Unit 2 4 Chemical Bonding Sections 4 1

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

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 3

Solvation or dissolution, is the process in which a solute dissolves into a solvent. • A solvated ion or molecule is surrounded by solvent molecules arranged in a specific manner 4

Solubility is the property of a substance (solute) to dissolve in another substance (solvent). • Measured in terms of the maximum amount of solute dissolved in a solvent at a specific temperature • The resulting solution is called a saturated solution • Certain substances are soluble in all proportions with a given solvent, this property is known as miscibility. • Solubility ranges widely from infinitely soluble to poorly soluble; in a relevant sense scientists refer to substances as being soluble, slightly soluble, or insoluble • Insoluble is often applied to poorly soluble compounds, though strictly speaking there are very few cases where there is absolutely no material dissolved 5

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Predicting Solubility in a particular solvent “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) 7

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Example 12. 1 Predict the relative solubility 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

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 10

Solubility Curve shows the temperature dependence on the solubility of a substance 11

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. 12

Crystallization is the process in which dissolved solute comes out of solution and forms crystals; describes the separation of excess solid substance from a supersaturated solution. Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. 13

Hydration is the process in which a molecule or ion is surrounded by water molecules arranged in a specific manner. d- d+ H 2 O 14

An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity. A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity. nonelectrolyte weak electrolyte strong electrolyte 15

A separation process is a method to convert a substance or mixture into its components. Physical separation techniques are based on the physical properties of the substances such as state, density, melting/boiling point, and solubility • Filtration • Chromatography • • Paper • Colum Distillation 16

Filtration is the process of separating suspended solid particles in a fluid (liquid or gas) based on a difference in size. Solid particles are removed by the use of a porous medium that permits the fluid to pass through, and retains the solid particles. 17

Chromatography is the process of separating a mixture into its individual components, typically so that the individual components can be thoroughly analyzed. • separation is based on differential affinities (strength of adhesion) of the various components of the analyte towards the stationary and mobile phases • Affinity is dictated by two properties of the molecule: adsorption (how well a component of the mixture sticks to the stationary phase) and solubility (how well a component of the mixture dissolves in the mobile phase) 18

Paper Chromatography 19

Column Chromatography 20

Distillation is the process of separating the components of a homogeneous mixture of liquids based on differences in boiling point. 21

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 22

Example 12. 2 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 Molality (m) m = moles of solute mass of solvent (kg) 24

Preparing a Solution of Known Concentration 25

Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution. Dilution Add Solvent Moles of solute before dilution (i) = Moles of solute after dilution (f) Mi V i = Mf V f 26

Example 4. 9 Describe how you would prepare 5. 00 × 102 m. L of a 1. 75 M H 2 SO 4 solution, starting with an 8. 61 M stock solution of H 2 SO 4.

Temperature and Solubility Solid solubility and temperature solubility increases with increasing temperature solubility decreases with increasing temperature 28

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 29

Temperature and Solubility O 2 gas solubility and temperature solubility usually decreases with increasing temperature 30

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 31

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 32

Fractional Distillation Apparatus 33

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 34

Freezing-Point Depression Adding solute lowers the freezing point of the solvent Ex: salt or sand on roads to prevent ice 35

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 van’t Hoff factor (i) = actual number of particles in soln after dissociation number of formula units initially dissolved in soln i should be nonelectrolytes Na. Cl Ca. Cl 2 1 2 3 36

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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 38

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Hydrophilic and Hydrophobic Colloids Hydrophilic: water-loving Hydrophobic: water-fearing Stabilization of a hydrophobic colloid 40

The Cleansing Action of Soap 41