CH1810 Lecture #2 Vapor Pressure of Liquids and Solutions

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1 CH1810 Lecture #2 Vapor Pressure of Liquids and Solutions

2 Vaporization and Condensation

3 Kinetic Energy and Temperature Molecules in a liquid are constantly in motion Types of motion: vibrational, and limited rotational and translational The average kinetic energy is proportional to the temperature. Some molecules have more kinetic energy than average, and others have less.

This will allow them to escape the liquid and become a vapor.

4 Vaporization If high energy molecules are at the surface, they may have enough energy to overcome the attractive forces. This will allow them to escape the liquid and become a vapor. Note: The larger the surface area, the faster the rate of evaporation.

5 Distribution of Thermal Energies Only a small fraction of the molecules in a liquid have enough energy to escape.

6 Distribution of Thermal Energies But, as the temperature increases, the fraction of the molecules with escape energy increases.

7 Distribution of Thermal Energies The higher the temperature, the faster the rate of evaporation.

8 Condensation Molecules of the vapor will lose energy through molecular collisions. Some of the molecules will get captured back into the liquid. Some may stick and gather together to form droplets of liquid.

9 Evaporation vs. Condensation Vaporization and condensation are opposite processes. In an open container, the vapor molecules generally spread out faster than they can condense, and there is a net loss of liquid. In a closed container, the vapor is not allowed to spread out indefinitely; at some time the rates of vaporization and condensation will be equal.

10 Effect of Intermolecular Attraction The weaker the attractive forces, the faster the rate of evaporation. Liquids that evaporate easily are said to be volatile. e.g., gasoline Liquids that do not evaporate easily are called nonvolatile. e.g., motor oil

11 Dynamic Equilibrium When two opposite processes reach the same rate so that there is no gain or loss of material, we call it a dynamic equilibrium. This does not mean there are equal amounts of vapor and liquid it means that they are changing by equal amounts.

12 Vapor Pressure The pressure exerted by the vapor when it is in dynamic equilibrium with its liquid is called the vapor pressure. The weaker the attractive forces between the molecules, the more molecules will be in the vapor. Therefore, the weaker the attractive forces, the higher the vapor pressure.

13 Vapor Pressure of Water at Various Temperatures

14 Vapor Pressure vs. Temperature Increasing the temperature increases the number of molecules able to escape the liquid. The net result is that as the temperature increases, the vapor pressure increases. Small changes in temperature can make big changes in vapor pressure.

15 Boiling Point When the temperature of a liquid reaches a point where its vapor pressure is the same as the external pressure, vapor bubbles can form anywhere in the liquid. This phenomenon is what is called boiling and the temperature at which the vapor pressure = external pressure is the boiling point.

16 Vapor Pressure Curves 760 mmhg Which of the liquids is the most volatile? Which liquid has the strongest intermolecular forces? Which of the liquids has the highest normal boiling point?

17 Normal Boiling Point The normal boiling point is the temperature at which the vapor pressure of the liquid = 1 atm. The lower the external pressure, the lower the boiling point.

Vapor Pressure and Temperature (a) The vapor pressures of water, ethanol, and diethyl ether show a nonlinear rise when plotted as a function of temperature.

18 Vapor Pressure and Temperature (a) The vapor pressures of water, ethanol, and diethyl ether show a nonlinear rise when plotted as a function of temperature. (b) A plot of ln Pvap versus 1/T (Kelvin) for water shows a linear relationship. (c) The slope of the plot of ln Pvap and (1/T) is proportional to the heat of vaporization of the liquid.

19 Clausius Clapeyron Equation The graph of vapor pressure vs. temperature is an exponential growth curve. The logarithm of the vapor pressure vs. inverse absolute temperature is a linear function. y = ax +b

20 Clausius Clapeyron Equation

21 Clausius Clapeyron Equation 2-Point Form The equation below can be used with just two measurements of vapor pressure and temperature. It can also be used to predict the vapor pressure if you know the heat of vaporization and the normal boiling point.

22 Vapor Pressure of a Liquid The vapor pressure of a liquid is the pressure exerted by a gas in equilibrium with its liquid phase in a sealed container. The rates of evaporation and condensation are equal.

23 Factors Affecting Vapor Pressure Temperature: as T increases, KE increases, V.P. increases Intermolecular forces: Stronger forces, higher KE needed to enter gas phase, V.P. decreases Presence of nonvolatile solute: Affects rate of evaporation, decreases V.P. of solution compared to pure solvent

24 Vapor Pressure of Solutions Vapor pressure lowering: The vapor pressure of solution of a nonvolatile solute is decreased because some of the surface area of the solution is occupied by solute molecules Raoult s Law: A colligative property of solutions The vapor pressure of solution is equal to the vapor pressure of the pure solvent multiplied by the mole fraction of the solvent in the solution. P solution = χ solvent P solvent Ideal solution: One that obeys Raoult s law

25 A Demonstration

26 Mixing of Volatile Solutes

27 Fractional Distillation Method of separating a mixture of compounds on the basis of their different boiling points. Vapor phase enriched in more volatile component

28 Fractional Distillation Boiling points: - Heptane = 98C - Octane = 126 C (a) Ideal situation. Fractional distillation of a mixture of heptane and octane produces two plateaus at the boiling points of the two components.

29 Boiling points: - Heptane = 98C - Octane = 126 C Fractional Distillation

30 Solutions of Volatile Components For mixtures containing more than one volatile component: Partial pressure of each volatile component contributes to total vapor pressure of solution. P total = χ 1 P 1 º + χ 2 P 2 º + χ 3 P 3 º + Where χ i = mole fraction of component i, and P i º = equilibrium vapor pressure of pure volatile component at a given temperature

31 Ideal Solutions In ideal solutions, the resulting solute-solvent interactions are equal to the sum of the broken dilutesolute and solvent-solvent interactions.

32 Real (Nonideal) Solutions Deviations from Raoult s Law: Due to differences in solute solvent and solvent solvent interactions (dashed lines = ideal behavior) Negative deviations Positive deviations

33 Practice Problem: Vapor Pressure of Solution 1) A solution contains g of water (MW = 18.0 g/mol) and g of ethanol (MW = 44.0 g/mol). What are the mole fractions of water and ethanol, and the vapor pressure of the solution at 25 o C? (Pwater = 23.8 torr; Pethanol = 58.7 torr) 1.00 mol H2O g H2O x = mol H2O g H2O 1.00 mol C2H6O g C2H6O x g C2H6O = mol C2H6O total moles 26.9 torr Pwater = (χwater)(23.8 torr) = ( )(23.8 torr) = 21.7 torr Pethanol = (χwater)(58.7torr) = ( )(58.7 torr) = 5.23 torr

Chapter 10. Intermolecular Forces II Liquids and Phase Diagrams

Chapter 10. Intermolecular Forces II Liquids and Phase Diagrams Chapter 10 Intermolecular Forces II Liquids and Phase Diagrams Liquids Properties & Structure Vaporization and Condensation Kinetic Energy and Temperature Molecules in a liquid are constantly in motion