6.2 - Effect of Concentration on Reaction Rate using Sodium Thiosulphate and Hydrochloric Acid

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Experiment Summary

This experiment investigates how the concentration of sodium thiosulphate affects the rate of reaction with hydrochloric acid.

The reaction produces sulfur, which forms a cloudy precipitate that obscures a cross marked on paper placed beneath the reaction flask. The time taken for the cross to disappear is used to measure the rate of reaction.

By varying the concentration of sodium thiosulphate while keeping other conditions constant, the relationship between concentration and reaction rate is examined.

The reaction is as follows:

\text{2HCl (aq) + Na}_2\text{S}_2\text{O}_3\text{(aq)} \rightarrow 2\text{NaCl (aq)} + \text{SO}_2\text{(aq)} + \text{S (s)} + \text{H}_2\text{O (l)}

Materials and Apparatus Required

Chemicals

0.1 M sodium thiosulphate solution
3 M hydrochloric acid

Apparatus

Conical flasks (250 cm³)
Graduated cylinders (100 cm³)
Stop-clock
White paper marked with a cross

Safety Precautions

Wear safety glasses throughout the experiment.
Hydrochloric acid is corrosive. Handle it carefully to avoid skin and eye contact.
Sulfur dioxide is produced in small amounts during the reaction, so ensure good ventilation in the lab to avoid inhalation.

Method

Measure 100 cm³ of 0.1 M sodium thiosulphate solution and pour it into a conical flask.
Add 10 cm³ of 3 M hydrochloric acid to the flask while starting the stop-clock immediately.
Swirl the flask and place it on a white piece of paper marked with a cross.
Observe the cross through the solution.
Record the time taken for the cross to completely disappear as the reaction progresses.
Repeat the experiment using 80 cm³, 60 cm³, 40 cm³, and 20 cm³ of sodium thiosulphate solution. In each case, add water to make the total volume up to 100 cm³, then mix thoroughly before adding hydrochloric acid.
For each concentration, calculate the reaction rate by taking the reciprocal of the time (1/time).
Record the results in a table and plot a graph of 1/time against concentration.

Results

Concentration of Sodium Thiosulphate (M)	Reaction Time (s)	1/Time (s⁻¹)
0.1	43	0.0233
0.08	52	0.0192
0.06	71	0.0141
0.04	114	0.0088
0.02	270	0.0037

Example Questions with Answers

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Q1: What effect does increasing the concentration of sodium thiosulphate have on the reaction time?

Increasing the concentration decreases the reaction time, meaning the cross disappears faster.

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Q2: How does increasing the concentration of sodium thiosulphate affect the reaction rate?

The reaction rate increases with higher concentrations of sodium thiosulphate.

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Q3: Why is the inverse of time (1/time) used as a measure of reaction rate?

The rate of reaction is inversely proportional to time, meaning that a faster reaction corresponds to a shorter time, and using 1/time allows for a direct comparison of rates.

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Q4: What is the significance of the graph of 1/time against concentration?

The graph shows that the reaction rate is directly proportional to the concentration of sodium thiosulphate, confirming that higher concentrations increase the rate of reaction.

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Q5: What safety precautions should be taken during the experiment?

Safety glasses must be worn, and the experiment should be carried out in a well-ventilated area to avoid inhaling sulfur dioxide gas produced during the reaction.

- Effect of Concentration on Reaction Rate using Sodium Thiosulphate and Hydrochloric Acid Simplified Revision Notes for Leaving Cert Chemistry

6.2 - Effect of Concentration on Reaction Rate using Sodium Thiosulphate and Hydrochloric Acid

Experiment Summary

Materials and Apparatus Required

Chemicals

Apparatus

Safety Precautions

Method

Results

Example Questions with Answers

Q1: What effect does increasing the concentration of sodium thiosulphate have on the reaction time?

Q2: How does increasing the concentration of sodium thiosulphate affect the reaction rate?

Q3: Why is the inverse of time (1/time) used as a measure of reaction rate?

Q4: What is the significance of the graph of 1/time against concentration?

Q5: What safety precautions should be taken during the experiment?

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