Reaction rates and collision theory
Explain how conditions change rate and use experimental data to calculate an average.
Before you begin
- Particles and reactions
- Ratios and graphs
Connect conditions to successful collisions
For a reaction to occur, particles must meet with sufficient energy and suitable orientation. Raising temperature usually increases collision frequency and the fraction with enough energy. Increasing concentration can increase collision frequency. Breaking a solid into smaller pieces increases exposed surface area without increasing its total mass.
Read rate from measurements
An average rate is a measured change divided by its time interval. On a product-volume graph, the slope describes production rate; a flattening curve indicates that the rate is decreasing. A catalyst provides a lower-activation-energy pathway. It can speed reaching a final state but does not necessarily increase the final amount produced from fixed starting reactants.
Worked example
A reaction produces 48 cm³ of gas in 24 s. Find the average production rate.
Show the worked solution
- Use the measured gas-volume change of 48 cm³.
- Divide by the corresponding time interval, 24 s.
- Keep volume per time as the rate unit.
2 cm³/s.
Try it yourself
Why can a powdered solid react faster than the same mass in one lump?
Common mistakes
- A steeper product-time graph means a faster rate, not automatically more final product.
What can you explain now?
A gas volume rises from 12 to 42 cm³ between 5 and 15 s. Find the average rate over that interval.
Compare with the explanation
3 cm³/s.
The change is 42 − 12 = 30 cm³ over 15 − 5 = 10 s. Use interval differences rather than the final readings alone.
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