This module covers how fast reactions happen and why: how to read a rate from a graph, how concentration, particle size and a catalyst change it, and how to judge whether a comparison is fair. Check the current 0620 syllabus on the Cambridge page for the wording you are examined on.
What is this topic asking you to do?
Questions here mix three skills: calculating from data, explaining with collision ideas, and evaluating an experiment. Marks are lost when one skill is used in place of another, for example explaining with energy when the question is about frequency.
What should you know first?
You need the particle model from particle models and changes of state and the mole ideas in relative masses and amounts. Gas volume from moles appears again in concentration, gas and yield calculations.
The mole and equation-ratio tutor and the equation balance reasoning trainer let you check the amounts behind a graph.
Orienting worked example
The numbers below are invented. A reaction makes 45 cm³ of gas in the first 30 s and a total of 60 cm³ by the end.
Step 1, average rate over the first 30 s: 45 ÷ 30 = 1.5 cm³/s.
Step 2, what would a catalyst change? It makes the 45 cm³ appear sooner, so the initial rate rises. The total stays 60 cm³, because the amount of product is fixed by the limiting reactant.
The example already uses the calculating and explaining skills together.
In what order should you study the lessons?
- Extract a rate from a supplied graph. Every later lesson gives you a graph or table, so the method comes first.
- Explain concentration effects using collision ideas. It gives you the reasoning chain you reuse.
- Compare powder and lumps without changing amount. It separates rate from amount.
- Describe a catalyst’s role without claiming it is used up. It adds activation energy and the wording examiners look for.
- Evaluate fair comparison in a fictional dataset. It lets you use all four ideas on one table.
Which traps catch most students?
- Dividing one volume by one time and calling it the rate at that time.
- Forgetting the unit, such as cm³/s or g/s.
- Saying concentration or a catalyst gives particles more energy.
- Saying powder or a catalyst gives more product.
- Saying a catalyst is used up.
- Comparing two runs that differ in more than one way.
- Reporting a mean that includes an anomalous result.
How should you use the practice set?
Work through the mixed practice set after the lessons. Cover the answers, show every line of working and mark method before number. Record any slip in the mistake log and retest queue.
The rate and energy graph interpreter can help you see a dataset as a graph before you write a rate.
Some students follow each lesson yet lose marks when a long question asks for a calculation and an explanation together. Our teachers watch that working live in online one-to-one Chemistry tuition and correct the step, not just the answer.