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.
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 :)
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
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
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 = +
To determine which process has a negative ΔS_sys (change in system entropy), let's analyze each option:
A) Evaporation of 1 mol of CCl4(l):
When a substance evaporates, its molecules transition from a condensed phase (liquid) to a gaseous phase. This phase transition generally corresponds to an increase in entropy. Therefore, it is unlikely that the entropy change during evaporation would be negative. So, option A is unlikely to have a negative ΔS_sys.
C) Mixing 5 mL of ethanol with 25 mL of water:
When two substances mix, the entropy change depends on factors such as the number of particles and the randomness of their arrangement. Generally, mixing two substances leads to an increase in entropy due to greater disorder, so it is unlikely that the entropy change would be negative. Thus, option C is not expected to have a negative ΔS_sys.
D) Raising the temperature of 100 g of Cu from 275 K to 295 K:
When the temperature of a substance increases, its entropy tends to increase as the thermal motion of the particles becomes more energetic. So, raising the temperature is unlikely to result in a negative ΔS_sys. Therefore, option D is not likely to have a negative ΔS_sys.
E) Grinding a large crystal of KCl to powder:
Grinding a crystal into a powder increases its surface area, which generally leads to an increase in entropy due to greater molecular disorder. Therefore, option E is also unlikely to have a negative ΔS_sys.
B) Compressing 1 mol of Ne at constant temperature from 0.5 atm to 1.5 atm:
When a gas is compressed, its particles move closer together, reducing the overall volume and therefore decreasing the entropy. So, in this case, the compression of Ne could result in a negative ΔS_sys.
Based on the analyses above, B is the most likely option to have a negative ΔS_sys because compressing a gas typically decreases entropy.