Energy transfer during exothermic and endothermic reactions: Everyday uses of exothermic reactions include self-heating cans and hand warmers.

The idea

Exothermic reactions release energy to the surroundings, and this energy transfer can be put to practical use. A self-heating can contains a compartment holding substances that react exothermically when mixed, for example calcium oxide reacting with water to form calcium hydroxide, so the drink warms without a cooker or microwave. A hand warmer often uses the oxidation of iron in air; when the sealed packet is opened, air reaches the iron and the exothermic oxidation releases heat for a period. In both cases the design controls when the reaction starts and how much heat is released, and the temperature rise of the surroundings shows energy transfer. The same principle explains why some reactions are chosen for heating purposes: they release useful energy safely and at a suitable rate.

What earns the marks

• States that self-heating cans and hand warmers use exothermic reactions to release energy to the surroundings.
• Describes a self-heating can as having a compartment where reactants mix and react exothermically, warming the drink.
• Describes a hand warmer as using oxidation of iron in air, releasing heat when the packet is opened.
• Links the everyday use to the temperature rise of the surroundings and to the idea that energy is transferred, not created.
• Compares the design features that control the reaction, such as a seal, a compartment or a permeable wrapper, so heat is released when needed.
• Evaluates a simple claim about a hand warmer or self-heating can using temperature change data or a described observation.

Common mistakes and how to put them right

• Saying the can or warmer 'makes' energy.

  Put it right: It transfers energy from chemical stores to the surroundings, and total energy is conserved.

• Confusing a hand warmer with an endothermic cold pack.

  Put it right: Hand warmers release heat, while cold packs absorb heat from the surroundings.

• Assuming the reaction continues forever.

  Put it right: The reactants are used up, so heat release stops when the reaction finishes or the oxidising agent is exhausted.

Exam tips

• Name the reaction type and the energy transfer when explaining a self-heating can or hand warmer, rather than only saying it gets hot.
• Use the words 'exothermic' and 'surroundings' in your answer to show the direction of energy transfer.
• If a question asks for a comparison, give one similarity and one difference between the two devices, such as the trigger mechanism or the reactants used.