• Support PF! Buy your school textbooks, materials and every day products Here!

A problem from thermodynamics -- Freezing of water at 273 K and 1 atm

  • #1

Homework Statement


Freezing of water at 273 K and 1 atm

which of the following is true for the above thermodynamics process
p) q=0
q)w=0
r)ΔSsys<0
s)ΔU=0
t)ΔG=0

Homework Equations


none[/B]


The Attempt at a Solution


[/B]
i got r, s ,t

since the reaction happens at constant temperature,internal energy is constant
since the reaction is open so it is isobaric in nature and since the volume changes due to phase change,so work done is not zero,so heat must be exchanged(first law of thermodynamics).
randomness decreases so ΔSsys<0

but answer given is r t q

i couldn't understand please help.
 

Answers and Replies

  • #2
Andrew Mason
Science Advisor
Homework Helper
7,566
329

Homework Statement


i got r, s ,t

since the reaction happens at constant temperature,internal energy is constant
since the reaction is open so it is isobaric in nature and since the volume changes due to phase change,so work done is not zero,so heat must be exchanged(first law of thermodynamics).
randomness decreases so ΔSsys<0

but answer given is r t q

i couldn't understand please help.
##\Delta U## includes potential energy. In order to form ice, the water molecules lose potential energy and rotational kinetic energy. Average translational kinetic energy does not change (i.e. temperature) but average potential energy per molecule decreases and there is loss of rotational kinetic energy. The process is exothermic (heat flows out of the water to form ice).

Since ##\Delta Q < 0##, ##\Delta S = \int dQ/T = \Delta Q/T < 0##.

Since P and T are constant, ##\Delta G = \Delta Q - T\Delta S = T\Delta S - T\Delta S = 0##

I am not sure about w = 0, however. Ice takes up more volume than liquid water (which is why ice floats). So there is a small amount of work done on the surroundings.

[later edits are in italics]

AM
 
Last edited:
  • #3
##\Delta U## includes potential energy. In order to form ice, the water molecules lose potential energy. Average kinetic energy does not change (i.e. temperature) but average potential energy per molecule decreases. The process is exothermic (heat flows out of the water to form ice).

Since ##\Delta Q < 0##, ##\Delta S = \int dQ/T = \Delta Q/T < 0##.

Since P and T are constant, ##\Delta G = \Delta Q - T\Delta S = T\Delta S - T\Delta S = 0##

I am not sure about w = 0, however. Ice takes up more volume than liquid water (which is why ice floats). So there is a small amount of work done on the surroundings.

AM
thank you very much sir,understood it.
 
  • #4
Andrew Mason
Science Advisor
Homework Helper
7,566
329
With respect to the internal energy, I should have said that the average translational kinetic energy does not change. The specific heat of water is greater than the specific heat of ice. This is because H2O molecules lose rotational kinetic energy when they freeze to form ice because they lose the rotational degree of freedom.

AM
 

Related Threads on A problem from thermodynamics -- Freezing of water at 273 K and 1 atm

Replies
1
Views
920
  • Last Post
Replies
19
Views
31K
  • Last Post
Replies
0
Views
2K
Replies
1
Views
1K
  • Last Post
Replies
8
Views
8K
Replies
3
Views
1K
  • Last Post
Replies
1
Views
4K
Replies
3
Views
2K
  • Last Post
Replies
1
Views
1K
Top