Atlas MCAT Course
Chemical Foundations → Rate Laws & Reaction Order
Rate Laws & Reaction Order
How conditions control reaction speed.
01The idea
Kinetics tells us that reactions face barriers.
Rate laws tell us how strongly a reaction responds when conditions change.
Two reactions can:
- Have the same ΔG
- Have similar activation energies
…and still respond very differently to changes in concentration.
It is discovered from data, not derived from stoichiometry.
02Build the model
AWhat a rate law represents
A rate law relates reaction rate to reactant concentrations:
Where:
- m and n are reaction orders
- Exponents describe sensitivity, not coefficients
BReaction order: sensitivity, not counting
B.1Zero-order reactions
- Rate is independent of reactant concentration
- Increasing concentration has no effect on rate
Interpretation:
- The system is saturated
- Something else is rate-limiting (often a catalyst or surface)
B.2First-order reactions
- Rate ∝ [A]
- Doubling concentration doubles rate
Common in:
- Unimolecular reactions
- Radioactive decay
- Many biological processes
B.3Second-order reactions
- Rate ∝ [A]² or [A][B]
- Rate is highly sensitive to concentration changes
Interpretation:
- Collision or interaction between particles is rate-limiting
B.4Overall reaction order
Overall order = sum of individual orders.
CRate laws vs. stoichiometry: a crucial separation
Stoichiometric coefficients:
- Describe balanced equations
- Do not determine rate laws
Rate laws:
- Reflect the rate-determining step
- Are determined experimentally
DIntegrated rate laws: time as a variable
Integrated rate laws describe:
- How concentration changes over time
Key MCAT focus:
- Shape of concentration vs time plots
- Linear relationships when graphed correctly
You are rarely asked to derive equations. You are often asked to recognize patterns.
EHalf-life: a powerful shortcut
Half-life is the time required for a quantity to decrease by half.
Key distinctions:
- First-order reactions: constant half-life
- Zero- and second-order reactions: variable half-life
This is heavily tested in biological and radioactive contexts.
03MCAT lens
The MCAT emphasizes
Qualitative rate changes
Pattern recognition
Identifying reaction order from behavior
You are rarely asked to compute numerical rates. You are often asked to predict outcomes after changes.
04Common traps
Trap 1: Rate laws can be read from balanced equations
Misconception
They must be measured.
Correct model
Kinetics depends on mechanism.
Trap 2: Higher order means faster reaction
Misconception
Order describes sensitivity, not absolute speed.
Correct model
Rate constant and Eₐ also matter.
Trap 3: Half-life always changes with concentration
Misconception
First-order half-life is constant.
Correct model
Order determines time behavior.
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 thisReaction order encodes sensitivity, not structure.
06Forward hook
Rate laws describe how conditions affect speed.
How do catalysts — especially enzymes — reshape rate behavior in biological systems?
That leads directly to enzyme catalysis and Michaelis–Menten kinetics.
Next lessonChemical Equilibrium & Le Châtelier’s Principle→