Effect of pressure on melting point.

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Discussion Overview

The discussion centers on the effect of pressure on the melting and boiling points of substances, specifically exploring how to quantify this relationship and the underlying principles governing phase transitions. Participants consider both theoretical and practical aspects, including calculations and specific substances like silicon dioxide.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about a quantifiable value that correlates pressure with changes in melting and boiling points for specific substances.
  • Another participant presents a formula for calculating the boiling point change with pressure, providing specific variables and constants involved in the calculation.
  • A subsequent reply questions the applicability of the provided equation to melting point changes, suggesting a need for clarification on its use.
  • Another participant mentions the necessity of using enthalpy values for phase transitions and references the Clausius–Clapeyron relation for further context.
  • Concerns are raised about the validity of using enthalpy of fusion in the calculations, with one participant noting discrepancies in their results when applying the values for that phase change.
  • It is pointed out that while there are no assumptions about the type of phase transition, there is an assumption that enthalpy remains constant, which may contribute to incorrect results.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of certain equations to melting point changes, with no consensus reached on the validity of the approaches discussed. The discussion remains unresolved regarding the correct method to quantify the effect of pressure on melting points.

Contextual Notes

Limitations include assumptions about enthalpy being constant and the lack of clarity on the specific phase transitions being considered. The applicability of the equations to melting points versus boiling points is also under debate.

ExcessRed
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Is there a quantifiable value for a given substance that correlates to the degree by which pressure has an effect on the element or substance melting/boiling point?

Allow me to elaborate.

If the substance in question is known (silicon dioxide, for example), how could we calculate the modified melting/boiling points given n bars of atmospheric pressure? Bonus points if you can explain why some chemicals skip the liquid phase under certain pressure conditions.
 
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Solved the problem myself, but thought you guys might like to know the solution:

T2 = 1/((1/(T1))-Math.log(Pressure/1)*R / (Hv * 1000));

R = 8.314
Pr = Pressure you're solving for (in bars.)
Hv = Heat of vaporization for the substance in kJ/mol.
T1 = Boiling point at 1 bar of pressure.
T2 = the solution!

Edit: To clarify, Math.log() in this context refers to log base e (2.718...)
 
Last edited:
This equation may be OK for boiling point change. But is it applicable to change in the melting point with change in pressure?
 
Are you sure it works with enthalpy of fusion? I've tried substituting the values for that phase change, but none of them are coming up correct.
 
ExcessRed said:
Are you sure it works with enthalpy of fusion? I've tried substituting the values for that phase change, but none of them are coming up correct.

There are no assumptions about what kind phase transition takes place, however, there is an assumption that enthalpy is constant (not a function of P, T) - it is quite possible that's why you get incorrect result.
 

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