Homework Help: Chemistry

Posted by Jason on Thursday, March 29, 2012 at 3:23pm.

The normal melting point of H2O is 273.15 K, and deltaH fusion is 6010 J/mol. Calculate the change in the normal melting point at 175 and 625 bar assuming that the density of the liquid and solid phases remains constant at 997 and 917 kg/m^3 respectively. Explain the sign of your answer.

So I had no problem calculating:
when P= 175 bar, deltaT= -1.25 K
when P= 625 bar, deltaT= -4.48 K

Assuming my answers are correct, can someone explain the negative signs in the context of the question?

Chemistry - DrBob222, Thursday, March 29, 2012 at 5:06pm
I think the negative sign simply means that at increased pressure the melting point decreases by 1.25 and 4.48 K respectively but I don't know if this is sufficient explanation or not.
Chemistry - Joe, Thursday, March 29, 2012 at 10:13pm
I think the negative sign implies that the slope between S and L is negative so as you increase Pressure you will get a small T. But what formula did you use to find delta T.
Chemistry - Jason, Thursday, March 29, 2012 at 10:55pm
I went
(dP/dT)fusion=deltaSm(fusion)/deltaVm(fusion)
=deltaHm(fusion)/Tfusionm(1/pliq. - 1/psolid)
=-140 barK^-1

So then deltaT=-deltaP/140

What is L? volume (litres?)
Chemistry - DrBob222, Friday, March 30, 2012 at 12:06am
I think Joe is referring to the phase diagram and the slope of the S(solid)/L(liquid) line which is negative.
Chemistry - Joe, Friday, March 30, 2012 at 11:46am
Yeah I'm referring to the phase diagram and the Solid/Liquid line

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