Required Practical: Rates of Reaction — AQA GCSE Chemistry
This required practical investigates how a factor affects the rate of a reaction, using two common methods.
Method 1: The disappearing cross (turbidity)
Used for the reaction of sodium thiosulfate + hydrochloric acid, which produces a cloudy yellow sulfur precipitate.
- Mix the reactants in a flask placed over a black cross drawn on paper.
- Time how long it takes for the cross to disappear from view (the solution turns cloudy).
- Repeat at different concentrations (or temperatures).
- The shorter the time, the faster the rate (rate ∝ 1 ÷ time).
Method 2: Measuring gas volume
Used for reactions that produce a gas, e.g. magnesium + hydrochloric acid or marble chips + acid.
- Collect the gas in a gas syringe (or measure mass loss on a balance).
- Record the volume of gas at regular time intervals.
- Plot a graph of volume against time; the steeper the curve, the faster the rate.
- Vary a factor such as concentration, temperature or surface area.
Analysing results
- A colorimeter can be used to measure how cloudy a solution becomes more precisely.
- Calculate mean rate = quantity ÷ time, and explain results using collision theory.
Exam tips
- Disappearing cross: measure time for the cross to vanish; rate ∝ 1/time.
- Gas method: use a gas syringe; steeper graph = faster rate.
- Control all variables except the one being investigated.
- Explain rate changes using collision theory.