What factors determine volatility?

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    Factors Volatility
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Discussion Overview

The discussion centers on the factors that determine the volatility of components in a binary mixture, specifically examining isopropanol and cyclohexane. Participants explore the relationship between boiling point, vapor pressure, and the concept of azeotropes in the context of volatility.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether boiling point is the sole factor in determining volatility, noting the close boiling points of isopropanol and cyclohexane.
  • Another participant suggests that boiling point serves as a proxy for vapor pressure, asserting that the component with the higher vapor pressure is more volatile, and concludes that cyclohexane is more volatile due to its lower boiling point.
  • A technical explanation is provided involving the Clausius-Clapeyron relation, detailing the relationship between changes in specific pressure and temperature during phase changes.
  • One participant raises the importance of considering azeotropes, indicating that isopropanol and cyclohexane form an azeotrope which affects their volatility characteristics.

Areas of Agreement / Disagreement

Participants express differing views on the factors influencing volatility, particularly regarding the role of boiling point versus vapor pressure. The discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Participants highlight the need to consider azeotropes and the specific conditions under which volatility is assessed, indicating that the relationship between boiling point and volatility may not be straightforward.

JeweliaHeart
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Is the boiling point of a pure component the only factor used to determine which component is more volatile in a binary mixture. Take, for example, the case of isopropanol and cyclohexane.

Isopropanol has a bp of 82.6 C, while cyclohexane's is 81.

The two are are very close in value; however cyclohexane's is lower so does that automatically make it more volatile?
 
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Boiling point is more of a proxy for the substance's vapor pressure. The component with the higher vapor pressure is the more volatile species. Check out the different ways to model vapor pressure for more insight.
In regards to your example,

As a component's vapor pressure rises, its boiling point decreases. Because cyclohexane has the lower boiling point, it has the highest volatility.
 
dp/dt = L/(t*deltaV)
change in specific pressure over change in temperature is equal to the latent heat of vaporization divided by the temperature times the change in volume associated with the phase change.
dp = change in specific pressure
dt = change in temperature
L = latent heat of vaporizaton = T*(S_g - S_L) = energy required to change a molecule from liquid to gas
t = temperature
deltaV = change in specific volume for one molecule from liquid to gas.

https://en.wikipedia.org/wiki/Clausius–Clapeyron_relation
 
Always check for azeotropes. Isopropanol and cyclohexane form an azeotrope at 67wt% cyclohexane with a boiling point of 68.6oC. The volatilities are reversed on either side of the azeotrope.
 

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