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Chemical Kinetics Part II: In great deeds, something abides!

Bvt. Maj. Gen. Joshua Lawrence Chamberlain

Bvt. Maj. Gen. Joshua Lawrence Chamberlain

With all due respect to Joshua Chamberlain; what about little deeds? By “little” I don’t mean “less important”, but “microscopic”.

When studying (and understanding) chemical kinetics, I’ve always found marvelous that a microscopic interpretation of some phenomena is not only possible, but simple too!

A certain number of factors (concentration, surface, temperature, catalysts) is able to modify the speed of a reaction. Even if “mathematical” explanations can use their formulas to achieve the result of predicting the changes, I believe a less numerical, and more practical point of view can obtain similar results, with an added value: it can help understand the issue in a much simpler way.

In the Physical chemistry textbook, both Chapter 1 and Chapter 9 (kinetics module) deal with molecular-kinetics models. These models can rationalize chemical facts, hypothesizing the existence of molecules which behave like what classical mechanics teaches; but simply at a smaller scale!

For starters, let’s try to understand the key factors able to modify the velocity of a reaction, in terms of “what happens to our tiny tiny molecules?”.

The factors I’ll cover are the following: reactants’ concentration; contact surface; temperature; catalysts.

Collision theory describes a world of particles which react, if and when they collide with sufficient energy and the right angle.

Concentration

is by far the easiest factor to analyze: if our reactants are concentrated, they will collide more often. Higher number of collisions means higher possibility to react. Higher possibility to react means higher velocity of reaction.

So simple!

I have to note that, of course, when reactants start consuming, their concentration lowers, and so the velocity of the reaction lowers as well. So simple (x2)!

Surface effects

aren’t much more difficult, either. Imagine a gold mine, where only one miner can work. As soon as the miner finds his fair amount of gold, he will run to the bank, leaving the place for another one to step in. There is no room for both to work simultaneously, so the speed of the process which depletes of gold our mine is limited by the speed of one working miner.

On the other hand, if we had room for ten or twenty miners, our mine would be salvaged in reasonably less time, with as many workers working simultaneously!

The same applies for our reactions: if the reactants can “touch” each other over small surfaces (interface between immiscible liquids, solids with big particle sizes, etc), they will react much more slowly than they would if they could share wider surfaces.

Temperature,

on the other hand, works in a really smooth and simple way. We know that there is a relationship between kinetic energy and temperature (Eq. 1.50 on page 1-35 of the Physical chemistry textbook): for this reason, it is easy to state that higher temperatures mean higher kinetic energies.

Relationship between kinetic energy and temperature

This has two effects:

  1. first of all, kinetic energy translates into “speed”, so molecules confined in a container, while increasing their speed, happen to collide more often;
  2. secondly (really important!), molecules need to “climb” an energy gap, called activation energy, to reach the transition state which will drive them to the products. And, obviously, kinetic energy is… well, energy!

Because of their utter importance, catalysts deserve a post on their own, so stay tuned for Part III.

I leave you with the thought that in little deeds, an explanation abides!

 

Written by: Gaetano Giammarino

Gaetano Giammarino has authored 11 more articles.

He has received a PhD in Computational Chemistry at Università degli Studi della Basilicata, where he teaches General Chemistry in the Faculty of Engineering.

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