Continuation of CHAPTER 11

CHEM 1212

Prentice Hall © 2003 Chapter 11

Vapor Pressure

• Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium.

• Dynamic Equilibrium - the point when as many molecules escape the surface as strike the surface.

What you have learned so far…….

• Some compounds are volatile (evaporates easily) because… (a) the molecules are held together by weak intermolecular forces therefore it doesn’t take much E to break the bonds and so they evaporate easily.

• Compounds that are volatile have lower boiling points.

Prentice Hall © 2003 Chapter 11

Prentice Hall © 2003 Chapter 11

Prentice Hall © 2003 Chapter 11

Clausius-Clapeyron Equation

• ln P = - DHvap + C RT

Where:T = absolute temperatureR = gas constant (8.314 J/K-mol)DH = heat of vaporizationP = vapor pressureC = constant

Prentice Hall © 2003 Chapter 11

Vapor Pressure and Boiling Pt.

• Sample Problem

• If the normal boiling point of water is 100. oC, what will be its boiling point at 735 torr? The heat of vaporization of water is 40.67 kJ/mole.

Prentice Hall © 2003 Chapter 11

Clausius-Clapeyron Equation

• To determine the effect of changing temperature or vapor pressure, use the following equation:

• ln [P1/P2] = - DHvap [1/T1 - 1/T2] R

Prentice Hall © 2003 Chapter 11

Clausius-Clapeyron Equation

So your very first equation in Chem 1212 is:

• ln [P1/P2] = DHvap [1/T2 - 1/T1] R

Problem

CCl4 has a vapor pressure of 213 torr at 40 oC and 836 torr at 80 oC. What is the normal boiling point of CCl4?

Prentice Hall © 2003 Chapter 11