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7.1 Exothermic and Endothermic Reactions

GCSE Chemistry · AQA · Chapter 7: Energy Changes

A free GCSE Chemistry lesson for AQA (8462) from OBA School: conservation of energy, exothermic and endothermic reactions, everyday examples and simple 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
  • Compare energy changes per mole in kJ/mol

Key words

  • Surroundings: Everything around the reacting chemicals: the solvent, the container, the air, the thermometer.
  • 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 of the universe is the same before and after a reaction.
  • Energy level diagram: A diagram that compares the energy stored in the reactants and the products.
  • kJ/mol: Kilojoules per mole: the unit used to compare the energy change for one mole of a reaction.

Energy is conserved

Chemicals store energy. During a reaction, energy is transferred between the 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 they store more, the extra energy has come from the surroundings.

Key fact: The temperature you measure is the temperature of the surroundings (for a reaction in solution, mostly the water). A rise in temperature means energy has been given out by the reaction.

Exothermic reactions

In an exothermic reaction, energy is transferred to the surroundings, so the temperature rises. Most reactions are exothermic. Examples:

  • combustion of fuels, e.g. methane: CH₄ + 2O₂ → CO₂ + 2H₂O
  • many oxidation reactions, e.g. iron rusting, and respiration in cells
  • neutralisation of acids by alkalis, e.g. HCl + NaOH → NaCl + H₂O
  • reactions of metals with acids, e.g. magnesium with hydrochloric acid

Everyday uses include hand warmers, self-heating cans, heating homes and cooking (see section 7.2).

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. CaCO₃ → CaO + CO₂ (needs continuous heating)
  • the reaction of citric acid with sodium hydrogencarbonate (the fizz of sherbet sweets on your tongue feels slightly cold)
  • photosynthesis, which takes in energy from sunlight
  • dissolving some salts, e.g. ammonium nitrate in water (a physical change, used in 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. The arrow shows the overall energy change.

Worked example: deciding from temperature readings

A student mixes two solutions. The starting temperature is 21.5 °C. The highest temperature reached is 29.0 °C.
Temperature change = final − initial = 29.0 − 21.5 = +7.5 °C.
The temperature has risen, so energy has been transferred to the surroundings.

Answer: Exothermic; temperature rise of 7.5 °C

Exam tip: Always write the temperature change with a sign, or say “rise” or “fall”. For an endothermic reaction the change is negative, e.g. 18.0 − 22.0 = −4.0 °C.

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

How big is the energy change? Energy per mole

The temperature change depends on how much of each chemical you use, so chemists compare energy changes per mole of reaction, in kilojoules per mole (kJ/mol). A negative sign means energy is given out (exothermic); a positive sign means energy is taken in (endothermic).

  • C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) (burning ethanol); Type exothermic; Energy change (kJ/mol) about −1370
  • MgCO₃(s) → MgO(s) + CO₂(g) (thermal decomposition); Type endothermic; Energy change (kJ/mol) about +100
  • H+(aq) + OH−(aq) → H₂O(l) (neutralisation); Type exothermic; Energy change (kJ/mol) about −57

Worked example: energy per mole

When 0.020 mol of butane burns completely, 57.6 kJ of energy is transferred to the surroundings.
Energy per mole = energy transferred ÷ number of moles = 57.6 ÷ 0.020 = 2880 kJ/mol.
Energy is given out, so the energy change is written −2880 kJ/mol.

Answer: 2880 kJ/mol transferred to the surroundings (−2880 kJ/mol)

Maths skill (Higher tier): If you are given a mass, first find the number of moles: moles = mass ÷ Mr. Then divide the energy by the moles.

Common mistakes

  • Saying an endothermic reaction “gives out cold”. It takes in energy from the surroundings.
  • Thinking the temperature on the thermometer is the temperature of the chemicals’ energy store. It measures the surroundings.
  • Drawing the products higher than the reactants for an exothermic reaction. They should be lower.
  • Saying energy is “used up” or “made” in a reaction. Energy is conserved; it is transferred.

Revision questions

Try these, then check your answers below each one.

Question 1. What happens in an exothermic reaction? (Choose from: Energy is transferred to the surroundings, so their temperature rises, Energy is taken in from the surroundings, so their temperature falls, Energy is created by the reaction, The total energy of the universe increases.)

Answer: Energy is transferred to the surroundings, so their temperature rises

Working: Energy is transferred to the surroundings. Energy is never created; the total energy is conserved.

Question 2. True or false? Photosynthesis is an endothermic reaction.

Answer: True

Working: True. Photosynthesis takes in energy from sunlight.

Question 3. A reaction mixture starts at 22.5 °C. The lowest temperature reached is 17.0 °C. Calculate the temperature change. Include a minus sign if it is a fall.

Answer: -5.5 °C

Working: 17.0 − 22.5 = −5.5 °C. The temperature fell, so the reaction is endothermic.

Related lessons

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This lesson is section 7.1 (AGCCH0701) 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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