AQA GCSE Chemistry · Rates & Equilibrium
Rates of reaction required practical
Investigate how changing the concentration affects the rate of reaction, by measuring the volume of gas produced.
Magnesium reacts with hydrochloric acid to give hydrogen. Collecting the gas in a gas syringe and reading it every 10 seconds shows the rate: fast at first, then slowing as the reactants are used up.
Equipment
- Conical flask with a bung and delivery tube
- Gas syringe (or an upturned measuring cylinder in water)
- Magnesium ribbon, cut to equal lengths
- Hydrochloric acid at different concentrations
- Measuring cylinder and stopwatch
Variables
- Independent you change
- Concentration of the hydrochloric acid
- Dependent you measure
- Volume of hydrogen gas produced over time
- Control you keep the same
- Same length (mass) of magnesium
- Same volume of acid
- Same temperature
- Same equipment
Method
- 1
Measure 50 cm³ of hydrochloric acid into a conical flask.
- 2
Connect the bung and delivery tube to a gas syringe, set at zero.
- 3
Add a 3 cm strip of magnesium ribbon, quickly put the bung in and start the stopwatch.
- 4
Record the volume of gas in the syringe every 10 seconds until it stops increasing.
- 5
Repeat with different concentrations of acid.
- 6
Plot volume against time for each concentration and compare the gradients.
Worked example
1.0 mol/dm³ hydrochloric acid with 3 cm of magnesium.
| Time (s) | Volume of gas (cm³) |
|---|---|
| 0 | 0 |
| 10 | 18 |
| 20 | 32 |
| 30 | 42 |
| 40 | 48 |
| 50 | 51 |
| 60 | 52 |
| 70 | 52 |
Working
Mean rate = volume of gas ÷ time
Over the first 60 s: 52 ÷ 60 = 0.87 cm³/s
In the first 10 s: 18 ÷ 10 = 1.8 cm³/s, the fastest part
Conclusion
The rate is fastest at the start, when there are most acid particles to collide with the magnesium, and slows as they are used up. The reaction stops at 52 cm³ when the magnesium has all reacted. With more concentrated acid, the curve is steeper because there are more particles in the same volume, so more frequent collisions.
Risks
- Hydrochloric acid irritates skin and eyesWear eye protection.
- Hydrogen is flammableKeep flames away from the apparatus.
- Gas syringe plunger can be pushed outUse small amounts of magnesium so the volume doesn’t exceed the syringe.
Common mistakes
- Losing gas before the bung goes in. Have the bung ready and put it in immediately.
- Using different lengths of magnesium. The amount of magnesium must stay the same.
- Saying particles collide “more”. Say they collide more frequently (more collisions per second).
Better results
- Use a gas syringe rather than collecting over water, as some gases dissolve in water.
- Repeat each concentration and calculate mean volumes.
- Alternative method: time how long sodium thiosulfate and acid take to hide a cross drawn under the flask.
Launching November 2026
7 exam questions on this practical.
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