Intermolecular Forces - Surface Tension - Intermolecular Forces - Cohesive vs Adhesive Forces -

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1 Intermolecular Forces - Surface Tension - Intermolecular forces holding molecules together. Intermolecular Forces - Cohesive vs Adhesive Forces - Cohesive forces: interactions between like particles. Adhesive forces: interactions between unlike particles. 1

2 Polarity and Solubility Two liquids that mix completely together are said to be MISCIBLE - otherwise they are IMMISCIBLE. CuSO 4 (aq) Nonpolar octane (C 8 18 ) Nonpolar CCl 4 Mixing of Polar Liquids - C 3 C 2 - O - - C 3 C 2 - O - - C 3 C 2 - O O O O Polar molecules can break up groups of other polar molecules through dipoledipole interactions, resulting in a thorough mixing 2

3 Mixing of Polar and Nonpolar Liquids - O - C - C - C - - O - O - C - C - C - The weak London forces present in the C 3 C 2 C 3 molecules are not strong enough to break the hydrogen bonds, so the two liquids are immiscible. Mixing Nonpolar Liquids Cl Cl C Cl Cl - C - C - C - C - C - C - London forces are the only interactions, but the strength of these forces between CCl 4 molecules is comparable to those between C 6 14 molecules, so the two liquids are miscible with each other. General Observation: 3

4 More Solubility Examples Combination of Forces More than one intermolecular force may need to be considered when examining solubility. Solubility decreases as relative energy of -bonding decreases and dispersion increases. 4

5 Combination of Forces cont d Octanol Extensive dispersion forces limit solubility of octanol in water. Solubility Behavior ydrophobic ( water-fearing ) Interaction that repels water, diminishes water solubility. ydrophilic ( water-loving ) Interaction that attracts water, promotes water solubility. ydrophobic ydrophilic 5

6 Sample Exercise 10.5 Which of these compounds should be very soluble in water and which should have limited solubility in water: carbon tetrachloride (CCl 4 ), ammonia (N 3 ), hydrogen fluoride (F), oxygen (O 2 )? Chemistry In Action: The Killer Lake 8/21/86 CO 2 Cloud Released 1700 Casualties Trigger? earthquake landslide strong Winds Lake Nyos, West Africa 6

7 Solubility of Gases in Water enry s law - the solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (). c = kp c is the concentration (M) of the dissolved gas P is the pressure of the gas over the solution k is a constant (mol/l atm) that depends only on temperature low P low c high P high c 7

8 Pressure Sample Exercise 10.6 Calculate the solubility of oxygen in water in moles per liter at 1.00 atm pressure and 20 C. The mole fraction of O 2 in air is (Remember that the sum of the mole fractions of all the gases in a mixture equals 1.) The enry s law constant for oxygen at 20 C is mol/l. atm. Phase Diagrams States of matter as a function of temperature and pressure solid liquid gas Temperature 8

9 Phase Diagram of Water Triple Point Critical Point Critical Temperature Critical Pressure Supercritical Fluid Equilibrium Lines Some Remarkable Properties of Water On the left is water; water is unusual because the solid is less dense than the liquid. For most substances, the solid is more dense than the liquid like for benzene (on the right). 9

10 Unique Property of Water - density decreases when it freezes - -bonding results in cage-like structure in solid state; less dense than liquid state. 10

11 Water and Aquatic Life Importance of Density: Lakes/rivers freeze from top down, allowing fish and aquatic life to survive below. As surface waters warm or cool, nutrient-rich bottom waters cycle to the surface; oxygen-rich surface waters cycle to the bottom. Phase Diagram for CO 2 11

12 1. Positive slope for the solid-liquid interface (normal) 2. Sublimation occurs at room temperature under 1 atm of pressure. 3. There is a critical point at 73 atm and 31 o C (varies from substance to substance). 4. Above the critical point the substance is known as a supercritical fluid (not a liquid; not a gas). Increasing the pressure normally would convert a gas to a liquid but doesn t happen. 5. Supercritical fluids have enhanced solvation abilities, i.e. the fluid will dissolve greater amounts of solute than normal. 12