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C7.2 Using Energy Transfers from Reactions

GCSE Combined Science · AQA · Chapter C7: Energy Changes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: hand warmers, self-heating cans and cold packs, and how to evaluate them using data. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how exothermic reactions are used in hand warmers and self-heating cans
  • Describe how endothermic changes are used in cold packs
  • Explain the difference between single-use and reusable products
  • Use temperature data to choose the best reaction for a job
  • Evaluate uses and applications of exothermic and endothermic reactions

Key words

  • Hand warmer: A small pack that uses an exothermic change to warm your hands.
  • Self-heating can: A can with a separate compartment where an exothermic reaction heats the food or drink.
  • Cold pack: A pack that uses an endothermic change to cool an injury.
  • Reversible: A change that can be made to go backwards, so a product can be reused.
  • Evaluate: Give advantages and disadvantages, then a justified conclusion.

Choosing a reaction for heating or cooling

Exothermic reactions can be used to warm things; endothermic changes can be used to cool things. To choose the best reaction for a product, chemists ask:

  • How big is the temperature change? Too hot could burn the user; too small is useless.
  • How quickly does it start, and how long does it last?
  • Are the chemicals safe, cheap and easy to package?
  • Can the product be reused, or is it single use? What happens to the waste?

Hand warmers

  • Disposable (iron powder); How it works Opening the packet lets air in. Iron powder is oxidised by oxygen (with water; salt speeds it up). This exothermic reaction is a fast, controlled form of rusting.; Good points stays warm for hours; Drawbacks single use; cannot be reversed; creates waste
  • Reusable (sodium ethanoate); How it works Clicking a metal disc makes a solution crystallise, which is exothermic. To reset it, heat it in boiling water so the crystals dissolve again.; Good points reused many times; warms up quickly; Drawbacks stays warm for a shorter time; must be boiled to reset

Worked example: reading the graph

Hand warmer B reaches its highest temperature (49 °C) after about 5 minutes: a rise of 49 − 19 = 30 °C. It has fallen to 25 °C after 60 minutes.
Hand warmer A rises more slowly to 44 °C (a rise of 25 °C) but is still at 39 °C after 2 hours.

Answer: B warms up faster and gets hotter; A stays warm for much longer

Self-heating cans

A self-heating can has two compartments. Pressing a button breaks a seal so water mixes with solid calcium oxide. The reaction is very exothermic and heats the food or drink in the outer part of the can:

calcium oxide + water → calcium hydroxide CaO(s) + H₂O(l) → Ca(OH)₂(s)

They are useful for camping, expeditions and emergencies. Drawbacks: the heating section takes up a lot of space, they cost more than normal cans, the food may not get hot enough on a very cold day, and they create extra waste. The food must never touch the chemicals, which are corrosive.

Cold packs

Instant cold packs contain water and a solid such as ammonium nitrate in separate sections. Squeezing the pack breaks the inner bag. The solid dissolves, which is endothermic, so energy is taken from the surroundings and the pack quickly gets cold. First-aiders use them to reduce swelling and numb pain from sports injuries where there is no ice. They are single use and stay cold for about 15–20 minutes. The same kind of endothermic dissolving is used in some self-cooling drinks cans, and the reaction of citric acid with sodium hydrogencarbonate could also be used, although the temperature fall is smaller.

  • Disposable hand warmer; Change used iron + oxygen (+ water) → hydrated iron(III) oxide; Exo or endo? exothermic; Reuse? no
  • Reusable hand warmer; Change used sodium ethanoate crystallising; Exo or endo? exothermic; Reuse? yes (boil to reset)
  • Self-heating can; Change used CaO + H₂O → Ca(OH)₂; Exo or endo? exothermic; Reuse? no
  • Instant cold pack; Change used ammonium nitrate dissolving in water; Exo or endo? endothermic; Reuse? no

Exam tip: In questions that ask you to “evaluate”, give advantages and disadvantages, use the numbers you are given, and finish with a justified conclusion.

Common mistakes

  • Saying a cold pack “releases cold”. Dissolving takes in energy from the surroundings.
  • Saying the hotter hand warmer is always better. Too hot can burn the skin, and how long it lasts also matters.
  • Writing calcium hydroxide as CaOH. It is Ca(OH)₂.
  • Comparing temperature changes from tests with different amounts of chemical or water. It is not a fair test.

Revision questions

Try these, then check your answers below each one.

Question 1. Which change is used in a disposable hand warmer? (Choose from: The exothermic oxidation of iron powder, The endothermic dissolving of ammonium nitrate, The thermal decomposition of calcium carbonate, The electrolysis of salt water.)

Answer: The exothermic oxidation of iron powder

Working: Iron powder reacts with oxygen (and water) in the air in an exothermic reaction – a fast, controlled form of rusting.

Question 2. Name the solid that reacts with water in a self-heating can.

Answer: calcium oxide

Working: Calcium oxide reacts exothermically with water: CaO + H₂O → Ca(OH)₂.

Question 3. True or false? Instant cold packs work because a solid dissolving in water takes in energy from the surroundings.

Answer: True

Working: True. Dissolving ammonium nitrate is endothermic, so the pack takes in energy and gets cold.

Related lessons

Continue in the full course

This lesson is section C7.2 (AGCSC2402) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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