Effect of Temperature on Rate

Temperature is one of the most important factors affecting the rate of a chemical reaction. In this lesson you will learn exactly why increasing temperature increases the rate, and you will study the core practical investigation that the Edexcel specification requires you to know. This is a topic that appears frequently in exams, both as short-answer questions and as extended-response questions.

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Why Does Increasing Temperature Increase the Rate?

When the temperature of a reaction mixture is increased, two things happen:

1. Particles Move Faster — More Frequent Collisions

At a higher temperature, particles have more kinetic energy. This means they move faster and collide with each other more often. The increased frequency of collisions means there are more opportunities for a reaction to take place in any given time period.

2. More Particles Exceed the Activation Energy — More Successful Collisions

This is the more important effect. At a higher temperature, the distribution of kinetic energies shifts so that a greater proportion of particles have energy equal to or greater than the activation energy (Eₐ). This means a larger fraction of the collisions that occur are successful — they have enough energy to break bonds and form products.

Both effects work together to increase the rate of reaction.

Effect	Explanation
More frequent collisions	Higher temperature → more kinetic energy → particles move faster → collide more often
More successful collisions	Higher temperature → greater proportion of particles have energy ≥ Eₐ → more collisions overcome the activation energy barrier

Exam tip: When explaining the effect of temperature on rate, you must mention both effects for full marks: (1) more frequent collisions and (2) a greater proportion of particles have energy ≥ activation energy. Many students only mention one and lose marks.

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The Temperature–Rate Relationship

As a rough guide, for many reactions:

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

This is only an approximation, and the exact change depends on the specific reaction and its activation energy, but it illustrates how sensitive rate is to temperature changes.

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Core Practical: Investigating the Effect of Temperature on Rate

The Sodium Thiosulfate and Hydrochloric Acid Experiment (Disappearing Cross)

This is one of the most commonly examined practicals in the Edexcel GCSE Chemistry specification. You must know the method, the observations, and how to interpret the results.

The Reaction

Sodium thiosulfate reacts with hydrochloric acid to produce sulfur, water, sodium chloride, and sulfur dioxide:

Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g) + S(s)

The sulfur produced is a yellow solid that forms as a fine precipitate, making the solution go cloudy.

Method

1. Draw a bold black cross on a piece of white paper.
2. Place a conical flask on top of the cross.
3. Measure 50 cm³ of sodium thiosulfate solution using a measuring cylinder and pour it into the flask.
4. Heat the sodium thiosulfate solution in a water bath to the desired temperature (e.g. 20 °C, 30 °C, 40 °C, 50 °C, 60 °C).
5. Use a thermometer to check the temperature.
6. Add 5 cm³ of dilute hydrochloric acid to the flask and start a stopwatch immediately.
7. Look down through the solution at the cross from above.
8. Stop the stopwatch when the cross can no longer be seen through the cloudy solution.
9. Record the time taken.
10. Repeat for each temperature.
11. Calculate the rate for each temperature using rate = 1/t.

Variables

Variable	Details
Independent variable	Temperature of sodium thiosulfate solution
Dependent variable	Time taken for the cross to disappear
Control variables	Volume and concentration of sodium thiosulfate, volume and concentration of HCl, same cross and same observer

Observations

• At low temperatures (e.g. 20 °C), the solution goes cloudy slowly and it takes a long time for the cross to disappear.
• At high temperatures (e.g. 60 °C), the solution goes cloudy much faster and the cross disappears quickly.
• The higher the temperature, the shorter the time for the cross to disappear.

Results Table (Typical)

Temperature (°C)	Time for cross to disappear (s)	Rate = 1/t (s⁻¹)
20	200	0.0050
30	120	0.0083
40	65	0.0154
50	35	0.0286
60	18	0.0556

Interpreting the Results

• As temperature increases, the time decreases.
• As temperature increases, the rate (1/t) increases.
• A graph of rate (1/t) against temperature shows an upward curve, demonstrating that the rate increases more steeply at higher temperatures.

Sources of Error and Improvements

Source of Error	Improvement
Judging when the cross disappears is subjective	Use a light sensor and datalogger connected to a computer to detect the change in light transmission — removes human judgement
Temperature may change during the reaction	Use a water bath to maintain a constant temperature throughout
Timing errors (human reaction time)	Use a datalogger or light sensor for automatic timing

Exam tip: If you are asked to suggest improvements to this experiment, always mention using a light sensor or datalogger to remove subjectivity. This is the key improvement that examiners look for.

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Explaining the Results Using Collision Theory

At higher temperatures:

1. The sodium thiosulfate and HCl particles have more kinetic energy and move faster.
2. They collide more frequently.
3. A greater proportion of the collisions have energy equal to or greater than the activation energy.
4. Therefore there are more successful collisions per second.
5. The sulfur precipitate forms faster, and the cross disappears in a shorter time.

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Plotting and Interpreting Graphs from this Practical

There are two common graphs you might be asked to draw or interpret from the disappearing cross experiment:

Graph 1: Time against Temperature