Example:

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Chapter 17

• Part 2

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• Example Sahara Desert Oases in the Sahara
  support life with water from ancient rivers and
  lakes (satellite observations support this). The
  Sahara used to be quite lush but the earth tilted
  away from the sun and changed it into a desert
  (What defines a desert? Rainfall). When the
  Sahara was closer to the sun it got more
  rainfall. This allowed for increased evaporation
  which lowered the average temperature even though
  the Sahara was closer to the sun.

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Saharan Desert
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Saharan Oasis
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Sahara was once fertile grasslands
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Reversible Reactions

• Most chemical reactions are reversible.
• Depending on conditions.

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H2O(l) ? H2O(g)

• Evaporation and condensation are reversible
  processes.
• An equilibrium occurs when two opposing processes
  occur at the same rate.

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Equilibrium H2O(l) ? H2O(g)
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H2O(l) ? H2O(g)

• Evaporation and condensation are reversible
  processes.
• An equilibrium occurs when two opposing processes
  occur at the same rate.
• At equilibrium the concentrations of the
  substances involved in the reaction will remain
  constant.
• concentration.

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Equilibrium H2O(l) ? H2O(g)
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Vapor Pressure

• Suppose we conduct an experiment in which we
  place a quantity of ethanol (C2H5OH) in an
  evacuated, closed container (a).
• The ethanol will quickly begin to evaporate.
• The pressure exerted by the vapor will begin to
  increase.
• After a short time the pressure will attain a
  constant value, called the vapor pressure of
  ethanol (b).

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Vapor Pressure

• At any given temperature, for a particular
  substance, there is a pressure at which the gas
  of that substance is in equilibrium with its
  liquid or solid form. This is the vapor
  pressure of that substance at that temperature.
• The equilibrium vapor pressure is an indication
  of a liquid's evaporation rate.
• A substance with a high vapor pressure at normal
  temperatures has a high rate of evaporation and
  is often referred to as volatile.

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Attractive Forces

• Two types of attractive forces are
• Intramolecular forces
• Intermolecular forces

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Intramolecular Forces

• Intramolecular forces attractive forces that
  hold particles together using ionic, covalent, or
  metallic bonds
• Intra means within
• Intramolecular forces bonds

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Intermolecular Forces(Van der Waals forces)

• Inter means between
• Intermolecular forces are forces between
  molecules
• Larger polar molecules have greater Van der Waals
  forces.