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

Required practical 5: Investigating effect of concentration on rate of reaction

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, Na₂S₂O₃ (aq)

• 2.0 mol/dm³ dilute hydrochloric acid, HCl (aq)

• 10 cm³ measuring cylinder

• 100 cm³ measuring cylinder

• 100 cm³ conical flask

• White paper with cross marked on it

• Stopwatch

Method:

1. Measure 50 cm³ of Na₂S₂O₃ solution into the conical flask

2. Place the white paper with cross marked on it underneath the conical flask

3. Measure 10 cm³ of dilute HCl into a measuring cylinder

4. Add the hydrochloric acid into the flask and immediately start the stopwatch

5. Look down at the cross and stop the timing when the cross can no longer be seen

   • Solid sulfur is formed which precipitates in solution, turning cloudy:

   • Na₂S₂O₃ + 2HCl → 2NaCl + S + SO₂ + H₂O

6. Record the time, in seconds, in the results table

7. Repeat steps 1 - 6 using the different concentrations of sodium thiosulfate solution described in the table:

   Sodium thiosulfate (cm3)	Water (cm3)	Concentration (g / dm3)
   50	0	40
   40	10	32
   30	20	24
   20	30	16
   10	40	8

8. Repeat each experiment three times

Diagram

Practical Tip

• To avoid the reaction starting before the chemicals are mixed in steps 6 and 7:

  • Make sure that you use the 25 cm³ measuring cylinder for the sodium thiosulfate and the 10 cm³ measuring cylinder for the hydrochloric acid

  • Wash / rinse the conical flask thoroughly between experiments 

Results

• Record your results for each test carefully in a suitable table like the one below: 

Evaluation

• Calculate the mean time for each of the thiosulfate concentrations and record it in the final column

  • Do not include anomalous values in your calculations

• Plot a line graph of the results

  • x-axis = sodium thiosulfate concentration in g/dm³

  • y-axis = mean time taken for the cross to disappear in seconds

• Draw a smooth curved line of best fit

• Describe what the graph shows

  • e.g. as the temperature increases, the time taken for the cross to disappear decreases

• If it is appropriate, give a more detailed description 

  • e.g. as the temperature increases, the time taken for the cross to disappear decreases until 50 ^oC when the time for the cross to disappear remains at 5 seconds

• Use particle theory to explain the description

• Other questions:

  • Identify the control variables

  • Were your results repeatable?

  • Is the investigation reproducible?

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