Transcript Allotropes - MullisChemistry

Water: The Universal Solvent




Polarity makes water special.
O atom is an electron “hog” in the covalent
bond it forms with 2 H atoms.
Water has a positive end and a negative end.
This makes it a good solvent for ionic solids.
Slightly negative
O
H
H
Slightly positive
Mullis
1
Water’s Properties



Hexagonal crystal shape
Molecule is polar.
Hydrogen bonding




Ice floats.
Expands during freezing until -4.0 º C.
Solid form is less dense than liquid
Surface tension:


Water “beads” on smooth surfaces.
Insects walk on water surfaces.
Mullis
2
Surface tension



Force that pulls adjacent parts of a liquid
surface together.
The higher the attractive forces between
particles in the liquid, the higher the surface
tension.
Hydrogen bonds make water have higher surface
tension than most liquids.
Soap
Water droplet
Mullis
3
Solutions




Same as a homogeneous mixture
Solvent = substance that takes in, or
dissolves, another substance
Solute = substance that is taken in, or
dissolved by the first substance
The 3 phases of matter can be paired
together in 9 ways to make solutions.
Mullis
4
Solutions and phases of matter
Solute (dissolved)
Gas
Gas
Gas
(in)
Solvent
Gas
Liquid
Solid
Example
O2 in N2 - Air
CO2 in water
Liquid
Liquid
Liquid
Gas
Liquid
Solid
Water in air
Solid
Solid
Solid
Gas
Liquid
Solid
Mothballs in air
Salt water
Metal alloys - jewelry
Hydrogen fuel storage
Acid in water (vinegar)
Amalgam dental fillings
(Hg in Ag)
Mullis
5
Factors which affect solution
rate and solubility

Solution rate:




Temperature
Particle size
Movement
Solubility:


Temperature
Pressure
Mullis
6
Solubility




Solids: Increased temp increases solubility.
Gases: Solubility in water decreases when
temperature increases.
Gases: Increased pressure increases
solubility.
More fizz in a soda bottle if you store with lid
on (increased pressure) and in the
refrigerator (decreased temperature).
Mullis
7
Concentration



1.
2.
3.
Relationship between solute and solvent:
Concentration = volume solute/volume solvent
Relative concentration of solutions:
Unsaturated: More solute can be dissolved at
a given temp. and pressure. May be dilute or
concentrated.
Saturated: Maximum solute a solvent will
dissolve at a given temp. and pressure.
Supersaturated: More than the maximum
solute a solvent will dissolve at a given temp.
and pressure. (Usually obtained by heating a
saturated solution to very high temp and
cooling slowly.)
Mullis
8
Suspensions and Colloids




Remember: A solution is a homogeneous mixture.
Some mixtures are not solutions—It depends on the size
of the particles.
Large particles usually don’t make homogeneous
mixtures.
The names of 2 types of mixtures with large particles are
suspensions and colloids.
Mullis
9
Suspensions





A mixture in which some particles will settle out is a
suspension.
Gravity will eventually make large particles sink.
Particles are large enough to be filtered out.
Examples: Muddy water and salad dressing.
Particles are large enough to reflect light, so a beam of
light will be visible in a suspension.
Mullis
10
Comparing mixtures



Solution = homogeneous, small particles
Suspension = heterogeneous, large particles
Colloid = in-between suspension and solution.


Is not completely homogeneous.
Examples: fog, homogenized milk, toothpaste.
Mullis
11
Colloids



How can you tell is a mixture is a colloid?
A beam of light will be visible when shone
through a colloid (like a suspension).
Particles do not separate or settle out after
standing.
Mullis
12
Colloids: 5 types

Sol



Solid is dispersed in solid or liquid.
Examples: Clay, toothpaste, red glass
Gel
Long particles that trap liquid inside.
 Examples: Jam, jelly, gelatin dessert, hair gel
Aerosol
 Solid is dispersed in air.
 Examples: Fog, some hair sprays
Foam
 Gas is dispersed in liquid.
 Examples: Marshmallows, whipped cream
Emulsion
 Liquid is dispersed in liquid.
 Examples: Milk, butter, some
cosmetics
Mullis




13
Solutes: Electrolytes




Electrolyte = Substance that dissolves in
water to give a solution that conducts
electricity
Mobile charged particles conduct electricity
(ions)
Soluble ionic compounds are electrolytes
(Na+ and Cl-)
Many acids are electrolytes
(HCl =>H3O+ and Cl-)
Mullis
14
Solute-Solvent Interactions
Like dissolves like

