CHEM 1405

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CHEM 1405

• Class Meeting 17

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Assignments and Reminders

• Reading Assignment
• Chapter 8 by Thursday
• Homework Problems due Thursday Mar 23rd
• Chapter 7 Problems 3, 4, 6, 8, 9,10, 11, 12,
  15,16,17,20,22
• Homework Problems due Thursday Mar 30th
• Chapter 8 Problems 1 through 29 except 3 and
  4
• Class website
  http//iws.ccccd.edu/jstankus/

Please use only one side of the page when
submitting Homework
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Chemistry Help Resources

• My office hours
• Tuesdays after class in Lecture room
• Thursdays 1-2 in Math Lab
• Free Tutoring through college
• Students must submit a tutor request form in
  order to receive detailed information about the
  available tutoring services.  The form is
  available on Collin's website and in the
  following offices
• CPC room A108 (ask for Sonia Castillo)
• PRC room F109 (ask for Shontel Penny or Mary
  Eldridge)
• SCC rooms G200 and G141
• There are group tutoring services available for
  the following courses (SUBJECT TO CHANGE!)
• CHEM 1405, 1411, 1412, 2423,
•  
• Also available will be online tutoring in the
  following courses (SUBJECT TO CHANGE!)
• CHEM 1412 below

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Exam II Statistics
Average Grade 68 Average Time taken 128
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Objectives

• 3.How are intermolecular forces important in
  determining the physical properties of
  substances, such as melting point, boiling point,
  viscosity, and surface tension?
• 4.What happens when a liquid vaporizes? Why is
  vaporization important in maintaining body
  temperature?
• 5.How is distillation used to purify a liquid?
• 6.How are the structures of crystalline solids
  described?
• 7.What happens when a solid melts?
• 8.What are the unique properties of water?

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Intermolecular Forces
Intramolecular Forces (bonds within a molecule)
Intermolecular Forces (molecules interacting with
other molecules)
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Intermolecular Forces

• These forces are usually negligible in gases but
  of prime importance in liquids and solids.
• They determine
• Boiling point
• Melting point
• Surface Tension
• Viscosity

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Consequences of Ionic forces

• Relative melting points
• MgO and NaCl
• Mg2 O2- Na Cl-
• Greater charges mean greater forces
• Therefore MgO has higher melting point

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Dipole forces

• Weaker interactions than ionic

Remember a dipole has charge separation
Opposite Charges Attract
Like Charges repel
Slightly Positive
Slightly Negative
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Dipole Forces
Spontaneously Align to put opposite charges
adjacent
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Hydrogen Bonding

• Anomaly in boiling and freezing points of water
  is due to another intermolecular forces

d-
Greater Electron Density
d
Lesser Electron Density
d
Similar to ionic bonding
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Hydrogen bonding
A hydrogen bond is a type of intermolecular force
in which a hydrogen atom covalently bonded in one
molecule is simultaneously attracted to a
nonmetal atom in a neighboring molecule.
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Hydrogen bonding
Hydrogen Bonding in ice
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Dispersion Forces

• Induced dipole

Unpolarized Molecule has uniform charge
distribution
Fluctuation in charge distribution can cause it
to be slightly polarized
This slight polarization can induce a dipole in
and adjacent molecule
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Dispersion Forces

• A dispersion force is an attractive force between
  an instantaneous dipole and an induced dipole
• Dispersion forces, to a large extent, determine
  the properties of non-polar compounds

Dispersion Forces are also called London Forces
Not after the city but after Fritz London who
discovered them
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Gases, Liquids and Solids

• Gases
• Molecules are widely spaced, motion is chaotic,
  and disorder is at a maximum.
• Liquids
• Molecules continue to undergo translational
  motion but are in much closer proximity
• Solids
• There is a high degree of order among the
  structural particles the motion of the particles
  is vibrational.

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States of Matter

• 3 phases of matter solid, liquid, and gas

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The Liquid State

• Molecules in a liquid are closer than in a gas
• Molecules are in constant motion, but restricted
  by neighboring molecules

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Viscosity
Viscosity resistance to flow of
liquids Related to - intermolecular forces of
attraction -Size and shape of constituent
particles Low viscosity (this easily flowable
liquids) - weak intermolecular
forces - small symmetrical molecules High
Viscosity (difficult to flow liquids) -Strong
intermolecular forces - large or unsymmetrical
molecules
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Viscosity Temperature Dependence

• Viscosity generally decreases with increasing
  temperature
• Example heating cooking oil
• Room temperature thick and viscous
• High temperature in Frying pan thin and low
  viscosity

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Viscosity in action

• Motor oils
• 20W Motor oil is a particular viscosity oil
• Lubricating ability is related to viscosity
• Multiweight (multiviscosity) oils
• Problem
• oil viscosity that is right at low temp may be
  too thin at high temp
• Oil viscosity that is right at high temp may be
  too thick at low temp
• Solution
• Oil with components that increase viscosity
  with temperature

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Surface Tension
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Surface Tension

• The surface tension of water enables the steel
  needle, though denser than water, to float on the
  surface of the water .
• It also supports the water strider.

