Water and Aqueous Systems Ch. 17

Liquid Water and its Properties 17-1

The Water Molecule, in review… The water molecule, H – O – H, is made up of covalent bonds that are highly polar. It has a bent shape with 105° bond angles. Oxygen has a slightly negative charge (more electronegative), while Hydrogen has a slightly positive charge. Water molecules attracted to each other through hydrogen bonds.

DRAW!

Many unique and important properties of water are due to it’s hydrogen bonds! Including its… High surface tension Low vapor pressure High specific heat capacity High heat of vaporization High boiling point Ice is less dense than water 17-1 17-2

Surface Properties 1. Surface tension = inward force, or pull, that tends to minimize the surface area of a liquid. Due to the hydrogen bonding (attraction) between water molecules. Water has HIGH surface tension, holds drops in a spherical shape. Surfactant = wetting agent, soap or detergent, that decreases surface tension (interferes with H-bonding) 2. LOW vapor pressure; hydrogen bonds keep water in liquid form from escaping into gaseous form.

Specific Heat Capacity 3. Specific Heat Capacity (C) = it takes 4.18 J (1 cal) of heat energy to raise the temperature of 1g of water 1°C. Due to hydrogen bonding, water has a HIGH specific heat capacity Cwater = 4.18 J/(g°C) C = Q = heat (J or cal) m x ΔT mass (g) x (°C)

Water Vapor and Ice 17-2

Evaporation + Condensation Heat of Vaporization = amount of E needed to convert 1g of a substance from liquid to gas at the boiling point. Water has a HIGH heat of vaporization due to H-bonding Heat of Vaporization = Heat of Condensation Hvap of water = 2.26 kJ/g or 2260 J/g Hfus of water = 334 J/g Phase change use: H = m x Hvap/fus Temp. change use: H = m x ΔT x C

Remember… When to use which H equation: Temp. Change Phase Change H = m x ΔT x Csteam Exothermic H = m x Hvap 100°C Temperature (°C) H = m x ΔT x Cwater H = m x Hfus 0°C Endothermic H = m x ΔT x Cice Heat Energy

Boiling Point Usually molecular compounds with a low molar mass have a low boiling point. 5. Water has a HIGH boiling point due to hydrogen bonding!

Ice Typically the density of a solid is greater than it’s liquid. 6. Ice is less dense than water! Hydrogen bonding holds water molecules in a rigid framework, crystallize (spaced far apart – larger volume, less dense) As it melts the framework collapses (closer together, more dense).

Aqueous Solutions 17-3

Solvents + Solutes Aqueous solution = water containing dissolved substance. Solute = dissolved particle. Solvent = dissolving medium; always water in an aqueous solution. Ionic compounds + polar covalent molecules dissolve easily. Nonpolar covalent molecules do not dissolve easily.

The Solution Process Solvation = the process that occurs when a solute dissolves. Ionic + Polar = negative and positive ions or poles are attracted to different regions of water molecule and are pulled apart. “Like dissolves like” – Ionic + polar substances can dissolve together. Ex: salt water (NaCl is ionic; water is polar). Nonpolar and nonpolar substances can dissolve together. Ex: gasoline + oil dissolve together, both are nonpolar!

Why don’t oil + water mix??

Electrolytes + Nonelectrolytes Electrolytes = compounds that conduct electric current in aqueous solutions or in a molten state. ALL ionic compounds and most polar molecules are electrolytes. Nonelectrolytes = compounds that do not conduct electric current in either aqueous or molten state. Many molecular (nonpolar) compounds are nonelectrolytes since they are not made up of ions

Weak electrolyte = only a fraction of solute exists as ions in solution (not dissolve completely, usually polar). Ex: HgCl2, NH3, sugar Strong electrolyte = almost all of solute exists as separate ions in solution (dissolves completely, usually ionic). Ex: NaCl, HCl, KCl

Water of Hydration Water of Hydration = the water in a crystal. Hydrate = a compound that contains water of hydration. Ex: CuSO4(aq)  CuSO4•5H2O(s) blue liquid blue crystals (hydrate) CuSO4•5H2O (s)  CuSO4(s) + 5H2O(g) white anhydrous powder evaporates heated

CuSO4(s) – Anhydrous Copper CuSO4•5H2O(s) Copper (II) Suflate Pentahydrate CuSO4(s) – Anhydrous Copper (II) Sulfate

Energy Source + Global Warming Pollutant Methane hydrate… a solid form of water that contains a large amount of methane within its crystal structure [that] occur both in deep sedimentary structures, and as outcrops on the ocean floor. “Fire Ice” Energy Source + Global Warming Pollutant

Hydrate Formula Writing a hydrate formula: Name of compound, Prefix (# of water) “-Hydrate” Ex: MgSO4•7H2O Magnesium sulfate heptahydrate Ex: Ba(OH)2•8H2O Barium hydroxide octahydrate Ex: Sodium sulfate decahydrate Na2SO4•10H2O Ex: Calcium chloride dihydrate CaCl2•2H2O Writing an anhydrous formula: Ex: MgSO4•7H2O(s) MgSO4(s) + 7H2O(g)

Hydrates Hydrates lose or gain water easily because forces keeping them together are weak. A hydrate which evaporates easily will effloresce – lose water of hydration Hydrate  Anhydrous Hydrates that remove water from air easily (gain water) are called hygroscopic Anhydrous  Hydrate Used as drying agents – dessicants.

Deliquescent compounds remove sufficient water from the air to dissolve completely and form solutions!

% Mass % Mass = grams part x 100% grams whole Ex: Calculate the % mass of water in CuSO4•5H2O. % mass of water = 5H2O x 100% CuSO4•5H2O = 90g x 100% = 36.0% water 249.6g

Heterogeneous Aqueous Systems 17-4 NO CLASS IS DOING THIS SECTION!!

Suspensions This chapter has focused on homogeneous solutions, now we will look at heterogeneous mixtures. Suspensions = mixture from which particles settle out upon standing. Particles are fairly large Can be separated through filtration.

Colloids Colloids = heterogeneous mixture containing particles of intermediate size. Can be in solid, liquid, or gas phase Ex: glue, gelatin, paint, aerosol sprays, smoke Do not settle out with time and cannot be separated by filtering. Exhibit Tyndall effect, scattering of visible light in all directions. Brownian motion = chaotic movement of colloidal particles; caused by collisions of particles with water molecules – prevents particles from settling!

Tyndall Effect

Brownian Motion