Aqueous Reactions and
Solution Stoichiometry
1. The Water Molecule
2. Hydrogen bonding
3. Dissolution process
4. Ways of Expressing Concentration,
Dilution
SolutionSolution A homogeneous mixture composed
of a two or more substances, in which each
substance retains its chemical identity
Components of solution
Solute - Substance being dissolved
Solvent - The substance, which acts as a
dissolving medium.
Characteristics of Solution
• Distribution of particles is uniform
• Components in solution do not separate upon
standing
• Components cannot be separated by filtration.
• Solute/Solvent mixes in ratios - up to the
solubility limit .
• Solution is almost always transparent.
• Compounds of solution may be separated by
different methods, like distillation or
chromatography.
The Water Molecule
If solvent is water, then solution is considered Aqueous
•Water is a simple triatomic
molecule.
•Each O-H bond is highly
polar, because of the high
electronegativity of the
oxygen (3.5)
•bond angle = 105 o, due to
the bent shape, the O-H
bond polarities do not
cancel. This means water as
a whole is polar.
Hydrogen Bonding is a link formed by a hydrogen
atom lying between two strongly electronegative atoms
Only N, O and F are sufficiently
electronegative to take part in
hydrogen bonding in neutral
molecules.
Strong
effective
electropositive charge and the
absence of core e layers allows
close approaching of the other
atom to H to the distances~ lengths
of atomic bonds.
Due to 2 unshared e HB is not only
electrostatic attraction, but donoracceptor directed bond. O-H length
in
water
is
0.096nm,
O…H=0.176nm
• Water’s bent shape and ability to
hydrogen bond gives water many
special properties, like: high surface
tension, low vapor pressure, high heat
of vaporization, high specific heat,
and high boiling point
• Water (due to high polarity of the
molecule) acts as an excellent solvent
Why is water unique?
-Strong Hydrogen Bonding
• Intramolecular forces are the forces that hold
atoms together in a molecule (ionic and covalent
bonds).
• Intermolecular forces are the forces that hold
MOLECULES together.
• Remember that the intermolecular forces are: hydrogen
bonding, ion –dipole, dipole-dipole, ion & dipole-induced
dipole and dispersion
Surface
Tension-is
a
measure of the force that must
be
applied
to
surface
molecules so that they
experience the same force as
molecules in the interior of
the liquid
Capillary action –the
rise of liquid up narrow
tubes. Water curves up
along the side of glass,
because adhesion forces
are stronger than cohesion.
∆Hsolution = ∆Hsolute + ∆Hsolvent + ∆Hmixing
–∆Hsolute = the energy required to break the
intermolecular forces between solute molecules, or the
ionic forces in ions, ∆Hsolute > 0
–∆Hsolvent = the energy required to break intermolecular
forces in the solvent and create “cavities” for the solute
particles, ∆Hsolvent > 0
–∆Hmixing = energy released in forming interactions
between solute and solvent, ∆Hmixing < 0
A substance is soluble when the energy required to break solutesolute and solvent-solvent interactions is less than, or not much greater
than, the energy released when solute-solvent interactions are formed.
∆Hsolvation = ∆Hmix + ∆Hsolvent
The solute is said to be solvated because it is
surrounded by, and interacting with, solvent
molecules
When the solvent is water, the process is called
hydration
Ionic versus covalent solute
1. Ion solutes dissolve by dissociation into their
ions.
2. Covalent solutes dissolve by H-bonding to
water, intermolecular forces become weak
NaCl and ethanol dissolving
Water of Hydration
• Many ionic compounds have water of hydration when
purchased, and the molecules of water must be taken into
account when calculating molecular weights
• Water molecules are chemically bonded to solid salt
molecules (not in solution)
• These compounds have fixed amounts of water
• The water can be driven off by heating
• Since heat can drive off the water, the forces holding it
are not so strong
• If a hydrate has a vapor pressure higher than that of water
vapor in air, the hydrate will lose the water of hydration
•CuSO4.5H2O
+ heat
CuSO4 + 5H2O
copper(II)sulfate pentahydrate
Qualitative Terms:
• A concentrated solution has a high proportion of
solute to solution
• A dilute solution has a low proportion of solute to
solution
• A saturated solution has the maximum amount of
solute that will dissolve in the solvent
– Depends on temp
• An unsaturated solution has less than the saturation
limit
• A supersaturated solution has more than the
saturation limit
– Unstable
The main ways of expressing concentration:
–Mass percent (% m/m)m/m) (grams solute / Total
grams of solution) * 100
–Volume (% v/v)v/v) (grams solute / Total grams of
solution) * 100
–Molarity(M) - moles solute / Liter solution
–Molality* (m) - moles solute / kg solvent
–Normality(N) - Number of equivalent / Liter
solution
–mole fraction(χA) - moles solute / Total moles
solution
–part per million (ppm), - 1 µg of solute in 1 g
of sample
Dilution
M 1V1 = M 2V2
MdiluteVdilute = MconcentratedVconcentrated
1. Factors Affecting Solubility
2. Precipitation reactions
3. Solubility Guidelines for Ionic
Compounds
Why does water not mix with oil ?
Yet water mixes with alcohol (‫)כוהל‬.
• Solute-solvent interactions are comparable to
solute-solute and solvent-solvent interactions
only if they are all of the same type
• This is summarized by the rule that like dissolves
like, i.e., ionic substances and polar molecules
are more soluble in polar solvents, while
nonpolar molecules are more soluble in
nonpolar solvents
Solute-Solvent Interactions
Polar organic compounds that are very soluble in water are
the ones with a polar -OH group in them and can have
strong polar (electrostatic) interactions with water
Examples: sucrose, alcohols
HOCH2
H
H
OH
HO
H
HOCH2
H H
OH
O
H
H
O
OH
HO
H
O
CH2OH
•Factors Affecting Solubility:
Solvent type, Concentration, Temperature
•Coulomb’s law gives us some idea of predicting
solubility: ion charges typically range from
+3 to –3, therefore 1<⏐q1q2⏐<9
Highly charged ions should not be as soluble as
ions with charges of +1 and –1
Solubility Guidelines for Ionic Compounds
Solubility Rules: memorize this list of
INSOLUBLE compounds and their exceptions -
Precipitation Reactions: Will a Precipitate Form?
KCl(aq+NH4NO3 (aq)
KCl(aq) + NH4NO3 (aq)
Na2SO4 (aq)+Ba(NO3)2 (aq)
K+(aq)+Cl-(aq)+NH4+(aq)+NO3-(aq)
No Reaction!
BaSO4 (s)+2 NaNO3 (aq)