Ch 12. Learning Objectives
• Properties of Matter
•Intermolecular Forces
• Problem solving to determine
amount of heat transfer related
to changes of state
•Hydrogen bonding
And properties of water
The Physical States of Matter
• Matter can be classified as solid, liquid, or gas based on
what properties it exhibits.
State
Shape
Volum e
Com press
Flow
Solid
Fixed
Fixed
No
No
Liquid
Indef.
Fixed
No
Y es
G as
Indef.
Indef.
Y es
Y es
•In solids, the intermolecular forces are strong thus
keeping the particles fixed into place. This is why solids
are rigid.
1
In liquids, the particles can move about somewhat freely
as allowed by the intermolecular forces. Liquids do not have
a fixed shape but they are non-compressible, i.e they have a
definite volume.
Properties and Attractive Forces
Phase
Density
Shape
Volume
Relative
strength of
attractive
forces
Gas
Low
Indefinite
Indefinite
Weakest
Liquid
High
Indefinite
Definite
Moderate
Solid
High
Definite
Definite
Strongest
2
Physical Changes of Matter
Melting
• Generally, we convert a material in the solid state into a liquid
by heating it.
• Adding heat energy increases the amount of kinetic energy of
the molecules in the solid.
• Eventually, they acquire enough energy to partially overcome
the attractive forces holding them in place.
• This allows the molecules enough extra freedom to move
around a little and rotate.
Boiling
•Generally, we convert a material in the liquid state into a gas
by heating it.
•Adding heat energy increases the amount of kinetic energy of
the molecules in the liquid.
•Eventually, they acquire enough energy to completely
overcome the attractive forces holding them together.
•This allows the molecules complete freedom to move around
and rotate.
Dynamic Equilibrium and Vapor Pressure
Dynamic equilibrium is reached when the rate of forward reaction
equals the rate of reverse reaction.
Vapor pressure of liquids is the sum of pressures exerted by
individual molecules of a substance on the surface of the liquid.
In dynamic
equilibrium
Q: What are the effects of temperature and surface area of a liquid ?
3
Boiling point
Boiling point is the temperature at which the vapor pressure of liquids
equals the atmospheric pressure. Also known as the normal boiling
point. (at 1 atm).
How is boiling point affected with altitude?
What will happen if we continuously
supply heat to a boiling liquid?
Trivia:
Water boils at 68oC on Mount
Everest.
Melting point
Melting point occurs at the temperature at which the solid and
liquid phases of a substance are in dynamic equilibrium.
H2O (s) Æ H2O(l). Endothermic process.
When considered as the temperature of the reverse change from
liquid to solid, it is referred to as the freezing point.
Trivia:
In organic chemistry Carnelley’s Rule established in 1882 by
Thomas Carnelley, states that high molecular symmetry is
associated with high melting point. He based his rule on
examination of 15,000 chemical compounds !!!
4
Heats of Vaporization & Fusion
Water vapor
temp. continues to rise
till entire mass of
H2O reaches b.p
(∆Hvap)
b.p of H2O
Liq. water
ice
m.p of H2O
temp. continues to rise till
entire mass of H2O(s) is liquid.
(∆Hfusion)
5
Molar heat of vaporization is the quantity of heat energy needed
to convert 1 mol of a liquid to its vapor phase. In kJ/mol.
Vaporization and evaporation are endothermic processes.
Heats of Vaporization of Liquids
at Their Boiling Points and at 25 °C
∆Hvap at
Liquid
Water
Isopropyl
C3H7OH
alcohol
Acetone C3H6O
Diethyl
C4H10O
ether
∆Hvap at
boiling
point,
(kJ/mol)
+40.7
25 °C,
(kJ/mol)
+44.0
82.3
+39.9
+45.4
56.1
+29.1
+31.0
34.5
+26.5
+27.1
Normal
Chemical boiling
formula point, °C
H2O
100
Tro's Introductory Chemistry,
Chapter 12
36
Molar heat of fusion (∆Hfus)is the quantity of heat needed to change
1 mol of a solid to its liquid. (In kJ/mol)
In general, ∆Hvap > ∆Hfus because vaporization requires
breaking all attractive forces.
Heats of Fusion of Several Substances
Liquid
Water
Isopropyl alcohol
Acetone
Diethyl ether
Chemical
formula
H2O
C3H7OH
C3H6O
C4H10O
Melting
point, °C
0.00
-89.5
-94.8
-116.3
Tro's Introductory Chemistry,
Chapter 12
∆Hfusion,
(kJ/mol)
6.02
5.37
5.69
7.27
50
6
Surface Tension
•Molecules in the interior of a
liquid experience attractions to
surrounding molecules in all
directions.
