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“Water and Aqueous Systems”. “Aqua” Latin = water. Liquid Water and it’s Properties. 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.
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“Water and Aqueous Systems” “Aqua” Latin = water
Liquid Water and it’s Properties • 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 • Water is a simple tri-atomic molecule. • Each O-H bond is highly polar, because of the high electronegativity of the oxygen • bond angle = 104,5 o • 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 • Water’s bent shape and ability tohydrogen bond gives water many special properties. • Water molecules are attracted to one another. • This gives water: high surface tension, low vapor pressure, high specific heat, high heat of vaporization, and high boiling point
High Surface Tension • liquid water acts like it has a skin • glass of water bulges over the top • Water forms round drops • spray water on greasy surface • All because water hydrogen bonds
O H H O H H Surface Tension d- • One water molecule hydrogen bonds to another. • Also, hydrogen bonding occurs to other molecules all around. d+ d+ d- d+ d+
Surface Tension • A water molecule in the middle of solution is pulled in all directions.
Surface Tension • Not true at the surface. • Only pulled down and to each side. • Holds the molecules together. • Causes surface tension.
Surface Tension • Water drops are round, because all molecules on the edge are pulled to the middle- not to the air.
Surface Tension • Glass has polar molecules. • Glass can hydrogen bond. • Attracts the water molecules. • Some of the pull is up a cylinder.
Meniscus • Water curves up along the side of glass. • This makes the meniscus, as in a graduated cylinder • Plastics are non-wetting; no attraction
Meniscus In Plastic In Glass
Surface tension • All liquids have surface tension • water is higher than most others • 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 • 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 • Water has a high heat capacity (also called specific heat). • It absorbs 4.18 J/gºC, while iron absorbs only 0.447 J/gºC. • Remember: SHC = heat energy mass x DT • 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 • OBJECTIVES: • Account for the high heat of vaporization and the high boiling point of water, in terms of hydrogen bonding.
Water Vapor and Ice • OBJECTIVES: • Explain why ice floats in water.
Evaporation and Condensation • Because of the strong hydrogen bonds, it takes a large amount of energy to change water from a liquid to a vapour. • 2260 J/g is the heat of vaporization. • This much energy to boil 1 gram water • You get this much energy back when it condenses. • Steam burns, but heats things well.
Ice • Most liquids contract (get smaller) as they are cooled. • They get more dense. • When they change to solid, they are more dense than the liquid. • Solid metals sink in liquid metal. • But, ice floats in water. • Why?
Ice • Water becomes more dense as it cools until it reaches 4ºC. • Then it becomes less dense. • As the molecules slow down, they arrange themselves into honeycomb shaped crystals. • These are held together by hydrogen bonds.
H H H O O H H H H H O O H O H O H H H H O H O O H H H H H H O O O H H H Liquid Solid
Ice • 10% greater volume than water. • Water freezes from the top down. • The layer of ice on a pond acts as an insulator for water below • It takes a great deal of energy to turn solid water to liquid water. • Heat of fusion is: 334 J/g.
Aqueous Solutions • OBJECTIVES: • Explain the significance of the statement “like dissolves like”.
Aqueous Solutions • OBJECTIVES: • Distinguish among strong electrolytes, weak electrolytes, and non-electrolytes, giving examples of each.
Solvents and Solutes • Solution - a homogenous mixture, that is mixed molecule by molecule. • Solvent- the dissolving medium • Solute -the dissolved particles • Aqueous solution- a solution with water as the solvent. • Particle size about 1 nm; cannot be separated by filtration
Aqueous Solutions • Water dissolves ionic compounds and polar covalent molecules best. • The rule is: “like dissolves like” • Polar dissolves polar. • Non-polar dissolves non-polar. • Oil is non-polar. • Oil and water don’t mix. • Salt is ionic- makes salt water.
How Ionic solids dissolve • Called solvation. • Water breaks the + and - charged pieces apart and surrounds them. • In some ionic compounds, the attraction between ions is greater than the attraction exerted by water • Barium sulphate and calcium carbonate
H H H H O O O H H H H O O H H O O H H H H H H O H O H How Ionic solids dissolve
Solids will dissolve if the attractive force of the water molecules is stronger than the attractive force of the crystal. • If not, the solids are insoluble. • Water doesn’t dissolve non-polar molecules because the water molecules can’t hold onto them. • The water molecules hold onto each other, and separate from the non-polar molecules. • Non-polars? No repulsion between them
Electrolytes and Non-electrolytes • Electrolytes- compounds that conduct an electric current in aqueous solution, or in the molten state • all ionic compounds are electrolytes (they are also salts) • barium sulphate- will conduct when molten, but is insoluble in water!
Electrolytes and Non-electrolytes • Do not conduct? Non-electrolytes. • Many molecular materials, because they do not have ions • Not all electrolytes conduct to the same degree • there are weak electrolytes, and strong electrolytes • depends on: degree of ionization
Electrolyte Summary • Substances that conduct electricity when dissolved in water, or molten. • Must have charged particles that can move. • Ionic compounds break into charged ions: NaCl ® Na+and Cl- • These ions can conduct electricity.
Non-electrolytes do not conduct electricity when dissolved in water or molten • Polar covalent molecules such as methanol (CH3OH) don’t fall apart into ions when they dissolve. • Weak electrolytes don’t fall completely apart into ions. • Strong electrolytes do ionise completely.
+ heat - heat Water of Hydration(or Water of Crystallization) • Water molecules chemically bonded to solid salt molecules (not in solution) • These compounds have fixed amounts of water. • The water can be driven off by heating: • CuSO4.5H2O CuSO4 + 5H2O • Called copper(II)sulphate pentahydrate.
Hydrates • Since heat can drive off the water, the forces holding it are weak • 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 • 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 • 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 • OBJECTIVES: • Explain how colloids and suspensions differ from solutions.
Heterogeneous Aqueous Systems • OBJECTIVES: • Describe the Tyndall effect.
Mixtures that are NOT Solutions • Suspensions: mixtures that slowly settle upon standing. • Particles of a suspension are greater in diameter than 100 nm. • Can be separated by filtering • Colloids: heterogeneous mixtures with particles between size of suspensions and true solutions (1-100 nm)
Mixtures that are NOT Solutions • The small particles are the dispersedphase, and are spread throughout the dispersion medium • The first colloids were glues. Others include mixtures such as gelatin, paint, aerosol sprays, and smoke
Mixtures that are NOT Solutions • Many colloids are cloudy or milky in appearance when concentrated, but almost clear when dilute • do not settle out • cannot be filtered out • Colloids exhibit the Tyndall effect- the scattering of visible light in all directions. • suspensions also show Tyndall effect
Mixtures that are NOT Solutions • Flashes of light are seen when colloids are studied under a microscope- light is reflecting- called Brownian motiontodescribe the chaotic movement of the particles
Mixtures that are NOT Solutions • 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. • Oil and vinegar dressing? • Mayonnaise? Margarine?