3.2.5Physical Chemistry
Practical- Effect of Surface Area on Rate of Reaction — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 19
Part 1 of 5Aim and Variables
Aim
- To investigate how changing the surface area of marble chips affects the rate of reaction with dilute hydrochloric acid.
Independent variable
- Surface area of the calcium carbonate, changed by using different sized chips or powder of the same mass.
Dependent variable
- Rate of reaction, measured as the volume of carbon dioxide gas collected in a fixed time of 30 seconds.
Control variables
- Mass of calcium carbonate, volume and concentration of acid, and temperature must be kept the same for a fair test.
Equation
- CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g), where the carbon dioxide is the gas collected.
Part 2 of 5Method
1
Place a fixed volume of dilute hydrochloric acid into a conical flask and connect it with a delivery tube to an inverted measuring cylinder.
2
Add a known mass of marble chips to the acid, close the bung immediately and start the timer.
3
Record the volume of gas collected after 30 seconds.
4
Repeat the test with different sized chips and with powder, keeping all the control variables the same each time.
5
Compare the volumes of gas, then repeat each test to check that the results are reproducible.
Part 3 of 5Results and Conclusion
Results and Conclusion
Trend
- Smaller pieces give a faster rate, so more gas is produced in the same 30 seconds.
Explanation
- A greater surface area exposes more calcium carbonate to the acid, giving more frequent successful collisions.
Safety
- Hydrochloric acid is corrosive, so wear eye protection and wipe up any spills promptly.
Improvement
- A gas syringe or a balance measuring mass loss gives more precise readings than a measuring cylinder.
| Size of marble chip | Relative rate |
|---|---|
| Large lumps | Slowest |
| Small chips | Faster |
| Powder | Fastest |
Part 4 of 5Worked Example
Question
- In one test a student uses a fixed mass of large marble lumps and 50 cm³ of acid.
- 40 cm³ of carbon dioxide is collected in 30 seconds.
- Calculate the rate of reaction, and explain why lumps give a lower rate than powder.
- Rate = volume of gas ÷ time = 40 cm³ ÷ 30 s
- Rate = 1.3 cm³/s (to 2 significant figures)
- Lumps have a small surface area, so fewer particles are exposed to the acid and fewer collisions happen each second
Answer
- The rate is about 1.3 cm³/s.
- Lumps have less surface area than powder, so the rate is lower.
Part 5 of 5Exam Checklist & Tips
3.2.5 – Key Points for the Exam
- Surface area The total area of the exposed outside of a solid that is available to react.
- Rate of reaction The change in amount of reactant or product per unit time.
- Independent variable The factor deliberately changed by the experimenter.
- Dependent variable The factor measured as the result of the change.
- Control variable A factor kept constant so that the test is fair.
Common Mistakes
- Changing the mass of marble chips between tests, which makes the comparison unfair.
- Starting the timer after the chips have been added, so the first seconds of gas are missed.
- Stating that powder reacts faster because it has more mass; the cause is its greater surface area.
- Reading the gas volume at a different time from the fixed 30 seconds used in the other tests.
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