Exothermic and Endothermic Reactions: Question 1

Worked solution

Step 1: Read the temperature change

Before mixing, the water is at $17\,^{\circ}\text{C}$. After the calcium chloride dissolves, the reading rises to $34\,^{\circ}\text{C}$, an increase in the temperature of the surroundings (the water).

Step 2: Apply the definitions

The syllabus definitions are:

• An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases.
• An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases.

Since the temperature of the water rose, thermal energy must have been transferred out of the reacting particles and into the water. This is an exothermic change.

Why the other options are wrong

• Option B correctly identifies the temperature rise but wrongly labels it endothermic; a temperature rise can only be caused by an exothermic transfer of energy to the surroundings.
• Option C describes the wrong direction of temperature change altogether. The reading rose, it did not fall.
• Option D correctly says “exothermic” but then gives the endothermic description (taking energy in) as the reason, which contradicts the rising temperature.

Final answers

• The change is exothermic, because it transfers thermal energy to the surroundings, raising their temperature, $\boxed{\text{option A}}$.