Questions

# Classify the following phase changes by the signs of the system\’s ΔH and ΔS.?

## Option:

1) ΔH=+ ΔS=+

2) ΔH=+ ΔS=-

3)ΔH=- ΔS=+

4)ΔH=- ΔS=-

• liquid to gas 1
• solid to gas 1
• liquid to solid 4
• gas to liquid 4
• solid to liquid 1
• gas to solid 4

If dH = positive; then dH = Hfinal – Hinitial > 0, i.e. Hfinal > Hinitial, so there must be a change of phase which requires energy; examples: solid to liquid, liquid to gas, solid to gas

If dH negative, then dH = Hfinal – Hinitial < 0, i.e. Hfinal < Hinitial, so there must be a change of phase which releases energy; examples: liquid to solid , gas to liquid, gas to solid

For entropy, recall entropy is related to chaos, so the more excited the state, the more enrtopy

Case 1)

positive S + positive H

solid to liquid

solid to gas

liquid to gas

Case 2)

negative S + negative S

gas to liquid

liquid to solid

gas to solid

positive S + negative H

Case 3)

positive S + negative H

Impossible, since you can’t have chaos creation without positive H

Case 4)

negative S + positive H

Impossible, since enthalpy addition will provide chaos

Phase changes are transitions between different states of matter, such as solid, liquid, and gas. The signs of the system’s enthalpy (δh) and entropy (δs) can provide information about the nature of the phase change.

Here are some common phase changes and the signs of their δh and δs:

1. Fusion (melting): This is the phase change from solid to liquid. During fusion, the system absorbs heat (δh is positive) and the system’s entropy increases (δs is positive).
2. Vaporization (boiling): This is the phase change from liquid to gas. During vaporization, the system absorbs heat (δh is positive) and the system’s entropy increases (δs is positive).
3. Solidification (freezing): This is the phase change from liquid to solid. During solidification, the system releases heat (δh is negative) and the system’s entropy decreases (δs is negative).
4. Condensation: This is the phase change from gas to liquid. During condensation, the system releases heat (δh is negative) and the system’s entropy decreases (δs is negative).
5. Sublimation: This is the phase change from solid to gas without passing through the liquid phase. During sublimation, the system absorbs heat (δh is positive) and the system’s entropy increases (δs is positive).

I hope this helps! Let me know if you have any questions.