Activation energy, denoted by \$E_a\$, is the minimum amount of energy that colliding particles must possess to overcome the energy barrier and react. Think of it as a hill that the particles must climb before they can transform into products.
In chemistry, we often write:
\$E_a = \text{minimum energy required for reaction}\$
Exam Tip: Remember that only collisions with energy ≥ \$E_a\$ lead to a reaction. Use the analogy of cars climbing a hill to explain this concept.
In an exothermic reaction, the products have lower energy than the reactants. The excess energy is released as heat (or light). The enthalpy change, \$ΔH\$, is negative.
\$ΔH < 0 \quad \text{(exothermic)}\$
Example: Combustion of methane
\$\text{CH}4 + 2\text{O}2 \rightarrow \text{CO}2 + 2\text{H}2\text{O} \quad ΔH = -890 \text{ kJ mol}^{-1}\$
Endothermic reactions absorb energy from the surroundings. The products have higher energy than the reactants, so \$ΔH\$ is positive.
\$ΔH > 0 \quad \text{(endothermic)}\$
Example: Photosynthesis
\$6\text{CO}2 + 6\text{H}2\text{O} \rightarrow \text{C}6\text{H}{12}\text{O}6 + 6\text{O}2 \quad ΔH = +2800 \text{ kJ mol}^{-1}\$
An energy diagram shows the relative energies of reactants, products, and the transition state (the top of the hill). The vertical distance from reactants to the transition state is \$E_a\$.
\$E_a = \text{height of the hill (energy barrier)}\$
The difference between reactants and products is the net enthalpy change, \$ΔH\$.
\$ΔH = H{\text{products}} - H{\text{reactants}}\$
Key points:
A catalyst provides an alternative pathway with a lower activation energy. It is like a ramp that helps cars climb the hill more easily.
\$\text{Catalyst lowers } E_a \quad \text{but } ΔH \text{ remains unchanged}\$
Exam Tip: When asked about catalysts, mention that they reduce \$E_a\$ (increase rate) but do not affect the heat of reaction (\$ΔH\$). Use the “ramp” analogy to illustrate this.
Heat flow, \$Q\$, is related to the enthalpy change:
\$Q = -ΔH\$
Thus:
| Reaction Type | ΔH Sign | Heat Flow | Emoji |
|---|---|---|---|
| Exothermic | \$ΔH < 0\$ | \$Q > 0\$ (heat released) | 🔥 |
| Endothermic | \$ΔH > 0\$ | \$Q < 0\$ (heat absorbed) | ❄️ |
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