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Thermodynamics: Entropy, Energy & the Direction of Chemical Reactions - Leejohnston - 11-17-2025
Thread 4 — Thermodynamics: Entropy, Energy & the Direction of Chemical Reactions
The Deep Rules That Govern All Chemical Change
Chemical reactions don’t happen randomly.
They follow strict universal laws that determine:
• whether a reaction is possible
• whether it releases or absorbs energy
• whether it happens slowly, rapidly, or not at all
• what direction equilibrium will shift
This thread explores the *thermodynamic foundations* of chemistry —
the invisible laws that decide everything.
1. Energy in Chemistry — Enthalpy (ΔH)
Enthalpy measures the heat absorbed or released during a reaction.
Exothermic (ΔH < 0)
• releases heat
• feels hot
• often favourable
Example: combustion of methane
Endothermic (ΔH > 0)
• absorbs heat
• feels cold
• requires energy input
Example: photosynthesis
Enthalpy alone does NOT determine whether a reaction will happen —
but it is one piece of the puzzle.
2. Entropy (ΔS) — The Tendency Toward Disorder
Entropy measures the number of possible arrangements of a system.
Key rules:
• gases have more entropy than liquids
• liquids have more entropy than solids
• more particles = higher entropy
• spreading out = higher entropy
• mixing increases entropy
Entropy increases in:
• melting
• evaporation
• dissolving
• reactions that produce more gas particles
Nature tends to move toward higher entropy —
but entropy alone still doesn’t decide everything.
3. Free Energy (ΔG) — The REAL Decision Maker
Gibbs Free Energy tells you whether a reaction is spontaneous.
ΔG = ΔH – TΔS
This combines:
• enthalpy
• entropy
• temperature
A reaction is spontaneous when:
ΔG < 0
A reaction is non-spontaneous when:
ΔG > 0
At equilibrium:
ΔG = 0
This single equation determines whether a reaction “wants” to happen.
4. Temperature Can Flip the Direction of a Reaction
Because ΔG depends on temperature, the same reaction can be:
• non-spontaneous at low temperature
• spontaneous at high temperature
Example:
ice melts (spontaneous) at high T
ice freezes (spontaneous) at low T
This is why some industrial reactions require heat —
they need the entropy term (TΔS) to overpower ΔH.
5. Reaction Coupling — Biology’s Genius Trick
Many reactions in biology have ΔG > 0 (not favourable).
So cells “pay” for them using highly favourable reactions.
The classic example:
ATP → ADP + Pi
This releases a large amount of energy (ΔG < 0).
Cells couple this with:
• DNA synthesis
• protein formation
• ion pumping
• muscle contraction
Life exists because cells combine unfavourable steps with favourable ones.
6. Equilibrium — The Balance Point of a Reaction
Chemical equilibrium is not “stopping.”
It’s a dynamic balance where forward and reverse rates match.
The equilibrium constant K determines the position:
• K >> 1 → products favoured
• K << 1 → reactants favoured
• K = 1 → balanced
The link between thermodynamics and equilibrium:
ΔG° = –RT ln(K)
Meaning:
• large K → very negative ΔG° (highly spontaneous)
• small K → very positive ΔG° (not spontaneous)
Thermodynamics and equilibrium are mathematically identical.
7. Why Thermodynamics Matters
Thermodynamics predicts:
• which reactions are possible
• how hot or cold a reaction must be
• whether a reaction will release or absorb energy
• how cells power their processes
• how batteries work
• how materials change state
• the limits of machines
Every chemical process — industrial or biological — is governed by ΔH, ΔS, and ΔG.
8. Master Equation Summary
The three laws of chemistry’s direction:
1. ΔH — heat flow
2. ΔS — disorder
3. ΔG — possibility
If you understand these, you understand why anything happens at all.
Written by Leejohnston & Liora — The Lumin Archive Research Division