Energy Level Diagrams Exam Questions
Practice IGCSE Chemistry exam questions on energy level diagrams. Covers drawing and interpreting energy profiles for exothermic and endothermic reactions, activation energy, and enthalpy changes with mark schemes and examiner notes.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
These questions cover energy level diagrams. Write full answers before checking.
Question 1 (4 marks, Core)
Draw an energy level diagram for an exothermic reaction. Label:
- The energy of the reactants
- The energy of the products
- The enthalpy change (delta H)
- The activation energy (Ea)
Mark scheme
- x-axis labelled “progress of reaction” / “reaction coordinate” and y-axis labelled “energy” [1]
- Reactants drawn at a higher energy level than products [1]
- Arrow pointing downward between reactants and products, labelled delta H (negative) [1]
- A “hump” above the reactant energy level, with an arrow from reactants to the top of the hump labelled Ea (activation energy) [1]
Examiner note: For exothermic: reactants are ABOVE products (energy is released). The activation energy arrow goes from the reactant level UP to the peak — it is the minimum energy needed to start the reaction. Delta H points downward and is negative.
Question 2 (3 marks, Core)
Draw an energy level diagram for an endothermic reaction. Label the enthalpy change.
Mark scheme
- Axes correctly labelled (energy vs progress of reaction) [1]
- Reactants drawn at a LOWER energy level than products [1]
- Arrow pointing upward between reactants and products, labelled delta H (positive) [1]
Examiner note: For endothermic: products are ABOVE reactants (energy is absorbed). Delta H is positive. A common error is drawing an endothermic diagram with products below reactants — that would be exothermic.
Question 3 (3 marks, Core)
An energy level diagram for a reaction shows the reactants at 200 kJ and the products at 130 kJ. The activation energy is 80 kJ.
(a) Calculate the enthalpy change for this reaction. (1)
(b) State whether the reaction is exothermic or endothermic. (1)
(c) State the energy of the transition state / top of the activation energy peak. (1)
Mark scheme
(a) Delta H = 130 - 200 = -70 kJ [1]
(b) Exothermic (delta H is negative / products are at lower energy) [1]
(c) Energy at peak = 200 + 80 = 280 kJ [1]
Examiner note: Delta H = energy of products - energy of reactants. A negative value means exothermic. The activation energy is measured from the reactant level to the peak, so the peak = reactant energy + Ea.
Question 4 (3 marks, Supplement)
Explain what activation energy is and why reactions need it even if they are exothermic.
Mark scheme
- Activation energy is the minimum energy that colliding particles must have to react / the energy needed to break the bonds in the reactant molecules to start the reaction [1]
- Even exothermic reactions need activation energy to break the existing bonds before new bonds can form [1]
- Once started, the energy released by forming new bonds sustains the reaction / the reaction releases more energy than was needed to start it [1]
Examiner note: Activation energy is like a hill that must be climbed before you can roll down the other side. All reactions have an activation energy barrier, but exothermic reactions release more energy than they absorb once they get going.
Question 5 (4 marks, Supplement)
A catalyst is added to a reaction. Describe the effect on the energy level diagram and explain how the catalyst increases the rate of reaction.
Mark scheme
- The catalyst provides an alternative reaction pathway [1]
- with a lower activation energy (the peak on the diagram is lower) [1]
- More particles have energy greater than or equal to the lower activation energy [1]
- So more collisions are successful / the rate of reaction increases [1]
Examiner note: A catalyst does NOT change the enthalpy change (delta H) — the energy of reactants and products stays the same. It only lowers the peak. On a diagram, draw a second, lower hump with the same start and end levels.
Question 6 (2 marks, Core)
Look at the energy level diagram described below: the reactants are at 50 kJ, the products are at 120 kJ.
(a) State the enthalpy change. (1)
(b) Classify the reaction. (1)
Mark scheme
(a) Delta H = 120 - 50 = +70 kJ [1]
(b) Endothermic [1]
Examiner note: Products are higher than reactants, so energy has been absorbed. Delta H is positive. If you see products higher on the diagram, the reaction is endothermic.
Question 7 (4 marks, Supplement)
The combustion of methane is represented by:
CH4 + 2O2 → CO2 + 2H2O, delta H = -890 kJ/mol
(a) Draw and label the energy level diagram for this reaction. (3)
(b) Explain in terms of bond energies why this reaction is exothermic. (1)
Mark scheme
(a) Axes labelled (energy vs progress of reaction) [1]; reactants (CH4 + 2O2) at a higher level than products (CO2 + 2H2O) [1]; delta H labelled as -890 kJ/mol, arrow pointing downward [1]
(b) The total energy released in forming the bonds in CO2 and H2O is greater than the total energy required to break the bonds in CH4 and O2 [1]
Examiner note: The diagram must show specific labels — write the formulae of reactants and products, not just “reactants” and “products.” Include the numerical value of delta H. The exothermic nature is explained by bond energies: energy out (forming bonds) > energy in (breaking bonds).
What to revise if you scored below 5
If diagrams were drawn incorrectly, memorise: exothermic = products LOWER, endothermic = products HIGHER. Always label both axes, delta H, and Ea. Revisit the energy level diagrams notes.
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