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Intermolecular forces

• Cohesive forces forces holding liquid together
• Adhesive forces force attracting a liquid to
  another surface
• Wetting of surfaces

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Adhesive Forces
Formation of a meniscus
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Capillary Action
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From Liquid to Gas Vaporization

• Liquid ? Vapor Vaporization
• molecules in the liquid gain sufficient kinetic
  energy to escape the liquid
• Vapor ?Liquid Condensation
• Molecules in gas state contact the liquid and are
  captured by the intermolecular forces

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Vaporization and Condensation Equilibrium
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Vapor Pressure

• Partial pressure of vapor molecules above the
  surface of a liquids at equilibrium

• Dependent on kinetic energy of the molecules
• Higher temperature ? higher vapor pressure

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Boiling Point

• The boiling point is the temperature at which the
  vapor pressure of a liquid becomes equal to the
  atmospheric pressure.

The molecules have sufficient kinetic energy to
form bubbles of vapor, which rise and burst on
the surface
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Boiling Points Pressure Effects

• Since B.P. is temperature where Vapor Pressure of
  liquid equals pressure above
• Reducing pressure above the liquid can reduce the
  boiling point temperature
• Increasing the pressure above can increase the
  boiling point temperature

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Pressure Cookers

• Application of changing the boiling point through
  higher pressures.
• Higher pressure leads to higher boiling
  temperature
• Higher temperature leads to quicker cooking

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Energy Considerations

• Molar heat of vaporization is the quantity of
  heat that must be absorbed to vaporize 1 mol of a
  given liquid at a constant temperature.

Energy added Without temperature change
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Heats of Vaporization

• Molar heat of vaporization
• Energy needed to vaporize 1 mole of a liquid
• Molar heat of vaporization of water 9700 cal/mol
• Heat of vaporization
• Energy needed to vaporize a specific mass of a
  liquid
• Heat of vaporization of water is 2260 J/g

Watch your units, what is given and what is asked
for
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Evaporative Cooling

• Example Compare the amount of cooling experienced
  by an individual who drink 400 mL of ice-water
  (0C) and an individual who sweats out 400 mL
  of water
• Ice water
• Energy needed to go from 0C ? 37C
• Heat capacity Problem
• Sweat
• Energy required to evaporate 400 mL water
  at 37C
• Heat of vaporization problem

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Evaporative Cooling Example cont

• Ice water Energy needed to go from 0C
  ? 37C
• heat used to heat liquid Mass x Heat Capacity
  x DT
• 400 mL x 1.00 g/mL (density) 400 g of water
• Heat capacity of water is 1.00 cal/gC
• DT 37C - 0C 37C
• heat used to heat ice water
• 400g x 1.00 cal/gC x 37 C 14800 cal
  14.8 kcal

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Evaporative Cooling Example cont

• Sweat Energy required to evaporate 400 mL
  water at 37C
• Heat used to vaporize liquid moles x molar heat
  of vaporization
• 400 g x ( 1 mol /18.02 g) 22.2 mol H2O
• Molar Heat of vaporization 9700 cal/mol
• Heat used to vaporize sweat 22.2 moles H2O x
  9700 cal/mol
• 
  215000 cal 215 kcal

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Evaporative Cooling Example cont

• Ice water
• Energy needed to go from 0C ? 37C
  
• 
  14.8 kcal
• Sweat
• Energy required to evaporate 400 mL water at
  37C
• 
  215 kcal
• Evaporating sweat is much more efficient at
  cooling
• But dont forget you need to replace that water
  that is lost through evaporation

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Distillation

• Distillation is the separation of components of a
  solution by boiling off the most volatile
  compounds such as alcohol or water and then
  condensing their vapors. Solids and high-boiling
  compounds are left behind.

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Solids

• Motion is mostly limited to vibration about a
  fixed point
• Amorphous solids - glass
• Has no well defined ordered structure
• Crystalline solid examples salt crystals
• Well defined ordered structure

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Packing items together
1D
2D
In a crystal lattice, the fundamental units
making up the crystalatoms, ions, or
moleculesare assembled in a regular, repeating
manner, extending in three dimensions through the
crystal
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Crystal Lattice Structure 3D
Body Centered Cubic
Face Centered Cubic
Cubic
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From Solid to Liquid melting

• Melting point of a solid (or freezing point of a
  liquid is the temperature where solid and liquid
  exist in equilibrium
• As a solid is heated the molecules vibrate more
  in place until they have enough energy to
  overcome the intermolecular forces holding them
  together

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Energy Considerations

• Molar heat of fusion is the quantity of heat that
  must be absorbed to melt 1 mol of a solid at a
  constant temperature.

Energy added Without temperature change
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Example heat of fusion

• The heat of fusion of benzene is 33 cal/g. How
  many calories are required to melt 20.0 g of
  solid benzene?
• Heat used to melt mass x heat of fusion
• 20.0 g x 33 cal/g
• 660 cal

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Heats of Fusion

• Molar heat of fusion
• Energy needed to melt 1 mole of a solid
• Molar heat of fusion of water 6.01 kJ/mol
• Heat of fusion
• Energy needed to melt a specific mass of a solid
• Heat of fusion of water is 80 cal/g

Watch your units, what is given and what is asked
for
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Sublimation

• Sublimation is the direct passage of molecules
  from the solid state to the vapor state
• Dry Ice (frozen Carbon Dioxide) goes directly
  from solid to liquid

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Phase Change Summary
Sublimation
Deposition
11.8
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Water unique properties

• Low density of solid form
• Ice floats
• High heat capacity
• Water acts as a thermostat for the earth
• High heat of vaporization
• Enables efficient cooling through sweat

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Liquid Crystals another phase of matter
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Forms of Carbon
Diamond
Graphite
Bucky Ball