Experiment: Effect of concentration on the rate of reaction

Concentration is the first factor most students meet in the Form 4 Rate of Reaction chapter, and it is a favourite Paper 3 practical. The investigation is simple to set up, gives a clear result, and lets you practise every science process skill the practical assessment rewards, stating a hypothesis, identifying variables, tabulating results and drawing a rate against concentration relationship. This guide sets out the method, the observations you should expect, and the skills the examiner looks for.

Aim

To investigate how the concentration of hydrochloric acid affects the rate of reaction between the acid and marble (calcium carbonate), measured by how quickly carbon dioxide gas is produced.

Apparatus and materials

• Conical flask
• Gas syringe, or a burette and trough for collecting gas over water
• Delivery tube with a stopper to fit the conical flask
• Stopwatch
• Measuring cylinder (for example, 50 cm³)
• Electronic balance
• Marble chips (calcium carbonate) of similar size
• Dilute hydrochloric acid of two or more known concentrations
• Distilled water

Procedure

1. Measure a fixed volume of dilute hydrochloric acid of the first concentration into the conical flask using the measuring cylinder.
2. Weigh out a fixed mass of marble chips of similar size and record the mass.
3. Add the marble chips to the acid, immediately fit the stopper and delivery tube connected to the gas syringe, and start the stopwatch at once.
4. Record the volume of carbon dioxide collected in the gas syringe at regular time intervals, for example every 30 seconds.
5. Continue until the reaction stops and no more gas is collected.
6. Wash out the flask and repeat steps 1 to 5 using acid of the next concentration, keeping the volume of acid, the mass and size of the marble chips, and the temperature the same.
7. For each concentration, plot the volume of gas against time on the same axes, and compare the steepness of the curves.

Expected observations

In every run, effervescence (bubbling) is seen as carbon dioxide is released, the marble chips slowly get smaller, and the gas syringe plunger moves out. With the more concentrated acid, the bubbling is more vigorous at the start, the gas is collected more quickly, and the curve of gas volume against time is steeper in its early part. Because the same mass of marble is used each time, the reaction eventually produces the same final volume of gas in every run, the more concentrated acid simply reaches that volume sooner.

Inference and conclusion

The steeper the curve at the start, the faster the rate of reaction. A higher concentration of acid gives a steeper curve, so a higher concentration gives a faster initial rate. This is explained by collision theory: a more concentrated acid has more acid particles in the same volume, so the particles collide with the marble surface more frequently, giving more effective collisions per second. The conclusion is that the rate of reaction increases when the concentration of the acid increases, while the final volume of gas depends only on the amount of marble that reacts.

Science process skills (Paper 3 style)

Stating a hypothesis. When the concentration of the acid increases, the rate of reaction increases. A good hypothesis states the relationship between the manipulated and responding variables.

Identifying variables. The manipulated variable is the concentration of the hydrochloric acid. The responding variable is the rate of reaction, measured as the volume of gas collected in a fixed time or the time for a fixed volume of gas. The controlled variables are the volume of acid, the mass and size of the marble chips, and the temperature.

Tabulating data. Draw a table with a column for time and separate columns for the volume of gas collected at each concentration, all volumes to the same precision as the gas syringe allows. A second table can record the initial rate for each concentration.

Plotting a graph. Plot volume of gas (y-axis) against time (x-axis) for each concentration on the same axes. The steeper curve shows the faster reaction. The gradient of the tangent at the start gives the initial rate.

Making an operational definition. The rate of reaction is defined operationally as the volume of carbon dioxide collected per unit time, or as one divided by the time taken to collect a fixed volume of gas.

Making an inference. The more concentrated acid reacts faster because its particles are more crowded and collide with the marble more often, a direct use of collision theory.

Safety precautions

• Wear safety goggles, because dilute hydrochloric acid is irritant and can damage the eyes if it splashes.
• Add the marble chips carefully and fit the stopper without forcing it, so acid is not splashed out and the flask does not crack.
• Do not seal the flask completely without the delivery tube attached, because the pressure of the gas produced could build up.
• Wash any acid splashes off the skin at once with plenty of water, because the acid is irritant.

Common errors

• A delay in fitting the stopper. Gas escapes before the syringe is connected, so the early readings are too low; add the chips and connect the apparatus quickly, starting the stopwatch as the chips go in.
• Marble chips of different sizes. Different sizes change the surface area, which is a second variable; use chips of similar size so only concentration changes.
• Reading the gas volume with parallax. Read the syringe at eye level so the volume is correct.
• Letting the temperature drift. Warmer acid reacts faster; keep every run at the same room temperature so the comparison is fair.
• Comparing final volumes instead of rates. The final volume is the same for the same mass of marble; compare the steepness of the curves, not where they finish.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this classic practical to drill the difference between rate and amount, a distinction many students blur. We show why the curves for different concentrations end at the same volume but rise at different speeds, and we rehearse the collision-theory explanation in the exact words the marking scheme rewards. Because rate questions appear across SPM Chemistry and this is a standard Paper 3 practical (Paper 3 is a practical test assessing science process skills), securing the method and the reasoning here protects marks in both the theory papers and the practical test.