W2 key Worksheet Week 2

Vapor Pressure and Clausius-Clapeyron Equation

1. Clausius-Clapeyron Equation

The Clausius-Clapeyron equation is pivotal in thermodynamics as it quantitatively relates vapor pressure, enthalpy of vaporization, and temperature. This equation provides insights into phase transitions and is integral for predicting how vapor pressure changes with temperature.

a. Enthalpy of Vaporization Calculation

  • Two-point form: This form of the equation is used to calculate the enthalpy of vaporization from the vapor pressures at two different temperatures.

  • Given Information: The slope of the natural logarithm of vapor pressure (ln(P)) versus the inverse of temperature (1/T) plot is provided as -3.92 x 10^3 K.



  • Key Equation:[ \Delta H_{vap} = -R \times slope ]Where:

    • ( R = 8.314 : J/(mol \cdot K) ) is the universal gas constant.

  • Calculation Steps:


    1. Substitute the given values into the equation:[ \Delta H_{vap} = - (8.314) \times (-3.92 \times 10^3) ]

    2. Calculate ( \Delta H_{vap} ) to yield a value in J/mol.

    3. Convert the final result to kJ/mol for practical use.

b. Vapor Pressure of Carbon Disulfide

  • Initial Conditions: The vapor pressure of carbon disulfide (CS2) at 25°C is noted as 363 torr and its normal boiling point is 46.3°C.

  • Objective: The goal is to find the enthalpy of vaporization (( \Delta H_{vap} )) for carbon disulfide.

  • Application: Utilize the Clausius-Clapeyron equation by inputting known vapor pressure values and corresponding temperatures, allowing for the derivation of ( \Delta H_{vap} ) specific to this compound.

c. Boiling Point of Benzene under Different Pressure

  • Given Parameters: The known enthalpy of vaporization for benzene is 30.72 kJ/mol, with a normal boiling point of 80.1°C under 1 atm pressure. The external pressure is given as 445 torr.

  • Goal: Find the adjusted boiling point of benzene when subjected to this external pressure.

  • Consideration: After calculations, critically evaluate whether the computed boiling point aligns with the established understanding of benzene's behavior under varying pressures, particularly concerning boiling point elevation or suppression.

Page 2: Comparative Properties of Substances

2. Order of Increasing Vapor Pressure

  • Lowest Vapor Pressure: SF6 (sulfur hexafluoride)

  • Intermediate Vapor Pressure: SiH4 (silane)

  • Highest Vapor Pressure: SF4 (sulfur tetrafluoride)

3. Order of Increasing Normal Boiling Point

  • Lowest Normal Boiling Point: Ne (neon)

  • Intermediate Normal Boiling Point: O2 (oxygen)

  • Highest Normal Boiling Point: Cl2 (chlorine)

4. Order of Increasing Surface Tension

  • Lowest Surface Tension: CH3CH2CH3 (propane)

  • Intermediate Surface Tension: H3CH2CH2Cl (ethyl chloride)

  • Highest Surface Tension: H2O (water)

5. Order of Increasing Viscosity

  • Lowest Viscosity: CH3CH2CH3 (propane)

  • Intermediate Viscosity: CH3(CH2)4CH3 (hexane)

  • Highest Viscosity: CH3(CH2)8CH3 (octadecane)

Polarity and Molar Mass Influence

  • Polarity and Molar Mass Trends: Generally, substances with higher molar masses exhibit increased viscosity and boiling points.

  • Behavior of Nonpolar vs. Polar Substances: Nonpolar substances tend to showcase distinct trends relative to their polar counterparts in properties such as surface tension and viscosity due to differences in intermolecular forces (e.g., van der Waals forces in nonpolar substances versus hydrogen bonding in polar substances).

This comprehensive view of the relationships between vapor pressure, enthalpy of vaporization, and substance properties enables a deeper understanding of thermodynamic principles and their applications in practical scenarios.