Practical: Effect of Surface Area on Rate of Reaction

Section 1 of 7Aim and variables

Aim

• Aim To find out how changing the surface area of calcium carbonate affects its rate of reaction with dilute hydrochloric acid.

Variables

• Variables Independent variable: the size of the marble chips (large, medium, small or powder).
• Dependent variablethe volume of carbon dioxide produced in a fixed time, for example 30 seconds.

Control variables

• Control variables Keep constant the mass of marble, the volume and concentration of acid, the temperature and the apparatus used.

Section 2 of 7Method

1

Measure a fixed volume of dilute hydrochloric acid with a measuring cylinder and pour it into a conical flask.

2

Fill a measuring cylinder with water, invert it in a trough of water and connect it to the flask with a delivery tube.

3

Weigh a fixed mass of marble chips of one size, drop them into the acid and immediately close the bung. Start the stopwatch.

4

Read the volume of carbon dioxide collected in the measuring cylinder after 30 seconds.

5

Repeat with the same mass of chips of a different size, using fresh acid each time, and repeat each size at least twice to find a mean.

Section 3 of 7The reaction and what to expect

1Equation

• Equation CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

2Smaller pieces

• Smaller pieces Crushing the marble into smaller pieces increases the total surface area of the same mass.

3More exposed particles

• More exposed particles More calcium carbonate particles can be hit by acid particles at the same time.

4Faster rate

• Faster rate Collisions are more frequent, so more gas is collected in 30 seconds.

Section 4 of 7Recording and processing results

Size of marble chip	Volume of gas in 30 s (cm3), run 1	Run 2	Mean (cm3)
Large
Medium
Small
Powder

Section 5 of 7Conclusion, safety and improvements

Surface area and rate

Conclusion

• Conclusion The smaller the pieces, the larger the surface area and the faster the reaction.
  • This is because more particles of marble are exposed to the acid, giving more frequent successful collisions.

Safety

• Safety Wear eye protection.
  • Dilute hydrochloric acid is an irritant, so wipe up any spills and avoid pushing the bung in hard.

Improvements

• Improvements Use a gas syringe for more precise readings, take volume readings at regular intervals to plot a graph, and work out the rate in cm³/s.

Sources of error

• Sources of error Some gas may escape before the bung is fitted, and carbon dioxide dissolves slightly in water, so volumes can be a little low.

Section 6 of 7Key terms

Term	Meaning
Surface area	The total area of the outer surface of a solid that is exposed to a reactant.
Independent variable	The variable you deliberately change.
Dependent variable	The variable you measure as the outcome.
Control variable	A factor kept the same so the test is fair.
Rate of reaction	How quickly reactants are used up or products are formed.

Section 7 of 7Exam technique

1

Using the same size of chips but different masses. Mass of marble must be kept constant while size changes.

2

Saying smaller chips 'have more surface' without explaining it is the surface area of the same mass, which leads to more collisions per second.

3

Starting the timer late. Start the stopwatch as the chips go in and the bung is closed.

4

Reusing acid from the previous run, which has lower concentration.

5

Writing that smaller chips make the reaction produce more gas overall. The final volume is the same; it is just reached sooner.