Question

1. For which process is dS system negative?
A)evaporation of 1 mol of CCl4(l)
B)compressing 1 mol of Ne at constant temperature from 0.5 atm to 1.5 atm
C)mixing 5 mL of ethanol with 25 mL of water
D)raising the temperature of 100 g of Cu from 275 K to 295 K
E)grinding a large crystal of KCl to powder

The answer is B but I want to know why the other answers aren't the correct ones.

Answer 1

T dS = d U + d W

A. T is constant, dU you put heat in so +, no work so dW=0
B. T is constant so U is constant but you squeezed so dW is - (its pressure is opposite to motion)
C. easy way to think is no temp change, no U change, but ethanol did work expanding from 5mm to 25 mm, so dW is positive. However Google entropy change with mixing.
D. dU is big +, got hot. But dV tiny and constant pressure so no dW
E. orderly crystal ---> slovenly powder, increased entropy :)

Answer 2

I was a bit glib about E. To see what I mean check for example:
http://hyperphysics.phy-astr.gsu.edu/hbase/Therm/entrop.html

Answer 3

As a chemist I would do this slightly different (but chemists and physicists get the same answer).
dS is + when the system gets more randomly separated. It is negative when the atoms/molecules are more confined, thus less random.

A)evaporation of 1 mol of CCl4(l)
B)compressing 1 mol of Ne at constant temperature from 0.5 atm to 1.5 atm
C)mixing 5 mL of ethanol with 25 mL of water
D)raising the temperature of 100 g of Cu from 275 K to 295 K
E)grinding a large crystal of KCl to powder

<b> A. dS is + because you are going from a liquid to a gas. Gas molecules are further apart so they are more randomly oriented.
B. dS is -. You are going from larger volume (more random) to a smaller volume(less random).
C. Dilution of the ethanol means more randomness so dS = +
D. Increase in T means more motion to the molecules, thus more random. dS = +
E. Going from a crystal to a powder is more randomness. dS = +