Evaporation

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Chapter 17
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Evaporation
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Evaporation

• Occurs when high kinetic energy particles at the
  surface of a liquid break free from the
  attractive forces of the neighboring particles
  and become vapor.

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Vapor

• A vapor is the gaseous state of a substance that
  is liquid or solid under ordinary conditions.

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Evaporation

• Evaporation is a cooling process.

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Why doesnt evaporating water freeze?
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Evaporation is a cooling process
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Evaporation is a cooling process
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Heat Index
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Evaporation is a cooling process
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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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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.

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Intermolecular forces vs. Intramolecular forces
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What happens when the water evaporates?

• Intermolecular attractions (van der Waals forces)
  are broken.
• Intramolecular attractions (bonds) are not broken.

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Demo

• Squirt Water and Alcohol on chalkboard

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alcohol vs. water

• How do the vapor pressures, rates of evaporation,
  and van der Waals forces compare?
• How would the boiling point of alcohol compare to
  water?

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Why is it called rubbing alcohol?
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Substances with weaker Van der Waals Forces

1. Are easier to evaporate
2. Have higher vapor pressure
3. Are more volatile
4. Have lower boiling points

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Equilibrium

• A state of equilibrium is the most stable state
  for a reversible system.

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LeChatliers Principle

• If a stress is placed on a system in equilibrium
  the system will tend to readjust so that the
  stress is reduced.
• 3 Stresses are changing the
• Concentration
• Temperature
• Pressure
• LeChatliers Principle Do the opposite

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I smell gasolineC8H18(l) ? C8H18(g)
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2NO2(g) ?N2O4(g)brown colorless
How does applying LeChatliers Principle explain
that this reaction is exothermic? Rule An
increase in temperature will always shift a
reaction in the endothermic direction. What
other rule could we use? Note Apply the rule,
it accounts for doing the opposite. So dont do
the opposite of the rule.
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H2O(l) ? H2O(g)
Is the reaction above exothermic or endothermic?
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Predict the effect of the following changes on
the reaction

• 2SO3(g) ? 2SO2 (g) O2 (g) ?H 197.78 kJ
• (a) Increasing the temperature of the reaction.

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Predict the effect of the following changes on
the reaction

• 2SO3(g) ? 2SO2 (g) O2 (g) ?H 197.78 kJ
• (b) Increasing the pressure on the reaction.

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Predict the effect of the following changes on
the reaction

• 2SO3(g) ? 2SO2 (g) O2 (g) ?H 197.78 kJ
• (c) Adding more O2.

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Predict the effect of the following changes on
the reaction

• 2SO3(g) ? 2SO2 (g) O2 (g) ?H 197.78 kJ
• (d) Removing O2.

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Homework

• Worksheet 1 Chapter 17.

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Solid-Vapor Equilibrium

• Sublimation is a process in which molecules go
  directly from the solid into the vapor phase
  (deposition is the reverse process).

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Sublimation
Iodine and dry ice are substances that commonly
undergo sublimation.
Which of these can reach equilibrium?
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Deposition

• The formation of frost.

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Boiling

• A substance boils when the vapor pressure of the
  liquid is equal to the atmospheric pressure.

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How could I boil this water?
AA
A
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What is temperature of the water?
AA
A
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Why does water boil at 100C?
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Is there another way to boil the water?
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Is there another way to boil the water?
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Is there another way to boil the water?
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What is the pressure inside the bell jar if the
water is boiling at 20C?
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We can make water boil at any temperature.
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234
Elevation 29,035 ft.
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Volcanic Vents
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• LONDON Scientists using a remote-controlled
  submarine have discovered the deepest known
  volcanic vent and say the superheated waters
  inside could contain undiscovered marine species
  and perhaps even clues to the origin of life on
  earth.
• Experts aboard the RRS James Cook said they found
  the underwater volcanic vent more than three
  miles (five kilometers) beneath the surface of
  the Caribbean.
• Volcanic vents are areas where sea water seeps
  into small cracks that penetrate deep into the
  earth's crust some reaching down more than a
  mile (two kilometers.) Temperatures there can
  reach 750 degrees Fahrenheit (400 degrees
  Celsius), heating the water to the point where it
  can melt lead.
• The blazing hot mineral-rich fluid is expelled
  into the icy cold of the deep ocean, creating a
  smoke-like effect and leaving behind towering
  chimneys of metal ore, some two stories tall. The
  spectacular pressure 500 times stronger than
  the earth's atmosphere keeps the water from
  boiling.

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Which beaker of water has had more heat added to
it?
Boiling for 5 minutes
Boiling for 30 minutes
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Which beaker of water is hotter?
Boiling for 5 minutes
Boiling for 30 minutes
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What would happen to this water as it boiled?
It gets colder
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How can we continue to boil the water as it cools?
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What happens when the vapor pressure of water
reaches 4.6 mm Hg.
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What is the boiling point of ethanol at normal
air pressure?
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What is the boiling point of water at an air
pressure of 400 torr?
82C
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At what air pressure will Diethyl ether boil at
20C?
450 torr
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How do these substances compare in terms of Van
der Waals forces, rate of evaporation, vapor
pressure, and volatility ?
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How do these substances compare in terms of Van
der Waals forces, rate of evaporation, vapor
pressure, and volatility?
Diethyl ether has the least Van der Waals forces,
and the greatest rate of evaporation, vapor
pressure, and volatility. Ethylene glycol has
the greatest Van der Waals forces, and the lowest
rate of evaporation, vapor pressure, and
volatility.
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Which would you expect to have the higher
boiling point?
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Which would you expect to have the higher
boiling point?
He -269C Rn -62C
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Demonstrations

• Boiling water with by applying hands.
• Freezing water by boiling.

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Homework

• Summarize the Equilibrium Lab (due tomorrow).
• Worksheet 1 Chapter 17 (due tomorrow).
• Worksheet 2 Chapter 17 (due in two days).
• Study Guide Chapters 15 17 (due in two days).