Question 1 Medium
On a reaction profile, what is shown by the difference in height between the reactants and the products?
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Answer: the overall energy change
- It shows the energy transferred.
- This is the overall energy change.
Exothermic and endothermic reactions: GCSE Combined Science Chemistry practice questions
A second set of questions on exothermic and endothermic reactions. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
How does a catalyst change a reaction profile?
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Answer: it lowers the peak
- It gives an easier route.
- The peak gets lower.
Question 3 Medium
Bonds broken need 2000 kJ/mol and bonds made release 2500 kJ/mol. Calculate the energy change in kJ/mol.
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Answer: -500 kJ/mol
- Energy change = 2000 − 2500.
- = −500 kJ/mol.
Question 4 Medium
On a reaction profile for an endothermic reaction, are the products higher or lower than the reactants?
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Answer: higher
- An endothermic reaction takes in energy.
- So the products have more energy and sit higher.
Question 5 Hard
Calculate the energy change for H₂ + Cl₂ → 2HCl. Bond energies in kJ/mol: H–H 436, Cl–Cl 243, H–Cl 432.
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Answer: -185 kJ/mol
- Broken: 436 + 243 = 679.
- Made: 2 × 432 = 864.
- 679 − 864 = −185.
Question 6 Hard
Calculate the energy change for CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies in kJ/mol: C–H 413, O=O 498, C=O 805, O–H 464.
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Answer: -818 kJ/mol
- Broken: 4 × 413 + 2 × 498 = 2648.
- Made: 2 × 805 + 4 × 464 = 3466.
- 2648 − 3466 = −818.
Question 7 Hard
In a reaction, bonds broken need 4200 kJ/mol. The energy change is −350 kJ/mol. Calculate the energy released when the new bonds form.
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Answer: 4550 kJ/mol
- Energy change = broken − made.
- Made = 4200 − (−350) = 4550 kJ/mol.
Question 8 Hard
A student mixes citric acid and sodium hydrogencarbonate and the temperature falls by 8 °C. Explain what this shows about the reaction.
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Answer: it is endothermic because it takes in energy from the surroundings
- The mixture gets colder.
- The reaction takes in energy from the surroundings, so it is endothermic.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What is an exothermic reaction?
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A reaction that transfers energy to the surroundings.
Check 2
What is an endothermic reaction?
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A reaction that takes in energy from the surroundings.
Check 3
Give two examples of exothermic reactions.
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Combustion and neutralisation.
Check 4
Give two examples of endothermic reactions.
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Thermal decomposition and photosynthesis.
Check 5
What is activation energy?
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The minimum energy that particles need to react.
Check 6
What does a catalyst do to activation energy?
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It lowers it.
Check 7
Where are the products on an exothermic profile?
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Lower than the reactants.
Check 8
What is the unit for bond energy?
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kJ/mol.
Check 9
Is breaking bonds exothermic or endothermic?
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Endothermic.
Check 10
Is making bonds exothermic or endothermic?
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Exothermic.
Check 11
What is the equation for energy change from bond energies?
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Energy to break bonds − energy released making bonds.
Check 12
What sign does the energy change have for an exothermic reaction?
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Negative.
Check 13
Give one use of an exothermic reaction.
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Hand warmers or self-heating cans.
Check 14
Give one use of an endothermic reaction.
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Instant cold packs.
Check 15
Why use a lid and insulation in a temperature change experiment?
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To reduce energy loss to the surroundings.
Check 16
What are the surroundings in a reaction?
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Everything around the reacting chemicals, such as the air and the container.
Check 17
What happens to the temperature of the surroundings in an exothermic reaction?
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It rises.
Check 18
What happens to the temperature of the surroundings in an endothermic reaction?
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It falls.
Check 19
What does the vertical axis of a reaction profile show?
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Energy.
Check 20
What does the height of the peak on a reaction profile show?
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The activation energy.
Check 21
What does the gap between reactants and products on a profile show?
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The overall energy change.
Check 22
On an endothermic reaction profile, where are the products?
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Higher than the reactants.
Check 23
Does a catalyst change the overall energy change of a reaction?
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No. It only lowers the activation energy.
Check 24
What is bond energy?
Show answer
The energy needed to break a bond, measured in kJ/mol.
Check 25
How many C–H bonds are in one molecule of methane, CH₄?
Show answer
Four.
Check 26
What does a positive energy change tell you about a reaction?
Show answer
It is endothermic.
Keep going with Exothermic and endothermic reactions
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