Vapor Pressure and Boiling Point Concepts

Vapor Pressure and Boiling Point

• Boiling Point (BP) defined as temperature at which vapor pressure equals atmospheric pressure.

• Standard (normal) boiling point occurs at 1 atm (760 mmHg).

• Normal boiling point of water is 100°C at 1 atm pressure.

Vapor Pressure Characteristics

• Vapor pressure is temperature dependent; increases with temperature.

• Relationship is nonlinear and represented as a curve (logarithmic function).

• Different substances have unique vapor pressure properties due to intermolecular forces.

Clausius-Clapeyron Equation

• Used for relating vapor pressure to temperature and heat of vaporization ($\Delta H_{vap}$).

• R = 8.314 J/(mol·K).

• Equation:
  $\ln\left(\frac{P<em>1}{P</em>2}\right) = \frac{\Delta H<em>{vap}}{R} \left( \frac{1}{T</em>2} - \frac{1}{T_1} \right)$

• Units must be consistent: pressures in the same units, $\Delta H_{vap}$ in J or kJ, temperatures in Kelvin.

Problem-Solving Steps

1. Define variables for known pressures and temperatures.

2. Ensure units are consistent.

3. Plug values into the Clausius-Clapeyron equation and solve for the desired variable.

4. Use significant figures appropriately.

5. Follow order of operations carefully (multiplication/division before addition/subtraction).

Example Problems

• Diethyl Ether Problem:

  • P1 = 760 mmHg, T1 = 35°C (308.15 K)

  • P2 = 400 mmHg, T2 = 18°C (291.15 K)

  • Solve for $\Delta H_{vap}$, which results in 28.2 kJ/mol.

• Water Pressure Problem:

  • Given T2 = 65°C (338.15 K) and $\Delta H_{vap}$ for water.

  • Use normal boiling point relation: P1 = 1 atm at T1 = 100°C (373.15 K).

  • Solve for P2, resulting in approximately 0.257 atm.