Edexcel IGCSE Chemistry · Spec 3.5C-3.7C
Energy Level Diagrams & Bond Energies
Energy level diagrams and calculating energy changes from bond energies.
Chemistry revision video
Energy Level Diagrams & Bond Energies
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Reading and drawing the diagram
An energy level diagram plots energy on the vertical axis against the progress of the reaction along the horizontal. It shows three things at once: where the reactants start, where the products finish, and the hump in between.
The two shapes
Exothermic: the products sit lower than the reactants. Energy has left the chemicals and gone to the surroundings, so the surroundings get warmer and the energy change is negative.
Endothermic: the products sit higher than the reactants. Energy has been taken in from the surroundings, so they get colder and the energy change is positive.
Whichever it is, the reaction still has to climb the hump first. Even an exothermic reaction needs energy to start, which is why a fuel needs a spark.
What each part of the diagram is
The height of the peak above the reactant level is the activation energy: the minimum energy colliding particles need for a reaction to happen.
The vertical gap between the reactant level and the product level is the overall energy change. Measure it between the two levels, not from the peak.
Drawing a catalyst adds a second, lower hump between the same two levels. The start and finish do not move, because a catalyst changes the route rather than the energy of the substances.
Drawing it so it earns the marks
Label both axes, and label the reactants and products on their levels.
Draw the activation energy as a single headed arrow pointing upwards, from the reactant level to the top of the peak. Draw the overall energy change as a single headed arrow between the two levels, pointing downwards for exothermic and upwards for endothermic.
Label each arrow. An unlabelled arrow in the right place tells the examiner nothing about which quantity you meant, and double headed arrows are not accepted for either.
Make the arrows long enough to reach properly between the levels they are measuring. A short arrow floating in the middle of the diagram does not identify a distance.
Bond energies
The calculation behind the diagram is a comparison of two totals.
Breaking bonds takes energy in; making bonds gives energy out. So the energy change equals the total energy needed to break all the reactant bonds minus the total energy released making all the product bonds.
A negative answer means more energy came out than went in, so the reaction is exothermic and the products sit lower. A positive answer means the opposite. The sign and the shape of the diagram are the same fact told two ways, so they must agree, and checking that they do is a useful last step.
Counting the bonds
The commonest error is not the arithmetic but the counting. Methane has four carbon to hydrogen bonds. Oxygen gas has one double bond per molecule, and a balanced equation with 2O2 has two of them. Water has two oxygen to hydrogen bonds, and 2H2O has four.
Draw the displayed formulae if you are unsure, and multiply each bond energy by the number of bonds of that type across the whole balanced equation before adding anything up.
Spec 3.5C-3.7C
What you need to know
- Draw energy level diagrams for both types of reaction
- Mark on the activation energy
- Use bond energies to find the energy change
Active recall
Quick check
Answer each question before opening the answer.
How do you calculate the energy change using bond energies?
Energy change = energy to break bonds (bonds in reactants) − energy released making bonds (bonds in products).
On an energy profile, where are the products for an exothermic reaction?
Below the reactants (energy is released).
Chemistry revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.