Atlas MCAT Course
Physical Foundations → Gibbs Free Energy & Spontaneity
Gibbs Free Energy & Spontaneity
When processes actually occur.
01The idea
The First Law tells us energy is conserved. The Second Law tells us entropy increases.
Neither alone tells us whether a specific process will occur under biological conditions.
THREAD · Energy vs. rate
Gibbs free energy integrates both into a single criterion.
02Build the model
AWhat Gibbs free energy represents
Gibbs free energy (G) represents the portion of a system’s energy that is available to do useful work at constant temperature and pressure — the conditions of most chemical and biological systems.
Conceptually:
This equation is not a formula to memorize — it is a competition.
- ΔH → energetic favorability
- ΔS → entropic favorability
- T → weighting factor
BThe three outcomes
- ΔG < 0 → spontaneous (exergonic)
- ΔG = 0 → equilibrium
- ΔG > 0 → non-spontaneous (endergonic)
CTemperature dependence
Temperature determines how much entropy contributes.
- At low T → enthalpy dominates
- At high T → entropy dominates
This explains why:
- Some reactions become spontaneous only at high temperature
- Others become non-spontaneous as temperature rises
DFour canonical ΔH / ΔS scenarios
| ΔH | ΔS | Spontaneity |
|---|---|---|
| − | + | Always spontaneous |
| + | − | Never spontaneous |
| − | − | Spontaneous at low T |
| + | + | Spontaneous at high T |
EFree energy and equilibrium
At equilibrium:
- ΔG = 0
- Forward and reverse rates are equal
This idea underlies:
- Chemical equilibria
- Biological steady states
FBiological coupling: making the unfavorable happen
Many biological reactions have ΔG > 0 and would not occur alone.
Cells solve this by coupling:
- An unfavorable reaction
- To a highly favorable one (e.g., ATP hydrolysis)
If the total ΔG is negative, the process proceeds.
03MCAT lens
The MCAT emphasizes
Sign reasoning over calculation
Biological relevance of coupling
ΔG as a predictor of direction, not rate
You are rarely asked to compute ΔG. You are often asked to interpret what a sign or change implies.
04Common traps
Trap 1: Negative ΔG means fast
Misconception
Kinetics are separate.
Correct model
ΔG predicts direction, not speed.
Trap 2: Endergonic reactions cannot occur
Misconception
Coupling enables them.
Correct model
Net ΔG governs behavior.
Trap 3: Equilibrium means no activity
Misconception
It means no net change.
Correct model
Dynamic balance persists.
05Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
If something feels unclear, the missing idea is usually thisSpontaneity reflects the trade-off between energy release and entropy gain.
06Forward hook
Gibbs free energy tells us whether a process can occur. The next question is:
How fast does it occur, and what controls the rate?
That leads directly to reaction kinetics, activation energy, and catalysis.
Next lessonOscillations & Simple Harmonic Motion→