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Chapter 15 Notes
1
CHAPTER 15Water and
Aqueous Systems
15.1 Water And Its Properties
• Bent Triatomic molecule• Covalent bonds• Polar
• Bond angle 105°• Causes polar bonds.• Dipolar interaction
polar molecules attracted to each other
• Hydrogen bondsAttraction between Two polar molecules
• What is a polar bond?
• What is a polar molecule?
Polar Molecule Non Polar Molecule
HHO HH C
H
H
HHO
HHO
HHO
Chapter 15 Notes
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Polar Bond in Water Molecules• Each OH bond is very polar.• “O” becomes slightly negative. (O§ )• “H” becomes slightly positive. (H§+ )• Causes attraction between water molecules called hydrogen bonding.• Hydrogen bonding is the reason that water has many unique properties.
Hydrogen Bonds• The O§ in one water molecule is attracted to
the H§+ another.
• High Surface tension• Low vapor pressure• High specific heat• High boiling point• High heat of vaporization
Energy required to change Temperature
Water• Covers 75% of earth’s surface.• Foundation of all living things.• Earth is the water planet.
Surface Tension • The inward force or pull which tends to minimize the surface area of any liquid.• Water’s surface tension is higher than most due to the strong Hydrogen bonds, but all liquids have some surface tension.
HHO
HHO
HHO
HHO
HHO
HHO
HHO
HHO
HHOHH
O
HHO
HHO
HHO
HHO
HHO
HHO
HHO
HHO
HHO
HHO
HHO
HHO
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Spherical Shape of Water
• The higher the surface tension, the more spherical the drop of that particular liquid.• Gravity tends to flatten out most liquids.
• Surface tension is explained by waters ability to form Hbonds
• Surface molecules have uneven attraction • H bonds only form on one side of a drop
because there are no molecules on the other side.
• Hydrogen bonding makes it difficult for water molecules on the surface to escape because water molecules are not attracted to the air.• This causes the water molecules on the surface to be drawn into the liquid and decreases surface area.
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• The surface tension of water may be decreased by adding a wetting agent such as soap or detergent.
• Soaps or detergents are surfactants.• Surfactants interfere with the hydrogen bonding between water molecules and stop them from beading.
Heat Capacity of Water• 4.18 J/g x ° C (Specific Heat of Water)• 0.447 J/g x °C (Specific Heat of Iron)• Raise 1g of a substance 1°C• What gets hotter in the sun, Iron or water?
Heat Capacity of Water (cont.)• On warm days, water absorbs heat and lowers air temperature.• On cold nights, water releases absorbed hear to raiser air temp.• Water is a storage medium for solar energy. (Deserts are hot!)
Water has Low Vapor Pressure• Hydrogen bonding is responsible for water’s low vapor pressure
• Hydrogen bonds prohibit the surface molecules from escaping easily.
• Lakes would evaporate!
Heat of Vaporization• Amount of energy needed to convert 1g of a substance from liquid to gas.• Water absorbs energy and warms• Water = 2.26 kj/g• Ammonia = 1.37 kj/g• Methane = 0.51 kj/g
Heat of VaporizationHow much heat is absorbed when 8.2 grams of liquid water evaporates?• It takes 2.26 kJ of energy to convert 1
gram of liquid water into gas.
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Heat of Condensation• Reverse of Vaporization• Releases energy and cools• As steam condenses, it releases energy and
will burn you.• Tropics would be warmer and poles would
be colder.
Substances similar to water• Most substances similar to water are gases at normal temperature.• Due to HBonding.• If Hbonding were weak in water, like it is in Ammonia, there would be no liquid water.
Water in the Solid State
• Because the density of a typical solid is greater than that of the liquid, the solid sinks in its own liquid.
• As water is cooled, it first behaves like a typical liquid. Its density gradually increases until the temperature reaches 4°C.
• Below 4°C the density of water starts to decrease. At 0°C, ice begins to form.
• Ice has about 10% greater volume and therefore a lower density than water. As a result, ice floats on water. Ice is one of the very few solids that float in their own liquids. Became less dense
• The structure of ice is a very regular open framework of water molecules arranged like a honeycomb. Hydrogen bonds hold the water molecules in place.
• Important consequence for aquatic life
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• Water molecules require a considerable amount of kinetic energy to return to a liquid state from a frozen state.• The heat absorbed when 1 g of water changes from a solid to a liquid is 334 J/g.• 1 kJ = 1000 J
How much heat is absorbed when 4.5 g of ice melts?
