Entropy (DP IB Physics): Revision Note

Entropy

• The entropy S 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

  • When a system becomes more disordered, its entropy will increase

• 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

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