Required Practical: Investigating the Effect of Concentration on Rate of Reaction (AQA GCSE Chemistry: Combined Science)

Part 5a - Measuring turbidity

Objective:

To investigate the effect of changing concentration on the rate of reaction by measuring the formation of a precipitate

Hypothesis:

A precipitate will be formed quicker if the concentration of the reactants is greater

Materials:

• 40 g/dm³ sodium thiosulfate solution
• 1.0 mol/dm³ sdilute hydrochloric acid
• Conical flask (100 cm³)
• Black cross on paper
• White paper or white tile
• Stopwatch or timer

Diagram showing the apparatus needed to investigate reaction rate in a precipitation reaction

Practical Tip:

The same person should be the one recording the time taken for the cross to disappear as this measurement is open to bias because it is based on human perception

Method:

1. Measure 50 cm³ of Na₂S₂O₃ solution into a flask
2. Measure 5 cm³ of dilute HCl into a measuring cylinder
3. Draw a dark cross on a piece of white paper and put it underneath the flask
4. Add the acid into the flask and immediately start the stopwatch
5. Solid sulfur is formed which precipitates in solution, turning cloudy:
   • Na₂S₂O₃ + 2HCl → 2NaCl + S + SO₂ + H₂O

6. Look down at the cross and stop the timing when the cross can no longer be seen
7. Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)

Results:

Record your results in a suitable table, e.g.:

Required Practical 5a Results Table

Evaluation:

Plot a graph of the results with time on the x-axis and the concentration of Na₂S₂O₃ on the y-axis. With an increase in the concentration of a solution, the rate of reaction will increase so the time for the cross to disappear decreases

Conclusion:

As there are more reactant particles in a given volume collisions occur more frequently, increasing the rate of reaction

Part 5b - Measuring Gas Volume

Objective:

To investigate the effect of changing concentration on the rate of reaction by measuring the volume of gas given off

Hypothesis:

The same amount of gas will be produced in less time if the concentration of reactants is increased

Materials:

• Magnesium ribbon cut into 3 cm lengths
• Sulfuric acid 1 mol/dm³ and 1.5 mol/dm³
• Conical flask (100 cm³)
• Safety goggles
• Gas syringe
• Stop clock

Diagram showing the apparatus needed to investigate reaction rate by measuring gas volume

Practical Tip:

Magnesium and sulfuric acid will react immediately so the flask must be connected to the gas syringe straight after adding the magnesium to avoid gas escaping

Method:

1. Measure 50 cm³ of 1.0 mol/dm³ H₂SO₄ solution into a flask
2. Add the magnesium ribbon to the flask and connect it to the gas collection equipment
3. Start the stopwatch and record the volume of gas every 10 seconds
4. When the reaction is complete, repeat using 1.5 mol/dm³ sulfuric acid

Results: Record your results in a suitable table, e.g.:

* It may be necessary to extend the table past 60 seconds.

Evaluation:

Plot a graph of the results with time on the x-axis and the concentration and the volume of gas on the y-axis. With an increase in the concentration of a solution, the rate of reaction will increase so the volume of gas increases

Conclusion:

As there are more reactant particles in a given volume collisions occur more frequently, increasing the rate of reaction