Atlas MCAT Course
Equations & How to Use Them → Equations: Reactions
Equations: Reactions
Spontaneity, free energy, and rates.
01The idea
Energy transfers, equilibrium, or spontaneity: will it happen? And separately: how fast, what changes the rate, catalysis, temperature effects.
THREAD · Energy vs. rate
02Build the model
AGibbs free energy: will a process occur?
Use when: spontaneity is questioned; temperature matters; chemical or biochemical reactions.
Passage cues: “Spontaneous”, “Coupled reactions”, “Equilibrium”.
Don’t use it for: rate questions (kinetics!).
BEquilibrium and free energy: why reactions stop
Use when: the reaction has reached equilibrium; forward and reverse rates match.
Passage cues: “Equilibrium”, “No net change”.
CFree energy and concentration: how concentration gradients drive reactions
Use when: non-standard conditions; concentration effects dominate; biochemical pathways.
Passage cues: “Not standard conditions”, “Cellular environment”.
DChemical potential (conceptual): why particles diffuse
ERate law (general form): rate depends on reactant concentrations
Use when: “rate doubles when…” style problems; determining reaction order from data (often conceptual).
FArrhenius equation: how temperature affects the rate constant
Use when: temperature dependence of rate is explicit; activation energy comparisons.
Don’t use it for: pure “equilibrium/spontaneity” questions (that’s thermo).
GMichaelis–Menten: enzyme kinetics relationship
Use when: enzyme saturation; “at low [S] vs high [S]” reasoning.
03See the connections
The lesson names these links itself. Read the map left to right.
Whether
- ΔG = ΔH − TΔS
- ΔG = ΔG° + RT ln Q
- ΔG = 0 at equilibrium
How fast
- Rate law
- Arrhenius
Enzymes
- Michaelis–Menten
Next: equilibrium, acids, and solutions.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A passage asks whether a reaction will occur at a given temperature. Which equation?
Think first.
Temperature rises and a reaction speeds up. Which equation explains the change in k?
Think first.
A reaction with positive ΔG° runs forward in a cell. How?
Think first.
Where do the exponents in a rate law come from?
05MCAT lens
MCAT shortcuts
Negative ΔG → allowed to happen
Equilibrium = no driving force left
Concentration can overpower standard ΔG
Exponents come from experiment, not stoichiometry
Higher T → larger k; higher Ea → smaller k
06Common traps
Trap 1: Confusing spontaneity with speed
Misconception
Thermodynamics ≠ kinetics.
Correct model
ΔG says whether; rate laws say how fast.
Trap 2: Forgetting temperature units
Misconception
Gas and entropy equations require Kelvin.
Correct model
Convert to K before using T.
Trap 3: Assuming equilibrium means equal concentrations
Misconception
It means equal rates, not amounts.
Correct model
ΔG = 0, not equal amounts.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
How ΔG predicts spontaneity
Check against 02 · A.
How concentration changes ΔG
Check against 02 · C.
How temperature changes rate
Check against 02 · F.
Why thermodynamics ≠ kinetics
Check against the traps.
If something feels unclear, the missing idea is usually thisThermodynamics says whether; kinetics says how fast.
08Remember
REMEMBER · if you keep only a few things
- ΔG = ΔH − TΔS; negative = spontaneous.
- ΔG = ΔG° + RT ln Q; ΔG = 0 at equilibrium.
- Gradients = stored free energy.
- rate = k[A]^m[B]^n; exponents from experiment.
- Arrhenius: higher T → larger k.
- Michaelis–Menten: saturation at high [S].
09Forward hook
Block IV connects to biochemistry (ATP coupling), physiology, chemistry (equilibrium, acid–base), and physics (energy conservation).
Which equation applies when a reaction can go forward and backward, or pH matters?
That leads to equilibrium, acids, and solutions.
Next lessonEquations: Equilibrium, Acids & Solutions→