Chapter 17 Water and Aqueous Systems

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Chapter 17Water and Aqueous Systems

• Charles Page High School
• Dr. Stephen L. Cotton

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Section 17.1Liquid Water and its Properties

• OBJECTIVES
• Describe the hydrogen bonding that occurs in
  water.

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Section 17.1Liquid Water and its Properties

• OBJECTIVES
• Explain the high surface tension and low vapor
  pressure of water in terms of hydrogen bonding.

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The Water Molecule

• Water is a simple triatomic molecule.
• Each O-H bond is highly polar, because of the
  high electronegativity of the oxygen
• bond angle 105 o
• due to the bent shape, the O-H bond polarities do
  not cancel. This means water as a whole is
  polar.
• Fig. 17.2, p.475

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The Water Molecule

• Waters bent shape and ability to hydrogen bond
  gives water many special properties!
• Water molecules are attracted to one another.
• This gives water high surface tension, low vapor
  pressure, high specific heat, high heat of
  vaporization, and high boiling point

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High Surface Tension

• liquid water acts like it has a skin
• glass of water bulges over the top
• Water forms round drops
• spray water on greasy surface
• All because water hydrogen bonds.
• Fig. 17.4, p.476

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Surface Tension
d-

• One water molecule hydrogen bonds to another.
• Also, hydrogen bonding occurs to other molecules
  all around.

d
d
d-
d
d
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Surface Tension

• A water molecule in the middle of solution is
  pulled in all directions.

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Surface Tension

• Not true at the surface.
• Only pulled down and to each side.
• Holds the molecules together.
• Causes surface tension.

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Surface Tension

• Water drops are round, because all molecules on
  the edge are pulled to the middle- not to the air!

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Surface Tension

• Glass has polar molecules.
• Glass can hydrogen bond.
• Attracts the water molecules.
• Some of the pull is up a cylinder.

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Meniscus

• Water curves up along the side of glass.
• This makes the meniscus, as in a graduated
  cylinder
• Plastics are non-wetting no attraction

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Meniscus
In Plastic
In Glass
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Surface tension

• All liquids have surface tension
• water is higher than most others
• How to decrease surface tension?
• Use a surfactant - surface active agent
• a wetting agent, like detergent or soap
• interferes with hydrogen bonding

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Low vapor pressure

• Fig. 17.6, p.477
• Hydrogen bonding also explains waters unusually
  low vapor pressure.
• Holds water molecules together, so they do not
  escape
• good thing- lakes and oceans would evaporate very
  quickly!

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Specific Heat Capacity

• Water has a high heat capacity (also called
  specific heat).
• It absorbs 4.18 J/gºC, while iron absorbs only
  0.447 J/gºC.
• Remember SH heat Mass x DT
• If we calculate the heat need to raise the
  temperature of both iron and water by 75ºC -
  water is almost 10 x more!

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Section 17.2Water Vapor and Ice

• OBJECTIVES
• Account for the high heat of vaporization and the
  high boiling point of water, in terms of hydrogen
  bonding.

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Section 17.2Water Vapor and Ice

• OBJECTIVES
• Explain why ice floats in water.

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Evaporation and Condensation

• Because of the strong hydrogen bonds, it takes a
  large amount of energy to change water from a
  liquid to a vapor.
• 2,260 J/g is the heat of vaporization.
• This much energy to boil 1 gram water
• You get this much energy back when it condenses.
• Steam burns, but heats things well.

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Ice

• Most liquids contract (get smaller) as they are
  cooled.
• They get more dense.
• When they change to solid, they are more dense
  than the liquid.
• Solid metals sink in liquid metal.
• But, ice floats in water.
• Why?

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Ice

• Water becomes more dense as it cools until it
  reaches 4ºC.
• Then it becomes less dense.
• As the molecules slow down, they arrange
  themselves into honeycomb shaped crystals.
• These are held together by hydrogen bonds. (Fig.
  17.9, p.481)

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Liquid
Solid
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Ice

• 10 greater volume than water.
• Water freezes from the top down.
• The layer of ice on a pond acts as an insulator
  for water below
• It takes a great deal of energy to turn solid
  water to liquid water.
• Heat of fusion is 334 J/g.

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Section 17.3Aqueous Solutions

• OBJECTIVES
• Explain the significance of the statement like
  dissolves like.

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Section 17.3Aqueous Solutions

• OBJECTIVES
• Distinguish among strong electrolytes, weak
  electrolytes, and nonelectrolytes, giving
  examples of each.

