Calculating Boiling Point of HF: ΔH and ΔS Equation Analysis

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Discussion Overview

The discussion revolves around calculating the boiling point of hydrofluoric acid (HF) using thermodynamic equations, specifically focusing on the relationship between enthalpy change (ΔH) and entropy change (ΔS) during the phase transition from liquid to gas. Participants explore the validity of the equations used and the conditions under which they apply.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a calculation for the boiling point of HF using the equation T=-ΔH/ΔS, questioning whether the result should be positive.
  • Another participant challenges the equation ΔH=-ΔS*T, suggesting that at phase equilibrium, ΔG should equal zero, which leads to a different formulation involving ΔG=H-ΔS*T.
  • A participant expresses uncertainty about the origin of the equation ΔH=-ΔS*T and whether it is applicable in this context.
  • Another participant asserts that ΔG=0 is a condition for equilibrium at constant temperature and pressure, referencing its derivation from thermodynamic principles.
  • There is a suggestion that while an equation like ΔH=-ΔS*T could exist, it has not been encountered by the participants in this context.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the validity of the equations used or the conditions for equilibrium. Multiple competing views remain regarding the appropriate equations and their applicability.

Contextual Notes

There are unresolved questions about the derivation and applicability of the equations discussed, as well as the assumptions underlying the conditions for equilibrium.

Dell
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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?
 
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Where does ΔH=-ΔS*T come from? Surely ΔG=0 at a phase equilibrium at constant temperature and pressure.
 
i don't 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?/
 
Dell said:
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.

Dell said:
is there no equation that ΔH=-ΔS*T?/

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

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