Mistake Master
Reaction energy profiles
A reaction energy profile is a picture of the whole reaction in one curve: how high the barrier is, where the peak sits, and whether you end up lower or higher than you started. The trap is mixing up the barrier with the net change.
§1
Reading the profile.
▸
A reaction energy profile plots potential energy along the reaction coordinate. Reactants sit at one energy, climb to a peak — the transition state — then descend to products.
The climb from reactants up to the peak is the activation energy, Eₐ (the barrier height). The difference between reactants and products is the enthalpy change, ΔH. These are two different features of the same curve.
The sign of ΔH tells the thermochemistry: products lower than reactants means exothermic (ΔH < 0); products higher means endothermic (ΔH > 0). Eₐ, meanwhile, controls the rate, not the thermochemistry.
§2
Extracting Eₐ and ΔH.
▸
Measure the barrier and the net change separately.
- Locate reactants, peak, and products. Identify the starting energy, the transition-state peak, and the final energy.
- Read Eₐ as the climb to the peak. Activation energy is the height from reactants up to the transition state.
- Read ΔH as reactants to products. The enthalpy change is the final energy minus the initial energy.
- Classify the thermochemistry. Products lower → exothermic; products higher → endothermic.
§3
The pieces you'll meet.
▸
Two distinct features of one curve.
§4
Worked example: read a profile.
▸
Profile. Reactants sit at 50 kJ, the peak (transition state) at 120 kJ, and products at 30 kJ.
Activation energy. Eₐ = 120 − 50 = 70 kJ (reactants up to the peak).
Enthalpy change. ΔH = 30 − 50 = −20 kJ (products below reactants), so the reaction is exothermic.
Key point. Eₐ (70 kJ) and ΔH (−20 kJ) are read from different parts of the curve; they are not the same number and answer different questions (rate vs thermochemistry).
§5
Mistakes that cost real points.
▸
"The activation energy is the same as the enthalpy change."
Eₐ is the climb from reactants to the transition state (the barrier); ΔH is the difference between reactants and products (the net change). They are separate features, and a reaction can have a large Eₐ but a small ΔH, or vice versa.
Fix. Read Eₐ from reactants to the peak, and ΔH from reactants to products. Do not equate them.
"If products are lower than reactants, there is no activation barrier."
Even an exothermic reaction (products lower) still has an activation-energy barrier to climb first. Being downhill overall does not remove the peak; the reaction still needs enough energy to reach the transition state.
Fix. Include the barrier regardless of ΔH's sign: exothermic reactions still climb to a transition state before descending.
"The transition state is a stable intermediate you can isolate."
The transition state is the fleeting peak of the energy curve — the least stable point along the path — not a stable species you can isolate. (Intermediates, which sit in valleys, are different.)
Fix. Treat the transition state as the momentary maximum on the curve, not an isolable intermediate.
§6
Skill Check.
▸
Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.