Activation energy and reaction pathways
Activation energy
- Particles don't react every time they meet.
- The activation energy ($E_a$) is the least energy colliding particles must have to react.
- It is like a hill the particles must get over.
Practice
The activation energy is:
Activation energy is the energy barrier (the "hill") that particles must get over to react.
Reaction pathway diagrams
- A reaction pathway diagram plots energy (up) against the progress of the reaction.
- It starts at the reactants, rises over a hill (the height = $E_a$), then ends at the products.
- The gap between reactant and product levels = $\Delta H$.
- Exothermic: products lower than reactants ($\Delta H < 0$). Endothermic: products higher ($\Delta H > 0$).
Practice
On a pathway diagram for an exothermic reaction, the products are:
Exothermic reactions release energy, so the products sit lower than the reactants (ΔH negative).
Practice
The enthalpy change ΔH is the gap between the reactant and product energy levels.
ΔH is the difference between the energy of the products and that of the reactants.
You've got it
Key idea
- activation energy $E_a$ = the minimum energy to react (the "hill")
- a pathway diagram: reactants → over the $E_a$ hill → products
- exothermic ends lower ($\Delta H<0$); endothermic ends higher ($\Delta H>0$)