Energy changes in reactions Cambridge IGCSE Co-ordinated Sciences (9–1) revision
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In plain words
Some reactions give out heat, like a fire: the surroundings get hotter. These are exothermic (think "exit"). Others take heat in, like a sports cold pack: the surroundings get colder. These are endothermic (think "enter").
Three things to know
- Exothermic reactions transfer thermal energy to the surroundings, so the temperature rises: burning, neutralisation. Endothermic reactions take it in, so the temperature falls: thermal decomposition.
- Breaking bonds takes energy in. Making bonds gives energy out. If more is given out than taken in, the reaction is exothermic and ΔH is negative.
- The activation energy is the minimum energy the colliding particles need for a reaction to happen.
Worked example
H₂ + Cl₂ → 2HCl. Bond energies in kJ/mol: H–H 436, Cl–Cl 242, H–Cl 431. Find the energy change.
- Bonds broken (energy in): 436 + 242 = 678 kJ.
- Bonds made (energy out): 2 × 431 = 862 kJ.
- Energy change = in − out = 678 − 862 = −184 kJ/mol. It is negative, so the reaction is exothermic.
Tips and tricks
- Energy change = energy to break bonds − energy from making bonds. Always "in minus out".
- A minus sign means exothermic. On an energy diagram, exothermic products sit lower than the reactants.
It lands in your notebook with its questions as flashcards.
Energy changes in reactions: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
In which type of reaction does the temperature of the surroundings fall?
An endothermic reaction takes thermal energy in from the surroundings.
Which of these is exothermic?
All burning gives out heat.
What happens when chemical bonds are broken?
Pulling atoms apart always needs energy.
In an exothermic reaction, how does the energy of the products compare with that of the reactants?
The difference has been given out to the surroundings.
What is the activation energy?
Collisions with less energy than this do not lead to a reaction.
What is the sign of ΔH for an exothermic reaction?
The chemicals lose energy to the surroundings.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Energy changes in reactions
Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 8 marks · papermunch.org
Name ______________________________ Date ______________
When two solutions are mixed the temperature rises from 20 °C to 27 °C. State whether the reaction is exothermic or endothermic, and explain how you know.[2]
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Exothermic: the temperature of the surroundings rose, so the reaction gave out thermal energy.
Explain, in terms of bonds, why a reaction is exothermic.[2]
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More energy is released when the new bonds form than is taken in to break the old bonds.
State what is meant by activation energy.[1]
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The minimum energy that colliding particles must have in order to react.
CH₄ + 2O₂ → CO₂ + 2H₂O. The bonds broken need 2648 kJ and the bonds made release 3466 kJ. Calculate the energy change and state what its sign shows.[3]
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−818 kJ/mol. 2648 − 3466. The negative sign shows the reaction is exothermic.
Answers: Energy changes in reactions
- 1. Exothermic: the temperature of the surroundings rose, so the reaction gave out thermal energy.
- 2. More energy is released when the new bonds form than is taken in to break the old bonds.
- 3. The minimum energy that colliding particles must have in order to react.
- 4. −818 kJ/mol. 2648 − 3466. The negative sign shows the reaction is exothermic.