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7.2 Using Energy Transfers from Reactions

GCSE Chemistry · AQA · Chapter 7: Energy Changes

A free GCSE Chemistry lesson for AQA (8462) 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 processes 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 the 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 food or drink can with a separate compartment where an exothermic reaction heats the contents.
  • 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.
  • Crystallising: Crystals forming from a solution. In some hand warmers this releases energy.

Choosing a reaction for heating or cooling

Chemists choose reactions for products using these questions:

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

Hand warmers

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

In the disposable type, iron powder is oxidised to hydrated iron(III) oxide – the same change as rusting, but much faster because the iron is a fine powder and sodium chloride (salt) acts as a catalyst. A supersaturated solution in the reusable type holds more dissolved solid than it normally could at room temperature, so it only needs a “seed” to start crystallising.

Worked example: reading the graph

Hand warmer B reaches its highest temperature (52 °C) in about 5 minutes: a rise of 52 − 20 = 32 °C. It is below 30 °C after about 50 minutes.
Hand warmer A rises more slowly to 46 °C (a rise of 26 °C) but is still at 42 °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. This reaction is very exothermic and heats the food or drink in the outer part of the can:

CaO(s) + H₂O(l) → Ca(OH)₂(s)

They are useful for camping or emergencies. Drawbacks: the heating compartment takes up a large part of the can, they cost more than normal cans, they may not get hot enough on a very cold day, and it can be hard to get the food hot enough without the can becoming too hot to hold. The food must never touch the calcium oxide or calcium hydroxide, which are corrosive.

Cold packs

Instant cold packs contain water and a solid such as ammonium nitrate, NH₄NO₃, 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. They are used by first-aiders to reduce swelling and numb pain from sports injuries, especially where there is no ice. Most are single use and stay cold for about 15–20 minutes. The same endothermic dissolving can be used to chill drinks cans.

Summary: comparing the products

  • Disposable hand warmer; Change used iron + oxygen (+ water) → hydrated iron(III) oxide; Exo or endo? exothermic; Reuse? no; Typical use warm for hours outdoors
  • Reusable hand warmer; Change used sodium ethanoate crystallising from a supersaturated solution; Exo or endo? exothermic; Reuse? yes (boil to reset); Typical use quick warmth for a short time
  • Self-heating can; Change used CaO + H₂O → Ca(OH)₂; Exo or endo? exothermic; Reuse? no; Typical use hot food or drink with no stove
  • Instant cold pack; Change used ammonium nitrate dissolving in water; Exo or endo? endothermic; Reuse? no; Typical use sports injuries; first aid

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

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 the self-heating reaction as CaO + H₂O → CaOH. Calcium hydroxide is Ca(OH)₂.
  • Comparing temperature changes from tests with different amounts of chemical or water. It is not a fair comparison.

Revision questions

Try these, then check your answers below each one.

Question 1. Which substances react in a self-heating can? (Choose from: Calcium oxide and water, Ammonium nitrate and water, Iron and oxygen, Sodium ethanoate and water.)

Answer: Calcium oxide and water

Working: CaO(s) + H₂O(l) → Ca(OH)₂(s) is very exothermic. Ammonium nitrate is used in cold packs.

Question 2. Name the product of the reaction between calcium oxide and water.

Answer: calcium hydroxide

Working: CaO + H₂O → Ca(OH)₂, calcium hydroxide.

Question 3. True or false? Instant cold packs work because dissolving ammonium nitrate in water is endothermic.

Answer: True

Working: True. Dissolving ammonium nitrate takes in energy from the surroundings, so the pack cools.

Related lessons

Continue in the full course

This lesson is section 7.2 (AGCCH0702) of our free GCSE Chemistry 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 Chemistry course.

 

 

 

 

 

 

 

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