Question

In a bimolecular reaction, the steric factor P was experimentally determined to be 4. 5. The correct option(s) among the following is(are)

• A. The value of the frequency factor predicted by the Arrhenius equation is higher than that determined experimentally.
• B. The activation energy of the reaction is unaffected by the value of the steric factor.
• C. Since P = 4.5, the reaction will not proceed unless an effective catalyst is used.
• D. The experimentally determined value of the frequency factor is higher than that predicated by the Arrhenius equation.

Answer 1

Answer: (B), (D)

B and D

Answer 2

In collision theory, the rate constant is given by

$$k = P\,Z\,\exp\left(-\frac{E_a}{RT}\right)$$

and compared with the Arrhenius form

$$k = A\,\exp\left(-\frac{E_a}{RT}\right)$$

we have

$$A = P\,Z.$$

Since the steric factor $P = 4.5$ (i.e. $P > 1$), it implies:

• $A$ (experimentally determined) is $4.5$ times larger than the collision frequency $Z$ (which is the value assumed if $P = 1$). Therefore, the experimentally determined frequency factor is higher than the one predicted by the ideal (or “predicated”) Arrhenius expression when assuming $P=1$.
• The activation energy $E_a$ comes solely from the energy profile of the reaction and remains unaffected by $P$.