Gibbs free energy
Gibbs free energy combines enthalpy and entropy: ΔG = ΔH − TΔS. A reaction is spontaneous if ΔG is negative, at equilibrium if ΔG = 0, and non-spontaneous if ΔG is positive. T is in kelvin.
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Analogy
A balance of two forces: heat release pushes a reaction forward, entropy gain pushes it too, and temperature sets how much entropy counts.
Example
ΔH = +178 kJ, ΔS = +161 J/KΔG = 0 when T = ΔH/ΔST = 178000 / 161T ≈ 1106 K (833 °C)
In practice
Chemists use ΔG to predict the temperature at which limestone breaks down into lime and CO₂ in a kiln.
Practice
Flashcards
- Give the Gibbs equation.ΔG = ΔH − TΔS.
- When is a reaction spontaneous at all temperatures?When ΔH is negative and ΔS is positive.
Interview questions
- Explain how ΔH and ΔS combine to decide whether a reaction is spontaneous at different temperatures.
- ΔG = ΔH − TΔS
- Spontaneous if ΔG is negative
- Four sign combinations
- Temperature matters only when ΔH and ΔS have the same sign
Questions people ask
What happens when ΔH and ΔS are both positive?
The reaction absorbs heat but gains entropy. At low temperature the ΔH term dominates, so ΔG is positive and the reaction is non-spontaneous. At high temperature the −TΔS term wins and ΔG turns negative. Such reactions become spontaneous when hot, like decomposing limestone.
Does negative ΔG mean a reaction is fast?
No. ΔG only tells you whether a reaction is thermodynamically favourable, not how fast it is. A large activation energy can make a favourable reaction extremely slow. Petrol in air has a large negative ΔG, but it does not burn until you light it.