The steric factor p in the collision theory rate constant is always ≤ 1. Which statement best explains why?
AMolecules are approximately spherical, so all orientations are equally reactive.
BOnly collisions with favorable relative geometry can lead to bond rearrangement, so most collisions are unproductive even above the energy threshold.
CTemperature limits the fraction of molecules with enough kinetic energy to react.
DThe reactive cross-section equals the geometric cross-section for most molecules.
Energy above E_a is necessary but not sufficient. For a reaction to occur, the attacking atom or group must approach the correct site on the target molecule — a geometric constraint captured by p. For small, symmetric molecules p ≈ 1, but for complex molecules where a specific bond must be attacked, p can be many orders of magnitude smaller than 1.
Question 2 True / False
According to collision theory, raising the temperature increases the reaction rate solely because more collisions occur per unit time.
TTrue
FFalse
Answer: False
Temperature does increase collision frequency, but only as Z ∝ T^(1/2) — a weak effect. The dominant contribution is through the Boltzmann factor exp(−E_a/RT): as T rises, the fraction of collisions with energy exceeding E_a grows exponentially. This exponential term is why even small temperature increases can dramatically accelerate reactions.
Question 3 Short Answer
Collision theory predicts a pre-exponential factor A from first principles, yet experimental A values for complex bimolecular reactions are often far smaller than the collision-theory prediction. What physical quantity accounts for this discrepancy, and what does it represent?
Think about your answer, then reveal below.
Model answer: The steric factor p (0 < p ≤ 1). It represents the fraction of collisions that have the correct relative orientation of reactants for the transition to products. A_experimental = p × A_collision theory. For reactions requiring precise alignment — such as an SN2 backside attack — p can be ≪ 1, explaining the large gap between predicted and observed pre-exponential factors.
Collision theory counts all collisions above E_a regardless of approach geometry. Introducing p as a correction factor salvages the Arrhenius form but exposes collision theory's key limitation: it does not model the orientational requirements from a potential energy surface. Transition state theory handles this more rigorously.