•However, molecules on the
surface experience an imbalance
in attractions, effectively pulling
them in.
•To minimize this imbalance and
maximize attraction, liquids try to
minimize the number of
molecules on the exposed surface
by minimizing their surface area.
•Stronger attractive forces
between the molecules = larger
surface tension.
Interesting Facts about Surface Tension
Water has the highest surface tension among common liquids
(mercury is higher).
when you look at a glass of water, the water "rises" where it touches
the glass (the "meniscus").
Plants are happy that water has a high surface tension because they
use capillary action to draw water from the ground up through their
roots and stems.
7
Viscosity
•Some liquids flow more easily
than others.
•The resistance of a liquid’s flow
is called viscosity.
•The stronger the attractive
forces between the molecules, the
more viscous the liquid is.
•Some liquids are more viscous
because their molecules are long
and get tangled in each other,
causing them to resist flowing
Effect of the Strength of Intermolecular
Attractions on Properties
• The stronger the intermolecular attractions are, the
more energy it takes to separate the molecules.
• Substances with strong intermolecular attractions have
higher boiling points, melting points, and heat of
vaporization; they also have lower vapor pressures.
8
Hydrogen Bonding
• HF, or molecules that have OH
or NH groups have particularly
strong intermolecular
attractions.
– Unusually high melting and
boiling points.
– Unusually high solubility in
water.
• This kind of attraction is called a
hydrogen bond.
Properties and H-Bonding
Name
Ethane
Formula
C2 H 6
Molar
mass
(g/mol)
30.0
Boiling
point,
°C
Melting
point,
°C
Solubility
in water
H
-88
-172
Immiscible
H
64.7
-97.8
Miscible
Structure
H
H
H
C
C
H
H
H
Ethanol
CH4O
32.0
H
C
O
H
9
Hydrogen Bonding
• When a very
electronegative atom is
bonded to hydrogen, it
strongly pulls the bonding
electrons toward it.
• Since hydrogen has no
other electrons, when it
loses the electrons, the
nucleus becomes
deshielded.
– Exposing the proton.
• The exposed proton acts as
a very strong center of
positive charge, attracting
all the electron clouds from
neighboring molecules.
H-Bonds vs. Chemical Bonds
• Hydrogen bonds are not chemical bonds.
• Hydrogen bonds are attractive forces between
molecules.
• Chemical bonds are attractive forces that make
molecules.
10
Practice Problems In Class
1. You put 1.00 L of water (4 cups) in a pan at 100oC
And the water evaporates. How much heat must have been
supplied to vaporize the water? ∆Hvap at 100oC = 40.6 kJ/mol
Ans: 2.26 X 103 kJ
2. Calculate amount of water in g that can be vaporized at its b.p
with 155 kJ of heat? Ans: 68.6 g
3.What is the energy required to melt 81.1 g of ice at
0 °C? The molar heat of fusion of water is 6.02 kJ/mol.
ANS: 27.1 kJ
∆Hfus at 0oC = 6.02 kJ/mol
(ice has negative calories)
4. An 8.5 g of ice cube is placed into 255 g of water.
Calculate the temperature change of water in complete
melting of the ice? (12.89)
5. How much heat in kJ is evolved in converting 1.00 mol of steam
At 145 C to ice at –50 C? Heat capacity of steam = 1.84 J/g. oC
And of ice is 2.09 J/g oC (12.93)
5.
Which of the following has the least solubility in
water? And why?
1.
2.
3.
4.
5.
CH3CH2CH3
CH3CH2NH2
CH3OCH3
H2O
CH3CH2OH
Check for this is in the review
worksheet handed out in
Class for Ch 12,13 and 14
11
6.
why?
1.
2.
3.
4.
5.
Which of the following high boiling point? And
CH3CH2CH3
CH3CH2NH2
CH3OCH3
CH4
CH3CH2OH
Check for this is in the review worksheet handed out in
Class for Ch 12,13 and 14
‘Unusual’ properties of water
Water is unique in that it is the only natural substance
that is found in all three states, liquid, solid (ice), and
gas (steam) at the temperatures normally found on
Earth.
Water becomes less dense upon freezing.
This results in an unusual phenomenon: water's solid form, ice,
floats upon water, allowing organisms to survive inside a
partially-frozen water body.
12
‘Unusual’ properties of water
Water has a high specific heat index. This means that water can
absorb a lot of heat before it begins to get hot.
•is valuable to industries and in car radiators as a coolant.
•helps regulate the rate at which air changes temperature,
which is why the temperature change between seasons is gradual
rather than sudden, especially near the oceans.
Water has a very high surface tension.
In other words, water is sticky and
elastic, and tends to
clump together in drops.
Hydrogen bonding
between water molecules
13