Entropy (DP IB Physics): Revision Note

Entropy

• The entropy of a given system is a measure of the number of possible arrangements of the particles and their energies

  • In other words, it is a measure of how disordered a system is

  • As a system becomes more disordered, its entropy increases

• The order of entropy for the different states of matter, from most disordered to least, is:

gas > liquid > solid

• The entropy of a substance changes during a change in state

• Entropy increases when a substance melts (solid → liquid) or boils (liquid → gas)

  • Increasing the temperature of a substance causes the particles to vibrate more

  • The particles in a gas can now freely move around and are far apart from each other

  • The entropy increases as the particles become more disordered

• Similarly, entropy decreases when a substance condenses (gas → liquid) or freezes (liquid → solid)

  • The particles are brought together and become arranged more regularly

  • The particles become less able to move as they become more ordered

  • There are fewer ways of arranging the energy, hence the entropy decreases

The entropy of a substance increases when the temperature is raised, as particles become more disordered

• Entropy can also increase when 

  • a solid dissolves in a solvent

  • a gas diffuses in a container

• In both cases, entropy increases because:

  • the particles become more spread out

  • there is an increase in the number of ways of arranging the energy

When a solid is dissolved in a solvent to form a dilute solution, the entropy increases as the particles become more disordered

When the partition of the container is removed, the gas diffuses and fills the entire space, causing the entropy of the system to increase

Real Isolated Systems

• In thermodynamics, the distinction between reversible and irreversible processes at the macroscopic level is very important

• A reversible process is defined as:

A process where there is no overall change in entropy as the system and its surroundings are returned to their original states

• Whereas, an irreversible process is defined as:

A process which results in an increase in entropy as the system and its surroundings cannot return to their original states 

• Processes in real isolated systems are almost always irreversible, and consequently, the entropy of a real isolated system always increases

Non-Isolated Systems

• While the entropy of an isolated system must always increase, the entropy of a non-isolated system can decrease

• An isolated system is defined as:

A system in which neither matter nor energy can be transferred in or out

• Whereas in a non-isolated system, matter and energy can be transferred in or out

  • In other words, a non-isolated system can be thought of as one which is part of a larger isolated system

• This means that the entropy of a non-isolated system can decrease locally, but this is compensated by an equal or greater increase in the entropy of the surroundings