Activation Energy
Simple Explanation
Activation energy (Ea) is the minimum energy that colliding particles must have for a reaction to occur — it is the energy "barrier" that must be overcome, even for reactions that release energy overall.
Why Do We Need It?
Activation energy explains why some exothermic reactions (like the combustion of wood) don't happen spontaneously at room temperature, even though they release energy overall — the reaction still needs an initial energy input (like a spark) to get started.
Formula
The Arrhenius Equation
k = A·e^(−Ea/RT)
How the rate constant of a reaction depends on temperature and activation energy — the mathematical basis for why raising temperature speeds up reactions so dramatically.
- k
- — the rate constant of the reaction
- A
- — the pre-exponential (frequency) factor — related to how often molecules collide with the correct orientation
- Ea
- — activation energy, in J/mol
- R
- — the gas constant, 8.314 J/(mol·K)
- T
- — absolute temperature, in kelvin (K)
When to use it: Whenever you need to explain or calculate how much a reaction rate changes with temperature, or compare the activation energies of different reactions.
Worked Example
Compare how a catalyst affects a reaction's activation energy
A reaction has an activation energy of 75 kJ/mol without a catalyst. With a catalyst, the activation energy drops to 50 kJ/mol. Explain the effect on reaction rate.
Why Does This Work?
Even reactions that release energy overall need to first break some existing bonds before new, stronger bonds can form — the activation energy is the energy needed to reach that unstable, in-between arrangement (the transition state), after which the reaction can proceed downhill in energy toward the products.
Real-Life Example
Why a match is needed to light a candle
A candle doesn't spontaneously combust sitting on a table, even though burning wax releases a lot of energy.
The combustion reaction has a significant activation energy — the heat from a lit match provides enough energy to get a small number of wax/oxygen collisions over that barrier, and the heat released by those reactions then supplies enough energy to sustain the reaction on its own.
Practice
Why doesn't a highly exothermic reaction like combustion happen spontaneously at room temperature without a spark?
MediumCommon mistake
Confusing activation energy with the overall enthalpy change (ΔH) of a reaction — Ea is always positive (it's a barrier that must be climbed) regardless of whether the reaction is exothermic or endothermic overall.
Quick Review
- Ea is the minimum energy needed for a collision to result in reaction.
- Even exothermic reactions have an activation energy barrier.
- Lowering Ea (e.g. with a catalyst) increases the reaction rate.