Transcript Chem 150 Unit 3 - Physical Properties & Intermolecular Interactions

Chem 150
Unit 3 - Physical Properties &
Intermolecular Interactions
When discussing the physical properties of molecules the
discussion often focuses on topics such as melting points,
boiling points and solubilities in various solvents. These
are considered physical properties because they do not
change the composition or identity of the molecules
involved. At the heart of these discussions is what causes
molecules to be attracted to and repelled by one another.
This discussion is critical to our understanding biological
systems.
States of Matter
For molecular substances there are basically three states or
phases of matter.
2
•
Solids (s): Molecules are held in place by intermolecular
interactions.
•
Liquids (l): Molecules are held next to one another by
noncovalent, intermolecular interactions, however, these
interactions are not strong enough to prevent the
molecules from flowing past one another.
•
Gases (s): The intermolecular interactions are too weak to
hold the molecules next to one another, so the molecules
wander off on their own.
States of Matter
Water vapor, H2O(g)
Ice, H2O(s)
3
Liquid water, H2O(l)
States of Matter
The strength and numbers of the noncovalent intermolecular
interactions determine which state a molecular substance is
in.
• The predominant noncovalent interaction between water
molecules is the hydrogen bond:
4
Questions (Clickers)
If you were interested in disrupting the noncovalent
interactions between water molecules in ice you could:
5
A)
Shine light on the ice.
B)
Stick the ice in a freezer.
C)
Hit the ice with a hammer.
D)
Add heat to the ice.
States of Matter
These interactions can be disrupted by adding heat.
• Adding heat increases the kinetic energy of the molecules
This is most readily observed with gases by looking at the
Ideal Gas Law equation:
• As the temperature of a gas increases, so does its kinetic
energy.
Cl2 +
2 Na
6
–2 e
–
2 Cl
2 Na+
+
2 e
–
States of Matter
Ideal Gas Law Simulation
7
States of Matter
A as heat is added to a molecular substance, it warms until
reaching one of the phase transition temperature.
• At that point the heat (kinetic energy) that is added to the
substance is used to break the noncovalent, intermolecular
interactions.
8
States of Matter
For a more detailed description of phase transitions, along
with an animation of the process,
see the Chem 150
Elaboration - States of Matter
9
Questions (Clickers)
When heat is added to liquid propane, (CH3CH2CH3), it
warms until reaching the boiling point, and then changes into
a gas. Which of the following statements most accurately
describes what is going on:
10
A)
The added kinetic energy is causing the carbon and
hydrogen atoms in propane to separate from one another.
B)
The added kinetic energy is causing the disruption of both
hydrogen bonds and London dispersion forces between the
propane molecules.
C)
The added kinetic energy is causing the disruption of the
London dispersion forces between the propane molecules.
D)
The added kinetic energy is causing the propane to change
into methane gas.
Questions (Clickers)
When heat is added to liquid ethanol, (CH3CH2OH), it warms
until reaching the boiling point, and then changes into a gas.
Which of the following statements accurately describes what
is going on:
11
A)
The added kinetic energy is causing the carbon and
hydrogen atoms to separate from one another.
B)
The added kinetic energy is causing the disruption of both
hydrogen bonds and London dispersion forces between the
ethanol molecules.
C)
The added kinetic energy is causing the disruptions of the
London dispersion forces between the ethanol molecules.
D)
The added kinetic energy is causing the ethanol to change
into methane gas and water vapor.
Questions (Clickers)
Both propane (M = 44 g/mol) and ethanol (M = 46 g/mol)
have comparable molecular weights. Which do you predict
has the higher boiling point? Explain why.
12
A)
They should have similar boiling points.
B)
The ethanol should have the higher boiling point.
C)
The propane should have the higher boiling point.
D)
It is hard to tell without doing the experiment.
Enthalpy, Entropy and Free Energy
Energy is defined as the ability to do work.
•
The heat energy we have been talking about is also called
Enthalpy (H)
•
It was used to do the work of breaking the noncovalent,
intermolecular interactions present in solids and liquids.
•
When Enthalpy is put into an object, such as an ice cube,
the change in Enthalpy for the ice cube increases.
