11.4-11.5 IMFs in Action, and Vaporization and Vapor Pressure

Surface Tension Surface tension is created due to the restriction to attract to fewer neighboring molecules at the surface of a liquid. A molecules ability to attract to other molecules lowers potential energy. Surface tension decreases as intermolecular forces decrease. A droplet of water on a leaf, beads up due to surface tension. Even though the water droplet should be a perfect sphere, it appears to be squashed and has more of an oblong appearance. Why do you think this is? The force of gravity distorts the shape.

Viscosity The resistance of a liquid to flow. Viscosity is greater in substances with stronger intermolecular forces, and depends on molecular shape. The longer the molecule, the higher the viscosity. Viscosity decreases with temperature.

Capillary Action The ability of a liquid to flow against gravity up a narrow tube. Results from a combination of two forces, a cohesive force (the force of attraction between molecules in a liquid) and an adhesive force (the force of attraction between the molecules and the surface of the tube). If the adhesive force is greater than the cohesive force, the liquid will be drawn up the tube and the meniscus formed will be concave. If the opposite is true, than the liquid will stay where it is (or drop) and the meniscus will be convex. As IMFs increase, capillary action decreases.

Vaporization and Condensation Vaporization is the process by which thermal energy can overcome intermolecular forces and produce a state change from liquid to gas. Condensation is the transition from gas to liquid. Liquids at the surface have a greater tendency to evaporate because they are less tightly held. The weaker the intermolecular force, the higher the temperature and the more surface area, the more easily the liquid will evaporate, increasing the rate of vaporization. Liquids that vaporize easily are known as volatile liquids. Those that do not vaporize easily are known as nonvolatile. Is vaporization endothermic or exothermic? Endothermic , it takes energy to break the intermolecular attraction between molecules or atoms. Condensation, on the other hand, is exothermic.

Heat of Vaporization Heat (enthalpy) of vaporization (ΔHvap) units (kJ/mol): The amount of heat required to vaporize one mole of a liquid to a gas. ΔHvap = q / m where m = mass and q = heat absorbed. The heat of vaporization is always positive since this process is endothermic. The heat of condensation is the exact opposite.

Let’s Try a Practice Problem! Calculate the amount of heat (in KJ) required to vaporize 2.58 kg of water at its boiling point. (ΔHvap H2O = 40.7 kJ/mol @ BP) ΔHvap = q / m q = ΔHvap m = (40.7 kJ/mol)((2580 g X (1.00 mol/ 18.0 g)) = 5834 kJ or 5.83X103 kJ

Vapor Pressure and Dynamic Equilibrium If a container of water is left open, the water at the surface starts to evaporate and leave the container. What happens of the water is in a sealed container? The water begins to evaporate, but then condenses on the seal. As the concentration of gaseous water molecules increases, so does the rate of condensation, until both are equal. Here, the rate of vaporization and condensation reach a dynamic equilibrium. Vapor pressure: the pressure of a gas in a dynamic equilibrium with its liquid. Weak intermolecular forces of volatile liquids result in high vapor pressure. When a system in dynamic equilibrium is disturbed, the system responds so as to minimize the disturbance and return to a state of equilibrium. (This is called Le Châteliers’s principle)

Quick Question! What happens to the vapor pressure of a substance when its surface area is increases at constant temperature? a.) The vapor pressure increases b.) The vapor pressure remains the same. c.) The vapor pressure decreases. a.) The vapor pressure increases.

Temperature Dependence of Vapor Pressure and Boiling Point As the temperature of a liquid increases, what happens to the vapor pressure? Why? The vapor pressure increases because more molecules gain enough energy to escape into the vapor state. A small increase in thermal energy equals a large increase in vapor pressure. Boiling Point: The temperature at which the liquids vapor pressure equals the external pressure. This is the point when even the internal liquid molecules gain enough energy to escape into the liquid state. Normal Boiling Point: the temperature at which the liquids vapor pressure equals 1 atm. What happens to the temperature of a liquid if you continue to heat the liquid past its boiling point? The temperature of the liquid remains constant, as boiling is a cooling process. What happens to the boiling point of a liquid as elevation increases? Why? The boiling point decreases, because pressure decreases with altitude, and if there is less external pressure acting on the liquid, the liquid molecules have an easier time escaping into the vapor state.

11.4-11.5 pg. 537 #’s 61, 66, 68, 70, 72 Read 11.6-11.8 pgs. 509-516

Clausius-Claperyon Equation The following is FYI: This equation is used to measure heat of vaporization in the laboratory, by plotting vapor pressure vs. temperature. The actual plot is the natural log (ln) of vapor pressure vs. the inverse of temperature (1/T). Clausius-Claperyon Equation (Follows the equation for a straight line: y = mx + b) -Δhvap 1 ln Pvap = ------------- x --- + ln β (where β is a constant that depends on the gas) R T