Rate of Dissolving and Solubility
Chemistry Literature Connection
• “Water does not resist. Water flows.
When you plunge your hand into it, all
you feel is a caress. Water is not a solid
wall, it will not stop you. But water
always goes where it wants to go, and
nothing in the end can stand against it.
Water is patient. Dripping water wears
away a stone. Remember that, my child.
Remember you are half water. If you
can't go through an obstacle, go around
it. Water does.”
― Margaret Atwood, The Penelopiad
Explaining Solutions
• Why are some substances soluble and some
not? Why do only some chemicals dissolved
in water?
• For Ionic Compounds – the solvents ability to ‘break’
ionic bonds determines their solubility.
• In covalent compounds a solute’s ability to dissolve in
a solvent is dependent on the potential
intermolecular forces (DDF, LDF, H-Bonding) that can
occur between them.
Let’s Review Intra & Intermolecular
Forces…
Intramolecular Forces:
– The attractive forces between atoms and ions within a
molecule
• e.g Ionic bond, covalent bonds (e.g. polar, nonpolar, double, triple)
• Relatively strong
Intermolecular Forces:
– The attractive forces between molecules
• E.g. Van der Waals forces: London dispersion forces, dipole-dipole
forces, hydrogen bonds
• Weak (in comparison to intramolecular forces)
• I.e. much less energy to melt H2O (inter) than for it to decompose
into H2 and O2 (intra)
London Dispersion
• Attractive forces between all
molecules, including nonpolar
molecules
• Result of temporary
displacements of the electron
cloud around atoms in a
molecule (extremely shortlived dipoles)
• Therefore weaker than dipoledipole
Hydrogen Bonding
H-bond
• Strong dipole-dipole force
between the positive
hydrogen atom of one
molecule and highly
electronegative atom of
another molecule (O, N, F)
Discussion/Recap:
• Rank the intermolecular forces from weakest to
strongest.
• How might intermolecular forces affect physical
state?
• How might intermolecular forces affect melting
point?
• So WHY do some substances (solutes) dissolve
to form solutions but others don’t?
– All comes down to POLARITY
• General rule of thumb – like dissolves like
– Polar solvents will dissolve polar solutes.
– Non-polar solvents will dissolve non-polar solutes
Polarity of Water Molecules
Electronegativity difference
O = 3.5
H = 2.1
3.5 – 2.1 = 1.4
Recall: between 0.5 and 1.6 = bond is polar covalent
Polarity of Water Molecules
Dipole moment =measure the polarity of a
chemical bond, occurs whenever there is a
separation of positive and negative charges
Ionic Compounds in Water
Ionic Compounds in Water
Polar Molecules in Water
Hydrogen Bonding
Polar Molecules (Sugar) in Water
Polar Molecules (Sugar) in Water
Non-Polar Molecules in Water
Learning Check:
Is it Soluble? Why or Why Not?
① ammonia in water
① Water in hexane
② Carbon tetrachloride in hexane
Dissociation
• Water is known as the “universal solvent” – WHY?
– Both DDF and H-Bonding occurs in water, so it is able to
‘dissolve’ a number of solutes
• (plus it is really small)
• Ionic Compounds will dissociate in polar solvents (not
dissolve) to form their ions because they are able to form
ion-dipole bonds. (slide 11 & 12)
– Need to break ionic bonds to ‘dissociate a salt
– Eg. NaCl(s)  Na+(aq) + Cl-(aq)
– We write dissociation equations to represent dissociation of
ionic compounds in aqueous solution.
