WATER AND AQUEOUS SYSTEMS
A. The Water Molecule
1. Triatomic molecule with two O-H single covalent bonds
a. Very polar covalent bonds (3.5 - 2.1 = 1.4)
b. H-O-H bond angle of 105 degrees
1. Very polar molecule
2. Intermolecular attraction due to Hydrogen Bonds
3. Many unique properties from this info
a. High surface tension
b. High specific heat capacity
c. High boiling point
d. high heat of vaporization
e. Low vapor pressure
B. Surface Properties
1. Surface tension -- the inward force, or pull, that tends to minimize the surface area of a liquid
a. the surface acts like a skin
2. Reason: Hydrogen bonds between molecules
a. Molecules on the surface are not completely surrounded by other water molecules, so they draw towards the center more
b. Helps to keep liquids in a spherical shape
1. Greater surface tension, more spherical drop
3. Lower surface tension with a surfactant -- a wetting agent that interferes with hydrogen bonds
4. Low vapor pressure of water is also a result of high surface tension
a. Since molecules are held together more, less will be able to excape into the vapor phase
C. Specific Heat Capacity
1. For water = 4.18 J/g C
a. very high compared to other compounds
2. Reason: Hydrogen bonds
b. Example: Iron = .447 J/g C
c. If you add 10 J of energy to 1 g or each at room temperature, which one will have the higher final temperature?
1. Answer: Iron, lower heat capacity, greater temperature change
II. Water vapor and Ice
A. Evaporation and Condensation
1. Heat of Vaporization for water = 2.26 kJ/g
a. very high
2. Reason: Hydrogen bonds
a. It takes 2.26 kJ of energy to break H-bonds in 1 g of liquid water to change it to steam
b. Example: Ammonia = 1.37 kJ/g --> less hydrogen bonding
Methane = 0.510 kJ/g --> no hydrogen bonding
3. Heat of Condensation for water = 2.26 kJ/g
a. Opposite of Vaporization (heat energy is released)
4. Allows water to be a liquid at moderate temperatures
a. Moderates the temperature of the entire planet
B. Ice
1. Most liquids contract slightly as they become a solid
a. solid is more dense than the liquid, so it will sink
2. water does this until it reaches 4 C
a. Below 4 C, the density decreases while forming ice
b. Ice has 10% greater volume than water
1. D(Ice) = .92 g/mL, D(water) = .99 g/mL
c. Ice floats in water!
3. Reason: Hydrogen bonds
a. As the ice forms, the H-bonds hold the molecules in a "honeycomb" looking framework
b. Need 334 J/g to break the rigid framework of ice
1. Very high heat of Fusion
4. Very important for aquatic life during winter months!
III. Aqueous Solutions
A. Solvents and solutes
1. Aqueous solutions -- water samples containing dissolved substances
a. solvent -- the dissolving medium (ex. -- water)
b. solute -- the dissolved particles (ex. -- salt)
2. Solutions are homogeneous mixtures
3. in water, solutes can be ionic or polar molecular compounds
a. Nonpolar molecular compounds do not dissolve in water
b. solvents/solutes can be solids, liquids, or gases
B. The Solution Process
1. Solvation -- the process that occurs whena solute dissolves
a. water molecules collides with solid crystals
b. water molecules attract the solute ions
c. water molecules surround each ion (solvated ion)
1. negative end of water molecules attracts to positive ions
2. Positive end of water molecules attracts to negative ions
2. Some ionic compounds have very strong ionic bonds, and are therefore insoluble
3. Polar solvents dissolve polar solutes and ionic solutes
4. Nonpolar solvents dissolve nonpolar solutes
5. "Like dissolves like"
a. Reason why oil and water don't mix!
C. Electrolytes and Nonelectrolytes
1. Electrolyte -- compounds that conduct electricity in solution
a. Strong electrolytes -- all solute becomes ions
b. Weak electrolytes -- some solute becomes ions
c. Nonelectrolytes -- compounds do not conduct electricity in solution
Strong electrolytes Weak electrolytes Nonelectrolytes
Inorganic acids (HCl) Heavy Metal Halides (HgCl2) Most organic compounds
Inorganic bases (NaOH) Inorganic bases (NH3) (Glucose, Glycerol)
soluble salts (KCl) Organic acids (Acetic)
Organic bases (aniline)
D. Water of Hydration -- the water attached to crystal structures
1. Effloresce -- hydrate loses water of hydration when it's vapor pressure is greater than the vapor pressure in the air
2. Hygroscopic -- crystals with low wapor pressure remove water from moist (humid) air
3. Desiccants -- Hygroscopic substances used as drying agents
4. Deliquescent -- remove enough water from the air to make solutions
5. Example -- Na2CO3.10H2O Percent Water = ?
Use periodic table to find mass of water = 10 x 18 g/mol = 180 g/mol
Use periodic table to find mass of total hydrate = 286 g/mol
% water = (180 g/mol)/(286 g/mol) x 100% = 62.9 %
IV. Heterogeneous Aqueous Systems
A. Suspensions and Colloids
1. suspensions -- mixtures in which the particles settle out when left alone (clay water)
a. Particles are too large to dissolve, instead they float in the water
b. Particles can be filtered out by basic filtration methods
c. considered a Heterogeneous mixture
2. Colloids -- mixtures in which particles are smaller than suspensions, but larger than solutions (milk, condiments)
a. Particles are too large to dissolve, but small enough that they do not settle out
b. Particles can not be filtered out by basic methods
c. considered a Heterogeneous mixture
3. Tyndall Effect -- the scattering of visible light in all directions by particles
a. You see the Tyndall effect all the time in dusty movie theaters!
b. Colloids and Suspensions exhibit the Tyndall Effect
c. Solutions do not exhibit the Tyndall Effect
4. Emulsions -- colloids with liquids in liquids (mayo, ketchup)
a. Can settle out after long periods of time
Outline based upon:
Matta, M. S., Staley, D. D., Waterman, E. L., & Wilbraham, A. C. (2000). Chemistry, Addison-Wesley. (5th ed.). Menlo Park, CA: Prentice Hall, pp. 475-495.
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