A spontaneous reaction is a chemical or physical process that occurs naturally on its own without needing continuous external energy once it has started.
Key points:
- Does not mean “fast” → a reaction can be slow but still spontaneous.
- Driven by the thermodynamic favorability of the reaction.
- Determined by Gibbs free energy (G):
- If ΔG < 0 → reaction is spontaneous.
- If ΔG > 0 → reaction is non-spontaneous.
Examples:
- Rusting of iron → occurs slowly in the presence of oxygen and moisture.
- Ice melting at room temperature → spontaneous because it increases entropy.
- Combustion of gasoline → spontaneous when ignited (requires initial spark).
Key concept:
Spontaneous reactions tend to increase the disorder (entropy) of the universe and/or release free energy.