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
- 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:
- Measure 50 cm3 of Na2S2O3 solution into a flask
- Measure 5 cm3 of dilute HCl into a measuring cylinder
- Draw a dark cross on a piece of white paper and put it underneath the flask
- Add the acid into the flask and immediately start the stopwatch
- Solid sulfur is formed which precipitates in solution, turning cloudy:
- Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H2O
- Look down at the cross and stop the timing when the cross can no longer be seen
- 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
| Concentration Na2S2O3 (g dm-3) | Time for cross to disappear (s) |
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:
- Measure 50 cm3 of 1.0 mol/dm3 H2SO4 solution into a flask
- Add the magnesium ribbon to the flask and connect it to the gas collection equipment
- Start the stopwatch and record the volume of gas every 10 seconds
- When the reaction is complete, repeat using 1.5 mol/dm3 sulfuric acid
Results: Record your results in a suitable table, e.g.:
| Time (s) | Volume of hydrogen gas produced (cm3) | |
| 1 mol / dm3 acid | 1.5 mol / dm3 acid | |
| 10 | ||
| 20 | ||
| 30 | ||
| 40 | ||
| 50 | ||
| 60 * | ||
* 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
