Transcript 8.2 Factors That Affect Rate of Dissolving and Solubility

8.2 Factors That
Affect Rate of
Dissolving and
Solubility
Factors that Affect the
Rate of Dissolving
• Rate of Dissolving – how quickly a solute
dissolves in a solvent.
• Depends on :
1) Temperature
2) Agitation
3) Particle Size
Temperature
• For most solid solutes the rate of
dissolving is greater at higher temperatures.
• WHY?
• At higher temperatures the solvent molecules
have greater kinetic energy.
• This causes them to collide with the undissolved
solid molecules more frequently.
• This increases their rate of dissolving.
Agitation
• Agitating a mixture by stirring or by shaking
the container increases the rate of dissolving.
• Why?
• This brings fresh solvent into contact with
undissolved solid.
Decreasing Particle Size
• A large lump of solid sugar dissolves slower
than and equal mass of powdered sugar.
• WHY?
• When you break up the large mass into many
small masses, you increase the surface area
that is in contact with the solvent.
• This allows the solid to dissolve faster,
increases solubility.
3 Steps for Dissolving
The Process of Dissolving at the Molecular Level
1. The forces between the particles in the solid
must be broken. This step always requires
energy. In an ionic solid, the forces that are
holding the ions together must be broken
(intramolecular forces). In a molecular solid,
the forces between the molecules must be
broken (intermolecular forces).
Continued…
2. Some of the intermolecular forces between
the particles in the liquid must be broken.
This step also requires energy.
3. There is an attraction between the particles of
the solid and the particles of the liquid. This
step always gives off energy.
Polar and Non-Polar Substances
• Things to remember from last class…
Non-polar molecules can contain either:
a) non-polar bonds (from the ∆EN)
b) Polar bonds but the dipoles cancel
out. Example CO2
Polar and Non-Polar Substances
Polar molecules can only be non-polar if:
a) If some of the polar bonds result in a net
dipole (dipole ≠ zero)
b) This usually happens when the molecule is
unsymmetrical or
“Like Dissolves Like”
Polar
compounds
Polar
Solvents
NO!
Non-Polar
Compounds
Non-Polar
Solvents
A. Solubility and Intermolecular Forces
• Dipole-Dipole Attractions –
Occurs between the opposite
charges on 2 different polar
molecules.
+
-
+
-
• This force is only 1% as strong as
Ionic or covalent bonding.
Solubility and Intermolecular Forces
• The Water Molecule…
B. Solubility and Intermolecular Forces
• Hydrogen Bonding- Occurs between an H and
a highly electronegative atom.
Examples H + N, O, F, Cl
Ion-Dipole Interactions
C. Solubility and Intermolecular Forces
• Ion-Dipole InteractionsAre attractive forces between ions and polar
molecules. Crystal molecules are made up of
cations and anions in a repeated fashion.
How does an Ionic Compound Dissolve?
The negative end of the water molecule attracts the
+ ions and the positive end of water attracts the –
ions. Thus the crystal breaks down (dissolves).
Molecular Size and Solubility
• Small molecules are often more soluble than
larger molecules.
• The more carbon atoms added, the less
soluble it will be.
Homework