Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Particle explanation
Increasing solution concentration increases the number of solute particles per unit volume. Reacting particles are closer on average and collide more frequently. If temperature and other variables remain constant, successful collisions occur more often and the rate increases.
Graph patterns
A more concentrated reactant usually gives a steeper initial gradient. If the total amount of limiting reactant is unchanged, both experiments may reach the same final product quantity. If concentration is changed by adding more moles in the same volume, the final amount may also change.
Fair comparison
To isolate concentration, control temperature, total reacting moles where appropriate, surface area of solids, catalyst and apparatus. Changing concentration by dilution also changes water volume, so total solution volume should be considered.
Example
In a sodium thiosulfate and acid experiment, higher thiosulfate concentration produces sulfur particles faster, so a marked cross disappears sooner. Rate can be represented by 1/time for a fixed visibility endpoint.
Practical or data skill
Plan a concentration-rate investigation using a gas syringe or disappearing-cross method. Use at least five concentrations and repeat each condition.
Examination tip
State ‘more particles per unit volume’, not merely ‘more particles’.
Review questions and suggested answers
Question 1
Why does concentration increase rate?
Suggested answer
There are more reacting particles per unit volume, causing more frequent successful collisions.
Question 2
Why might two curves reach the same plateau?
Suggested answer
They may contain the same amount of limiting reactant and therefore form the same total product.
Question 3
What should be kept constant in a concentration experiment?
Suggested answer
Temperature, other reactant amount or concentration, apparatus and any solid surface area.
