8.3 The Effect of Temperature

A free GCSE Chemistry lesson for AQA (8462) from OBA School: why reactions speed up when they are heated: more frequent collisions and more energetic collisions. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the effect of increasing temperature on the rate of reaction
• Explain the effect using collision theory, in terms of both the frequency and the energy of collisions
• Calculate rates as 1 ÷ time and interpret data and graphs about temperature
• Apply ideas about temperature to everyday and industrial situations

Key words

• Kinetic energy: The energy a particle has because it is moving. Heating gives particles more kinetic energy.
• Activation energy: The minimum energy that particles must have to react when they collide.
• Successful collision: A collision with at least the activation energy, so the particles react.
• Water bath: A container of water kept at a set temperature, used to keep reaction mixtures at a steady temperature.
• Proportion: The fraction of the whole: here, the fraction of collisions that are successful.

Two effects of heating

Increasing the temperature increases the rate of almost every reaction. Heating gives the particles more kinetic energy, and this has two effects:

• More frequent collisions. Particles move faster, so they collide more often each second.
• More energetic collisions. A greater proportion of particles have energy equal to or greater than the activation energy, so a greater proportion of collisions are successful.

Key fact: Of the two effects, the increase in the energy of collisions is much more important than the increase in their frequency. A small temperature rise gives many more particles enough energy to get over the activation energy barrier.

How big is the effect?

For many reactions at around room temperature, a rise of about 10 °C roughly doubles the rate. This is only a rule of thumb, but it shows how sensitive rates are to temperature. Cooling a reaction has the opposite effect: the rate falls.

• Food in a fridge (about 4 °C); Temperature change lower; Effect on rate Reactions in microorganisms and food go more slowly, so food lasts longer.
• Cooking in a pressure cooker; Temperature change higher (water boils above 100 °C); Effect on rate Food cooks faster.
• Industrial processes; Temperature change higher; Effect on rate Faster production, but heating costs money and energy, so a sensible temperature is chosen.

Investigating the effect of temperature

Sodium thiosulfate solution reacts with dilute hydrochloric acid to make a yellow precipitate of sulfur: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l) Warm both solutions separately in a water bath to the chosen temperature, mix them in a flask over a pencil cross, and time how long it takes for the cross to disappear. Keep the volumes and concentrations the same each time.

• Time for cross to disappear (s); 20 124; 30 66; 40 35; 50 19; 60 11
• Rate = 1 ÷ time (s⁻¹); 20 0.0081; 30 0.015; 40 0.029; 50 0.053; 60 0.091

Worked example: rate from a time

At 40 °C the cross disappeared after 35 s.
rate = 1 ÷ time = 1 ÷ 35 = 0.02857… s⁻¹

Answer: 0.029 s⁻¹ (2 s.f.)

Getting reliable results

• Control variables: the volumes and concentrations of both solutions, the same flask and the same cross. Only the temperature changes.

• Measure the temperature of the mixture just after mixing, not just the water bath. The mixture cools while you time it, especially above about 50 °C.

• Judging the end point by eye causes random error. Repeat each temperature at least three times, leave out anomalies and take a mean. A light sensor gives more precise timings.

Safety: Sulfur dioxide is toxic and can trigger asthma attacks. Use small volumes in a well-ventilated room, and pour the mixture away in a fume cupboard or sink with plenty of water.

Effect on a rate graph

If a reaction that gives off a gas is repeated at a higher temperature (with the same amounts of reactants), the graph of volume against time is steeper at the start and levels off sooner. It levels off at the same final volume, because the amount of limiting reactant has not changed.

Exam tip: In “explain” questions, write both points: particles move faster so collide more frequently, and more of the collisions have energy ≥ activation energy, so the frequency of successful collisions increases.

Common mistakes

• Only saying “the particles move faster”. You must link this to more frequent collisions and to more collisions having enough energy.
• Saying heating lowers the activation energy. Heating gives particles more energy; the activation energy stays the same (only a catalyst changes it).
• Saying a higher temperature makes more product. It makes the same amount of product faster.
• Forgetting to heat both solutions to the right temperature before mixing them.

Revision questions

Try these, then check your answers below each one.

Question 1. Which statement best explains why increasing the temperature increases the rate of a reaction? (Choose from: The activation energy decreases, Particles move faster, so they collide more frequently, and a greater proportion of collisions have energy equal to or greater than the activation energy, There are more particles in the same volume, The particles become bigger, so they collide more often.)

Answer: Particles move faster, so they collide more frequently, and a greater proportion of collisions have energy equal to or greater than the activation energy

Working: Higher temperature means more kinetic energy: more frequent collisions and more energetic collisions, so the frequency of successful collisions increases.

Question 2. True or false? Increasing the temperature lowers the activation energy of a reaction.

Answer: False

Working: False. The activation energy stays the same. More particles have enough energy to reach it. Only a catalyst lowers the activation energy.

Question 3. In a disappearing cross experiment at 45 °C, the cross disappeared after 25 s. Calculate the rate as 1 ÷ time.

Answer: 0.04 s⁻¹

Working: 1 ÷ 25 = 0.04 s⁻¹.

Related lessons

• Previous lesson: 8.2 Collision Theory and Surface Area
• Next lesson: 8.4–8.5 The Effect of Concentration, Pressure and Catalysts
• In this chapter: 8.1 Rate of Reaction
• In this chapter: 8.6–8.7 Reversible Reactions and Energy Changes
• In this chapter: 8.8–8.9 Dynamic Equilibrium and Altering Conditions

Continue in the full course

This lesson is section 8.3 (AGCCH0803) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.