Water and Aqueous Systems Chapter 15 Water and Aqueous Systems

15.1 Water and Its Properties Molecule has net polarity Polar bonds δ– δ+ Water in the Liquid State Water is a polar molecule. That means part of the water molecule has a partial positive charge while the other part of the water molecule has a partial negative charge. This explains why water has a “bent” appearance This also allows water molecules to attract to other nearby water molecules (+ side of one to – side of another) The oxygen atom acquires a partial negative charge (δ–).

Hydrogen Bonding These strong attractive forces between 2 different water molecules is called Hydrogen Bonding Hydrogen Bonding causes water to have several unique properties: High Surface Tension Low Vapor Pressure High Boiling Point Liquid water Hydrogen bond

Surface Tension Have you ever noticed that water forms nearly spherical droplets on a leaf? The inward force, or pull, that tends to minimize the surface area of a liquid is called surface tension. All liquids have a surface tension, but water’s surface tension is higher than most. The surface tension of water tends to hold a drop of liquid in a spherical shape.

Vapor Pressure Hydrogen bonding between water molecules also explains water’s unusually low vapor pressure. An extensive network of hydrogen bonds holds the molecules in liquid water to one another. These hydrogen bonds must be broken before water changes from the liquid to the vapor state, so the tendency of these molecules to escape is low and evaporation is slow.

Boiling Point Molecular compounds of low molecular mass are usually gases or liquids with low boiling points at normal atmospheric pressure. Ammonia (NH3) has a molar mass of 17.0 g/mol and boils at about – 33˚C. Water has a molar mass of 18.0 g/mol, but it has a boiling point of 100˚C. The difference between the boiling points of ammonia and water is due to hydrogen bonding, which is more extensive in water than in ammonia. It takes much more heat to disrupt the attractions between water molecules than those between ammonia molecules.

Water in the Solid State Ice cubes float in your glass of iced tea because solid water has a lower density than liquid water. This situation is not usual for liquids. When the temperature of the water falls below 4˚C, the density of water actually starts to decrease. Density of Liquid Water and Ice Temperature (˚C) Density (g/cm3) 100 (liquid water) 0.9584 50 0.9881 25 0.9971 10 0.9997 4 1.0000 0 (liquid water) 0.9998 0 (ice) 0.9168

Density of Ice Why is ice less dense than liquid water? The structure of ice is a regular open framework of water molecules in a hexagonal arrangement.

15.3Heterogeneous Aqueous Systems Suspensions – A mixture where particles will settle upon standing. Heterogeneous Mixtures Large sized particles Ex: Muddy Water Colloids – A heterogeneous mixture where particles are intermediate in size. Colloids can look cloudy or milky or might even look clear. Tyndall Effect – The scattering of light by medium –large sized particles. Can be used to differentiate between colloids and solution Flashlight Solution Colloid Suspension

Some Colloidal Systems Colloids The first substances to be identified as colloids were glues. Some Colloidal Systems System Type Example Dispersed phase Dispersion medium Gas Liquid Foam Whipped cream Solid Marshmallow Emulsion Milk, mayonnaise Aerosol Fog, aerosol Smoke Dust in air Sols, gels Egg white, jelly, paint, blood, starch in water, gelatin Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

15.2 Homogeneous Aqueous Systems Solutions Solvent – In a solution, the dissolving material Ex: The Water Solute – The dissolved particles in a solution. Ex: The Salt Aqueous Solution – A solution where water is the solvent. (as opposed to alcohol or oil)

3 Types of solutions Gaseous solutions– air Liquid solutions – vinegar (acetic acid dissolved in water); soft drinks (solutions of a gas, CO2, dissolved in water. Solid solutions– alloys such as sterling silver – 92% silver, 8% copper; white gold – gold containing nickel, tin, zinc or copper.

