Rates of Reaction: Gas Collection Method

Aim

To investigate the effect of surface area (or concentration or temperature) on the rate of reaction between calcium carbonate (marble chips) and dilute hydrochloric acid, by measuring the volume of carbon dioxide gas produced over time.

Apparatus

• Conical flask (250 cm3) with a bung fitted to a delivery tube or connected to a gas syringe
• Gas syringe (100 cm3) or a trough of water with an inverted measuring cylinder
• Stopwatch
• Electronic balance (if using mass loss method as an alternative)
• Measuring cylinder (50 cm3)
• Marble chips (calcium carbonate, CaCO3) — large chips and powdered for the surface area investigation
• Dilute hydrochloric acid (HCl, 1 mol/dm3)
• Cotton wool (if using mass loss method)

Method

Gas syringe method

1. Measure 50 cm3 of dilute hydrochloric acid using a measuring cylinder and pour it into the conical flask.
2. Weigh out 5 g of marble chips (large pieces) using the balance.
3. Add the marble chips to the acid, quickly fit the bung connected to the gas syringe, and start the stopwatch.
4. Record the volume of gas in the syringe every 30 seconds until the reaction stops (no further change in volume for at least two readings).
5. Rinse the flask and repeat with 5 g of powdered marble and the same volume and concentration of acid.
6. Repeat each experiment at least twice.

Water displacement method (alternative)

1. Fill a trough with water and fill a measuring cylinder with water. Invert the measuring cylinder in the trough without letting air in.
2. Set up the conical flask with a delivery tube leading under the water and into the inverted measuring cylinder.
3. Add the acid and marble chips as above. Gas displaces water from the measuring cylinder. Record the volume every 30 seconds.

Expected results

The reaction is: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)

Both experiments produce the same total volume of gas (same mass of reactant), but powdered marble reacts faster than large chips. The graph for powdered marble has a steeper initial gradient (faster rate at the start) but levels off at the same final volume.

The graph shape: a curve that rises steeply at first, then gradually levels off to a plateau. The gradient is steepest at time zero (highest rate) because the concentration of acid is at its maximum.

Analysis

Plot volume of gas (y-axis) against time (x-axis) for both experiments on the same axes.

Compare the graphs:

• Initial gradient — steeper for powdered marble (higher initial rate)
• Final volume — the same for both (same mass of CaCO3 used)
• Time to completion — shorter for powdered marble

To calculate the rate at a specific time, draw a tangent to the curve at that point. The gradient of the tangent (change in volume / change in time, in cm3/s) equals the rate at that moment.

The rate decreases over time because the concentration of hydrochloric acid decreases as it is used up.

Sources of error

• Gas leaks: If the bung is not fitted quickly enough, gas escapes before recording begins. Practise fitting the bung rapidly or use a side-arm flask.
• Gas syringe friction: If the syringe plunger sticks, the recorded volume will be too low. Ensure the syringe moves freely before starting.
• Inconsistent marble chips: Natural marble varies in composition. Using the same batch reduces this variation.
• Temperature fluctuations: The reaction is exothermic, so the temperature of the mixture rises slightly during the reaction, which increases the rate. This is unavoidable but should be noted.
• Timing errors: Starting the stopwatch at exactly the moment the reactants are mixed is difficult. Use the same person to add reactants and start timing each time.

Exam tips

Examiners often ask you to sketch two curves on the same axes — one for large chips and one for powder. The powder curve must be steeper initially but finish at the same height. Drawing the powder curve ending at a higher volume is a common mistake: the total product depends on mass of reactant, not surface area.

If asked to explain the shape of the curve, state: the initial steep part is where acid concentration is highest, giving the fastest rate; the curve levels off as acid concentration falls; the plateau means the reaction has stopped because one reactant (usually the acid) is used up.

When asked about improvements, suggest: using a datalogger connected to a pressure sensor for more precise continuous readings, using excess acid so all the calcium carbonate reacts (allowing the mass of marble to be the independent variable), or repeating and averaging to improve reliability.