Practical Investigation: Effect of Concentration on Reaction Rate

Key Concept: Hypothesis

• Hypothesis: A proposal explaining a fact or observation.
• Must be testable in scientific studies.

Experiment 1: Disappearing Cross Reaction

• Objective: Explore effect of concentration on reaction rate using sodium thiosulfate and hydrochloric acid.
• Reaction: Sodium thiosulfate + Hydrochloric acid → Sulfur (solid, causes cloudiness or turbidity).

Method

1. Measure 10 cm³ of sodium thiosulfate solution into a conical flask.
2. Place flask on a printed black cross.
3. Add 10 cm³ of hydrochloric acid to the flask.
4. Swirl solution and start stopwatch.
5. Observe the cross from above; stop clock when cross is no longer visible.
6. Repeat with lower concentrations of sodium thiosulfate.
7. Calculate mean values, excluding anomalous results.

Key Concept: Reproducibility

• Definition: A measurement is reproducible if consistent results are obtained by different people or with different techniques/equipment.
• Issue: Variability in human eyesight may affect reproducibility, though standard cross size may mitigate this.

Experiment 2: Gas Volume Measurement

• Objective: Measure hydrogen gas volume produced in reaction with magnesium and hydrochloric acid.

Method

1. Use a measuring cylinder to place 50 cm³ of hydrochloric acid into a conical flask.
2. Attach a bung and delivery tube to flask.
3. Submerge delivery tube in water and place an upturned measuring cylinder filled with water over it.
4. Add a 3 cm strip of magnesium to the acid and start stopwatch.
5. Measure hydrogen gas volume every 10 seconds until no more is produced.
6. Repeat with varying concentrations of hydrochloric acid.

Conclusion

• Both experiments demonstrate that higher concentration leads to faster reaction rate.
• Reproducibility validated by consistent results across different methods.

Additional Resources

• Revision workbook with additional practice questions available for further study.