An enthalpy level diagram tells you where a reaction starts and finishes. An energy profile tells you what happens on the way — and that hump in the middle explains why a jar of petrol can sit safely next to a jar of air until you strike a match.
📚 What you need to know
An energy profile shows how the energy of the system changes as the reaction proceeds.
The peak is the transition state: bonds are partly broken and partly formed. It is unstable and cannot be isolated.
Activation energy (Ea) is the minimum energy needed for a successful collision — measured from reactants to the peak.
ΔH is measured from reactants to products.
Exothermic profiles end lower than they start; endothermic profiles end higher.
For the same reaction, Ea(reverse) = Ea(forward) – ΔH.
Why there is a hump at all
Before new bonds can form, existing ones have to start breaking — and breaking bonds costs energy. So on the way from reactants to products the system has to climb to a high-energy arrangement in which old bonds are stretched and new ones are only partly made.
That arrangement is the transition state. It sits at the top of the hump, it is extremely unstable, and it cannot be isolated or bottled — it exists for an instant as the reaction passes through it.
Activation energy
the minimum energy colliding particles need for a successful reaction
This is why exothermic reactions still need a spark. The reaction gives out far more energy than it takes in — but something has to pay the entry fee first. Once a few molecules react, the energy they release pushes the next ones over the hump, and it becomes self-sustaining.
Reading a profile
Two measurements, one curve. Ea goes up to the peak; the enthalpy change ignores the peak entirely.
Two different measurements come off the same curve, and confusing them is the classic error:
Ea runs from the reactant level up to the peak.
ΔH runs from the reactant level to the product level, ignoring the peak entirely.
Because an exothermic reaction’s products sit lower, its reactants start closer to the transition state, so exothermic reactions generally have a smaller activation energy than comparable endothermic ones.
Forward and reverse
Run the reaction backwards and the same curve serves, read right to left. The peak has not moved, but you are now climbing to it from the other side.
Read the same curve from the right and you get the reverse activation energy. The difference between the two barriers is the enthalpy change.
🧩 Getting all three values off one diagram
Ea(forward) = peak energy – reactant energy.
ΔH = product energy – reactant energy. Negative if the products are lower.
Ea(reverse) = peak energy – product energy.
Check: Ea(forward) – Ea(reverse) should equal ΔH.
A useful consequence. If the forward reaction is endothermic, the reverse must be exothermic by the same amount — same magnitude, opposite sign. The curve is identical; only the direction of travel changes.
WORKED EXAMPLE
On a profile, the reactants are at 0 kJ mol–1, the transition state at +130 kJ mol–1 and the products at +50 kJ mol–1. Find ΔH, Ea(forward) and Ea(reverse).
A reaction has ΔH = –210 kJ mol–1 and Ea(forward) = +65 kJ mol–1. Determine the activation energy of the reverse reaction.
The products lie 210 kJ BELOW the reactantsThe peak is 65 kJ above the reactants, so it is 65 + 210 above the products.Ea(reverse) = 65 + 210Ea(reverse) = +275 kJ molA large reverse barrier — which is why very exothermic reactions rarely run backwards.
WORKED EXAMPLE
Sketch an energy profile for the combustion of methane, given ΔH = –890 kJ mol–1 and Ea = +2650 kJ mol–1.
Exothermic, so products below reactantsLabel reactants CH₄(g) + 2O₂(g) and products CO₂(g) + 2H₂O(l).The hump is very tall compared with the dropEa is roughly three times the size of ΔH, so draw the peak high above BOTH levels.Ea arrow: reactants → peak, +2650ΔH arrow: reactants → products, −890That tall barrier is exactly why methane is safe to store but burns fiercely once lit.
💡 Exam tip
Label both axes. Vertical is energy or enthalpy, horizontal is reaction coordinate or extent of reaction.
Draw arrows with clear start and end points on the correct levels — a vague arrow gets no mark.
Mark the transition state at the top of the hump if the question mentions it.
Keep the sketch roughly to scale. If Ea is much bigger than ΔH, make it look that way.
⚠️ Common mix-up
Ea is not measured from the products for the forward reaction, and it is not measured from zero on the axis.
ΔH does not include the hump. It is purely the difference between the two flat levels.
Activation energy is always positive, for both the forward and the reverse reaction.
The transition state is not an intermediate. It cannot be isolated.
A large Ea makes a reaction slow, not impossible — and says nothing about the sign of ΔH.
Up next: Standard Enthalpy Changes — pinning down the exact conditions so that two people measuring the same reaction get the same number.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
