Core practical - Rate of reaction (AQA GCSE Chemistry): Revision Notes
Core practical - Rate of reaction
What is this experiment about?
This core practical investigates how changing the concentration of reactants affects how fast a chemical reaction happens. You'll use sodium thiosulfate and hydrochloric acid to create a precipitation reaction and time when a cross becomes invisible.
The chemical reaction
When sodium thiosulfate reacts with hydrochloric acid, it produces a yellow sulphur precipitate:
The sulphur forms as tiny particles that make the solution cloudy. As more sulphur forms, the solution becomes so cloudy you can't see through it.
Equipment you need
- Eye protection (safety goggles)
- Dilute hydrochloric acid
- Sodium thiosulfate solution
- Measuring cylinders
- Conical flask
- Black cross drawn on white paper
- Timer
- Water for dilution
Method step by step
Step 1: Use a measuring cylinder to measure 50 cm³ of sodium thiosulfate solution. Pour this into a conical flask.
Step 2: Measure 10 cm³ of dilute hydrochloric acid in a separate measuring cylinder.
Step 3: Place the flask on top of the black cross drawn on white paper.
Step 4: Add the hydrochloric acid to the sodium thiosulfate. Start timing immediately and gently swirl the flask.
Step 5: Look down through the flask and stop the timer when you can no longer see the cross through the cloudy mixture.
Step 6: Repeat the experiment using different concentrations of sodium thiosulfate by mixing it with water. Keep the total volume the same each time.
Safety first
- Wear eye protection throughout the experiment
- Work in a well-ventilated room because sulphur dioxide gas is produced, which can cause breathing difficulties
- The reaction mixture becomes hot, so handle carefully
Understanding your results
The key pattern you'll see is that higher concentration leads to faster reaction rates. This means:
- When sodium thiosulfate is more concentrated, the cross disappears quickly (shorter time)
- When sodium thiosulfate is more dilute, the cross disappears slowly (longer time)
You'll create a results table showing different concentrations and the time taken for each. The relationship shows that as concentration increases, the time decreases.
Why does this happen?
When the concentration is higher, there are more particles in the same space. This means more frequent collisions between reactant particles, so the reaction happens faster and the precipitate forms more quickly.
Dealing with uncertainty
Your timing won't be perfectly accurate because:
- It's hard to judge exactly when the cross disappears
- Reaction times can vary slightly between repeats
- Different people might judge the disappearance point differently
You can reduce uncertainty by taking repeat measurements and calculating an average.
Key Variables
- Independent variable: Concentration of sodium thiosulfate (what you change)
- Dependent variable: Time taken for cross to disappear (what you measure)
- Control variables: Temperature, volume of solutions, size of cross, viewing angle
Key Points to Remember:
- This experiment shows that higher concentration means faster reaction rate
- The cross-disappearance method is a simple way to measure reaction rates
- Always work safely with chemicals and ensure good ventilation
- Repeat measurements help reduce experimental errors
- The pattern shows an inverse relationship - as concentration goes up, time goes down