Polar dissolves polar
Nonpolar dissolves nonpolar

Immiscible liquids




(oil and vinegar)
Do not mix
Liquid solutes and solvents that are not soluble in each other
Miscible liquids


Water + NaCl
CCl4 + motor oil
(gas and oil)
Mix
Liquids that dissolve freely in one another in any proportion
Mullis
15
Strong Electrolytes ( 100% ionized)
A.
Strong Acids: HCl, HBr, HI, H2SO4, HNO3, HClO4, HClO3
B.
Strong Bases: Hydroxides of group IA and II A, Except Be and Mg
C.
Soluble Salts ( ionic compounds: metal/nonmetal)
Always Soluble if these are in a compound
Except with
NO3-, Group IA, NH4+, CH3COO-, ClO4-, ClO3-
No Exceptions
Cl- Br-, I-
Pb, Ag, Hg22+
SO42-
Ag, Pb, Hg22+
Ca, Sr, Ba
Mullis
16
Mullis
17
Henry’s Law
Solubility of gas in liquid


The solubility of a gas is directly proportional to
the partial pressure of that gas on the surface
of the liquid.
Soda bottle:




High pressure at the surface while the bottle is
closed, so lots of CO2 in the liquid
Open bottle, pressure on surface lowers to room
atmosphere and CO2 leaves the liquid
High pressure = High gas concentration
Low pressure = low gas concentration
Mullis
18
Phase changes depend on forces holding
molecules together
Mullis
19
Vaporization


Enough energy is added to overcome the
forces holding molecules together in the
liquid (or solid) state
Boiling, evaporation or sublimation
Covalent bond
Mullis
20
Intermolecular forces:
Generalizing properties
Low boiling point = particles are more likely to leave
liquid solution
 Weaker IM forces = lower boiling point
 Lower boiling point = more vapor = higher vapor
pressure
 High boiling point = slow evaporation
 If IM forces are the same, look at formula weight.
Heavier molecules have higher boiling points.
 Strength of IM forces:
Hydrogen bond>dipole-dipole>London dispersion

Mullis
21
Boiling Point ↑: Vapor Pressure↓
Mullis
22
Dispersion Forces


Weakest forces attracting molecules together
Dependent on momentary polarity induced by
protons and electrons
23
Dispersion Forces
More electrons= more polarizability
 Heavier molecules have stronger IMFs
because they are more polarizable. (If they
have same type of forces)
24
Dipole-Dipole Forces

Between molecules made up of atoms with
differing electronegativities: Typically
molecules containing polar covalent bonds
Mullis
25
Dipole-Dipole forces, cont.


As with dispersion forces, when comparing two
substances having dipole-dipole interactions as their
strongest IMFs, the most polarizable will have stronger
IMFs.
HCl (36.5 amu, 17 electrons in Cl) should have higher
boiling point than HBr (81 amu, 35 electrons in Br)
Mullis
26
Hydrogen “bonds”


These are NOT bonds
Attraction between H and N,O or F
27
Intermolecular Forces
Interacting molecules
or ions
Polar molecules?
No
Ions involved?
Yes
Are polar molecules
and ions both present?
Yes
No
Are H atoms bonded to
N,O or F atoms?
No
London Forces
only
Ex. Ar(l), I2(s)
Dipole-Dipole
Ex. H2S
Yes
No
Yes
Hydrogen
Bonding
Ex. NH3, H2O
Ion-dipole
Forces
Ex. KBr in H2O
Ionic
bonding
Ex. NaCl
Chart from Chemistry: The Central Science by Brown-LeMay et. al
Mullis
28
Phases of matter: Comparison
Property
Solid
Liquid
Gas
Particles
Closely packed
High density
(But…Water is
different!)
More densely
packed than in
gas
Most
compressibleleast densely
packed
Particle
movement
Vibrate weakly
around fixed
positions
Lowest kinetic
energy
Can change
positions with
other particles
Can change
positions with
other particles
Highest kinetic
energy
Intermolecular
forces
Most effective
(strongest)
Stronger than in
gases
Least effective
(weakest)
Shape and
volume
Both definite
Definite volume
only
No definite
shape or volume
Mullis
29
Changing states



Equilibrium: When there is no net change
in a system.
Dynamic equilibrium:
When a vapor is in equilibrium with its liquid
as one molecule leaves the liquid to become
a vapor, another molecule leaves the vapor
to become a liquid. In other words, an equal
number of molecules will be found moving in
both directions.
Mullis
30
Boiling Point





Vapor pressure: Pressure exerted by a vapor =
Pressure of the liquid at given temperature
Liquid boils when its vapor pressure equals
pressure of the atmosphere.
Boiling is the conversion of a liquid to vapor
within the liquid as well as at its surface.
Boiling point is the temperature at which the
equilibrium vapor pressure of the liquid equals
the atmospheric pressure.
Volatile liquids are liquids that evaporate readily.
Mullis
31
Boiling Point, cont.




