Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Factor-to-rate chains
For concentration and gas pressure, focus on particles per unit volume and collision frequency. For surface area, focus on exposed solid particles. For temperature, include increased kinetic energy and a greater fraction above Ea. For catalysts, use an alternative lower-Ea pathway.
Rate versus amount
A condition can change rate without changing total product from fixed limiting reactant amounts. Catalysts, temperature and particle size usually change time taken, not stoichiometric yield. Concentration changes may alter both if the number of moles also changes.
Interpreting unfamiliar contexts
Identify the reacting phases and decide which rate factor is relevant. Stirring may increase contact in a heterogeneous mixture but is not one of the five named syllabus factors; explain it through transport and contact rather than inventing a new law.
Multi-mark answers
Write linked causal sentences. Avoid repeating ‘more collisions’ without specifying why. Use graph evidence where provided, including gradients, plateaux and final amounts.
Practical or data skill
Complete a mixed set of rate questions requiring graph comparison, experimental evaluation and collision-theory explanation.
Examination tip
Before writing, identify whether the question asks about rate, final amount, or both.
Review questions and suggested answers
Question 1
Does a catalyst change theoretical yield?
Suggested answer
No.
Question 2
Why might higher concentration change both rate and final amount?
Suggested answer
It can increase particles per volume and may also increase total moles present.
Question 3
What is the final link in a collision-theory explanation?
Suggested answer
The number of successful collisions per second changes, so reaction rate changes.
