Investigating the Rate of a Chemical Reaction

Aim

To investigate how temperature, concentration and surface area affect the rate of a chemical reaction.

Learning Outcome

• Explain what reaction rate means.
• Describe collision theory in simple terms.
• Investigate factors that affect reaction rate.
• Measure reaction time.
• Calculate average reaction rate.
• Interpret simple reaction-rate graphs.

Time Allocation

Approximately 60 to 90 minutes.

How Does It Work?

The rate of a chemical reaction describes how fast reactants are used up or how fast products are formed.

For a reaction to happen, particles must collide successfully. A successful collision happens when particles collide with enough energy.

Reaction rate increases when there are more successful collisions.

Scientific Background

Reaction rate can be measured by observing how quickly gas is produced, how quickly a solid disappears, how quickly a colour changes, how quickly reactants are used up, or how quickly products are formed.

Examples of units include:

• cm³/s for gas produced
• g/s for mass used
• ml/s for liquid produced or used

A reaction is usually fastest at the beginning because there are more reactant particles available. As the reaction continues, reactants are used up and the reaction slows down.

Hypothesis

If temperature, concentration or surface area is increased, then the rate of reaction will increase because more successful collisions will occur.

Variables

Independent Variable

Temperature, concentration or surface area.

Dependent Variable

Reaction rate or reaction time.

Control Variables

• Same volume of reactants.
• Same type of chemicals.
• Same measuring method.
• Same apparatus.
• Same amount of solid reactant.

Apparatus

• Beakers
• Measuring cylinders
• Stopwatch
• Thermometer
• Safety goggles
• Vinegar or dilute hydrochloric acid
• Baking soda, calcium carbonate chips or effervescent tablets
• Water
• Hot water
• Cold water
• Mortar and pestle, optional
• Gas syringe and delivery tube, optional for advanced measurement

Safety Precautions

Experiment Part A: Effect of Temperature

Aim

To investigate how temperature affects reaction rate.

Method

1. Pour equal volumes of vinegar or dilute acid into two beakers.
2. Place one beaker in warm water.
3. Place the other beaker in cold water.
4. Measure and record the temperature of each solution.
5. Add equal amounts of baking soda or calcium carbonate to each beaker.
6. Start the stopwatch immediately.
7. Record the time taken for the reaction to slow down or finish.
8. Compare the reaction speeds.

Results Table: Temperature

Trial	Temperature (°C)	Time Taken (s)	Observation
Cold solution
Warm solution

What Learners Should Observe

• The warm solution reacts faster.
• Bubbles form more quickly in the warm solution.
• The cold solution reacts more slowly.

Explanation

Higher temperature makes particles move faster. Particles collide more often and with more energy, increasing the number of successful collisions.

Conclusion

Increasing temperature increases the rate of reaction.

Experiment Part B: Effect of Surface Area

Aim

To investigate how surface area affects reaction rate.

Method

1. Fill two beakers with equal volumes of water.
2. Take two identical effervescent tablets.
3. Leave one tablet whole.
4. Crush the second tablet into smaller pieces.
5. Add the whole tablet to the first beaker.
6. Add the crushed tablet to the second beaker.
7. Start the stopwatch immediately.
8. Record the time taken for each reaction to finish.
9. Compare the reaction speeds.

Results Table: Surface Area

Tablet Type	Time Taken (s)	Observation
Whole tablet
Crushed tablet

What Learners Should Observe

• The crushed tablet reacts faster.
• Bubbles form more quickly with the crushed tablet.
• The whole tablet takes longer to react.

Explanation

Crushing the tablet increases its surface area. More particles are exposed to react at the same time, increasing the number of successful collisions.

Conclusion

Increasing surface area increases the rate of reaction.

Experiment Part C: Effect of Concentration

Aim

To investigate how concentration affects reaction rate.

Method

1. Prepare one concentrated vinegar or acid solution.
2. Prepare one diluted solution by adding water.
3. Pour equal volumes of each solution into separate beakers.
4. Add equal amounts of baking soda or calcium carbonate to each beaker.
5. Start the stopwatch immediately.
6. Record the time taken for each reaction to slow down or finish.
7. Compare the reaction speeds.

