Question

Some chemical reactions absorb energy rather than release it. Which of these phenomena are caused by chemical reactions that absorb energy? If you're not sure, make a guess.

1. Plants use sunlight and atmospheric gases to make food.
2. A person activates a cold pack by hitting it against a surface.
3. A rechargeable battery is plugged in to charge.
4. Heat is used to separate a compound into its elements.

Answer 1

When a person activates a cold pack by hitting it against a surface, they are causing a chemical reaction to occur inside the pack. The chemical reacts with water and absorbs heat energy, quickly cooling down the pack. Cold packs typically use ammonium nitrate, which absorbs heat from the surroundings when it dissolves.

Step-by-step explanation:

An endothermic reaction is one that absorbs heat from its surroundings. When a person activates a cold pack by hitting it against a surface, they are causing a chemical reaction to occur inside the pack. The chemical reacts with water and absorbs heat energy, quickly cooling down the pack. This reaction is endothermic because it requires heat from the surroundings.

Cold packs typically use ammonium nitrate, which absorbs heat from the surroundings when it dissolves:

NH4NO3 (s) + H2O (l) → NH4+ (aq) + NO3− (aq)

This endothermic process helps to limit swelling and provide relief for muscle strains and sore joints.