Post on 05-Jan-2016
Water and Solutions
A topic that’s “all wet”
Unique properties of water
Polarity O-H bond is polar – oxygen
attracts electrons more strongly than hydrogen
Water molecule is asymmetric due to its bent shape, therefore the molecule is also polar
Unique properties of water
Hydrogen bonds – strong attractions between the positive and negative ends of the water molecules
Ice floats! Solid water is less dense than liquid water Hydrogen bonds hold ice together in a crystal structure In the crystal the water molecules are held farther apart
than in liquid water Water is a liquid at RT!
Most other compounds of similar molar mass are gases Water has surface tension!
This causes water to bead up, to form a meniscus, and allows bugs to skate across the surface
Unique properties of water
Water exhibits capillarity! Capillarity is the tendency of a liquid to rise in a
tube of narrow diameter Water has a high specific heat!
Specific heat (cP) is the heat necessary to raise the temperature of one gram of stuff one degree Celsius.
Metals (conductors) have a low heat capacity (easy to heat up), water & other nonmetals have a high heat capacity (hard to heat)
Unique properties of water Stuff cP (J/gºC) Gold 0.129 Iron 0.450 Cement 0.88 Wood 1.76 Ethanol 2.46 Water 4.18 High specific heat is due to hydrogen bonds,
which take a lot of energy to overcome
Unique properties of water
Water acts as a heat buffer, which moderates temperature extremes on earth
Water has a high Hv! Hv is the “heat of vaporization” – the heat
needed to vaporize one gram of liquid at its boiling point
Water – the universal solvent – it will dissolve a wide range of different materials
Solutions The dissolving process: Ionic substances Polar water molecules are attracted to the +
and – ions in the crystal lattice
The dissolving process
It takes several water molecules to break an ion free
Free ions are solvated – surrounded by water molecules
Dissolving process
The process of breaking up a salt crystal is called dissociation – chemical bonds are broken
Polar covalent molecular substances Dissolving does not require dissociation – the
molecules remain intact
glucose
Dissolving process
The water molecules are attracted to the partially charged portions of the glucose molecule
These interactions overcome the interactions of the crystal structure
Dissolving molecular substances
Dissolving molecular substancesConcentration Nonpolar covalent substances are not
soluble in water – weak interactions with water
Solubility rule – like dissolves like Concentration Solutions consist of a solute and a solvent Concentrated solution – a lot of solute
dissolved Dilute solution – little solute dissolved
Saturation
Saturated solution – maximum amount of solute dissolved
Unsaturated solution – less than the maximum amount dissolved
Supersaturated solution – more than the maximum amount dissolved
Made by slowly cooling a hot saturated solution
Solute will crystallize if solution is disturbed
Solubility curve
Temperature and solubility – generally, increasing temperature increases solubility – except for gases
Concentration
Units of solubility – g/100g H2O Units of concentration – molarity Symbol – M M = moles solute liters solution Review – moles = mass/(molar mass), mass = molesx(molar mass)
Making solutions
You need to make 2.3 liters of a 0.15 M solution of NaCl in water. How many g salt must you weigh?
“Solution”: M = moles/liter 0.15M = xmol/2.3L x = 0.15x2.3 = 0.345mol molar mass of NaCl = 58.5g/mol mass = molesx(molar mass) 0.345mol x 58.5g/mol = 20.1825g You would have to weigh about 20.2g NaCl, and
dilute it to 2.3L with water.
Colligative properties
Freezing point depression Freezing point of a solution is lower
than that of the pure solvent Solute interferes with solvents’ ability
to form crystals FP depression depends on
concentration
Colligative properties
If the solute is a salt, the effect is increased by the number of ions in the salt
NaCl Na+ + Cl- (2 ions) CaCl2 Ca+2 + 2Cl- (3 ions) Therefore calcium chloride causes the
greater FP depression for each mole of salt. FP depression is the basis of de-icing
compounds for aircraft, anti-freeze, and salts sold for melting ice on sidewalks.
Colligative properties
Boiling point elevation Solutions have a higher boiling point than the
pure solvent Solutes take up room at the surface of the
solution, lowering the vapor pressure Lower vapor pressure means that more heat
is needed to boil solution BP elevation is basis of anti-freeze and
candy making
Colligative properties
Osmosis Semi-permeable
membrane – a thin film that allows only certain materials to pass (i.e. water but not salt)
Water will move so as to dilute the solute
Colligative properties
Colloids
Consist of particles permanently suspended in a medium
Particles are 10-100 times as large as those in a solution
Tyndall effect – colloidal particles scatter light Examples – smoke in air, fog/smog, soapy
water, blood, milk, toothpaste