Lesson focus: collision theoryThe idea that particles only react when they collide with at least the activation energy. explains why higher temperature increases reaction rate through collision frequencyHow many times something happens each second; for a wave, how many waves pass each second, measured in hertz (Hz). and collision energy.
Use collision theory to explain temperature effects and design a valid sodium-thiosulfate rate investigation.
Build on earlier learning
Energy-level diagramsActivation energy was represented as a barrier. Temperature changes how many particle collisions can cross that barrier.
Practical
Temperature and rate
- Place a conical flask of sodium thiosulfate solution on a black cross.
- Add hydrochloric acid and start timing.
- Stop when the cross disappears; repeat at different temperatures.
Record: A table and graph of temperature against time for the cross to disappear.
Safety: Work in a ventilated room; the reaction gives off sulfur dioxide.
Collision theory
Particles react only when they collide with enough energy (the activation energy). Heating gives more particles that energy; a higher concentration gives more collisions.
Reactant A Reactant B Enough energy to react
Controls
Speeds are exaggerated so the effect of temperature shows within a minute.
You will be successful when you can…
- Define a successful collision.
- Explain both temperature effects.
- Calculate reciprocal-time rate.
- Evaluate the disappearing-cross method.
Words that carry the science
- collision theory
- A model in which reaction occurs when particles collide with sufficient energy and suitable orientation.
- successful collision
- A collision that leads to reaction.
- activation energy
- The minimum collision energy needed for reaction.
- frequency
- How often an event occurs per unit time.
- reciprocal rate
- A rate proxy calculated as 1 divided by the measured time.
- endpoint
- The defined observation used to stop timing an investigation.
Connect temperature to successful collision frequency
01Particles collide more often when warmer
Raising temperature increases average particle kinetic energy. Particles move faster, so in a given volume they collide more frequently.
This contributes to a higher reaction rate, but collision frequencyHow many times something happens each second; for a wave, how many waves pass each second, measured in hertz (Hz). alone is not the most important temperature effect for many reactions.
02A larger fraction crosses activation energy
At higher temperature, the distribution of kinetic energies shifts so a much larger fraction of collisions have energy at least equal to activation energyThe smallest amount of energy particles need when they collide to make a reaction start.. The number of successful collisions per second therefore rises.
This explains why a modest temperature increase can cause a substantial rate increase rather than only the small increase expected from collision frequency.
03The disappearing cross is a human endpoint
Sodium thiosulfate and dilute hydrochloric acid form sulfur particles that make the mixture cloudy. Time until a cross is no longer visible provides a rate proxy, 1/time.
The endpointThe clear sign you agree in advance that tells you when to stop the timer or the test, such as a cross disappearing from view. depends on observer, lighting, flask depth and cross design. Wear eye protection and use dilute, microscale quantities with the same observer or video analysis, equal volumes and concentrations, controlled temperatures and repeats. Sulfur dioxide forms: work in strong ventilation or a fume cupboard where the risk assessment requires it, keep the vessel covered when appropriate, stop and dispose of mixtures using the laboratory procedure, and do not repeat unnecessarily.
Calculate reciprocal rate
The cross disappears after 40 s.
- Use rate proxy = 1 ÷ time.
- Substitute 1 ÷ 40 s.
- Calculate 0.025 s⁻¹.
- Describe it as a reciprocal-time rate proxy, not a direct concentration change.
How to make the reasoning visible: A shorter time gives a larger reciprocal value and indicates a faster reaction under the fixed endpoint.
Analyse temperature-rate data
Each time is the mean of three repeats at the same concentrations and volumes.
| Temperature / °C | Mean time / s | 1/time / s⁻¹ | Pattern | Reliability note |
|---|---|---|---|---|
| 15 | 96 | 0.010 | Slowest | Small reciprocal |
| 25 | 61 | 0.016 | Faster | Clear endpoint |
| 35 | 38 | 0.026 | Faster | Matched volumes |
| 45 | 24 | 0.042 | Fastest | Timing start is harder |
Graph skills: SLAPUK and choosing a scaleRate data with awkward values: choose a scale that is not the raw data.
Why plot 1/time rather than time as the rate measure?
Why should both solutions reach target temperature before mixing?
Check your understanding
Answer from memory, then check against the suggested answer.
What is a successful collision?
What two effects does higher temperature have?
How is reciprocal-time rate calculated?
Explain why cooling food slows spoilage reactions.
Why is “particles move twice as fast, so rate doubles” usually invalid?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Sodium thiosulfate reacts with acid to form a sulfur precipitate. The reaction is faster at 35 °C than at 20 °C.
Explain, using particle theory, why the rate increases.
(4)Describe how the faster rate is observed.
(1)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., include both collision frequency and the fraction exceeding activation energy.