Molecular Size versus Chemical Reactivity

Molecular Size versus Chemical Reactivity

Normally, for every 10 degrees C increase in temperature, a reaction approximately doubles in rate (this is known as Q₁₀). This pertains to reactions involving both large and/or small molecules. And this leads to our question:

THE question here is: Does the reactivity of a large molecule depend more on the kinetics of its overall mass or on the kinetics of its constituent atoms? By 'kinetics' it is meant the force of potential collisions, which of course can be determined by the simple physics equation:

F = mass x velocity²

But what 'mass' - is it that predominately of the whole molecule or just that of the individual atoms that make up the molecule? Stating it another way with an example: in an enzymatic reaction does Q₁₀ depend on the large enzyme molecule's increased bouncing around and bumping into substrate molecules as temperature increases, or is it due to the increase vibrations of the atoms that make up the active site?

Surprisingly, there is a simple way to attack this question - by comparing the specific heat capacities of a macromolecular compound with two small molecule compounds one with heavy atoms and the other with light atoms. Thus compare - say wax, polyethylene or glass with lead. If you find that the specific heat capacity of your macromolecular substance is greater than the lead, then it must be that the macromolecules store heat mainly in their constituent atoms, but if it is less than lead, then most of the heat is stored in the overall vibration of the large molecules.

However, first you need to know how to do simple heat capacity measurements. Then apply those simple methods to answering THE question as to which 'mass' is most influenced by rising temperature.