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C7.1 Exothermic and Endothermic Reactions

GCSE Combined Science · AQA · Chapter C7: Energy Changes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: conservation of energy, exothermic and endothermic reactions, everyday examples and energy level diagrams. 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

  • State that energy is conserved in chemical reactions
  • Describe exothermic and endothermic reactions in terms of energy transfer and temperature change
  • Give examples of exothermic and endothermic reactions
  • Draw and interpret simple energy level diagrams
  • Use temperature readings to decide whether a reaction is exothermic or endothermic

Key words

  • Surroundings: Everything around the reacting chemicals: the water they are dissolved in, the container, the air.
  • Exothermic: A reaction that transfers energy to the surroundings, so the temperature of the surroundings increases.
  • Endothermic: A reaction that takes in energy from the surroundings, so the temperature of the surroundings decreases.
  • Conservation of energy: Energy cannot be created or destroyed, only transferred. The total energy is the same before and after a reaction.
  • Energy level diagram: A diagram that compares the energy stored in the reactants with the energy stored in the products.

Energy is conserved

Chemicals store energy. During a reaction, energy is transferred between the reacting chemicals and their surroundings. The amount of energy in the universe at the end of a reaction is the same as before the reaction. If the products store less energy than the reactants, the difference has been transferred to the surroundings. If the products store more, the extra energy has come from the surroundings.

Key fact: The thermometer measures the temperature of the surroundings (for a reaction in solution, mostly the water). A rise in temperature means the reaction has given out energy.

Exothermic reactions

In an exothermic reaction, energy is transferred to the surroundings, so the temperature rises. Most reactions are exothermic. Some give out only a little energy (a rise of a few degrees); others give out so much that they burst into flame.

  • combustion of fuels, e.g. methane in natural gas: CH₄ + 2O₂ → CO₂ + 2H₂O
  • oxidation reactions, e.g. iron rusting and respiration in living cells
  • neutralisation of acids by alkalis, e.g. HNO₃ + KOH → KNO₃ + H₂O
  • reactions of metals with acids, e.g. zinc with sulfuric acid

Everyday uses include hand warmers and self-heating cans (C7.2).

Endothermic reactions

In an endothermic reaction, energy is taken in from the surroundings, so the temperature falls. Endothermic reactions are less common. Examples:

  • thermal decomposition, e.g. heating copper carbonate: CuCO₃ → CuO + CO₂ (needs continuous heating)
  • the reaction of citric acid with sodium hydrogencarbonate (the fizz of sherbet on your tongue feels cool)
  • photosynthesis, which takes in energy from sunlight
  • dissolving some salts, e.g. ammonium nitrate in water (used in sports injury cold packs)
  • Energy transfer; Exothermic to the surroundings; Endothermic from the surroundings
  • Temperature of surroundings; Exothermic increases; Endothermic decreases
  • Energy of products compared with reactants; Exothermic lower; Endothermic higher
  • Examples; Exothermic combustion, oxidation, neutralisation; Endothermic thermal decomposition, citric acid + sodium hydrogencarbonate

Energy level diagrams

An energy level diagram shows the energy stored in the reactants and products as two horizontal lines. An arrow from the reactants to the products shows the overall energy change.

Memory aid: Exothermic: products below reactants, arrow points down. Endothermic: products above reactants, arrow points up.

Deciding from temperature readings

Worked example: exothermic or endothermic?

Two solutions are mixed. The starting temperature is 20.5 °C. The highest temperature reached is 26.0 °C.
Temperature change = final − initial = 26.0 − 20.5 = +5.5 °C.
The temperature has risen, so energy has been transferred to the surroundings.

Answer: exothermic; temperature rise of 5.5 °C

Exam tip: Always give the temperature change with a sign, or say “rise” or “fall”. For an endothermic reaction the change is negative, e.g. 15.5 − 21.0 = −5.5 °C.

If you keep reading the thermometer after the reaction has finished, the temperature slowly returns to room temperature as energy moves between the cup and the air. Use the highest (or lowest) temperature, not the last one.

Common mistakes

  • Saying an endothermic reaction “gives out cold”. It takes in energy from the surroundings.
  • Saying energy is “used up” or “made” in a reaction. Energy is conserved; it is transferred.
  • Drawing the products higher than the reactants for an exothermic reaction. They should be lower.
  • Using the final temperature instead of the highest or lowest temperature reached.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these reactions is endothermic? (Choose from: Thermal decomposition of calcium carbonate, Burning natural gas, Neutralising an acid with an alkali, Iron rusting.)

Answer: Thermal decomposition of calcium carbonate

Working: Thermal decomposition takes in energy, so it is endothermic. Combustion, neutralisation and oxidation (rusting) are exothermic.

Question 2. The diagram shows two energy level diagrams. Which one shows an endothermic reaction? (Choose from: Diagram P, Diagram Q, Both, Neither.)

Answer: Diagram P

Working: In P the products are higher in energy than the reactants: energy has been taken in from the surroundings, so P is endothermic. Q is exothermic.

Question 3. True or false? The total amount of energy in the universe is the same at the end of a reaction as at the start.

Answer: True

Working: True. Energy is conserved: it is only transferred between the chemicals and the surroundings.

Related lessons

Continue in the full course

This lesson is section C7.1 (AGCSC2401) 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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