Physical Properties of Solutions Chapter 12 1 Copyright

Physical Properties of Solutions Chapter 12 1 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount 2

A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature. An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature. A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature. Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. 3

Three types of interactions in the solution process: • solvent-solvent interaction • solute-solute interaction • solvent-solute interaction Molecular view of the formation of solution DHsoln = DH 1 + DH 2 + DH 3 4

“like dissolves like” Two substances with similar intermolecular forces are likely to be soluble in each other. • non-polar molecules are soluble in non-polar solvents CCl 4 in C 6 H 6 • polar molecules are soluble in polar solvents C 2 H 5 OH in H 2 O • ionic compounds are more soluble in polar solvents Na. Cl in H 2 O or NH 3 (l) 5

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Review of Concepts Which of the following would you expect to be more soluble in benzene than in water: C 4 H 10, HBr, KNO 3, P 4 8

Example 12. 1 Pg 523 Predict the relative solubilities in the following cases: (a) Bromine (Br 2) in benzene (C 6 H 6, = 0 D) and in water ( = 1. 87 D) (b) KCl in carbon tetrachloride (CCl 4, = 0 D) and in liquid ammonia (NH 3, = 1. 46 D) (c) formaldehyde (CH 2 O) in carbon disulfide (CS 2, = 0 D) and in water

Concentration Units The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. Percent by Mass mass of solute x 100% % by mass = mass of solute + mass of solvent mass of solute x 100% = mass of solution Mole Fraction (X) moles of A XA = sum of moles of all components 10

Example 12. 2 Pg 525 A sample of 0. 892 g of potassium chloride (KCl) is dissolved in 54. 6 g of water. What is the percent by mass of KCl in the solution?

Concentration Units Molarity (M) M = moles of solute liters of solution 12

Temperature and Solubility Solid solubility and temperature solubility increases with increasing temperature solubility decreases with increasing temperature 13

Fractional crystallization is the separation of a mixture of substances into pure components on the basis of their differing solubilities. Suppose you have 90 g KNO 3 contaminated with 10 g Na. Cl. Fractional crystallization: 1. Dissolve sample in 100 m. L of water at 600 C 2. Cool solution to 00 C 3. All Na. Cl will stay in solution (s = 34. 2 g/100 g) 4. 78 g of PURE KNO 3 will precipitate (s = 12 g/100 g). 90 g – 12 g = 78 g 14

Temperature and Solubility O 2 gas solubility and temperature solubility usually decreases with increasing temperature 15

Pressure and Solubility of Gases The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law). c is the concentration (M) of the dissolved gas c = k. P P is the pressure of the gas over the solution k is a constant for each gas (mol/L • atm) that depends only on temperature low P high P low c high c 16

Colligative Properties Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. Vapor-Pressure Lowering Adding a nonvolatile solute reduces the relative amount of solvent, which results in fewer molecules becoming vapor % 0 10 nt e solv % 0 <10 nt e v l o s 17

Fractional Distillation Apparatus 18

Boiling-Point Elevation Adding solute increases the boiling point of the solvent Ex: adding salt to a pot of water to cook pasta Explained by vapor pressure lowering 19

Freezing-Point Depression Adding solute lowers the freezing point of the solvent Ex: salt or sand on roads to prevent ice 20

Effect of Concentration Solute Added to Water Resulting Freezing Point Just water 0°C 1 mol sugar − 1. 86°C 1 mol Na. Cl − 3. 72°C 1 mol Ca. Cl 2 − 5. 78°C 21

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A colloid is a dispersion of particles of one substance throughout a dispersing medium of another substance. Colloid versus solution • colloidal particles are much larger than solute molecules • colloidal suspension is not as homogeneous as a solution • colloids exhibit the Tyndall effect 23

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Hydrophilic and Hydrophobic Colloids Hydrophilic: water-loving Hydrophobic: water-fearing Stabilization of a hydrophobic colloid 25

The Cleansing Action of Soap 26