Atlas MCAT Course
Chemical Foundations → Chemical Equilibrium & Le Châtelier’s Principle
Chemical Equilibrium & Le Châtelier’s Principle
Predicting direction, not speed.
01The idea
Many reactions in biology and chemistry do not go to completion. Instead, they reach a stable balance where reactants and products coexist. This balance is called equilibrium.
If you can predict directional shifts, you can reason through acid–base balance, gas exchange, ligand binding, metabolic reversibility, and physiological buffering.
THREAD · Energy vs. rate
02Build the model
AWhat equilibrium actually means
At equilibrium:
- Forward reaction rate = reverse reaction rate
- Concentrations are constant over time
- Molecules are still reacting
BThe equilibrium constant: position, not preference
The equilibrium constant (K) describes the ratio of products to reactants at equilibrium. It tells you whether products or reactants are favored once balance is reached.
It does not tell you how fast the reaction occurs, or whether the reaction is biologically useful.
CΔG vs. equilibrium: clearing a major confusion
| Idea | Tells you |
|---|---|
| ΔG | Direction right now |
| Equilibrium (K) | Final balance point |
A reaction can be spontaneous initially (ΔG < 0) and still stop once equilibrium is reached.
This distinction is critical for metabolism.
DReaction quotient (Q): where you are right now
The reaction quotient (Q) uses the same ratio as K, but reflects current conditions, not equilibrium.
- Q < K → reaction proceeds forward
- Q > K → reaction proceeds backward
- Q = K → system is at equilibrium
ELe Châtelier’s principle: how systems respond to stress
This is not a rule to memorize — it is a stability principle.
FHow equilibria respond to changes
| Change | Response | Changes K? |
|---|---|---|
| Add reactant | Shifts toward products | No |
| Remove product | Shifts toward products | No |
| Raise pressure (gases) | Favors fewer gas molecules | No |
| Lower pressure (gases) | Favors more gas molecules | No |
| Change temperature | Heat acts as reactant or product | Yes |
For concentration changes, the system moves to restore balance. Pressure effects will matter later for lung physiology and gas exchange.
GCatalysts: what they do not do
Catalysts lower activation energy and speed up the approach to equilibrium. They do not change K, and they do not shift equilibrium position.
This unifies enzyme kinetics, metabolism, and physiology.
HBiological systems exploit non-equilibrium
Living cells rarely sit at equilibrium. Instead, they maintain gradients, continuously input energy, and operate in steady states, not equilibrium.
This connects directly to ion gradients, membrane potentials, and ATP hydrolysis.
03See the connections
The lesson names these links itself. Read the map left to right.
Where it settles
- K (equilibrium position)
- Changed only by temperature
Where it is now
- Q compared with K
- ΔG: direction right now
How it responds
- Le Châtelier: counteract the stress
- Catalysts change speed, not position
Next: fluids, pressure, and flow.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A reaction at equilibrium has K much greater than 1. What can you conclude?
Think first.
For a reaction, Q is currently greater than K. What happens next?
Think first.
An enzyme is added to a reaction mixture. What changes?
Think first.
Which of these changes the equilibrium constant itself?
05MCAT lens
The MCAT emphasizes
Directional prediction
Conceptual Le Châtelier reasoning
The equilibrium vs. rate distinction
You are rarely asked to compute K numerically or solve ICE tables. You are often asked to predict shifts after a disturbance.
06Common traps
Trap 1: Equilibrium means equal concentrations
Misconception
It means equal rates.
Correct model
Ratios depend on K.
Trap 2: Catalysts favor products
Misconception
They favor speed.
Correct model
Position is unchanged.
Trap 3: Reactions stop at equilibrium
Misconception
They continue dynamically.
Correct model
Net change stops.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
What equilibrium represents
Check against 02 · A: equal forward and reverse rates, with reactions still running.
What K tells you (and what it doesn’t)
Check against 02 · B: position, not speed or usefulness.
How Q predicts direction
Check against 02 · D: compare Q with K.
How systems respond to stress
Check against 02 · E and F: they shift to counteract the disturbance.
Why biology avoids equilibrium
Check against 02 · H: life runs in energy-fed steady states.
If something feels unclear, the missing idea is usually thisEquilibrium describes balance — life describes controlled imbalance.
08Remember
REMEMBER · if you keep only a few things
- Equilibrium: equal rates, constant concentrations, still reacting.
- K tells position, not speed.
- ΔG gives direction now; the reaction stops changing at ΔG = 0.
- Q < K forward; Q > K backward; Q = K equilibrium.
- Le Châtelier: the system counteracts the disturbance.
- Only temperature changes K. Catalysts change speed only.
- Life stays away from equilibrium.
09Forward hook
You now have acid–base control, redox logic, metabolism, electrochemistry, and equilibrium and Le Châtelier. This completes the core chemistry for the Chem/Phys section. The next step is classical physics applied to biology:
Why do fluids move, and what controls how easily they move?
That starts with fluids, pressure, and flow.
Next lessonElectrochemistry→