Factors Affecting Rate Constants
High-Yield Summary
- A reaction mechanism is the actual step-by-step process behind a balanced equation; the slowest step (rate-determining step) controls the overall rate.
- Collision theory: rate depends on collision frequency, but only effective collisions (correct orientation + enough energy to clear Ea) produce product.
- Arrhenius equation: k = Ae^(−Ea/RT) — k rises with higher A (frequency factor) or higher T, falls with higher Ea.
- Transition state = high-energy, unstable peak on a free energy diagram; negative ΔG = exergonic, positive ΔG = endergonic.
- Five factors raise reaction rate: lower Ea, higher temperature, less steric hindrance, higher reactant concentration, and catalysts.
Key Terms
- Reaction mechanism
- The actual series of elementary steps by which reactants become products.
- Intermediate
- A species produced in one mechanism step and consumed in another; doesn't appear in the overall balanced equation.
- Rate-determining step
- The slowest step in a mechanism; controls the overall reaction rate.
- Effective collision
- A collision with correct orientation and sufficient energy (≥ Ea) to produce product.
- Transition state
- The high-energy, unstable configuration (activated complex) at the peak of the reaction coordinate.
Arrhenius Equation
k = Ae^(−Ea/RT)
- k = Rate constant
- A = Frequency factor — collisions with correct orientation; rises with concentration
- Ea = Activation energy
- R = Ideal gas constant
- T = Temperature (K)
- Higher A or higher T → higher k. Higher Ea → lower k.
Free Energy Diagram, Left to Right
- 1Reactants sit at a lower energy level on the x-axis (reaction progress).
- 2Energy climbs to a peak — the transition state — representing the activation energy barrier.
- 3Energy drops to the products' level; ΔG = energy(products) − energy(reactants).
- 4Negative ΔG → exergonic (spontaneous); positive ΔG → endergonic (non-spontaneous).
Five Factors Affecting Rate
| Factor | Effect on Rate |
|---|---|
| Activation energy (Ea) | Higher Ea → slower (fewer molecules clear the barrier); lower Ea → faster |
| Temperature | Higher T → more frequent/energetic collisions → faster (≈2× per 10°C, rough rule); extreme heat can denature catalysts |
| Steric hindrance | Bulky substituents block the reactive site → fewer effective collisions → slower |
| Reactant concentration | Higher concentration → more effective collisions → faster (except zero-order reactions) |
| Catalysts | Lower Ea (new pathway or stabilized transition state) without being consumed → faster |
Common MCAT Trap
- The 10°C-doubles-rate rule is only a rough generalization — don't treat it as an exact law for every reaction.
- Extremely high temperature can denature a catalyst (especially an enzyme) and sharply DECREASE rate — don't assume 'hotter is always faster' once a catalyst is involved.
- Catalysts are not consumed and don't appear in the overall balanced equation, but they DO appear in the mechanism (often as an intermediate-like species).
Quick Recall
What two conditions must a collision satisfy to be 'effective'?
According to the Arrhenius equation, what happens to k as Ea increases?
Why can raising temperature too high slow down an enzyme-catalyzed reaction?