C2 4.9 Using energy transfers from reactions

Learning Objectives and Key points:

How can we use the energy from exothermic reactions?

• Exothermic changes can be used in hand warmers and self-heating cans. Crystallisation of a supersaturated solution is used in reusable warmers. However, disposable, one-off warmers can give off heat for longer.
• The disposable hand warmers use the oxidation of iron to release energy. Iron turns into hydrated iron(III) oxide and sodium chloride(common salt) is used as a catalyst.
• Reusable hand warmers are based on the formation of crystals form solutions of salt(often sodium ethanoate). A supersaturated solution is prepared. We do this by soaking as much salt as possible in boiling water so it can be reused. A small metal disc is used to start the reaction and when you press it small particles of metal are scraped off and these start the crystallisation and these crystals spread throughout the solution giving off energy.
• Self-heating cans and the reaction used is usually                                                       Calcium oxide + water = calcium hydroxide                …