“Water and Aqueous Systems” “Aqua” Latin = water

Liquid Water and it’s Properties l OBJECTIVES: –Describe the hydrogen bonding that occurs in water. –Explain the high surface tension and low vapor pressure of water in terms of hydrogen bonding.

The Water Molecule l Water is a simple tri-atomic molecule. l Each O-H bond is highly polar, because of the high electronegativity of the oxygen l bond angle = 104,5 o l due to the angular (bent) shape, the O- H bond polarity vectors do not cancel out. This means water as a whole is polar.

The Water Molecule l Water’s bent shape and ability to hydrogen bond gives water many special properties. l Water molecules are attracted to one another. l This gives water: high surface tension, low vapor pressure, high specific heat, high heat of vaporization, and high boiling point

High Surface Tension l liquid water acts like it has a skin –glass of water bulges over the top l Water forms round drops –spray water on greasy surface l All because water hydrogen bonds

Surface Tension l One water molecule hydrogen bonds to another. l Also, hydrogen bonding occurs to other molecules all around. H H O ++ ++ -- H H O ++ -- ++

Surface Tension l A water molecule in the middle of solution is pulled in all directions.

Surface Tension l Not true at the surface. l Only pulled down and to each side. l Holds the molecules together. l Causes surface tension.

Surface Tension l Water drops are round, because all molecules on the edge are pulled to the middle- not to the air.

Surface Tension l Glass has polar molecules. l Glass can hydrogen bond. l Attracts the water molecules. l Some of the pull is up a cylinder.

Meniscus l Water curves up along the side of glass. l This makes the meniscus, as in a graduated cylinder l Plastics are non- wetting; no attraction

Meniscus In Glass In Plastic

Surface tension l All liquids have surface tension –water is higher than most others l How to decrease surface tension? –Use a surfactant - surface active agent –a wetting agent, like detergent or soap –interferes with hydrogen bonding

Low vapour pressure l Hydrogen bonding also explains water’s unusually low vapor pressure. –Holds water molecules together, so they do not escape –good thing- lakes and oceans would evaporate very quickly

Specific Heat Capacity l Water has a high heat capacity (also called specific heat). l It absorbs 4.18 J/gºC, while iron absorbs only J/gºC. Remember: SHC = heat energy mass x  T l If we calculate the heat need to raise the temperature of both iron and water by 75ºC - water is almost 10 x more!

Water Vapour and Ice l OBJECTIVES: –Account for the high heat of vaporization and the high boiling point of water, in terms of hydrogen bonding.

Water Vapor and Ice l OBJECTIVES: –Explain why ice floats in water.

Evaporation and Condensation l Because of the strong hydrogen bonds, it takes a large amount of energy to change water from a liquid to a vapour. l 2260 J/g is the heat of vaporization. –This much energy to boil 1 gram water l You get this much energy back when it condenses. l Steam burns, but heats things well.

Ice l Most liquids contract (get smaller) as they are cooled. l They get more dense. l When they change to solid, they are more dense than the liquid. l Solid metals sink in liquid metal. –But, ice floats in water. l Why?

Ice l Water becomes more dense as it cools until it reaches 4ºC. l Then it becomes less dense. l As the molecules slow down, they arrange themselves into honeycomb shaped crystals. l These are held together by hydrogen bonds.

H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O H H O LiquidSolid

Ice l 10% greater volume than water. l Water freezes from the top down. –The layer of ice on a pond acts as an insulator for water below l It takes a great deal of energy to turn solid water to liquid water. l Heat of fusion is: 334 J/g.

Aqueous Solutions l OBJECTIVES: –Explain the significance of the statement “like dissolves like”.

Aqueous Solutions l OBJECTIVES: –Distinguish among strong electrolytes, weak electrolytes, and non-electrolytes, giving examples of each.

Solvents and Solutes l Solution - a homogenous mixture, that is mixed molecule by molecule. l Solvent - the dissolving medium l Solute -the dissolved particles l Aqueous solution- a solution with water as the solvent. l Particle size about 1 nm; cannot be separated by filtration

Aqueous Solutions l Water dissolves ionic compounds and polar covalent molecules best. l The rule is: “like dissolves like” l Polar dissolves polar. l Non-polar dissolves non-polar. l Oil is non-polar. –Oil and water don’t mix. l Salt is ionic- makes salt water.

