Required Practical 5: Rates of Reaction

1. Hypothesis

  • A hypothesis is a testable explanation of an observation.

  • Example: “Increasing concentration will increase the rate of reaction.”

2. Method 1: Disappearing Cross (Sodium Thiosulfate + Hydrochloric Acid)

Reaction:

Sodium thiosulfate + Hydrochloric acid → Sulfur (s) + other productsSodium thiosulfate + Hydrochloric acid → Sulfur (s) + other products

Sulfur is a solid → makes solution cloudy (turbidity).

Steps:

  1. Place 10 cm³ sodium thiosulfate solution into a conical flask (using a measuring cylinder).

  2. Put the flask on top of a printed black cross.

  3. Add 10 cm³ hydrochloric acid, swirl gently.

  4. Start stopwatch.

  5. Look down through the flask → stop the timer when the cross is no longer visible.

  6. Repeat with different concentrations of sodium thiosulfate.

  7. Repeat each concentration at least 3 times, calculate a mean (ignore anomalies).

Issue:

  • Judging when the cross disappears is subjective (depends on eyesight).

  • But using the same cross size helps reduce this error.

3. Method 2: Gas Volume (Magnesium + Hydrochloric Acid)

Reaction:

Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen gasMagnesium + Hydrochloric acid → Magnesium chloride + Hydrogen gas

Steps:

  1. Place 50 cm³ hydrochloric acid in a conical flask.

  2. Attach a bung and delivery tube.

  3. Place delivery tube into a container of water with an upturned measuring cylinder filled with water.

  4. Add a 3 cm strip of magnesium ribbon to the acid and immediately start stopwatch.

  5. Every 10 seconds, record the volume of hydrogen gas in the measuring cylinder.

  6. Continue until no more gas is produced.

  7. Repeat with different concentrations of hydrochloric acid.

4. Key Idea: Reproducibility

  • A result is reproducible if another person or method gives the same pattern.

  • Both experiments show higher concentration = faster rate → reproducible finding.

5. Conclusion

  • Increasing concentration → more frequent collisions → faster reaction.

  • Both methods support this.

⚡ Exam Tips:

  • Always mention anomalies are excluded when calculating a mean.

  • Know the limitations of each method (subjective cross vs. precise gas volume).

  • Always link conclusion back to collision theory.