Activation energy and reaction pathways
Introduced| English | Meaning |
|---|---|
| activation energy/ˌæktɪˈveɪʃn ˈenədʒi/ | The minimum energy that reacting particles need for a successful reaction. |
| reaction pathway diagram/rɪˈækʃn ˈpæθweɪ ˈdaɪəɡræm/ | A diagram relating a reacting system's energy to progress along a reaction pathway. |
The energy hill
- Even reactions that release energy need a little push to get started.
- That initial push is the activation energy 活化能, shown on a reaction pathway diagram 反应进程图.
The activation energy is the:
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Ea is the minimum energy to start reacting.
Particles must collide with at least the activation energy to react.
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Less energetic collisions do not react.
The minimum energy needed to start a reaction is the ______ energy.
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Ea is the energy hill that must be climbed.
Activation energy
- The activation energy (Ea) is the minimum energy needed for a reaction to start.
- Particles must collide with at least this much energy to react.
Exothermic reactions end lower than they start; endothermic reactions end higher
Activation energy
Ea = the energy barrier
Every reaction must climb the activation-energy barrier before products can form.
A reaction pathway diagram plots energy against:
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The x-axis is reaction progress.
Reaction pathway diagrams
- A reaction pathway diagram plots energy against the progress of the reaction.
- It shows the reactant level, an "energy hill" (the peak = activation energy), and the product level.
In an endothermic pathway, the products are:
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Energy is absorbed, so products are higher.
Exothermic vs endothermic pathways
- Exothermic: products below reactants (energy released).
- Endothermic: products above reactants (energy absorbed). Both have an activation-energy hill.
Worked example. Petrol is stable in air at room temperature because the activation energy is high. A spark provides that energy, the particles get over the hill, and the exothermic reaction then races ahead.
You've got it
- activation energy (Ea) = the minimum energy needed to start a reaction
- a reaction pathway diagram shows reactants, the Ea "hill", and products
- both exo- and endothermic reactions must climb the activation-energy hill first