Rate of reaction & concentration
GCSE Chemistry (8462) · Required practical 5 — method, variables, the marks examiners report students losing.
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.
- 2Add the magnesium, immediately start the stopwatch and connect the gas syringe.
- 3Record the volume of gas collected at regular time intervals.
- 4Repeat with different concentrations of acid, keeping everything else the same.
- 5(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.
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
Using collision theory, explain why increasing the concentration of the acid increases the rate of reaction.
On a volume-time graph the curve is steepest at the start and then levels off. Explain what the levelling off shows.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
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