What affects entropy?

We can calculate exact entropy changes by looking up standard molar entropies and working out the change from there. Or, we can do experiments to work it out. However, it’s perfectly possible to guess what increases and decreases entropy.

Degrees of freedom

A a degree of freedom is something that is allowed to vary. It pops up in statistical contexts a lot. In a chemical context, however, we can think of it as being how free molecules are to move. This can be related to their structure, or the molecules’ ability to move around.

Processes that increase entropy

• Melting from solid (fixed positions) to liquids (freer positions)
• Vaporising from liquid to gas (fast moving, easily fills a container)
• Reactions that dissolve solids
• Reactions that increase the number of gas molecules
• Breaking molecules into two (increasing the number of molecules in gas or solution)
• Ring-opening – going from more fixed, rigid, ring-like structures to long, flexible chains

Processes that decrease entropy

• Freezing, and condensation
• Reactions that combine two molecules into one (addition)
• Reactions that reduce the number of gas molecules
• Reactions that make a molecule more rigid (going from long chains to inflexible rings)

Processes where we need to know more details

• Reactions that reduce the number of molecules or make them more rigid… but may free up solvent molecules in the process
• Reactions where the number of gas molecules don’t change

So we can see that where the freedom for molecules to move and rearrange themselves naturally lead to more possible combinations and arrangements of them. This results in higher entropy.

This comes from the statistical approach to entropy, where the energy associated with entropy is a function of the number of possible arrangements of the system. There are more ways of arranging chaotic, and loosely connected water molecules than there are in rigid and crystalline ice. Therefore liquid water has a higher entropy.