High elevation: Low atmospheric pressure
Low atmospheric pressure = lower boiling
point
High pressure in pressure cooker = increased
boiling point, faster cooking
If pressure above liquid increases, the liquid
temperature rises until it matches the new
pressure and boils again.
Mullis
32
Separation by Distillation



Distillation is the separation of liquid
substances according to their different boiling
points.
As a liquid mixture is heated, the substance
with the lower boiling point will vaporize first.
Distillate: Condensed liquid substance
Mullis
33
Phase Diagram
A phase diagram is a graph of pressure vs. temperature
that shows the conditions under which phases of matter exist.
Critical temp (Tc): Above this, the substance cannot exist in the liquid state.
Mullis
34
Phase Diagrams: Density


Negative liquid/solid slope shows density of solid is LESS than
liquid (like H2O). See previous slide.
Most substances will have a positive slope of this line since most solids
are more dense than the liquid:
http://wine1.sb.fsu.edu/chm1045/notes/Forces/
Phase/Forces06.htm
Mullis
35
Four major "points" on a phase diagram
1.
2.
Triple point, TP - All three phases can exist in
equilibrium at this temperature and pressure.
(The solid-liquid line and the liquid-vapor line meet.)
Normal boiling point, Tb - The temperature at which
the vapor pressure of a liquid is equal to standard
atmospheric pressure.
(Standard atmospheric pressure line crosses the liquid-vapor
line.)
3.
Normal melting point, Tm - The temperature at which
the vapor pressure of the solid and the vapor pressure
of the liquid are equal.
(Standard atmospheric pressure line crosses the solid-liquid
line.)
4.
Critical temperature, Tc - The temperature above
which no amount of pressure will liquefy a vapor.
(The liquid-vapor line becomes vertical.)
Mullis
36
Kinetic Energy and Equilibrium
Vapor Pressure
In the beginning:
# particles condensing to liquid phase =
# particles evaporating to gas phase
 Increase temp
Increase kinetic energy
 Now, more molecules have enough energy to
leave the liquid.
 More vapor molecules = higher vapor pressure
 Equilibrium will soon be established, but at a
higher vapor pressure.
Mullis
37
Colligative properties

van’t Hoff factor = i
i = number of ions one unit of substance will
dissociate into in solution

CaCl2 dissociates into Ca2+, Cl- and Cl-, so i= 3.
(for now)
Boiling Point Elevation
ΔT = kbmi
ΔT = increase in solution boiling point
kb = boiling point elevation constant for the solvent
m = molality
Mullis
38
Freezing Point Depression
ΔT = kfmi
ΔT = decrease in solution freezing point
kf = freezing point depression constant for the solvent
m = molality
Vapor Pressure Lowering
P = XPº
P = vapor pressure of the solution
X = mole fraction of the solvent
Pº = vapor pressure of the pure solvent
Add solute: Solution’s vapor pressure goes down.
Lower vapor pressure = Raise boiling point
Mullis
39
Vapor Pressure: Raoult’s Law



Vapor = gas formed by the boiling or
evaporation of a liquid or a solid
Vapor pressure of a liquid is the pressure
exerted by the vapor in equilibrium with
its liquid.
A solution containing a nonvolatile solute
has a lower vapor pressure than the pure
solvent.
Mullis
40
Raoult’s Law
Psolvent = XsolventP°solvent
 Psolvent: Vapor pressure of solvent in the solution
 Xsolvent: Mole fraction of solvent in solution
 P°solvent: Vapor pressure of the pure solvent
 As the mole fraction (%solvent) goes up, its
vapor pressure also goes up proportionally.
 If the solute is nonvolatile, then Psolvent = Psolution
Lowering of vapor pressure is defined:
ΔPsolvent=P°solvent– Psolvent OR Δ Psolvent =
XsoluteP°solvent
Mullis
41