Reaction spontaneity is a core idea in IB Chemistry. Many students assume that “spontaneous” means “fast,” but that is not true. Reaction spontaneity depends on thermodynamic factors—not reaction rate. This article explains exactly what determines spontaneity and how to evaluate it using Gibbs free energy, enthalpy, entropy, and temperature.
What Does “Spontaneous Reaction” Mean?
A spontaneous reaction is one that can proceed without continuous external energy input once it begins.
It does not necessarily mean the reaction happens quickly.
Key points:
- Spontaneity is a thermodynamic concept.
- Rate of reaction is a kinetic concept.
- A reaction can be spontaneous but very slow (e.g., rusting).
- A reaction can be non-spontaneous but fast when driven by energy (e.g., electrolysis).
Understanding spontaneity requires examining energy and disorder.
The Three Factors That Determine Spontaneity
Reaction spontaneity depends on:
1. Enthalpy change (ΔH)
Measures heat released or absorbed.
- Negative ΔH = exothermic = often (not always) favorable
- Positive ΔH = endothermic = may still be spontaneous if entropy helps
2. Entropy change (ΔS)
Measures disorder.
- Positive ΔS (more disorder) favors spontaneity
- Negative ΔS opposes spontaneity
3. Temperature (T)
Measured in Kelvin, temperature influences the entropy term’s importance.
To combine all three effects, chemists use Gibbs free energy.
