[sodium.dioxid]

Would it be correct to say that a lower activation energy does not necessarily equate to a faster reaction? The catalyst might actually by more sterically hindered even though it requires a lower activation energy to react. So, technically you can't deduce the faster pathway by simply comparing the catalyzed reaction energy diagram and the diagram without the catalyst. Correct?

[fledarmus]

No, the steric hindrance is built into the activation energy. Overcoming a steric barrier may be one part of the reason an activation energy is high.

The activation energy is just the energy required to get the reaction to occur; it includes the energy required to overcome any unfavorable interactions.

If the starting materials are the same, a lower activation energy always equates to a faster reaction.

[sodium.dioxid]

No, the steric hindrance is built into the activation energy. Overcoming a steric barrier may be one part of the reason an activation energy is high.

My book implies something different.

"Not all collisions with energies greater than E_a result in a reaction." There is "an additional factor, p, the steric factor" which has a hand in reaction rate.

[ramboacid]

We can consider this problem by examining the Arrhenius Equation:
$$k\,=\,Ae^{-\frac{E_a}{RT}}$$
The Arrhenius equation relates the rate, k, of a reaction to the activation energy, E_a, and the temperature, T. R is the ideal gas constant, and A is the Arrhenius constant, a pre-exponential factor.

It is the pre-exponential factor A that accounts for steric hindrance. According to collision theory, the value of A is short for
$$P\,\sigma\,\bar{v}_{rel}\,N_{A}^{2}$$
where

• P is a steric factor that accounts for the need for the molecules to be in the proper orientation
• [itex]\sigma[/itex] is the cross-sectional area of the molecule in question
• [itex]\bar{v}_{rel}[/itex] is the mean relative speed at which molecules approach each other in a gas
• [itex]N_{A}[/itex] is Avogadro's number

Therefore, the activation energy doesn't solely account for the rate of the reaction. Other factors are accounted for in the Arrhenius constant. (Chemical Principles: the Quest for Insight, 4th Ed., by Atkins and Jones)

I'm sorry if the [itex]\LaTeX[/itex] is difficult to read - this is the first time I'm using it on the forum.

[sodium.dioxid]

Therefore, the activation energy doesn't solely account for the rate of the reaction. Other factors are accounted for in the Arrhenius constant. (Chemical Principles: the Quest for Insight, 4th Ed., by Atkins and Jones)

That's what I was getting at.

[Jorriss]

No, the steric hindrance is built into the activation energy. Overcoming a steric barrier may be one part of the reason an activation energy is high.

My book implies something different.

"Not all collisions with energies greater than E_a result in a reaction." There is "an additional factor, p, the steric factor" which has a hand in reaction rate.

You guys are referring to two different things. He is referring to an energy barrier associated with bulky molecules and getting around electrostatics. The steric factor in your book refers to a steric factor associated with the geometry of the incoming molecule having to match the geometry of the reactant.