# Homework Help: Enthalpy of transition of water to ice

1. Nov 3, 2013

### Joel Martis

1.At constant temperature of 258K and constant pressure of 1.000 atm , super cooled liquid water (2.00mol) freezes irreversibly. Find the change in enthalpy(ΔH) by assuming that Cp,m of liquid water and ice are 75.48 KJ/mol and 37.15KJ/mol and are constant. (this problem was from my assignment sheet so there might be errors in the units of Cp,m. By the way , Cp,m is the specific heat capacity at constant pressure.)

2.the only equation i could think of is ΔH=ΔU+ΔPV

2. Nov 3, 2013

### Staff: Mentor

Hi Joel Martis. Welcome to Physics Forums.

In terms of the enthalpy and temperature, what is the defining equation for the heat capacity at constant pressure?

Chet

3. Nov 3, 2013

### Joel Martis

I think it is Cp,m= (∂H/∂T)p

4. Nov 3, 2013

### Staff: Mentor

Good. So now you have to define a path form the starting point to the final point along which you know all the properties needed to calculate the ΔH. The starting point is liquid water at -15C, and the end point is ice at -15 C. But, you only know the heat of melting/fusion at 0C. So, how do you get form the starting point to the final point if you only know the heat of fusion/melting at 0 C, and the heat capacities for the liquid water and the water ice?

Chet

5. Nov 4, 2013

### Joel Martis

Well, how about heating the supercooled water from 258K to 273K , freezing it, and then cooling the ice back to 258K?

6. Nov 4, 2013

### Staff: Mentor

Sounds great.

Chet