Reaction spontaneity is a core idea in IB Chemistry Topic 5 (Energetics) and Topic 15 (HL: Energetics and Thermodynamics). 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.
Gibbs Free Energy: The Key to Spontaneity
The formula that determines spontaneity is:
ΔG = ΔH – TΔS
Where:
- ΔG = Gibbs free energy change
- ΔH = enthalpy change
- T = temperature in Kelvin
- ΔS = entropy change
Interpretation:
- ΔG < 0 → spontaneous
- ΔG > 0 → non-spontaneous
- ΔG = 0 → equilibrium
This equation is one of the most important in IB HL chemistry.
How Enthalpy Influences Spontaneity
Exothermic reactions (ΔH negative)
These tend to be spontaneous because they release energy.
Examples:
- Combustion
- Neutralization
- Many redox reactions
Endothermic reactions (ΔH positive)
These can still be spontaneous if the entropy term (TΔS) is large enough.
Example:
- Melting of ice at room temperature
- ΔH is positive (absorbs heat)
- ΔS is positive (more disorder)
- At high enough T, TΔS outweighs ΔH → spontaneous
How Entropy Influences Spontaneity
Entropy increases when:
- Solids dissolve
- Gases form
- Number of particles increases
- Temperature increases
- Molecules gain more freedom of movement
A positive ΔS favors spontaneity because nature tends toward disorder.
Examples of positive entropy processes:
- Evaporation
- Sublimation
- Dissociation into ions
Entropy often determines spontaneity at high temperatures.
How Temperature Determines the Outcome
Temperature determines whether entropy or enthalpy dominates.
Four possibilities:
- ΔH negative, ΔS positive
- Always spontaneous
- ΔG always negative
- ΔH positive, ΔS negative
- Never spontaneous
- ΔG always positive
- ΔH negative, ΔS negative
- Spontaneous only at low temperatures
- ΔH positive, ΔS positive
- Spontaneous only at high temperatures
Understanding these cases is essential for Paper 2 questions.
Examples in IB Chemistry
Dissolving ammonium nitrate
- Endothermic (ΔH positive)
- Entropy increases (ΔS positive)
- Spontaneous because TΔS > ΔH
Rusting of iron
- Exothermic and accompanied by entropy changes
- Spontaneous but slow (high activation energy)
Photosynthesis
- Non-spontaneous (ΔG positive)
- Requires sunlight energy input
These examples show how thermodynamics and kinetics differ.
FAQs
Does spontaneous mean fast?
No. Spontaneity describes thermodynamic favorability, not speed. A spontaneous reaction can be slow.
Can a reaction be spontaneous at one temperature but not another?
Yes. Temperature changes the TΔS term, which can flip the sign of ΔG.
What happens when ΔG = 0?
The reaction is at equilibrium. The forward and reverse reactions occur at equal rates.
Conclusion
Reaction spontaneity depends on the combined effects of enthalpy, entropy, and temperature through the Gibbs free energy equation. A reaction is spontaneous when ΔG is negative, meaning the process can proceed without continuous energy input. By understanding the balance between energy release and disorder, IB Chemistry students can predict reaction behavior across a wide range of chemical systems.
