Required practical - Rate of reaction (AQA GCSE Chemistry Combined Science): Revision Notes
Required practical - Rate of reaction
What this practical measures
This experiment looks at how changing the concentration of a reactant affects how fast a chemical reaction happens. You'll be measuring the rate of reaction between sodium thiosulfate and hydrochloric acid.
This practical is designed to demonstrate the relationship between reactant concentration and reaction speed - one of the fundamental principles in chemical kinetics.
Aim of the experiment
To observe how changing the concentration of sodium thiosulfate solution affects the speed of its reaction with hydrochloric acid.
The chemical reaction
The reaction you'll be studying is:
Key observations during the reaction:
- Both starting solutions are clear and colourless
- The sulphur (S) product forms as a yellow precipitate that makes the solution cloudy
- This cloudiness is what you'll be measuring to determine when the reaction is complete
Equipment you need
- Eye protection (safety goggles)
- Dilute hydrochloric acid
- Sodium thiosulfate solution
- Measuring cylinders
- Conical flask
- Black cross drawn on white paper
- Timer
Safety First: Eye protection is mandatory throughout this experiment as you'll be working with acids and producing sulphur dioxide gas.
Method - step by step
Worked Example: Complete Experimental Procedure
Step 1: Measure 50 cm³ of sodium thiosulfate solution using a measuring cylinder and pour it 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 a black cross drawn on white paper
Step 4: Add the hydrochloric acid to the sodium thiosulfate and immediately start timing while swirling the flask
Step 5: Watch the mixture - a yellow sulphur precipitate will start to form
Step 6: Stop timing when the cross can no longer be seen through the cloudy mixture
Step 7: Repeat the experiment using different concentrations by mixing sodium thiosulfate solution with water
Different concentrations to test
| Sodium thiosulfate (cm³) | Water (cm³) | Total volume |
|---|---|---|
| 40 | 10 | 50 cm³ |
| 30 | 20 | 50 cm³ |
| 20 | 30 | 50 cm³ |
| 10 | 40 | 50 cm³ |
Critical for Fair Testing: Always use the same total volume (50 cm³) and the same amount of hydrochloric acid (10 cm³) each time. This ensures that concentration is the only variable being changed.
What results to expect
Expected Pattern in Results:
- Higher concentration of sodium thiosulfate = shorter time for cross to disappear
- Lower concentration of sodium thiosulfate = longer time for cross to disappear
This demonstrates that higher concentration leads to faster reaction rate.
Safety considerations
Essential Safety Measures:
- Wear eye protection throughout the experiment
- Work in a well-ventilated room - sulphur dioxide gas can cause breathing difficulties
- Handle acids carefully and report any spills immediately
- Wash hands thoroughly after the experiment
Understanding uncertainty in results
When students repeat this experiment, their results vary slightly due to human error and measurement limitations:
Understanding Measurement Uncertainty:
- Student 1 might get: 20s, 21s, 22s (average = 21s, uncertainty = ±1s)
- Student 2 might get: 18s, 21s, 24s (average = 21s, uncertainty = ±3s)
Uncertainty shows how reliable your measurements are. Smaller uncertainty means more reliable results and better experimental technique.
Key points about this practical
This practical demonstrates several important scientific concepts:
- You're measuring time as your dependent variable
- Concentration is your independent variable
- The cross disappearing is your end point for timing
- Results show that concentration directly affects reaction rate
- Higher concentration means more particles in the same space, leading to more frequent collisions between reactant molecules
The Science Behind the Results: When concentration increases, there are more reactant particles in the same volume. This leads to more frequent collisions between particles, resulting in a faster reaction rate and shorter time for the cross to disappear.
Remember!
Key Points to Remember:
- Higher concentration = faster reaction = shorter time
- Always use the same total volume of solution each time for fair testing
- The yellow precipitate blocks your view of the cross - this is your signal to stop timing
- Work safely in a well-ventilated area due to SO₂ gas production
- Repeat experiments to check your results and calculate uncertainty
- This practical demonstrates the fundamental relationship between concentration and reaction rate