# Chemistry- boiling point

find the boiling point of HF according to the following reaction

HF(L) --> HF(G)
ΔH=7.53 [KJ/mol]
ΔSsys=0.0257[KJ/(mol*K)]

i used the equation

ΔH=-ΔS*T
>>T=-ΔH/ΔS
>>T=-0.0257/7.53

T=-292.996°K

surely this is meant to be positive ???
have i used the wrong equation?? which other equation should i have used?

## Answers and Replies

Mapes
Science Advisor
Homework Helper
Gold Member
Where does ΔH=-ΔS*T come from? Surely ΔG=0 at a phase equilibrium at constant temperature and pressure.

i dont know, i have it written in my workbook, could it be a mistake??

if ΔG=0
ΔG=H-ΔS*T
H=ΔS*T

and then it all works fine,,, but how do i know that ΔG=0,?

is there no equation that ΔH=-ΔS*T?/

Mapes
Science Advisor
Homework Helper
Gold Member
how do i know that ΔG=0,?

This is a condition that applies to any two systems at equilibrium at constant pressure and temperature. It's derived in intermediate thermodynamics as a consequence of the first and second laws.

is there no equation that ΔH=-ΔS*T?/

There could be, but I haven't seen it, and it doesn't apply here.