Continuation of CHAPTER 11 CHEM 1212

Prentice Hall © 2003Chapter 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.

Prentice Hall © 2003Chapter 11 Explaining Vapor Pressure on the Molecular Level Vapor Pressure

If equilibrium is never established then the liquid evaporates. Volatile substances evaporate rapidly.  vapor pressure of substance is high The higher the temperature, the higher the average kinetic energy, the faster the liquid evaporates. Vapor Pressure and Volatility

Chapter 11

Liquids boil when the external pressure equals the vapor pressure. Temperature of boiling point increases as pressure increases. Vapor Pressure and Boiling Pt.

Two ways to get a liquid to boil: increase temperature or decrease pressure. –Pressure cookers operate at high pressure. At high pressure the boiling point of water is higher than at 1 atm. Therefore, there is a higher temperature at which the food is cooked, reducing the cooking time required. Normal boiling point is the boiling point at 760 mmHg (1 atm). Vapor Pressure and Boiling Pt.

Enthalpy of Vaporization aka heat of vaporization (  H vap ) Is the amount of heat needed to convert a liquid to a vapor at its normal boiling point

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.

Chapter 11

Clausius-Clapeyron Equation ln P = -  H vap + C RT Where: T = absolute temperature R = gas constant (8.314 J/K-mol)  H = heat of vaporization P = vapor pressure C = constant

IDEAL GAS CONSTANT R = liter. atm/K. Mole OR R = J/K. mole

Vapor Pressure and Boiling Pt. Sample Problem If the normal boiling point of water is 100. o C, what will be its boiling point at 735 torr? The heat of vaporization of water is kJ/mole.

Clausius-Clapeyron Equation To determine the effect of changing temperature or vapor pressure, use the following equation: ln [P 1 /P 2 ] = -  H vap [1/T 1 - 1/T 2 ] R

Clausius-Clapeyron Equation Or to remove the negative sign: ln [P 1 /P 2 ] =  H vap [1/T 2 - 1/T 1 ] R

Clausius-Clapeyron Equation So your very first equation in Chem 1212 is: ln [P 1 /P 2 ] =  H vap [1/T 2 - 1/T 1 ] R

Problem CCl 4 has a vapor pressure of 213 torr at 40 o C and 836 torr at 80 o C. What is the heat of vaporization of CCl 4 ?

Extra Problem The melting point of potassium is 63.2 o C. Molten potassium has a vapor pressure of 10.0 torr at 443 o C and a vapor pressure of torr at 708 o C. A.) Calculate the heat of vaporization of liquid potassium. B.) Calculate the normal boiling point of potassium. C.) Calculate the vapor pressure of liquid potassium at 100 o C.