Experiment: Effect of temperature on the rate of reaction

Temperature is one of the four factors the Form 4 Rate of Reaction chapter asks you to investigate, and it gives one of the clearest results in the whole syllabus. The method mirrors the concentration experiment but changes a different variable, so it is a good way to practise designing a fair test where only one thing is altered. This guide sets out the aim, the method, the expected observations, and the Paper 3 skills the practical assessment rewards.

Aim

To investigate how temperature affects the rate of reaction between marble (calcium carbonate) and dilute hydrochloric acid, measured by the time taken to collect a fixed volume of carbon dioxide gas.

Apparatus and materials

• Conical flask
• Gas syringe, or a measuring cylinder and trough for collecting gas over water
• Delivery tube with a stopper to fit the conical flask
• Thermometer
• Water bath or beaker of water for warming the acid
• Bunsen burner, tripod and gauze (or a hotplate)
• Stopwatch
• Measuring cylinder (for example, 50 cm³)
• Electronic balance
• Marble chips (calcium carbonate) of similar size
• Dilute hydrochloric acid of one fixed concentration

Procedure

1. Measure a fixed volume of dilute hydrochloric acid into the conical flask using the measuring cylinder.
2. Warm the acid in a water bath to the first chosen temperature and record the temperature with the thermometer.
3. Weigh out a fixed mass of marble chips of similar size.
4. 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.
5. Record the time taken to collect a fixed volume of carbon dioxide, for example 20 cm³.
6. Wash out the flask and repeat steps 1 to 5 at several different temperatures, keeping the volume and concentration of acid and the mass and size of the marble chips the same.
7. For each temperature, calculate the rate as one divided by the time taken, and plot rate against temperature.

Expected observations

At every temperature, effervescence is seen as carbon dioxide is released and the gas syringe plunger moves out. At a higher temperature the bubbling is more vigorous and the fixed volume of gas is collected in a shorter time. As the temperature increases, the time taken falls, so the calculated rate (one divided by time) increases. The graph of rate against temperature rises, showing that the reaction speeds up as the acid gets hotter.

Inference and conclusion

The shorter the time to collect the fixed volume of gas, the faster the rate of reaction. A higher temperature gives a shorter time and therefore a faster rate. Collision theory explains why: when the acid is heated, its particles gain kinetic energy and move faster, so they collide with the marble more frequently; at the same time a larger fraction of the collisions have energy equal to or greater than the activation energy, so more of the collisions are effective. The conclusion is that the rate of reaction increases as temperature increases.

Science process skills (Paper 3 style)

Stating a hypothesis. When the temperature of the acid increases, the rate of reaction increases. The hypothesis links the manipulated variable to the responding variable.

Identifying variables. The manipulated variable is the temperature of the acid. The responding variable is the rate of reaction, found from the time to collect a fixed volume of gas. The controlled variables are the volume and concentration of the acid and the mass and size of the marble chips.

Tabulating data. Draw a table with columns for temperature, time taken, and rate (one divided by time), recording temperatures to the precision of the thermometer and times to the precision of the stopwatch.

Plotting a graph. Plot rate (y-axis) against temperature (x-axis). The upward trend shows that the rate rises with temperature. A smooth line of best fit through the points shows the relationship.

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

Making an inference. The reaction is faster at a higher temperature because the particles move faster and a larger fraction of their collisions have enough energy to react, the collision-theory explanation the marking scheme expects.

Safety precautions

• Wear safety goggles, because dilute hydrochloric acid is irritant and can damage the eyes if it splashes.
• Handle hot acid and hot glassware with care, and use tongs or a heatproof mat, because burns and scalds are a risk when warming the acid.
• Do not heat the flask directly with the stopper sealed, because trapped gas and vapour build up pressure; warm the acid in a water bath first, then add the chips.
• Wash any acid splashes off the skin at once with plenty of water, because the acid is irritant.

Common errors

• Adding the chips before recording the temperature. The reaction starts before the temperature is known; record the temperature of the acid just before adding the marble.
• Letting the acid cool during the run. The temperature drifts, so the test is not fair; work quickly, or keep the flask in the water bath at the set temperature.
• A delay in fitting the stopper. Gas escapes and the time reading is wrong; add the chips and connect the apparatus quickly, starting the stopwatch as the chips go in.
• Changing the mass or size of the marble. This alters the surface area, a second variable; use the same mass and similar-sized chips each time.
• Confusing time with rate. A shorter time means a faster rate; plot rate (one divided by time), not time, if the question asks for rate against temperature.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this experiment to fix the two-part collision-theory answer that temperature questions demand: particles move faster and collide more often, and a larger fraction of collisions have enough energy to react. Many students give only the first half and lose a mark. We also rehearse turning time into rate, which trips up students who plot time by mistake. Because rate appears throughout SPM Chemistry and this is a standard Paper 3 practical (Paper 3 is a practical test assessing science process skills), a clean method and a full explanation here protect marks across the papers.