What is Activation Energy?
Activation energy (Ea) is the minimum energy barrier that reactants must overcome to form products in a chemical reaction. It is independent of whether the reaction is exothermic or endothermic, and catalysts work by lowering it.
Activation energy is the energy required to start a chemical reaction — the height of the energy barrier on a reaction-coordinate diagram. Lowering Ea (via a catalyst) speeds up reactions without changing ΔH.
Step-by-step worked examples
A reaction has Ea = 50 kJ/mol and ΔH = −100 kJ/mol. Is it exothermic? Will it be fast?
ΔH < 0 → exothermic (releases energy). Ea = 50 kJ/mol is moderate (not very low). Speed depends on Ea, not ΔH. This reaction is thermodynamically favorable but not necessarily fast at room temp.
Adding a catalyst lowers Ea from 80 kJ/mol to 30 kJ/mol. Does ΔH change?
ΔH is the difference: products − reactants energy. A catalyst changes the pathway, not the start/end points. ΔH remains the same; only Ea decreases → reaction speeds up.
Why doesn't wood burn at room temperature even though combustion is highly exothermic?
Combustion is exothermic (ΔH < 0) but has very high Ea. At room temp, molecules lack energy to overcome Ea. Heating (match/spark) provides energy to start the reaction.
Flashcards
Quick quiz
Q1.A reaction has ΔH = −50 kJ and Ea = 200 kJ. At room temp?
Q2.A catalyst lowers Ea. What else changes?
Q3.Why do hot reactants react faster?
Q4.Activation energy is
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Common mistakes
Lowering ΔH speeds up a reaction. — Correct: Lowering Ea speeds up a reaction. ΔH affects spontaneity, not speed.
Catalysts are consumed in reactions. — Correct: Catalysts are regenerated; they lower Ea but are not consumed.
High Ea means impossible reaction. — Correct: High Ea means slow at low temp. Heat or catalyst can enable it.
All exothermic reactions are fast. — Correct: Exothermic favors products (ΔH < 0) but speed depends on Ea.
FAQ
What is activation energy definition?
The minimum energy reactants must have to overcome the energy barrier and form products in a chemical reaction.
Why is activation energy important?
It determines how fast a reaction occurs. Lower Ea = faster reaction, independent of whether ΔH is favorable.
What are activation energy examples?
Combustion has high Ea (needs spark). Enzyme-catalyzed reactions have low Ea. Rusting has moderate Ea (slow at room temp).
How do catalysts lower activation energy?
Catalysts provide an alternative reaction pathway with a lower energy barrier, speeding up the reaction without changing ΔH.