A student investigated the change in mass when hydrated cobalt chloride was heated - AQA - GCSE Chemistry - Question 2 - 2022 - Paper 2

The mass of the test tube and contents decreased because water was produced as a gas during the heating process. This water vapor escaped from the test tube, leading to a reduction in the overall mass. Since the vapor was not measured, the observable decrease in mass is attributed to this loss.

The test tube and contents were heated until the mass did not change to ensure that the reaction was complete. This indicates that no further water vapor was being lost, confirming that all hydrated cobalt chloride had been converted into its anhydrous form.

To calculate the energy taken in when 2.00 g of hydrated cobalt chloride is heated, we use the proportionality based on the given information.

Using the formula:

$\text{Energy} = \frac{2.00 \times 88.1}{238}$

Calculating the above gives:

$\text{Energy} = 0.740 kJ$

Thus, the energy taken in during this reaction is 0.740 kJ, rounded to three significant figures.

The reaction that takes place when hydrated cobalt chloride is heated is an endothermic reaction. This type of reaction absorbs heat from the surroundings as the hydrated form decomposes into its anhydrous counterpart.