•
•
13
(ΔH > 0). the Δ symbol means “change in”.
Changes in nature can be either spontaneous (favorable),
or nonspontaneous (unfavorable).
Questions (Clickers)
When the ice melts from the surface of a pond on a warm
spring day, this change is
14
A)
spontaneous
B)
nonspontaneous
Questions (Clickers)
When the ice forms on the surface of a pond on a cold winter
day, this change is
15
A)
spontaneous
B)
nonspontaneous
Enthalpy, Entropy and Free Energy
16
•
Why are some changes spontaneous while others are
nonspontaneous?
•
Why, like the ice on a pond, are changes spontaneous
some of the time and nonspontaneous at other times?
•
Asking some questions about the energy changes that take
place can to help answer theses questions
Enthalpy, Entropy and Free Energy
Changes occur spontaneously in nature when energy is
released.
• The case of the rolling stone.
17
Enthalpy, Entropy and Free Energy
Although Enthalpy is a form of energy, it alone cannot be
used to answer these questions.
•
The melting of the ice from a pond on a warm spring day is
spontaneous
•
However, the ice is absorbing heat (ΔΗ > 0) (endothermic)
A second factor called Entropy (S), needs to also be
considered to determine if a change is spontaneous or
nonspontaneous.
18
Enthalpy, Entropy and Free Energy
Entropy is a measure of disorder.
19
•
When ΔS > 0, things become more disorder.
•
Nature prefers things to be disordered:
Enthalpy, Entropy and Free Energy
Enthalpy and Entropy can be combined to calculate another
type of energy called Free Energy (G).
•
ΔG = ΔΗ - ΤΔS
The change in Free Energy can be used to predict whether a
change is spontaneous or nonspontaneous.
20
•
When ΔG < 0, the change is spontaneous (favorable)
•
When ΔG > 0, the change is nonspontaneous
(unfavorable)
Enthalpy, Entropy and Free Energy
When Ice melts, ΔH > 0 and ΔS > 0
•
It gains heat and becomes more disordered
Above the freezing temperature, TΔS > ΔH and ΔG is
negative ( ΔG < 0)
•
Ice melts spontaneously.
Below the freezing Temperature, TΔS < ΔH and ΔG is
positive ( ΔG > 0)
•
21
Ice does not melt spontaneously.
Enthalpy, Entropy and Free Energy
When Ice freezes, ΔH < 0 and ΔS < 0
•
It loses heat and becomes more ordered
•
•
ΔH makes a negative contribution to ΔG
ΔS makes a positive contribution to ΔG
Below the freezing Temperature, the magnitude of TΔS < ΔH
and ΔG is NEGATIVE (ΔG < 0)
•
Ice freezes spontaneously.
Above the freezing temperature, the magnitude of TΔS > ΔH
and ΔG is positive (ΔG > 0)
•
22
Ice freezes nonspontaneously.
Enthalpy, Entropy and Free Energy
For a more detailed description using the enthalpy, entropy
an free energy changes to predict if a process is spontaneous
or not,
see the Chem 150
Elaboration - Ethalpy, Entropy & Free Energy
23
Enthalpy, Entropy and Free Energy
Figure 5.6, Raymond
ΔS < 0
(molecules are more ordered)
ΔS > 0
(molecules are more disordered)
24
Questions (Clickers)
Before going on a picnic on a hot sumer day, you stopby the
store and pick up a block of dry ice, CO2(s).
In terms of ΔH alone, is the sublimation of dry ice
25
A)
Spontaneous
B)
Nonspontaneous
Questions (Clickers)
Before going on a picnic on a hot sumer day, you stopby the
store and pick up a block of dry ice, CO2(s).
In terms of ΔS alone, is the sublimation of dry ice
26
A)
Spontaneous
B)
Nonspontaneous
Questions (Clickers)
Before going on a picnic on a hot sumer day, you stopby the
store and pick up a block of dry ice, CO2(s).
At 50°C (This is one hot day!!) the ΔG for the sublimation of
dry ice as a
27
A)
negative value.
B)
positive value.
