Entropy is the randomness of a system. At the molecular level, entropy can be described in terms of the possible number of different arrangements of particle positions and energies, called microstates. The more microstates the system has, the greater its entropy.

Microstates depend on molecular motion. Molecules may undergo three different types of motion: translational motion, vibrational motion, and rotational motion. During translational motion, the entire molecule moves in one direction. During vibrational motion, atoms in a molecule move toward and away from one another. During rotational motion, molecules rotate or spin. The number of molecules, the space available for the molecules, and the energy available to the molecules will impact the number of microstates.

Another way of looking at entropy is that the universe is moving toward a broader distribution of energy. The universe has a constant amount of energy as stated in the first law of thermodynamics. The universe started with low entropy (concentrated in the moment before the "big bang") and the entropy has since been constantly increasing by distributing this energy. Heat distribution from high temperature to low temperature is a another example of this phenomenon.

Part A

Rank these systems in order of decreasing entropy.

Rank from highest to lowest entropy. To rank items as equivalent, overlap them.

1 mol of
helium gas at
273 K and 20 L

1/2 mol of
helium gas at
100 K and 20 L

1 mol of
carbon tetrachloride gas at
273 K and 40 L

1 mol of
chlorine gas at
273 K and 40 L

1/2 mol of
liquid helium
at 100 K

1 mol of
helium gas at
273 K and 40 L

1/2 mol of
helium gas at
273 K and 20 L

Part B

Classify each process by its individual effect on the entropy of the universe, S.

Drag the appropriate items to their respective bins.

a process run infinitesimally slowly at equilibrium and reversed to its original state

a constant composition mixture of solid and liquid water at STP (273.15 K and 1 atm)

motion of a frictionless pendulum

a bag of red marbles and a bag of green marbles dumped together on a table top

melting of ice cream from solid to liquid at 25?C

isothermal expansion of a real gas

Increases S of the universe

Does not affect S of the universe

Decreases S of the universe