Liquids and Solids Intermolecular forces and Phase Changes

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Liquids and SolidsIntermolecular forces and
Phase Changes

• Chapter 14, sections 14.1-14.3

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Review

• What are some phases of matter?
• What are some properties of a gas?
• What are some things you know about a gas from
  the gas laws?

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Energy

• Think about water
• It can be a solid, liquid or a gas
• We have learned about the properties of a gas
• The properties of a liquid lie between those of a
  solid and a gas. (not exactly midway though)

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Energy cont

• Liquids and solids are far more closely related
  to each other than either of them to gases.
• It takes almost 7 times as much energy for water
  to go from being a liquid to a gas (41 kJ/mol)
  than from being a solid to liquid (6 kJ/mol)

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Intermolecular Forces

• Most substances that consisting of small
  molecules are gases, not liquids or solids at
  room temperature
• O2, CO2, CH4
• Water is a liquid. Why?
• Because of Intermolecular Forces
• Forces that occur between molecules

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Intermolecular Forces

• Remember that water has a dipole moment
• A slightly negative end and a slightly positive
  end.
• And the positive end is attracted to the negative
  end
• This is called Dipole-dipole attraction

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Intermolecular Forces

• Normal dipole-dipole forces are only about 1 as
  strong as covalent or ionic bonds, and they
  become weaker as the molecules get further apart.
  
• However, dipole-dipole forces are VERY strong
  when hydrogen is involved
• Examples H2O H-N or H-F
• This is called Hydrogen bonding

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Intermolecular Forces
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Intermolecular Forces

• Hydrogen bonding has a great affect on those
  molecules that have it.
• Water is so much higher because of the great
  difference in the electronegativity numbers of H
  and O

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Intermolecular Forces

• All molecules, even those with out dipole moments
  exert force on one another.
• Everything exists in a liquid or solid state at
  some temperature
• So there must be forces that hold even non-dipole
  atoms together in these states
• The forces that exist between non polar atoms or
  molecules are called London Dispersion Forces

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Intermolecular Forces

• We usually assume that electrons are even
  dispersed around a non polar atom or molecular.
• But sometime atoms can develop a instantaneous
  dipole

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Intermolecular Forces
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Intermolecular Forces

• Although the instantaneous dipole is very weak
  and very short lived, it slows the motion of the
  atoms down to allow the weak London Dispersion
  Forces to produce a solid or liquid
• London Dispersion Forces increase as the size of
  the atom increases and the polarity increases

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Water and Phase Changes

• Pure water behaves in a very specific way.
• It forms a solid at 0oC (at 1 atm of pressure)
• It forms a gas at 100oC (at 1 atm of pressure)
• Between 0oC and 100oC it is a liquid

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Water and Phase Changes
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Water and Phase Changes

• And water is one of the only substance that
  EXPANDS as it freezes
• The density of liquid water is 1.00 g/ml
• The density of ice is .917 g/ml
• So ice floats!

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Energy for change of state

• It takes energy to melt ice and to vaporize water.

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Energy for change of state

• The energy required to melt 1 mole of a substance
  is called the Molar Heat of Fusion
• The molar heat of fusion of water is 6.02 kJ/mol
• The energy required to change 1 mole of substance
  from liquid to a gas is called The Molar heat of
  Vaporization
• The molar heat of vaporization for water is 40.6
  kJ/mol

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Energy for change of state

• So, calculate the energy require to melt 8.5
  grams of ice.
• First figure out how many moles of water you have
• 8.5 grams/ 18.01 g/mol .47 moles of H2O
• Then take the moles multiplied by the heat of
  fusion
• . 47 mols x 6.02 kJ/mol 2.8 kJ

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Energy for change of state

• Calculate the amount of energy needed to heat 15
  grams of water to a gas
• 15 grams / 18.0 gram per mole .83 moles
• .83 moles x 40.6 kJ/mole 34 kJ

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Homework

• Read page 440-449
• Problems 1-6 on 449 and 22 on page 464