This question is about the reaction between sodium thiosulfate solution and hydrochloric acid - AQA - GCSE Chemistry - Question 8 - 2022 - Paper 2

To determine the rate of reaction at 30 seconds:

1. Draw a tangent to the curve at 30 seconds on Figure 7.
2. Identify the coordinates of the tangent at this point; let’s assume they are reasonable approximations, such as $x$ (30 seconds) and $y$ (percentage of light reached).
3. Use the formula:

$\text{Rate} = \frac{\text{change in mass of sulfur}}{\text{time}}$

4. Calculate the ratio based on the coordinates taken from the tangent. For example, if $\Delta y / \Delta x = 4$ (representative of a small square value), then use this to convert and find the moles produced over the specific time, factoring in the scale of the reaction.

The rate of reaction decreases between 0 and 60 seconds due to the concentration of reactants decreasing. As the reaction progresses, the concentrations of sodium thiosulfate and hydrochloric acid reduce as they are consumed, leading to fewer particles available to collide and react effectively.

The reaction stopped because one of the reactants was completely consumed. When all the sodium thiosulfate or hydrochloric acid has reacted, there are no more reactants left to continue the reaction.

The expected line would depict a steeper decrease in the percentage of light reaching the light sensor compared to the 0.10 mol/dm³ sodium thiosulfate solution, indicating a faster rate of reaction due to the higher concentration.

The student could improve the method by stopping light from other sources reaching the light sensor. This minimizes any external light interference that could affect the accuracy of the readings.

The results were repeatable, indicating that the same method under similar conditions produces consistent and reliable outcomes.

The correct expression that represents the stoichiometry is $V \propto m$, meaning that the volume of sodium thiosulfate solution is directly proportional to the mass of sulfur produced.

The simplest whole number ratio of the volumes can be determined from the reaction equation. Based on the stoichiometric coefficients, the ratio is 1 : 4, indicating that for every 1 volume of sodium thiosulfate, 4 volumes of hydrochloric acid are required.