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

## Aim

To investigate how the rate of reaction changes as the concentration of sodium thiosulphate is changed. In this practical, you will:

• React different concentrations of sodium thiosulfate with dilute hydrochloric acid. The equation for this reaction is:

Sodium thiosulfate + Hydrochloric acidSodium chloride + Water + Sulfur dioxide + Sulfur

Na2S2O3 (s) + 2HCl (aq)2NaCl (aq) + H2O (l) + SO2 (g) + S (s)

• Use a stopwatch to measure the time taken for a mixture to become cloudy at each concentration of sodium thiosulphate.
• Use your results to calculate how the rate of reaction changes as you change the concentration of sodium thiosulfate.

## Method

To carry out this practical, follow these steps:

1. Use a measuring cylinder to add 50 cm³ of dilute sodium thiosulfate solution to a conical flask.

2. Draw a black cross on a piece of paper and place the conical flask on the cross.

3. Use a measuring cylinder to add 5 cm³ of dilute hydrochloric acid to the conical flask. Swirl the flask, then start the stopwatch.

4. Look down at the cross through the mixture and record the time on the stopwatch when you can no longer see the cross.

5. Repeat steps 1-4 using different concentrations of sodium thiosulfate solution.

## Results

The following table shows the concentration of sodium thiosulfate (in g/dm³) and the time it took for the cross to disappear (in seconds).

## Analysis

Plot your results on a graph, with time (s) on the X-axis (horizontal) and the concentration of sodium thiosulfate (in g/dm³) on the Y-axis (vertical). After this, draw a curve of best fit through the points.

## Conclusion

By examining the graph you plot, you should expect to find that as the concentration of sodium thiosulfate increases, the time taken for the mixture to become cloudy decreases. This indicates that the rate of the reaction increases as the concentration of sodium thiosulfate increases.