IB Chemistry SL & HLTopic 5 — The Rate of Chemical ChangePaper 1 & 2Core skill~10 min read
Activation Energy
Even a reaction that releases loads of energy needs a push to get started. Petrol will happily burn, but it sits in the tank all day until a spark arrives. That starting push is the activation energy, and on an energy profile it shows up as a hill you have to get over before you can roll down the other side.
📘 What you need to know
Activation energy (Ea) is the minimum energy colliding particles need in order to react.
On an energy profile, Ea is measured from the reactants level up to the peak.
The peak is the transition state — a short-lived arrangement with old bonds half broken and new ones half formed.
ΔH is measured from the reactants level to the products level. It is negative for exothermic and positive for endothermic.
The transition state is always higher than both reactants and products.
For the reverse reaction, Ea(reverse) = Ea(forward) − ΔH.
Ea and ΔH are completely independent — a big Ea tells you nothing about ΔH.
What is actually happening at the top of the hill
To turn reactants into products, bonds have to break. Breaking bonds costs energy, and you have to pay that cost before you get any energy back from the new bonds forming.
So as two particles come together, there is a moment where the old bonds are stretched and weakened but the new ones have not properly formed yet. That awkward halfway arrangement is the transition state, and it is the highest-energy point on the whole journey. If a colliding pair cannot supply enough energy to reach it, they simply fall back and bounce apart.
Do not confuse the transition state with an intermediate. A transition state is a peak — it cannot be isolated and only exists for an instant. An intermediate sits in a dip between two peaks and is a real, if short-lived, substance.
The two shapes of energy profile
Every profile you meet is one of these two. The only difference is whether the products end up below or above the reactants.
The green arrows are Ea, drawn from the reactants line up to the peak. The coloured arrows are ΔH, drawn from the reactants line across to the products line. Notice they start from the same place but end somewhere different.
The mistake that costs marks. Ea is measured from the reactants, never from the bottom of the axis and never from the products. Draw the reactants level as a dashed line right across the diagram first, then measure up from it.
Getting Ea for the reverse reaction
Most reactions can go backwards, and the reverse journey uses the same peak — it just starts from the other side. So the reverse activation energy is the climb from the products level up to that same transition state.
Textbooks usually give you two separate rules for this, one for exothermic and one for endothermic. You do not need two. One signed formula covers both cases:
Reverse activation energy
Ea(reverse) = Ea(forward) − ΔH
Put the sign of ΔH in and the arithmetic looks after itself. For an exothermic reaction ΔH is negative, so subtracting it adds to Ea — the reverse barrier is bigger, which makes sense because you are starting from a lower valley. For an endothermic reaction ΔH is positive, so the reverse barrier is smaller.
Both arrows finish at exactly the same height, because there is only one transition state. The reverse arrow is longer purely because it starts further down.
A quick sanity check you can do in the exam: on an exothermic profile the reverse Ea must be the bigger of the two. If your arithmetic gives you a smaller one, you have subtracted when you should have added.
Worked examples
WORKED EXAMPLE
Reverse Ea for an exothermic reaction
A reaction has a forward activation energy of 60 kJ mol−1 and ΔH = −100 kJ mol−1. Calculate the activation energy of the reverse reaction.
Step 1: write the formula
Ea(reverse) = Ea(forward) − ΔH
Step 2: substitute, keeping the sign= 60 − (−100)= 60 + 100Ea(reverse) = 160 kJ mol⁻¹exothermic forwards, so the reverse barrier is bigger — that checks out
WORKED EXAMPLE
Reverse Ea for an endothermic reaction
A different reaction has a forward activation energy of 120 kJ mol−1 and ΔH = +80 kJ mol−1. Find Ea for the reverse reaction, and state whether the reverse reaction starts more easily.
Step 1: same formula, positive ΔH this timeEa(reverse) = 120 − (+80)Ea(reverse) = 40 kJ mol⁻¹Step 2: compare40 is smaller than 120, so yes — the reverse (exothermic) direction has the smaller barrier and starts more easily
WORKED EXAMPLE
Reading everything off one profile
On an energy profile, the reactants sit at −250 kJ, the transition state at −60 kJ and the products at −410 kJ. Find Ea(forward), ΔH, and Ea(reverse).
Step 1: Ea forward is reactants to peak−60 − (−250) = 190 kJ mol⁻¹Step 2: ΔH is reactants to products−410 − (−250) = −160 kJ mol⁻¹negative, so exothermic — matches the products being lowerStep 3: Ea reverse is products to the same peak−60 − (−410) = 350 kJ mol⁻¹Step 4: check with the formula190 − (−160) = 350 ✓Ea fwd = 190, ΔH = −160, Ea rev = 350 kJ mol⁻¹two routes to the same answer — use the second as a free check
💡 Exam tip
Label axes properly: “potential energy” or “enthalpy, H” going up, and “reaction coordinate” (not “time”) going across.
Draw dashed levels first, then add arrows. It stops you measuring from the wrong place.
Arrows need heads at both ends for Ea, and should touch exactly the two levels they connect.
ΔH needs a sign; Ea never does. Ea is always a positive number.
If the question gives absolute enthalpies, just subtract: always “where you finish minus where you start”.
Calculating Ea from experimental data is HL only, but reading it off a diagram is required at both levels.
⚠ Common mix-up
Measuring Ea from the x-axis. It runs from the reactants level to the peak, not from zero.
Measuring ΔH from the peak. ΔH connects reactants to products and ignores the peak completely.
Assuming a big Ea means endothermic. They are unrelated — an exothermic reaction can have a huge barrier.
Thinking the reverse reaction has the same Ea. Same peak, different starting level, so different climb.
Forgetting the sign of ΔH in the reverse formula. That single slip flips your answer from 160 to −40.
Calling the peak an intermediate. The peak is a transition state; intermediates sit in dips.
Labelling the x-axis “time”. It is the reaction coordinate — how far along the change you are.
Up next: Energy Profiles With and Without Catalysts — what happens to that hill when you add a catalyst, and why the products end up in exactly the same place.
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