Exothermic and Endothermic Reactions: Question 7

Worked solution

Step 1: Recall the typical exothermic and endothermic reaction types

Combustion reactions, neutralisation reactions, and most oxidation reactions transfer thermal energy to the surroundings, so they are exothermic. Thermal decomposition reactions, and dissolving certain salts, take in thermal energy from the surroundings, so they are endothermic.

Step 2: Classify each option

• Option A, burning magnesium ribbon, is a combustion (oxidation) reaction. This is exothermic, releasing thermal energy and raising the temperature of the surroundings.
• Option B, dissolving solid sodium hydroxide in water, is exothermic, this particular dissolving process transfers thermal energy to the water, raising its temperature.
• Option C, neutralising an acid with an alkali, is exothermic, neutralisation reactions transfer thermal energy to the surroundings, raising their temperature.
• Option D, heating solid calcium carbonate until it decomposes, is a thermal decomposition reaction. It needs a continuous supply of thermal energy to proceed, so it is endothermic. It takes in thermal energy from the surroundings (the Bunsen burner flame).

Final answers

• The endothermic reaction is heating calcium carbonate until it decomposes, $\boxed{\text{option D}}$.