Liquids
Liquids have various physical properties that reflect the
strength of the intermolecular interactions that hold the liquid
together
• Boiling point temperature
• Viscosity
•
28
•
Resistance to flow
Vapor pressure
Liquids
Viscosity
• Resistance to flow
29
Liquids
Vapor Pressure and Boiling Points are related
• The boiling point is the temperature at which the vapor
pressure is equal to the atmospheric pressure.
1 atm = 760 Torr
30
Liquids
Vapor Pressure and Boiling Points are related
• The boiling point is the temperature at which the vapor
pressure is equal to the atmospheric pressure.
1 atm = 760 Torr
31
Questions (Clickers)
You planning to do some surgery on your kitchen table and
know that you need to sterilize your instruments by heating
them to 120°C. You rummage around in the kitchen
cupboards and find a pressure cooker that can heat water to
a pressure of 1.4 atm. Will this be sufficient for sterilizing your
instruments? (You may use Table 5.6 in your book to answer
this question; see the previous slide.)
A)
Yes
B)
No
Explain you answer.
32
Questions (Answer)
•1.4 atm (760Torr/atm) = 1064 Torr
•This is less than the pressure required to reach 110°C (1075
Torr), therefore it is an insufficient pressure to reach 120°C.
•(120-100)/(125-100)*(1741-760)+760=~1544 Torr (interpolation)
1 atm = 760 Torr
33
Solutions
Biological systems are mixtures of substances
• Pure substances contain only one type of element or
compound
•
They contain only one type of atom or molecule:
‣
‣
‣
‣
‣
•
34
H2
Hg
O2
H2O
sucrose (C12H22O11)
Mixtures contain more than one type of pure substance
•
Heterogeneous mixture - components are not evenly mixed at the molecular
level.
•
Homogeneous mixture - components are evenly mixed at the molecular level.
Solutions
A solution is another name for homogeneous mixture.
•
•
35
Solvent - the major component in a solution
Solute - the minor component in a solution.
Solvent
Solute
Carbonated Water
Water
(l)
CO2
(g)
Air
Nitrogen
(g)
Oxygen
(g)
Salt Water
Water
(l)
Na+ & Cl- Ions
(s)
Dental Amalgam
Silver (Ag)
(s)
Mercury (Hg)
(l)
Vodka
(80 Proof)
Water
(l)
Ethanol
(l)
Solutions
A solution is another name for homogeneous mixture.
•
Liquid solutions should be clear (transparent).
•
Liquid solution’s solutes should not settle with time
This distinguishes solutions from suspensions and
colloids.
36
Solutions
In order form a solution to form
• the solute molecules have to be able to form similar
noncovalent interactions with the solute molecules as
•
•
37
the solute molecules form with themselves
the solvent molecules form with themselves.
States of Matter
Simulation of Glycerol and Propane
Dissolving in Water
38
Solutions
Solubility is a measure of how
much solute will dissolve in a
solvent.
Solubility depends on
temperature.
39
•
The solubility of gases
decrease with increasing
temperature
•
The solubility of solids and
liquids usually increase with
increasing temperature.
Solutions
When a solution is saturated, the solute dissolves and
precipitates at the same rate.
40
Solutions
In order form a solution to form
• the solute molecules have to be able to form similar
noncovalents with the solute molecules as
•
•
41
the solute molecules form with themselves
the solute molecules form with themselves.
Solubility of Gases in Water
Henry’s Law - The solubility of a gas in a liquid is
proportional to the pressure of the gas over the liquid.
•
The fizzing of soda when the cap is removed is an example
of the lowered solubility of CO2 in water when it’s pressure
above the soda is descrease.
•
The solubility of CO2 in water is very high, because it can
react with water to produce and even more soluble
product, H2CO3 (carbonic acid):
CO2 + H2O
•
42
H2CO3
We will see that this is a very important reaction in biochemistry
Organic Compounds
Nonpolar, organic solutes
will dissolve readily in
nonpolar, organic solvents.
• “Like dissolves Like”
43
Organic Compounds
The solubility is determined by the balance between the polar
and nonpolar portions of the molecule.
44
Questions (Clicker)
Pentanoic acid and 1-pentanol have the same number of
carbon atoms. Which one is expected to have the higher
solubility in water? Explain.
