This question is about the reaction between sodium thiosulfate and hydrochloric acid - AQA - GCSE Chemistry Combined Science - Question 3 - 2022 - Paper 2

The variable time is a dependent variable because it is affected by the concentration and temperature of the reactants.

To complete Figure 4, plot the data from Table 1 with temperature on the x-axis and time in seconds on the y-axis, ensuring all points are accurately positioned. Then draw a line of best fit to represent the trend of the data.

To calculate the gradient of the tangent at 0.4 mol/dm³, select two points on the tangent line, (x₁, y₁) and (x₂, y₂). The gradient (slope) is calculated using the formula:

$ext{slope} = \frac{y_2 - y_1}{x_2 - x_1}$

Substituting the coordinates of the points found from the graph will provide the gradient value.

The unit of gradient is s dm⁻¹ mol.

The rate of the reaction decreases as it proceeds because, as the reaction occurs, the concentration of sodium thiosulfate particles decreases. Fewer reactant particles result in a lower frequency of successful collisions between them, which slows the reaction rate.