Activation energy and the Arrhenius equation
Activation energy (Ea) is the minimum energy needed for a collision to react. The Arrhenius equation, k = Ae^(−Ea/RT), shows k grows with temperature and falls as Ea rises. A is the frequency factor.
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Analogy
A hill between two valleys: molecules need enough energy to climb over the top before rolling down.
Example
ln(k₂/k₁) = (Ea/R)(1/T₁ − 1/T₂)= (50000/8.314)(1.09×10⁻⁴)= 0.655k₂/k₁ ≈ 1.9 (about double)
In practice
A high Ea is why petrol does not ignite by itself in the tank but burns fast once a spark gives the first push.
Practice
Flashcards
- What is activation energy?The minimum energy needed for a collision to lead to reaction.
- How do you get Ea from a graph?Plot ln k against 1/T. Slope = −Ea/R.
Questions people ask
How do I find Ea from experimental data?
Measure k at several temperatures. Plot ln k against 1/T (in kelvin). The graph is a straight line with slope −Ea/R. Multiply the slope by −R, where R = 8.314 J/(mol·K), to get Ea in joules per mole. Divide by 1000 if you want kilojoules per mole.
Is activation energy the same as ΔH?
No. Ea is the height of the energy barrier from reactants to the peak. ΔH is the difference in energy between reactants and products. A reaction can be strongly exothermic yet have a large Ea, which is why it needs a spark to start.