Results Table: Concentration

Solution Type	Time Taken (s)	Observation
Concentrated solution
Diluted solution

What Learners Should Observe

• The concentrated solution reacts faster.
• More bubbles form quickly.
• The diluted solution reacts more slowly.

Explanation

Higher concentration means more reactant particles are present in the same volume. This increases the chance of successful collisions.

Conclusion

Increasing concentration increases the rate of reaction.

Optional Quantitative Method: Measuring Gas Volume

Aim

To measure reaction rate by collecting gas produced over time.

Method

1. Set up a conical flask with a delivery tube connected to a gas syringe.
2. Add a measured volume of dilute acid to the flask.
3. Add a measured amount of calcium carbonate chips or magnesium ribbon.
4. Quickly close the flask with the bung.
5. Start the stopwatch immediately.
6. Record the gas volume every 10 seconds.
7. Continue until the gas volume stops increasing.

Results Table: Gas Volume

Time (s)	Gas Volume (cm³)
0
10
20
30
40
50
60

What Learners Should Observe

• Gas volume increases quickly at first.
• Gas volume increases more slowly later.
• The volume eventually stops increasing.

Conclusion

The reaction is fastest at the beginning and slows down as reactants are used up.

Calculating Average Reaction Rate

The average rate of reaction can be calculated using:

Example 1: Gas Produced

A reaction produces 180 cm³ of hydrogen gas in 120 seconds.

Reaction Rate = 180 cm³ ÷ 120 s

Reaction Rate = 1.5 cm³/s

Example 2: Reactant Used

A reaction uses 3 g of magnesium in 240 seconds.

Reaction Rate = 3 g ÷ 240 s

Reaction Rate = 0.0125 g/s

Reaction Rate Graphs

Product Formed vs Time

• The graph starts at zero.
• It rises quickly at first.
• It becomes less steep.
• It levels off when the reaction is complete.

Reactant Remaining vs Time

• The graph starts high.
• It decreases quickly at first.
• It decreases more slowly later.
• It levels off when the reaction is complete.

Graph Interpretation

Expected Results

• Warmer solutions react faster.
• Concentrated solutions react faster.
• Crushed solids react faster.
• Reactions are fastest at the beginning.
• Reactions slow down as reactants are used up.

Questions for Learners

1. What is meant by reaction rate?
2. What is collision theory?
3. What is a successful collision?
4. Why does temperature increase reaction rate?
5. Why does concentration increase reaction rate?
6. Why does surface area increase reaction rate?
7. Which reaction was the fastest?
8. Which reaction was the slowest?
9. How can gas volume be used to measure reaction rate?
10. What does a steep graph show?

Common Mistakes

• Using unequal volumes of reactants.
• Starting the stopwatch too late.
• Measuring different amounts of solid.
• Crushing tablets unevenly.
• Mixing up concentrated and diluted solutions.
• Not recording observations immediately.
• Thinking the reaction rate stays constant throughout the whole reaction.

Teacher Notes

• Keep the main Grade 10 experiment practical and measurable.
• Avoid advanced concentration calculations.
• Use simple timing, gas volume, bubbling or reactant disappearance.
• Repeat trials where possible and calculate an average.
• The gas syringe method is stronger for quantitative work.
• Vinegar and baking soda are safer and easier for demonstrations.
• Dilute hydrochloric acid with calcium carbonate or magnesium gives better scientific measurements.

Teacher Tip

Ask learners to predict which reaction will be fastest before testing. Then ask them to explain the result using collision theory.

Extension Activity

• Investigate different acid concentrations.
• Investigate different temperatures.
• Investigate different particle sizes.
• Measure gas volume produced over time.
• Investigate how a catalyst affects reaction rate.

Real-World Application

Reaction rates are important in food production, medicine, mining, chemical manufacturing, digestion, fire safety, explosions and industrial chemistry.

Controlling reaction rates helps scientists and engineers make reactions safer, faster and more efficient.