How does molecular geometry affect the boiling point?

1 Answer
Apr 20, 2015

Because it effects the the Brownian Motion and hence, the Entropy of the liquid.

First, we need to define the boiling point. The boiling point is the temperature at which a liquid will start changing into vapor.

Second, we need to understand what determines the state of a substance. It is simply determined by the distance (or rather the attraction forces) between the molecules of the substance.

A) Solids: molecules are tightly packed together, making the the volume constant and the shape fixed.

B) Liquids: molecules move freely but are still influenced by the intermolecular forces, giving them constant volume but an undefined shape that takes the shape of its container.

C) Gases: molecules are loose and have very weak (or zero) intermolecular forces, letting them lose any definite volume or shape. It will completely occupy the container its in.

So we conclude that the easier the molecules can move away from each other, the easier it is for those molecules to change from liquid to gas.

The driving force that causes molecules can move freely and randomly in a container (aka the Brownian Motion) is called Entropy. This force is directly influenced by the temperature.

So where does the molecular geometry come in all this? It goes like this:

"The less complex the 3D structure of a molecule is, the easier for that molecule to move freely and randomly near other molecules of the same substance."

Think of it as trying to have a group of people run around together in a small room while:

1- Wearing light clothes.
2- Wearing heavy clothes.
3- Wearing heavy clothes and carrying food trays.
4- Wearing light clothes and carrying a few broomsticks.

That's exactly how molecular geometry effects boiling point.

I hope my answer wasn't too long and has given you a clear understanding of how matter works.