An enthalpy level diagram tells you where a reaction starts and finishes. An energy profile adds the bit in between — the hill the reactants have to climb before anything happens at all. This one diagram explains why petrol does not ignite in the tank, and it is drawn in almost every exam session.
📘 What you need to know
An energy profile plots potential energy against the reaction coordinate (how far the reaction has progressed).
The peak is the transition state: bonds are part broken and part formed. It is the highest energy point and cannot be isolated.
Activation energy, Ea, is the minimum energy colliding particles need to reach the transition state and react.
Measure Ea from the reactants level up to the peak. Measure ΔH from the reactants level to the products level.
For the reverse reaction, Ea is measured from the products up to the same peak: Ea(reverse) = Ea(forward) − ΔH.
Exothermic reactions have a smaller forward Ea than reverse; endothermic reactions are the other way round.
A catalyst lowers Ea by offering a different route. It does not change ΔH.
What the axes actually mean
The vertical axis is potential energy (you will also see it labelled enthalpy, H) — the energy stored in the bonds and arrangement of the particles.
The horizontal axis is the reaction coordinate. This is not time, and it is not concentration. It is simply “how far along the process of turning into products are these particles”. Left-hand end: untouched reactants. Right-hand end: finished products. Everything in between is the messy middle where old bonds are stretching and new ones are forming.
Do not read the horizontal axis as a stopwatch. A reaction can sit at the left of the profile for years and then cross the whole diagram in a millisecond.
The transition state and the hill
Two particles that collide do not slide smoothly into being products. First they must be forced into an awkward, unstable arrangement where the old bonds are stretched almost to breaking and the new ones are only half made. That arrangement is the transition state, and it sits at the top of the hill.
Getting there costs energy, and colliding particles pay for it out of their own kinetic energy. If a collision does not bring enough energy, the particles simply bounce apart unchanged. The minimum needed is the activation energy.
Definition to learn word for wordEa = the minimum energy that colliding particles must have for a successful collision
Two completely separate quantities live on one diagram. Confusing them is the most common error on this topic, so check the start of every arrow you draw.
Endothermic profiles
Nothing about the shape changes except where the right-hand plateau ends up. For an endothermic reaction the products finish above the reactants, so the reaction is uphill overall as well as uphill on the way.
Because the products are already high up, they only need a small push to get back over the peak. That is why endothermic reactions have a large forward activation energy and a small reverse one.
If you can draw one of these two profiles from memory you can draw the other — only the height of the right-hand plateau moves.
Reading numbers off a profile
Exam questions rarely just ask you to draw the diagram. They give you a sketch with a couple of energy values on it and ask you to extract Ea or ΔH. Both come out of the same three levels.
🧩 Extracting values from any profile
Write down the three heights: reactants, peak, products.
Ea forward = peak − reactants. Always positive.
ΔH = products − reactants. Sign tells you exo or endo.
Ea reverse = peak − products. Also always positive.
Check your work: Ea(reverse) should equal Ea(forward) − ΔH.
WORKED EXAMPLE
On an energy profile the reactants lie at 50 kJ mol−1, the transition state at 185 kJ mol−1 and the products at 20 kJ mol−1. Find Ea for the forward reaction, ΔH, and Ea for the reverse reaction.
A reaction has ΔH = +52 kJ mol−1 and a forward activation energy of 90 kJ mol−1. Deduce the activation energy of the reverse reaction and state which direction is faster at a given temperature.
Step 1: Use the relationshipEₐ(rev) = Eₐ(fwd) − ΔH = 90 − (+52)= +38 kJ mol⁻¹Step 2: Compare the two barriersthe reverse barrier is much lower, so more collisions can clear itEₐ(rev) = +38 kJ mol⁻¹, reverse reaction is fasteran endothermic forward reaction always has the taller barrier
What a catalyst does — and does not do
A catalyst provides an alternative pathway with a lower transition state. More of the colliding particles now have enough energy to get over the top, so the rate goes up.
What a catalyst absolutely cannot do is move the reactants or products levels. Those are fixed by the bonds in the substances themselves. So ΔH is completely unchanged, and both the forward and reverse activation energies fall by the same amount.
A common exam trap is a diagram where the catalysed curve also ends lower. That is wrong: a catalyst cannot change how much energy the bonds store.
Why this matters for exothermic reactions. An exothermic reaction is downhill overall, but the hill still has to be climbed first. That is precisely why a mixture of petrol and air sits harmlessly in a fuel tank: the activation energy has not been supplied. A spark supplies it to a few molecules, those release enough energy to activate their neighbours, and the reaction runs away.
WORKED EXAMPLE
The reaction from the first worked example (Ea = 135 kJ mol−1, ΔH = −30 kJ mol−1) is repeated with a catalyst that lowers the forward activation energy to 78 kJ mol−1. State the new ΔH and calculate the new reverse activation energy.
Step 1: What does a catalyst change?only the height of the peak, so the two levels stay putΔH = −30 kJ mol⁻¹ (unchanged)Step 2: New reverse barrierEₐ(rev) = 78 − (−30) = +108 kJ mol⁻¹Step 3: Sense-checkboth barriers dropped by 57 kJ mol⁻¹, the same amount, exactly as expectedΔH unchanged at −30; Eₐ(rev) = +108 kJ mol⁻¹
Drawing one under exam pressure
Marks for sketching a profile are handed out for very specific features. Include all of these and the marks are hard to lose.
Both axes labelled: potential energy (or enthalpy) upwards, reaction coordinate across.
A single smooth curve, not straight lines with a corner, and not a curve that dips below the products.
Reactants and products plateaus labelled, at the correct relative heights.
Ea arrow from the reactants plateau to the peak, and a ΔH arrow between the two plateaus.
Dashed horizontal guide lines so the examiner can see where your arrows start and stop.
If values are given, write them on the arrows with signs and units.
💡 Exam tip
Start the Ea arrow at the reactants line, not at the horizontal axis. Starting from the axis is the single most-penalised slip on this topic.
Use a ruler for the dashed guide lines, then draw the arrows on top of them.
If a question mentions a reversible reaction, expect to be asked for both activation energies from one diagram.
Label the peak transition state and be ready to add that it is unstable and cannot be isolated.
When you write the definition of Ea, the word minimum is worth a mark on its own.
A catalyst question is almost always testing whether you know ΔH stays the same. Say so explicitly.
⚠ Common mix-up
Measuring Ea from the axis to the peak. It is measured from the reactants level to the peak.
Confusing Ea with ΔH. One is the height of the hill, the other is the difference between the two ends.
Giving Ea a negative value. Activation energy is always positive, in both directions.
Drawing the catalysed curve finishing lower. A catalyst changes the route, never the levels.
Treating the horizontal axis as time. It is the progress of the reaction.
Saying the transition state is an intermediate. An intermediate sits in a dip and can sometimes be detected; a transition state sits at a peak and cannot.
Forgetting the sign of ΔH in the reverse-barrier formula. Subtracting a negative ΔH makes the reverse barrier bigger.
Up next: Standard Enthalpy Changes — before we can compare any two ΔH values fairly, we need to agree on the conditions they were measured under.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