How Ionic solids dissolve l Called solvation. l Water breaks the + and - charged pieces apart and surrounds them. l In some ionic compounds, the attraction between ions is greater than the attraction exerted by water –Barium sulphate and calcium carbonate

How Ionic solids dissolve H H O H H O H H O H H O H H O H H O H H O H H O H H O

l Solids will dissolve if the attractive force of the water molecules is stronger than the attractive force of the crystal. l If not, the solids are insoluble. l Water doesn’t dissolve non-polar molecules because the water molecules can’t hold onto them. l The water molecules hold onto each other, and separate from the non-polar molecules. l Non-polars? No repulsion between them

Electrolytes and Non-electrolytes l Electrolytes- compounds that conduct an electric current in aqueous solution, or in the molten state –all ionic compounds are electrolytes (they are also salts) l barium sulphate- will conduct when molten, but is insoluble in water!

Electrolytes and Non-electrolytes l Do not conduct? Non-electrolytes. –Many molecular materials, because they do not have ions l Not all electrolytes conduct to the same degree –there are weak electrolytes, and strong electrolytes –depends on: degree of ionization

Electrolyte Summary l Substances that conduct electricity when dissolved in water, or molten. l Must have charged particles that can move. l Ionic compounds break into charged ions: NaCl  Na + and Cl - l These ions can conduct electricity.

l Non-electrolytes do not conduct electricity when dissolved in water or molten l Polar covalent molecules such as methanol (CH 3 OH) don’t fall apart into ions when they dissolve. l Weak electrolytes don’t fall completely apart into ions. l Strong electrolytes do ionise completely.

Water of Hydration (or Water of Crystallization) l Water molecules chemically bonded to solid salt molecules (not in solution) l These compounds have fixed amounts of water. l The water can be driven off by heating: l CuSO 4. 5H 2 O CuSO 4 + 5H 2 O l Called copper(II)sulphate pentahydrate. - heat + heat

Hydrates l Since heat can drive off the water, the forces holding it are weak l If a hydrate has a vapour pressure higher than that of water vapor in air, the hydrate will effloresce by losing the water of hydration

Hydrates l Some hydrates that have a low vapour pressure remove water from the air to form higher hydrates- called hygroscopic –used as drying agents, or dessicants –packaged with products to absorb moisture

Hydrates l Some compounds are so hygroscopic, they become wet when exposed to normally moist air- called deliquescent –remove sufficient water to dissolve completely and form solutions

Heterogeneous Aqueous Systems l OBJECTIVES: –Explain how colloids and suspensions differ from solutions.

Heterogeneous Aqueous Systems l OBJECTIVES: –Describe the Tyndall effect.

Mixtures that are NOT Solutions l Suspensions: mixtures that slowly settle upon standing. –Particles of a suspension are greater in diameter than 100 nm. –Can be separated by filtering l Colloids: heterogeneous mixtures with particles between size of suspensions and true solutions (1-100 nm)

Mixtures that are NOT Solutions l The small particles are the dispersed phase, and are spread throughout the dispersion medium l The first colloids were glues. Others include mixtures such as gelatin, paint, aerosol sprays, and smoke

Mixtures that are NOT Solutions l Many colloids are cloudy or milky in appearance when concentrated, but almost clear when dilute –do not settle out –cannot be filtered out l Colloids exhibit the Tyndall effect- the scattering of visible light in all directions. –suspensions also show Tyndall effect

Mixtures that are NOT Solutions l Flashes of light are seen when colloids are studied under a microscope- light is reflecting- called Brownian motion to describe the chaotic movement of the particles

Mixtures that are NOT Solutions l Emulsions- colloids dispersions of liquids in liquids –an emulsifying agent is essential for maintaining stability –oil and water not soluble; but with soap or detergent, they will be. l Oil and vinegar dressing? –Mayonnaise? Margarine?