THE ‘DISSOLVING’ OF IONIC CMPDS
Hydration – the process where ions are surrounded by water molecules
Writing Dissociation Equations
• Write equations to represent the dissociation
of the following ionic compounds in aqueous
solution:
– Sodium fluoride
– Aluminum sulphate
– Cobalt (II) chloride hexahydrate
Some Terms to Remember…
• Electrolyte = a compound that in an aqueous
solution conducts electricity
– Mosty ionic compounds
• Nonelectrolyte = a compound that, in an
aqueous solution, does not conduct electricity
– Mostly molecular compounds
THE DISSOLVING OF MOLECULAR CPDS
MISCIBLE compounds are able to mix easily to
form a solution (ex. glucose)
IMMISCIBLE compounds are unable to mix to
form a solution (ex. oil)
Remember – ‘Like Dissolves Like’
for polar and non-polar substances
Involves Hydrogen Bonding
Dissolving Process
① Separation of Solute
– must overcome IMF or ion-ion attractions in solute
– requires energy, ENDOTHERMIC ( + DH)
② Separation of Solvent
– must overcome IMF of solvent particles
– requires energy, ENDOTHERMIC (+ DH)
③ Interaction of Solute & Solvent
– attractive bonds form between solute particles and solvent
particles
– “Solvation” or “Hydration” (where water = solvent)
– releases energy, EXOTHERMIC (- DH)
Rate of Dissolving
Increases with:
① Agitation
• Brings solvent in contact with solute
• Stirring, shaking or swirling
② Increased temperature
• Kinetic Molecular Theory
③ Particle size
• Surface Area
• Ex: crushing a solid using a mortar and
pestle
Factors Affecting Solubility
• Solubility - measures how much solute
dissolves in a given volume of solvent, at a
specific temperature, to form a saturated
solution.
– Depends on molecule size, temperature and
pressure.
• Let’s do a lab….
Factors Affecting Solubility
1. Nature of Solute / Solvent. - Like dissolves like
intermolecular forces
(IMF), size of particles
2. Temperature –
i.
Solids/Liquids- Solubility increases with Temperature
Increase kinetic energy (K.E.) increases motion and collision between
solute / solvent.
ii) gas - Solubility decreases with Temperature
Increase K.E. result in gas escaping to atmosphere.
3. Pressure Factor i) Solids/Liquids - Very little effect
Solids and Liquids are already close together, extra pressure will not
increase solubility.
ii) gas - Solubility increases with Pressure. Increase pressure squeezes gas solute into
solvent.
Solubility Curves
of several
1.Solubilities
Comparing
KI and KCl,
ionic
solid is
asmore
a function
of at 10
which
soluble
oC?
temperature.
MOST salts
2.have
How
much
KI would
greater
solubility
in
hotdissolve
water. in 100 g of water at
10 oC?
3.A few
How
much
would
salts
haveKInegative
dissolve
in 25 g of water at
heat
ofo solution,
10 C?
(exothermic
process) and
4. A chemist prepares a
theysolution
becomecontaining
less soluble45 g of
withNaNO
increasing
100 g of water 20
3 in
oC. Is the solution
temperature.
saturated?
Solubility curve learning check
• What mass of solute will dissolve in 100mL of water at the
following temperatures. Also determine which of the three
substances is most soluble in water at 15°C.
• 1.KNO3at 70°C
• 2.NaCl at 100°C
• 3.NH4Cl at 90°C
Solution: Solubility
1.KNO3at 70°C is 138g / 100mL
2.NaCl at 100°C is 38g / 100mL
3.NH4Cl at 90°Cis 72g / 100mL
4.NaCl is the most soluble at 15°C
Remember the Degrees of
Saturation?
GENERAL TREND:
Solubility curve learning check cont’d
1. What term - saturated, unsaturated, or supersaturated - best
describes:
• a solution that contains 70g of NaNO3 per 100 mL H2O at 30°C
• a solution that contains 60g of dissolved KCl per 100 mL H2O at
80°C
Solution
• The NaNO3 solution is unsaturated. At 30°C a saturated solution
would be able to dissolve approximately 95 g of NaNO3 . Since there
are only 70g in the solution, 25 more grams of NaNO3 could be
added and it would all dissolve.
• The KCl solution is supersaturated. At 80°C a saturated KCl solution
contains 50 g KCl per 100 mL H2O. This solution is holding 10 g of
excess KCl.
• If a small crystal of KCl is added to this supersaturated solution, the
excess KCl will immediately come out of solution.
Temperature & the Solubility of Gases
The solubility of gases DECREASES at higher temperatures
HOMEWORK:
Here’s a funny…
A physicist, a biologist, and a chemist went to
the ocean for the first time. The physicist was
fascinated and wanted to study the fluid
dynamics of the waves. He walked into the
ocean and never returned. The biologist
decided to study the flora and fauna. She
walked into the ocean and never returned.
The chemist waited for a long time. Finally he
wrote down his observation, "Physicists and
biologists are soluble in ocean water."