Practical Investigation of Concentration on Reaction Rate

Key Objectives

• Understand how to carry out practical investigations to explore the effect of concentration on reaction rates.
• Learn two different methods for the investigation.

Important Concepts

Hypothesis

• A hypothesis is a proposal that could explain a fact or observation.
• It must be testable in scientific contexts.

Reproducibility

• A measurement is considered reproducible if another person or different techniques/equipment result in the same outcome.

Method 1: Disappearing Cross Reaction

Objective

• Explore the effect of concentration on reaction rate using sodium thiosulfate and hydrochloric acid.

Chemical Reaction

• Reactants: Sodium thiosulfate solution + Hydrochloric acid
• Product: Sulfur (solid) causing solution to turn cloudy (turbidity).

Procedure

1. Use a measuring cylinder to add 10 cm³ of sodium thiosulfate solution into a conical flask.
2. Place the conical flask onto a printed black cross.
3. Add 10 cm³ of hydrochloric acid to the flask.
4. Swirl the solution and start the stopwatch.
5. Observe until the cross is no longer visible through the cloudy solution and stop the timer.
6. Repeat the experiment with varying concentrations of sodium thiosulfate.
7. Calculate mean values for each concentration, excluding anomalous results.

Considerations

• Eye sight variations in observers could affect results, but using the same size cross should mitigate this issue.

Method 2: Measuring Gas Production

Objective

• Measure the volume of gas produced in a reaction of magnesium and hydrochloric acid.

Chemical Reaction

• Reactants: Magnesium + Hydrochloric acid
• Product: Hydrogen gas

Procedure

1. Add 50 cm³ of hydrochloric acid to a conical flask using a measuring cylinder.
2. Attach a bung and delivery tube to the flask.
3. Place the delivery tube into a container filled with water.
4. Position an upturned measuring cylinder filled with water over the delivery tube.
5. Add a 3 cm strip of magnesium to the acid and start the stopwatch.
6. Measure the volume of hydrogen gas every 10 seconds.
7. Continue until no more hydrogen is produced.
8. Repeat with different concentrations of hydrochloric acid.

Conclusion

• Both experiments demonstrate that increased concentration leads to faster reactions.
• The reproducibility of findings is supported by consistent results from both methods.

Additional Resources

• Practice questions available in the provided workbook link.