Spontaneous and Non-Spontaneous Reactions
Understanding Natural Processes, Thermodynamic Criteria, and Real-World Examples
Reactant A
Reactant B
Product AB
Energy Input
Spontaneous – Reaction proceeds naturally
0% reaction complete
A spontaneous process occurs autonomously, transitioning from a non-equilibrium state to an equilibrium state without external intervention.
- (i) Water flow: Water flows from higher to lower elevation.
- (ii) Acid-base neutralisation: NaOH + HCl → NaCl + H₂O
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
A reaction is spontaneous if it requires initial energy but then proceeds autonomously. Combustion of coal is spontaneous once ignited.
Non-Spontaneous Processes
A non-spontaneous process does not occur on its own without external energy input.
- (i) Pumping water uphill (requires work).
- (ii) Refrigerator heat transfer (requires electrical work).
- (iii) N₂ + O₂ → 2NO (requires lightning energy).
N2(g) + O2(g) → 2NO(g) (non-spontaneous)
Thermodynamic Criterion
Spontaneity at constant temperature and pressure is determined by Gibbs free energy (ΔG):
ΔG = ΔH – TΔS
- ΔG < 0 → Spontaneous
- ΔG > 0 → Non-spontaneous
- ΔG = 0 → Equilibrium
Examples
- Spontaneous: Dissolution of salt, rusting of iron, expansion of gas.
- Non-spontaneous: Electrolysis of water, photosynthesis, charging a battery.
Complete Lecture: Spontaneous vs Non-Spontaneous Reactions
Key Takeaways
- Spontaneous processes occur naturally without external energy input after initiation.
- Non-spontaneous processes require continuous energy input from an external source.
- The sign of ΔG determines spontaneity.
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