• It takes 334 joules of energy to melt 1 gram.
Solid Liquid
Much more difficult to change
Phase than Temperature.15.2 Homogeneous Aqueous Systems
• Solvation occurs when a solute dissolves in a solvent• Ex. NaCl in water
* Uniform composition
Solvents and Solutes• Solvent: The dissolving medium
• Aqueous Solution: Water that contains dissolved substances
• Solute: Dissolved particles
Solvation• Negatively and positively charged ions become surrounded by solvent molecules.
• Polar solvents dissolve ionic and polar compounds.
• NonPolar solvents dissolve nonpolar compounds.
Chapter 15 Notes
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Na+ ClHH
O
HHO
H HO
HHO
H HO
HHO
HHO
Solvation (cont.)• Like dissolves like.• Salt and water: ionic & polar• Gasoline and grease: non polar
• In some ionic compounds, the attraction of the ions is stronger than the attraction from the water.
Electrolytes• Compounds that conduct an electric current in
aqueous solution or the molten state are electrolytes• All ionic compounds • Insoluble ionic compounds are ionic compounds in
molten state.• Barium sulfate only in molten state.
Nonelectrolytes• Do not conduct an electric current in either
aqueous solution or the molten state.• Many molecular compounds are nonelectrolytes
because they are not composed of ions. • Compounds made of carbon.
• When a weak electrolyte is in solution, only a fraction of the solute exists as ions
• (Ammonia, Water)• When a strong electrolyte is dissolved, almost
all of the solute exists as ions.• (Acids, Bases, Soluble Salts)
Chapter 15 Notes
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Why is more energy required to convert liquid to vapor than to convert ice into liquid?
What is the temperature at which ice can change to liquid?
What is the temperature at which liquid turns to vapor?
Hydrates• Water molecules are an integral part of the crystal structure of many substances.• The water in a crystal is called the water of hydration or water of crystallization
• Ex. CuSO4. 5H2O
• If a hydrate has a vapor pressure higher than that of the water vapor in air, the hydrate will effloresce by losing the water of hydration.
• Salts and other compounds that remove moisture from air are said to be hygroscopic
• Hygroscopic substances are used as drying agents, or desiccants.
• They have Less vapor pressure in the compound than in the air.
• These deliquescent compounds remove sufficient water from the air to dissolve completely and form solutions
What is the percentage of water in the hydrate CoCl2 6H2O?
Co: 1 x 59= 59gramsCl: 2 x 35.5 = 71grams
6H2O: 6 x 18grams = 108grams
130 grams
Total Mass of Hydrate:238grams
%CoCl2: 54.6 %%H2O: 45.4 %
Chapter 15 Notes
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How many grams of CoCl 2 will remain if 54.0grams of the hydrate CoCl 2 6H2O is heated until only the anhydrous salt remains?
%CoCl2: 54.6 %%H2O: 45.4 %
After heating 5.0grams of a hydrate, 3.9grams of anhydrous salt remained. What was the percentage of water in the hydrate?
15.3 Heterogeneous Aqueous Systems
• Suspensions: mixtures from which some of the particles will settle slowly upon standing
• Heterogeneous• Can be filtered• Ex. Clay and water
You can Tell the differenceparticle are large
No DissolvingColloids
• Mixtures containing particles that are intermediate in size between those of suspensions and true solutions.
• No settling.• Cannot be filtered.• Ex. Glue, jello, paint
Water molecules surround the Na+ and Cl ions.
Making the solution uniform when dissolving occurs.
Na+ ClHH
O
HHO
H HO
HHO
H HO
HHO
HHO
• The properties of colloids differ from those of solutions and suspensions.
• Many colloids are cloudy or milky in appearance but look clear when they are very dilute.
• The particles in a colloid cannot be retained by filter paper and do not settle out with time.
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• Colloidal particles exhibit the Tyndall effect, which is the scattering of visible light in all directions.
• Suspensions also exhibit the Tyndall effect, but solutions never do.
Why? No dissolving!!
Not uniform!!
• The chaotic movement of colloidal particles is called Brownian motion
• Brownian motion is caused by the water molecules of the medium colliding with the small, dispersed colloidal particles.
Solution Colloid Suspension
Dissolving
Cannot be filtered
Small Particles
Uniform Throughout
Homogeneous
No Settling
No Tyndall Effect
No Brownian Motion
• Emulsions are colloidal dispersions of liquids in liquids.
• Soaps and other emulsifying agents allow the formation of colloidal dispersions between liquids that do not ordinarily mix.
• • Oil & Water