Solvation Solvated ions Surface of ionic solid Solvation – The process by which ionic compounds dissolve in a solvent DRAW THE PICTURE

Electrolytes and Nonelectrolytes Electrolyte – A compound that conducts an electric current when dissolved in water. All ionic compounds are electrolytes Strong Electrolytes – all of the ionic compound breaks apart into ions Ex: NaCl Weak Electrolytes – only some of the ionic compound breaks apart into ions Ex: Vinegar Nonelectrolyte – A compound that does not conduct an electric current in solution. Solutes dissolve to form: Ions Molecules (electrolytes) (non-electrolytes)

Electrolyte Nonelectrolyte To (+) electrode To (–) electrode To (+) electrode To (–) electrode

Classification of Matter Chart Pure Substances Mixtures Elements Compounds Homogeneous Heterogeneous Solutions Colloids Suspensions

16.1 Properties of Solutions Solution Formation If you wish to dissolve a substance, you can help by… Stirring it Stirring sugar in cold tea vs not stirring at all Heating it Think sugar in cold tea vs sugar in hot tea Crushing it – this increases the surface area of the solute Think sugar packet instead of sugar cube

Solubility Solubility – The amount of solute that dissolves in a given amount of solvent at a specific temperature and pressure Saturated Solution – Contains the maximum amount of solute in a given amount of solvent If even more solute is added to the solution, the salt will just sink to the bottom. No amount of stirring will get the solute to dissolve. Unsaturated Solution – Contains less than the maximum amount of solute in a given amount of solvent. If more solute is added to the solution, it salt will dissolve.

Factors Affecting Solubility 1. Types of solvents and solutes –“Like dissolves like” Polar/ionic versus nonpolar water oil salt gasoline sugar Styrofoam

Factors Affecting Solubility 2. Temperature For most solids, solubility increases as temperature increases Graph for solids: Supersaturated Solution – Contains more solute than it can theoretically hold at a given temperature. How is this possible? These solutions are created by saturating a hot solution and allowing it to cool undisturbed. Sometimes, the extra salt will stay dissolved after the temperature has been cooled back down.

Supersaturated Solutions

Factors Affecting Solubility 2. Temperature For most solids, solubility increases as temperature increases Graph for solids: For gases, solubility decreases as temperature increases Graph for gases:

Factors Affecting Solubility 3. Pressure (gases only) As pressure increases, solubility increases Henry’s law: solubility is proportional to pressure.

Solubility Problems Ex1: What is the solubility of potassium chlorate at 50.0 oC in 100.0 ml of water? 20.0 g of potassium chlorate in 100.0 grams of water Ex2: What temperature will result in a saturated solution of 80.0 grams of sodium nitrate and 100.0 grams of water? 10.0 oC

Solubility Problems Ex3: If 40.0 grams of ammonium chloride are placed in 100.0 grams of water at 50.0 oC, is the solution saturated or unsaturated? If saturated, how much salt remains undissolved? If unsaturated, how much more salt can be dissolved? The solution is unsaturated and can hold 10.0 more grams of ammonium chloride Ex4: If 80.0 grams of potassium nitrate are placed in 100.0 grams of water at 44.0°C, is the solution saturated or unsaturated? If saturated, how much salt remains undissolved? If unsaturated, how much more salt can be dissolved? The solution is saturated with 5.0 grams of potassium nitrate undissolved

Solubility Problems Ex5: What is the solubility of lithium sulfate at 90.0 oC in 50.0 ml of water? At this temperature the 100.0 ml of water can hold 30.0 grams of this salt. So, if half as much water is present, half as much salt will dissolve. 30.0 g = x 100.0 ml 50.0 ml x = 15.0 grams of lithium sulfate in 50.0 grams of water

Solubility Problems Ex6: What is the solubility of lithium sulfate at 90.0 oC in 200.0 ml of water? 30.0 g = x 100.0 ml 200.0 ml x = 60.0 grams of lithium sulfate in 200.0 grams of water

Solubility Problems Ex7: What is the solubility of lithium sulfate at 90.0 oC in 68.2 ml of water? 30.0 g = x 100.0 ml 68.2 ml x = 20.5 grams of lithium sulfate in 68.2 grams of water