A)
O
Pentanoic acid
CH
CH2 CH2
B)3 CH
1 2Pentanol
C)
45
pentanoic
Neitheracid
C
OH
CH3 CH2 CH2 CH2 CH2 OH
1-pentanol
Biochemical Compounds &
Their Interactions with Water
Biological molecules are grouped into three categories.
•
Hydrophilic (water loving) molecules.
•
•
Hydrophobic (water fearing) molecules.
•
•
Polar molecules that can interact favorably with water
Nonpolar molecules that cannot interact favorably with water
Amphipathic molecules, which are conflicted about their
feelings towards water.
• Molecules containing both very polar and very nonpolar parts.
46
Biochemical Compounds &
Their Interactions with Water
Hydrophilic (water loving)
molecules.
47
•
Polar molecules that
can interact favorably
with water
•
Carbohydrates (sugars)
have lots of polar
hydroxyl groups
Biochemical Compounds &
Their Interactions with Water
Hydrophilic (water loving) molecules.
•
•
48
Polar molecules that can interact favorably with water
Amino acids have both an amino and a carboxylic acid
group, which are polar.
Biochemical Compounds &
Their Interactions with Water
Hydrophobic (water fearing) molecules.
•
•
49
Nonpolar molecules that cannot interact favorably with
water
The carboxylic acid groups, though polar, are dominated
by the long hydrocarbon portions
Biochemical Compounds &
Their Interactions with Water
Hydrophobic (water fearing) molecules.
•A nonpolar solute
"organizes" water
•The H-bond network of
water reorganizes to
accommodate the
nonpolar solute
•This is an increase in
"order" of water-This is a
decrease in ENTROPY
50
Biochemical Compounds &
Their Interactions with Water
Hydrophobic (water fearing) molecules.
H
kcal/mol
CH4 in benzene CH4 in water -2.8
CH4 in ether CH4 in water
-2.4
CH4 in CCl4 CH4 in water
-2.5
C3H8 liquid C3H8 in water
-1.8
Tra nsfer reaction (25ÞC)
Recall G=H-TS
51
S
cal/K mol
-18
-19
-18
-23
G
kcal/mol
+2.6
+3.3
+2.9
+5.1
Biochemical Compounds &
Their Interactions with Water
Amphipathic molecules, which are conflicted about their
feelings towards water.
•
52
Molecules containing both very polar and very nonpolar
parts.
Biochemical Compounds &
Their Interactions with Water
When placed in water,
amphipathic molecules,
form structures, such as
micelles, which attempt to
address the conflict.
53
Colloids and Suspensions
It is also possible to have mixtures which are not uniform at
the molecular level.
•
These are called heterogeneous mixtures.
When a heterogenous mixture involves the mixing of a solid
with a liquid, there are two possible situations:
•
•
Suspensions:
•
With time the solid settles out of the mixture
Colloids:
•
The solid stays suspended in the liquid indefinitely,
Both suspensions and colloids are cloudy.
54
Colloids and Suspensions
55
Diffusion and Osmosis
Within a solution, the solute and solvent molecules are
constantly moving
56
•
If the concentration of the solute is not uniform throughout
a solution, this movement will cause a net movement of
solute molecules from the regions of high concentration to
the regions of low concentration
•
In the end the concentration will be the same everywhere.
Diffusion and Osmosis
This movement is called diffusion.
57
Diffusion and Osmosis
If a semipermeable membrane that only allows solvent to
pass through it is used to separate a region of high solute
concentration from a region of low solute concentration
•
58
Solvent will move through the membrane from the region of
low solute concentration to the region of high solute
concentration in an effort to make the solute concentration
the same on both sides of the membrane.
Diffusion and Osmosis
This movement is called osmosis.
59
Diffusion and Osmosis
This movement can be stopped by
applying a pressure to the surface
of the solution on the high solute
concentration side of the
membrane.
• The pressure required to stop the
movement is called the osmotic
pressure.
60
Diffusion and Osmosis
Osmotic pressure is an important concept for understanding
biological systems because the cell membrane is a
semipermeable membrane
• If the solute concentrations are not equal on both sides of
the membrane, the cells can either shrivel up or swell up
and explode
61
The End