# 6.1.6 Required Practical: Investigating the Effect of Concentration on Rate of Reaction

### 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 dm-3 sodium thiosulfate solution
• 1.0 mol dm-3 sdilute hydrochloric acid
• 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 as 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, eg:

Required Practical 5a Results Table

Evaluation:
Plot a graph of the results with time on the x-axis and the concentration of Na2S2Oon 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 dm-3 and 1.5 mol dm-3
• 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 dm-3 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 dm-3 sulfuric acid

Results: Record your results in a suitable table, eg:

Required Practical 5b Results Table

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

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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