Atlas MCAT Course
Chemical Foundations → Reaction Kinetics & Activation Energy
Reaction Kinetics & Activation Energy
Why speed is separate from spontaneity.
01The idea
A reaction can be thermodynamically favorable and still not happen on a useful timescale.
Diamonds converting to graphite is favorable — yet effectively never happens.
THREAD · Energy vs. rate
The MCAT frequently tests whether you can keep these ideas separate.
02Build the model
AReaction rate: a measure of speed
Reaction rate describes how quickly reactants are converted to products.
Rate depends on:
- Frequency of effective collisions
- Energy of those collisions
- Proper molecular orientation
BActivation energy: the barrier to reaction
Activation energy (Eₐ) is the minimum energy required to reach the transition state.
Even favorable reactions must:
- Overcome this barrier
- Rearrange bonds and electron density
CThe transition state: the point of no return
The transition state:
- Is a high-energy, unstable configuration
- Is not a reaction intermediate
- Represents the peak of the energy barrier
Once reached, products form spontaneously.
DEnergy diagrams: visualizing kinetics
Energy diagrams show:
- Relative energy of reactants and products
- Height of the activation energy barrier
Key interpretations:
- Taller barrier → slower reaction
- ΔG determines favorability
- Eₐ determines rate
EFactors that affect reaction rate
E.1Temperature
Higher temperature:
- Increases average kinetic energy
- Increases fraction of molecules exceeding Eₐ
Result: faster reaction rates.
E.2Concentration
Higher concentration:
- Increases collision frequency
- Increases rate (for most reactions)
E.3Surface area
Greater surface area:
- Exposes more reactant particles
- Increases collision opportunities
E.4Catalysts: changing the path, not the destination
Catalysts:
- Lower activation energy
- Provide alternative reaction pathways
- Are not consumed
FEnzymes as biological catalysts
Enzymes:
- Are highly specific catalysts
- Stabilize the transition state
- Lower Eₐ dramatically
They achieve this by:
- Proper orientation
- Strain induction
- Microenvironment optimization
This sets the stage for enzyme kinetics.
03See the connections
The lesson names these links itself. Read the map left to right.
Two separate questions
- Thermodynamics: can it occur? (ΔG)
- Kinetics: how fast? (Eₐ)
Rate depends on
- Collision frequency
- Collision energy
- Molecular orientation
- Activation energy
Temperature, concentration, surface area, and catalysts change the rate.
Enzymes are biological catalysts
- Stabilize the transition state
- Lower Eₐ dramatically
- Orient, strain, and tune the microenvironment
This leads directly to rate laws, reaction order, and half-life.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
Diamond turning into graphite is thermodynamically favorable. Why doesn’t it happen on a useful timescale?
Think first.
A catalyst is added to a reaction. What changes?
Think first.
Two reactions release the same overall amount of energy. Reaction A has a taller activation energy barrier than reaction B. Which is faster?
Think first.
On an energy diagram, what is the transition state?
05MCAT lens
The MCAT emphasizes
Separating kinetics from thermodynamics
Interpreting energy diagrams
Understanding catalytic effects
You are rarely asked to calculate rates. You are often asked to explain why a reaction speeds up or slows down.
06Common traps
Trap 1: Favorable reactions occur quickly
Misconception
Kinetics is independent.
Correct model
Barriers matter.
Trap 2: Catalysts make reactions more favorable
Misconception
They affect rate, not ΔG.
Correct model
Pathway changes, endpoints don’t.
Trap 3: The transition state is a stable intermediate
Misconception
It is unstable and fleeting.
Correct model
It is a peak, not a valley.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why some favorable reactions are slow
Check against 02 · B and D: even favorable reactions must overcome the activation energy barrier. Eₐ determines rate; ΔG determines favorability.
Why increasing temperature speeds reactions
Check against 02 · E (Temperature): higher temperature increases the fraction of molecules exceeding Eₐ.
Why catalysts don’t affect equilibrium
Check against 02 · E (Catalysts): they lower Eₐ and open an alternative pathway, but do not change ΔG or equilibrium.
Why enzymes are essential for life
Check against 02 · F: enzymes are highly specific catalysts that stabilize the transition state and lower Eₐ dramatically.
If something feels unclear, the missing idea is usually thisReaction speed is controlled by barriers, not by favorability.
08Remember
REMEMBER · if you keep only a few things
- Thermodynamics (ΔG) says whether a reaction can occur. Kinetics (Eₐ) says how fast.
- Even favorable reactions have to climb the activation energy barrier.
- The transition state is a peak, not a valley, and not an intermediate.
- Higher temperature, higher concentration, and more surface area speed reactions up.
- Catalysts lower Eₐ. They don’t change ΔG or equilibrium.
- Enzymes are specific biological catalysts that stabilize the transition state.
09Forward hook
Reaction kinetics explains chemical speed. The next question is:
How do we quantitatively describe reaction rates and their dependence on conditions?
That leads directly to rate laws, reaction order, and half-life.
Next lessonRate Laws & Reaction Order→