v) The transition state resembles a mountain pass rather than the top of an energy
hill.
vi) The reactants and products appear to be separated by an energy barrier
resembling a mountain range.
vii) Transition state lies at the top of the route that requires the lowest energy
climb. Whether the pass is a wide or narrow one depends on ∆S‡.
viii) A wide pass means that there is a relatively large number of orientations of
reactants that allow a reaction to take place.
- Reaction rate versus temperature:
- Most chemical reactions occur much more rapidly at higher temperatures ⇒ For
many reactions taking place near room temperature, a 10 °C increase in
temperature will cause the reaction rate to double.
i) This dramatic increase in reaction rate results from a large increase in the
number of collisions between reactants that together have sufficient energy to
surmont the barrier (∆G‡) at higher temperature.
- Maxwell-Boltzmann speed distribution:
i) The average kinetic energy of gas particles depends on the absolute
temperature.
KEav = 3/2 kT
k: Boltzmann’s constant = R/N 0 = 1.38 × 10–23 J K–1
R = universal gas constant
N 0 = Avogadro’s number
ii) In a sample of gas, there is a distribution of velocities, and hence there is a
distribution of kinetic energies.
iii) As the temperature is increased, the average velocity (and kinetic energy) of
the collection of particles increases.
iv) The kinetic energies of molecules at a given temperature are not all the same
⇒ Maxwell-Boltzmann speed distribution:
