The spontaneity of a reaction—whether it occurs naturally without external energy—is determined mainly by thermodynamic factors. The key factors are:
1. Enthalpy Change (ΔH)
- Exothermic reactions (ΔH < 0) release heat, which usually favors spontaneity.
- Endothermic reactions (ΔH > 0) absorb heat and are less likely to be spontaneous unless other factors compensate.
2. Entropy Change (ΔS)
- Entropy measures the disorder or randomness of a system.
- Increase in entropy (ΔS > 0) favors spontaneity.
- Decrease in entropy (ΔS < 0) can make a reaction non-spontaneous unless compensated by enthalpy.
3. Temperature (T)
- Temperature influences the impact of entropy on free energy.
- High temperature favors reactions where entropy increases.
- Low temperature favors reactions where enthalpy decrease dominates.
4. Gibbs Free Energy (ΔG)
- ΔG combines enthalpy, entropy, and temperature to determine spontaneity.
- ΔG < 0 → reaction is spontaneous.
- ΔG > 0 → reaction is non-spontaneous.
- ΔG = 0 → reaction is at equilibrium.
5. Pressure and Concentration (for gases)
- Changes in pressure or reactant/product concentrations can shift equilibrium and affect spontaneity for gas-phase reactions.