Download Dissociation

Regents Chemistry
Chapter 14 Notes
Dissociation
— As we’ve already seen in the past, when a compound that is made up of ions dissolves in water, the ions separate from
each other
— This separation of ions that occurs when an ionic compound dissolves is known as dissociation (quite literally,
disassociation of the parts from each other)
— Remember that this would be indicated in a chemical equation by using the symbol (aq) (“aqueous,” meaning dissolved in
water)
e.g.
NaCl(s)  Na+(aq) + Cl–(aq)
H2O
HO
CaCl2(s) 
Ca2+(aq) + 2 Cl–(aq)
2
—
—
Notice that the number of ions produced in each case is directly related to the chemical formula of each of the substances
in question and that, as always, the equations must be balanced!
Assuming that 100% of each compound dissociates into ions (which we can’t always do, as you’ll see later on…), then
then:
1 mol of NaCl dissociates to yield 1 mol of Na+ ions and 1 mol of Cl– ions
1 mol of CaCl2 dissociates to yield 1 mol of Ca2+ ions and 2 mol of Cl– ions
Precipitation Reactions
— Although no compound is ever totally insoluble, compounds of very low solubility can be considered insoluble for most
practical purposes
— There is no easy method to predict whether a compound made up of a certain combination of ions will be soluble when put
into water
— However, general solubility guidelines do exist, which offer information in general about what substances will dissolve in
water
— Learn to use your reference tables — it’s fun and if you take advantage of this special limited time offer, it’s absolutely free
— The guidelines are useful in helping to predict what will happen if the solutions of two different soluble compounds are
mixed
— If the mixing results in a combination of ions that forms an insoluble compound, a double replacement reaction (remember
those?!?) will occur and the end result will be the formation of a precipitate
— Precipitation occurs because the forces of attraction between the insoluble ion pair combination is stronger than the
attraction between the ions themselves and the surrounding water molecules
— They are attracted so strongly to each other, the water just can’t compete in trying to keep them separated, so the ions
remain bound together as a unit (they don’t dissociate); crack out the cigars, because a precipitate has just been born.
Net Ionic Equations
— Reactions of ions in an aqueous solution are usually represented by a net ionic equation rather than by a formula equation.
— A net ionic equation displays only those substances and ions that actually undergo a chemical change in a reaction in an
aqueous solution; it does not display so-called spectator ions
— Spectator ions are ions that do not participate in a chemical reaction because they are found in the solution both before
and after the reaction has taken place (the ions do not participate in the reaction)
— The spectator ions cancel out from both sides of the equation, leaving the net ionic equation:
e.g.
Cd2+(aq) + 2NO3–(aq) + 2NH4+(aq) + S2–(aq)  CdS(s) + 2NO3–(aq) + 2NH4+(aq)
Overall Ionic Equation
spectator ions
Cd2+(aq) + S2–(aq)  CdS(s)
—
—
—
—
Net Ionic Equation
In addition to ionic compounds forming ions when being dissolved into a solution, some molecular compounds can also
form ions in solution
The process of ionization is when ions are formed from solute molecules by the molecular action of the solvent
Although it is a similar process to dissociation, they are not the same: when an ionic compound dissociates, ions that
already exist are simply separated as they enter into solution; with ionization, ions are formed where previously no ions
existed
This again has to do with the strength of the forces of the solvent molecules pulling on the solute molecules; if the solvent
pulls strongly enough, it can literally rip the solute molecules apart and as a result ions form where no ions previously
existed
e.g.
HCl  H+(aq) + Cl–(aq)
OVER
—
—
—
—
As we have seen previously, substances which can conduct an electrical current in solution (because they dissociate to
yield mobile ions) are known as electrolytes
Any compound which exists mostly as dissociated ions in solution, rather than just as dissolved (but undissociated)
molecules, is referred to as a strong electrolyte (almost all of a given amount of solute that dissolves breaks into its
components ions rather than staying as molecules)
Some compounds, when dissolving it solution, yield not only ions but also some dissolved molecules that do not dissociate
into ions (i.e., not all of a given amount of solute will break apart into ions); such a substance is known as a weak
electrolyte: only a relatively small amount of the dissolved compound exists as ions when in a solution
NOTE THAT the description of an electrolyte of being “strong” or “weak” has nothing to with a solution being
“concentrated” or “dilute” !!
 Strong and weak electrolytes differ in the degree to which they ionize/dissociate when put into solution
(how much of what I am dissolving will dissociate into ions?)
 Concentrated and dilute solutions differ in the amount of solute dissolved in a given quantity of solvent
(how much is being dissolved into a solution?)
Colligative Properties of Solutions
— Certain properties of solutions do not depend directly on the identity of the what is being dissolved into solution
— Colligative properties are properties of solutions that depend on the concentration of the solute particles in the solution
but not on the identity of the solute particles themselves
— Vapor Pressure Lowering:
 the molecules of the nonvolatile solute “block” the path of escape of the solvent molecules at the liquid’s surface,
while not having much tendency to want to escape themselves. So they just sit there and get in the way kinda like
Uncle Vern at the holidays
 The more of the solute dissolved into the solution (i.e. the greater its concentration), the greater the effect it will
have (the lower the vapor pressure will become)
— Boiling Point Elevation:
 Because the vapor pressure is lowered, the solution can remain a liquid to a higher temperature
(remember that it won’t vaporize until vapor pressure = atmospheric pressure ): it will boil at a temperature
higher than the pure substance
 The boiling point elevation (tb) is the difference between the boiling points of a pure solvent and a
nonelectrolyte solution of that solvent
 The molal boiling point constant (Kb) is the boiling point elevation of the solvent in a 1 molal solution of a
nonvolatile, nonelectrolyte solute.
 The boiling point elevation is directly proportional to the molal concentration of the solution:
tb = Kb  m
*remember m = molality = mol/Kg
 Remember that boiling point elevation has nothing to do with the nonvolatile, nonelectrolyte solute used, and
everything to do with the nature of the solvent only!
—
Freezing Point Depression:
 Because the vapor pressure is lowered, the solution can remain a liquid to a lower temperature: it will
freeze at a temperature lower than the pure substance
 The freezing point depression (tf) is the difference between the freezing points of a pure solvent and a
nonelectrolyte solution of that solvent
 The molal freezing point constant (Kf) is the freezing point depression of the solvent in a 1 molal solution of a
nonvolatile, nonelectrolyte solute.
 The freezing point depression is directly proportional to the molal concentration of the solution:
tf = Kf  m
*remember m = molality = mol/Kg
 Remember that freezing point depression has nothing to do with the nonvolatile, nonelectrolyte solute used,
and everything to do with the nature of the solvent only!
—
Osmotic Pressure:
 a semipermeable membrane allows the movement of some particles while blocking the movement of
others, based on the size of the particles
 The movement of solvent molecules through a semipermeable membrane from the side of lower solute
concentration to higher solute concentration is known as osmosis
 Osmotic pressure is the external pressure that must be applied in order to stop osmosis from spontaneously
occurring
 Because osmotic pressure is dependent upon the concentration of solute particles and not on the type of
particles, it is a colligative property
One final note: remember that colligative properties depend on the total concentration of solute particles; electrolytes cause
changes in colligative properties proportional to the total molality in terms of all dissolved particles instead of just
(undissociated) formula units. Don’t forget to take this into account!!!