Using collision theory, explain why increasing the concentration of the acid increases the rate of reaction.
[2 marks]
Total for this question: 2
Chemistry required practicals
Rate of reaction & concentration
AQA 4.6 · RP5
GCSE Chemistry (8462) · Required practical 5 — method, variables, the marks examiners report students losing.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
Investigate how changing the concentration of a reactant affects the rate of reaction, measured by gas volume or by how long a mixture takes to turn cloudy.
Apparatus
- Conical flask and bung
- Gas syringe (or measuring cylinder over water); or a printed cross for the turbidity method
- Stopwatch and measuring cylinders
- Reactants (e.g. magnesium + hydrochloric acid, or sodium thiosulfate + hydrochloric acid)
Method
- 1Measure a fixed volume of acid of a known concentration into the conical flask.
- 2Connect the gas syringe to the delivery tube and bung, check the assembly is airtight and the plunger moves freely, then set the syringe to zero.
- 3Add the magnesium, immediately push the bung home to seal the flask, and start the stopwatch in the same motion. Do not add the magnesium to an open flask and connect the syringe afterwards — the hydrogen produced in those first seconds escapes, so every reading is too low.
- 4Record the volume of gas collected at regular time intervals.
- 5Repeat with different concentrations of acid, keeping everything else the same.
- 6(Turbidity method: place the flask on a printed cross and time how long until the cross disappears through the cloudy mixture.)
Variables
Independent
Concentration of the acid
Dependent
Rate of reaction (gas volume per second, or 1/time for the cross to disappear)
Control
- Volume of acid
- Mass and surface area of the solid
- Temperature
Results & processing
- Plot volume of gas against time: a steeper initial gradient means a faster rate; the rate at any moment is the gradient of the tangent.
- Higher concentration gives a steeper curve. For the turbidity method use 1/time as a measure of rate.
Where students lose marks
Not linking the rate change to collision theory.
Fix: Higher concentration = more particles in the same volume = more frequent collisions = faster rate.
Misreading the gradient of the volume-time graph.
Fix: The rate at a given time is the gradient of the tangent to the curve; the initial rate is the steepest part.
Changing more than one variable.
Fix: Keep temperature, volume, and the mass / surface area of the solid the same so only concentration varies.
Starting the reaction before the collection apparatus is ready.
Fix: Connect the syringe first, then add the magnesium, seal the flask and start the timer immediately so the initial hydrogen is collected.
Improve the method
- Use a gas syringe for precise gas volumes and repeat readings.
- Keep the temperature constant (e.g. a water bath), since temperature also changes the rate.
Try it — exam-style
Medium
ORIGINAL
Medium
ORIGINAL
On a volume-time graph the curve is steepest at the start and then levels off. Explain what the levelling off shows.
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on rate of reaction & concentration?
Rates questions want collision theory in every answer and a graph read correctly — I drill both. Free intro call, then a free first lesson.