

1. Particles gain more kinetic energy and move faster.
2. Collisions happen more frequently (particles meet each other more often).
3. A greater proportion of collisions have energy equal to or above the activation energy (Eₐ).
4. Therefore there are more successful collisions per second and the rate increases.

Exam Tip: This is one of the most commonly asked 4–6 mark questions. You must give both reasons: (1) more frequent collisions AND (2) a greater proportion of particles have energy ≥ Eₐ. Giving only one reason limits you to a maximum of half the marks.

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Collision Theory Explanation in Detail

Reason 1: More Frequent Collisions

At a higher temperature, particles have more kinetic energy and move faster. They travel across the solution (or gas) more quickly, so they collide with other particles more often. This alone would increase the rate slightly.

Reason 2: More Energetic Collisions (the main reason)

At a higher temperature, the distribution of particle energies shifts — a much larger proportion of particles now have energy ≥ Eₐ. This means a higher fraction of collisions are successful.

This second reason is the more significant of the two. A 10 °C rise in temperature roughly doubles the rate of many reactions — and most of that increase comes from the greater proportion of energetic particles, not just the increased frequency.

graph TD
    A["Temperature increases"] --> B["Particles gain<br/>kinetic energy"]
    B --> C["Particles move<br/>faster"]
    C --> D["More frequent<br/>collisions"]
    B --> E["Greater proportion<br/>of particles have<br/>energy ≥ Eₐ"]
    D --> F["More successful<br/>collisions per second"]
    E --> F
    F --> G["Rate of reaction<br/>INCREASES"]

    style A fill:#e74c3c,color:#fff
    style G fill:#27ae60,color:#fff

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Energy Distribution and Temperature

The Maxwell–Boltzmann distribution shows the spread of particle energies in a sample. When temperature increases:

Feature	Lower temperature	Higher temperature
Peak of the curve	Higher and further left (most particles have moderate energy)	Lower and shifted right (energies are more spread out)
Area under curve	Represents total number of particles — stays the same	Same total area
Proportion ≥ Eₐ	Small	Much larger

The key takeaway: at higher temperature, the curve flattens and shifts right, meaning many more particles exceed Eₐ.

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Graphical Evidence

Graph: Volume of Gas vs Time at Different Temperatures

Feature	Higher temperature	Lower temperature
Initial gradient	Much steeper (faster rate)	Shallower (slower rate)
Total product	Same (if amounts of reactants are unchanged)	Same
Time to complete	Shorter	Longer

Both curves level off at the same total volume (assuming the same reactant quantities), but the higher-temperature experiment reaches the plateau much sooner.

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The 10 °C Rule (Approximate)

As a rough guide, for many reactions:

Increasing the temperature by 10 °C approximately doubles the rate of reaction.

This is not an exact rule — it varies from reaction to reaction — but it is a useful approximation at GCSE level.

Temperature (°C)	Relative rate (approx.)
20	1
30	2
40	4
50	8
60	16

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Worked Example

In an experiment, the reaction between sodium thiosulfate and hydrochloric acid is carried out at different temperatures. The time for the cross to disappear is recorded.

Temperature (°C)	Time (s)	1/Time (s⁻¹)
20	200	0.0050
30	102	0.0098
40	53	0.0189
50	27	0.0370
60	14	0.0714

Observations:

• As temperature increases, the time decreases significantly.
• 1/time (a measure of rate) approximately doubles with each 10 °C rise.
• A graph of 1/time against temperature gives a curve — rate increases more and more steeply (exponential relationship).

Exam Tip: If asked why the graph of rate vs temperature is a curve (not a straight line), explain that the relationship is not directly proportional — the effect of temperature on the proportion of particles exceeding Eₐ is exponential, not linear.

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Comparing Temperature with Other Factors

Factor	Increases frequency of collisions?	Increases proportion with energy ≥ Eₐ?
Concentration	Yes	No
Surface area	Yes	No
Temperature	Yes	Yes
Catalyst	No	Effectively yes (lowers Eₐ)

Temperature is unique because it affects both frequency and energy.

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Summary

• Increasing temperature gives particles more kinetic energy so they move faster.
• Collisions are more frequent.
• A greater proportion of particles have energy ≥ Eₐ, so more collisions are successful.
• The rate of reaction increases — roughly doubles for every 10 °C rise.
• On graphs, higher temperature gives a steeper initial curve but the same total product.
• Temperature is the only factor that affects both collision frequency and the proportion of energetic collisions.

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Extended Worked Example 1: Predicting Rate Change from a Temperature Rise

Question: A reaction takes 80 seconds to complete at 20 °C. Estimate how long it would take at 50 °C, using the rule that rate doubles for every 10 °C rise.