Question

2)

a) Describe in your own words and terms where the origin of the activation barrier comes from and what it represents in a chemical reaction. b) Given the same thermodynamic factors, consider the reaction of two small molecules or two large molecules with one another. Which pair should have the higher activation energy? Why?

Answer 1

The activation energy represents the probability that two molecules (for a bimolecular rxn) will react when they have a collision. The more reactive the two molecules the more likely they will react when they collide (smaller activation energy, higher probability of reaction). Since most rxns involve the breaking and making of chemical bonds between two parts of two different molecules, the smaller the molecule the fewer “nonreactive” bonds present and the higher the probability that when the two molecules collide the reactive portions of the molecules will come into contact and actually react. For two large molecules, there are many more bonds present that will not react, thus reducing the odds that the two reactive portions of the molecule will come together in the right way to react. Thus two larger molecules will have a lower probability of getting the right portions together to react and thus will have a higher activation energy relative to two small molecules. In biological systems that are composed of large complicated molecules, Mother Nature counters this effect to some extent by designing channels that guide small molecules to the reactive portion of the large enzyme. The use of opposite charges on two large proteins can also help guide the reactive portions of these molecules together, thus increasing the odds of proper rxn occurring.