Gibbs Free Energy Explained Simply

2. Entropy (ΔS)

• Positive ΔS → increased disorder → promotes spontaneity
• Negative ΔS → decreased disorder → opposes spontaneity

3. Temperature (T)

Temperature determines whether the entropy term (TΔS) outweighs the enthalpy term.

The Four Key Spontaneity Cases

IB Chemistry often tests these four scenarios:

1. ΔH negative, ΔS positive

• Always spontaneous
• ΔG is always negative
  Examples: combustion reactions

2. ΔH positive, ΔS negative

• Never spontaneous
• ΔG always positive
  Examples: decompression of gases in reverse, some precipitation reactions

3. ΔH negative, ΔS negative

• Spontaneous at low temperatures
• Enthalpy helps; entropy opposes
  Example: freezing water

4. ΔH positive, ΔS positive

• Spontaneous at high temperatures
• Entropy helps overcome endothermicity
  Example: melting ice

Temperature often determines which term dominates.

What Does ΔG Tell Us About a Reaction?

Gibbs free energy answers three essential questions:

1. Will the reaction occur on its own?

If ΔG < 0, the reaction is thermodynamically favorable.

2. How does temperature affect feasibility?

Raising or lowering temperature can change ΔG by affecting TΔS.

3. Is the system at equilibrium?

At equilibrium, ΔG = 0 and no net reaction occurs.

Calculating ΔG Using Standard Values

IB data booklet provides standard Gibbs free energies of formation (ΔGf°).

You can calculate reaction ΔG° using:

ΔG° = ΣΔGf°(products) – ΣΔGf°(reactants)

This method is common in Paper 2 questions.

Why Gibbs Free Energy Is Better Than ΔH Alone

A reaction may be:

• Endothermic (ΔH positive) but still spontaneous
• Exothermic (ΔH negative) but non-spontaneous if entropy decreases enough

ΔG considers both energy and disorder, making it more complete than enthalpy alone.

Real-World Examples

1. Melting Ice at 25°C

• ΔH positive
• ΔS positive
• TΔS > ΔH → ΔG negative → spontaneous

2. Rusting of Iron

• ΔG negative
• Very slow (high activation energy)
  Shows spontaneity ≠ speed

3. Photosynthesis

• ΔG positive
• Requires sunlight
  Example of a non-spontaneous reaction driven by external energy

FAQs

Does spontaneous mean fast?

No. Thermodynamics determines whether a reaction is possible; kinetics determines how fast it happens.

Can ΔG change with temperature?

Yes. Because of the TΔS term, spontaneity can switch depending on temperature.

What does ΔG = 0 mean?

The system is at equilibrium—no net change occurs.

Conclusion

Gibbs free energy brings together enthalpy, entropy, and temperature to determine whether a chemical reaction is spontaneous. By applying the equation ΔG = ΔH – TΔS, you can analyze reaction feasibility, predict temperature dependence, and better understand the thermodynamic forces that drive chemical change. Mastering this concept is essential for achieving high marks in IB Chemistry.

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