Free Energy
High-Yield Summary
- Chemical kinetics studies reaction rates and the factors that influence them — separate from thermodynamics, which studies whether a reaction occurs at all.
- Gibbs free energy: ΔG = ΔH − TΔS. Negative ΔG → spontaneous; positive ΔG → non-spontaneous.
- Spontaneous does NOT mean fast — spontaneity says nothing about reaction rate.
- Thermodynamics (will it happen?) and kinetics (how fast?) are independent questions, answered by different tools.
Gibbs Free Energy
ΔG = ΔH − TΔS
- ΔG = Free energy change; negative = spontaneous, positive = non-spontaneous
- ΔH = Enthalpy change (heat content)
- T = Temperature (K)
- ΔS = Entropy change (disorder)
Thermodynamics vs. Kinetics
| Thermodynamics | Kinetics |
|---|---|
| Whether a reaction will occur / is energetically favorable | How fast a reaction proceeds once it starts |
| Described by ΔG | Described by a rate equation |
| Tells you nothing about rate | Tells you nothing about spontaneity |
Must-Know Points
- Nearly every biochemical reaction that sustains life is spontaneous (ΔG < 0), yet many proceed too slowly to matter without a catalyst like an enzyme.
- A spontaneous reaction can take microseconds or millennia — ΔG's sign says nothing about timescale.
Common MCAT Trap
- Never assume a spontaneous (negative ΔG) reaction is automatically fast — spontaneity and rate are answered by completely separate branches of chemistry.
- Don't reach for a rate equation to judge favorability, or for ΔG to judge speed — each tool answers only its own question.
Quick Recall
If ΔG for a reaction is negative, can you conclude the reaction is fast?
Why do many spontaneous biochemical reactions require enzymes?
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