PRECIPITATION REACTIONS
A solid precipitate forms
Precipitation depends on solubility
GENERAL SOLUBILITY RULES
Soluble if more than 3g dissolves in 100 ml water
1. Most group 1 and ammonium (NH4+) compounds are soluble.
2. Most nitrates, acetates, and chlorates are soluble. (NO3-,
CH3COO-, ClO3-)
3. Most metal/halide (group 17) salts are soluble EXCEPT Ag,
Hg(I), Pb (chloride, bromide, iodide) - these three form
precipitates
4. Most sulfates (SO42-) are soluble EXCEPT Ag, Hg(I), Pb, Ba, Sr
(Ca partially) -these six form precipitates
5. Except with ions from group 1 or ammonium, most
carbonates, hydroxides, phosphates, and sulfides are
INsoluble (not soluble - form precipitate) (CO32-, OH-, PO43-, S2-)
EXAMPLES:
sodium sulfate and barium nitrate
copper (II) sulfate and sodium acetate
silver nitrate and ammonium chloride
Now we need the net ionic equations
NET IONIC EQUATIONS
Only show active participants in the reaction
Steps:
1. Write the balanced equation with subscripts (i.e. (s), (aq))
2. Split up aqueous salts (dissolved in water means present as
ions) into the ions (don’t forget to show the ion charges).
Solids or liquids stay together. This is the total or complete
ionic equation.
3. Cancel out spectator ions (didn’t participate – just
“watching”)
Example: silver nitrate and lithium iodide
Shortcut for precipitation reactions: Once you have found the
precipitate (solid) ,just write the ions that form it on the left of
the arrow and the solid on the right .
COLLIGATIVE PROPERTIES
Depend only on number of particles in solution
A. Properties of solutions that differ from the pure
solvent:
1. Vapor pressure * (causes all others)
2. Freezing Point
3. Boiling Point
4. Rate of diffusion through a membrane (osmosisosmotic pressure)
COLLIGATIVE PROPERTIES
B. Solute takes up some surface space so fewer solvent
molecules can evaporate which gives a lower vapor pressure
C. Fractional Distillation
1. separates by boiling point
2. fractionating tower takes advantage of v.p. differences
D. Solute effect on freezing and boiling points -interferes with
process
1. F.P. decreases
2. B.P. increases
OSMOTIC PRESSURE
A. osmosis - movement of solvent through
a semipermeable membrane to equalize
the concentration difference (moves
from high concentration of solvent to low
concentration)
B. at equilibrium, the number of particles crossing the
membrane in both directions is equal
OSMOTIC PRESSURE
1. Osmotic pressure - pressure at equilibrium after
osmosis (also the pressure needed to be applied to
stop osmosis)
2. results from:
a. greater concentration of solute on one side of
membrane
b. Solute cannot cross the membrane but solvent can
RED BLOOD CELLS
1. if salt concentration outside cell is greater
the water moves out of RBC- Crenation (shrivels)
2. if salt concentration inside cell is greater
the water moves into RBC- Hemolysis (bursts)
PLANT CELLS
1. Called turgor pressure (holds plant upright)
2. Cells don’t burst because of cell wall
CALCULATING BP AND FP CHANGES
A. Boiling Point Elevation (∆Tbp)
1. 1 mole of solute particles in 1 kg of water raises the
b.p. .515oC
2. Kbp = .515oC (boiling point constant)
3. ∆ Tbp = m Kbp
B. Freezing Point Depression (∆ Tfp)
1. 1 mole solute particles in 1 kg of water lowers the
f.p. 1.853oC
2. Kfp = 1.853oC (freezing point constant)
3. ∆ Tfp = m Kfp
STEPS TO CALCULATE: FORWARD
1. Find molality of solution (mol solute/kg solvent)
2. Multiply molality by number of particles produced by the solute
in solution to find the total number of moles of particles in
solution: 1 for molecules (nonmetals), count ions for ionic
compounds (look for metal &/or polyatomic ions from
flashcards)
3. Substitute the result from #2 into the equation as m and
calculate using the correct constant
4. Find new bp by adding ∆Tbp to normal boiling point of solvent
Find new fp. by subtracting ∆Tfp from normal freezing point of
solvent
STEPS TO CALCULATE: BACKWARD
1. Get the ∆Tbp by subtracting 100 from bp or get ∆Tfp by
making the fp positive (subtracting a negative from 0).
2. Get the molality of the solution: Divide by constant
and divide by number of particles.
3. Get the mass of the solute: Multiply by kg water and
multiply by formula mass of the solute from the
periodic table
EXAMPLES
What will the bp and fp be for a solution of 65.0 g sugar
C12H22O11 dissolved in 545 g H2O?
What will the bp and fp be for a solution of 52.0 g MgCl 2
in 682 g H2O?
How many grams of MgCl2 need to be dissolved in 250 g
H2O to have a freezing point of -2.3 oC?