Sodium thiosulfate reacts with dilute hydrochloric acid as shown - AQA - A-Level Chemistry - Question 1 - 2019 - Paper 3

The property of SO2 that causes pollution is its acidity, which can lower the pH of water bodies, harming aquatic life.

The reaction of SO2 with water is represented by the equation:

$SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)$

The shape of a water molecule (H2O) can be illustrated as follows:

    H
     \
      O
     /  \
    H    

In this diagram, there are two lone pairs of electrons on the oxygen atom.

The H-O-H bond angle is approximately 104.5°.

The shape of the water molecule is bent due to the presence of two lone pairs of electrons that repel the bonding pairs. According to VSEPR theory, lone pairs repel more strongly than bonding pairs, resulting in a smaller bond angle of 104.5° compared to the ideal tetrahedral angle of 109.5°.

To investigate the effect of temperature on the reaction between sodium thiosulfate and hydrochloric acid, I will outline the following method:

1. Prepare a series of solutions of sodium thiosulfate and hydrochloric acid at different temperatures (e.g., using a water bath).
2. Combine equal volumes of sodium thiosulfate solution and hydrochloric acid in a conical flask.
3. Start timing when the two solutions are mixed and observe the solution.
4. Record the time taken for the solution to turn opaque, indicating the formation of sulfur.
5. Repeat this experiment at various temperatures and compare the times taken to determine how temperature affects the reaction rate.

I will measure the time taken for a fixed amount of sulfur to be produced by observing the solution from above. When the reaction occurs, sulfur precipitates and the solution becomes cloudy. Using a stopwatch, I will time how long it takes to reach a specific turbidity level.

I will present the results in a graph where the x-axis represents the temperature (°C) and the y-axis represents the time taken for the solution to become opaque (seconds). This will allow for a visual representation of the relationship between temperature and reaction rate.

The expected graph would show a downward trend, indicating that as the temperature increases, the time taken for the sulfur to form decreases, reflecting a faster reaction rate:

Time
 |
 |\
 | \
 |  \
 |   \
 |    \
 |     \
 +-------------------------
           Temperature