IB Chemistry SLTopic 5 — The Rate of ReactionPaper 1 & 2Core idea~11 min read
Activation Energy
Even a reaction that releases enormous energy has to spend some first. Bonds must be stretched and broken before new ones can form, and that upfront cost is why petrol sits safely in a tank until something lights it.
📚 What you need to know
Activation energy, Ea: the minimum energy colliding particles need for a reaction to occur.
On an energy profile, Ea is measured upwards from the reactants to the peak.
The peak is the transition state — the highest-energy arrangement on the way through.
Ea is fixed for a given reaction pathway and is unaffected by temperature or concentration.
Reading an energy profile
Activation energy
the minimum energy that colliding particles must have for a reaction to take place
Two quantities, two different arrows. Ea goes reactants-to-peak; ΔH goes reactants-to-products and ignores the peak entirely.
The two arrows answer different questions, and the exam relies on you keeping them apart:
Ea is about rate. A big hill means few collisions can get over it, so the reaction is slow.
ΔH is about energy released or absorbed overall. It is the difference in level between start and finish, and says nothing at all about speed.
A reaction can be strongly exothermic and glacially slow. Diamond turning into graphite releases energy, but the activation energy is so enormous that a diamond will outlast everything you own. ΔH tells you whether a reaction can release energy; Ea tells you whether it will, in any useful timeframe.
WORKED EXAMPLE
On an energy profile, the reactants lie at 50 kJ mol–1, the peak at 190 kJ mol–1 and the products at 110 kJ mol–1. State whether the reaction is exothermic or endothermic, and find Ea and ΔH.
Step 1 — which way round?Products (110) are HIGHER than reactants (50), so energy has been absorbed.endothermicStep 2 — activation energy, reactants to peak190 − 50 = 140Eₐ = +140 kJ mol⁻¹Step 3 — enthalpy change, reactants to products110 − 50 = +60ΔH = +60 kJ mol⁻¹Always subtract in the order products minus reactants for ΔH, and peak minus reactants for Eₐ.
Going the other way
Every reaction can in principle run backwards, and the reverse reaction has to climb to the same peak — it just starts from a different level. For an exothermic reaction the products sit lower down, so the return journey is a bigger climb.
One peak, two climbs. The gap between the two activation energies is exactly ΔH.
The reverse activation energy
Ea(reverse) = Ea(forward) – ΔH
The formula handles both cases without you having to think about signs. For an exothermic reaction ΔH is negative, so subtracting it adds to Ea and the reverse reaction is harder. For an endothermic reaction ΔH is positive, so the reverse is easier.
WORKED EXAMPLE
For a reaction, Ea(forward) = 258 kJ mol–1 and ΔH = –92 kJ mol–1. Calculate the activation energy of the reverse reaction.
Step 1 — substituteEₐ(rev) = 258 − (−92)Step 2 — mind the double negative= 258 + 92Eₐ(reverse) = 350 kJ mol⁻¹Sensible? The forward reaction is exothermic, so the products are more stable and harder to push back uphill. A larger reverse Eₐ is exactly what you would expect.
WORKED EXAMPLE
A student says: “Heating the reaction lowers the activation energy, which is why it goes faster.” Identify the error and give the correct explanation.
The errorEₐ is a property of the reaction pathway. Temperature does not change it at all — the hill stays exactly the same height.What actually changesHeating changes the PARTICLES, not the barrier. A greater proportion of them now have energy ≥ Eₐ, and they collide more often.the barrier is fixed; the particles changeOnly a catalyst changes Eₐ, and it does so by offering a different pathway rather than by lowering the original one.
💡 Exam tip
Label energy profiles fully: reactants, products, transition state, Ea, ΔH, with an arrow for each quantity.
Ea starts at the reactant level, not at zero on the axis. Drawing it from the axis is a classic lost mark.
Give ΔH a sign; Ea is always positive.
Use Ea(reverse) = Ea(forward) – ΔH and let the signs do the work.
Calculating Ea from experimental data (the Arrhenius equation) is HL only — at SL you read it off a profile.
⚠️ Common mix-up
Measuring Ea from the x-axis instead of from the reactant level.
Believing temperature changes Ea. It changes how many particles clear it.
Confusing Ea with ΔH, or reading ΔH as the height of the peak.
Losing the double negative in Ea(forward) – ΔH for an exothermic reaction.
Assuming a big ΔH means a fast reaction. Energy released and energy required are unrelated.
Up next: Energy Profiles With and Without Catalysts — the one thing that genuinely does change the height of the hill, and exactly what it leaves untouched.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