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Solvents and Solutes

• Solution - a homogenous mixture, that is mixed
  molecule by molecule.
• Solvent - the dissolving medium
• Solute -the dissolved particles
• Aqueous solution- a solution with water as the
  solvent.
• Particle size about 1 nm cannot be separated by
  filtration!

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Aqueous Solutions

• Water dissolves ionic compounds and polar
  covalent molecules best.
• The rule is like dissolves like
• Polar dissolves polar.
• Nonpolar dissolves nonpolar.
• Oil is nonpolar.
• Oil and water dont mix.
• Salt is ionic- makes salt water.

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How Ionic solids dissolve

• Called solvation.
• Water breaks the and - charged pieces apart and
  surrounds them.
• Fig. 17.12, p. 483
• In some ionic compounds, the attraction between
  ions is greater than the attraction exerted by
  water
• Barium sulfate and calcium carbonate

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How Ionic solids dissolve
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• Solids will dissolve if the attractive force of
  the water molecules is stronger than the
  attractive force of the crystal.
• If not, the solids are insoluble.
• Water doesnt dissolve nonpolar molecules because
  the water molecules cant hold onto them.
• The water molecules hold onto each other, and
  separate from the nonpolar molecules.
• Nonpolars? No repulsion between them

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Electrolytes and Nonelectrolytes

• Electrolytes- compounds that conduct an electric
  current in aqueous solution, or in the molten
  state
• all ionic compounds are electrolytes (they are
  also salts)
• barium sulfate- will conduct when molten, but is
  insoluble in water!

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Electrolytes and Nonelectrolytes

• Do not conduct? Nonelectrolytes.
• Many molecular materials, because they do not
  have ions
• Not all electrolytes conduct to the same degree
• there are weak electrolytes, and strong
  electrolytes
• depends on degree of ionization

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Electrolytes and Nonelectrolytes

• Table 17.3, p.485 lists some common electrolytes
  and nonelectrolytes
• How do you know if it is strong or weak? Rules
  on handout sheet.

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Electrolyte Summary

• Substances that conduct electricity when
  dissolved in water, or molten.
• Must have charged particles that can move.
• Ionic compounds break into charged ions
• NaCl Na1 and Cl1-
• These ions can conduct electricity.

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• Nonelectrolytes do not conduct electricity when
  dissolved in water or molten
• Polar covalent molecules such as methanol (CH3OH)
  dont fall apart into ions when they dissolve.
• Weak electrolytes dont fall completely apart
  into ions.
• Strong electrolytes do ionize completely.

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Water of Hydration(or Water of Crystallization)

• Water molecules chemically bonded to solid salt
  molecules (not in solution)
• These compounds have fixed amounts of water.
• The water can be driven off by heating
• CuSO4.5H2O CuSO4 5H2O
• Called copper(II)sulfate pentahydrate.

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Hydrates

• Table 17.4, p.486 list some familiar hydrates
• Since heat can drive off the water, the forces
  holding it are weak
• If a hydrate has a vapor pressure higher than
  that of water vapor in air, the hydrate will
  effloresce by losing the water of hydration

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Hydrates

• Some hydrates that have a low vapor pressure
  remove water from the air to form higher
  hydrates- called hygroscopic
• used as drying agents, or dessicants
• packaged with products to absorb moisture

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Hydrates

• Some compounds are so hygroscopic, they become
  wet when exposed to normally moist air- called
  deliquescent
• remove sufficient water to dissolve completely
  and form solutions
• Fig. 17.17, p.487
• Sample Problem 17-1, p.488 for percent composition

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Section 17.4Heterogeneous Aqueous Systems

• OBJECTIVES
• Explain how colloids and suspensions differ from
  solutions.

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Section 17.4Heterogeneous Aqueous Systems

• OBJECTIVES
• Describe the Tyndall effect.

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Mixtures that are NOT Solutions

• Suspensions mixtures that slowly settle upon
  standing.
• Particles of a suspension are greater in diameter
  than 100 nm.
• Can be separated by filtering (p.490)
• Colloids heterogeneous mixtures with particles
  between size of suspensions and true solutions
  (1-100 nm)

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Mixtures that are NOT Solutions

• The small particles are the dispersed phase, and
  are spread throughout the dispersion medium
• The first colloids were glues. Others include
  mixtures such as gelatin, paint, aerosol sprays,
  and smoke
• Table 17.5, p.491 list some common colloidal
  systems and examples

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Mixtures that are NOT Solutions

• Many colloids are cloudy or milky in appearance
  when concentrated, but almost clear when dilute
• do not settle out
• cannot be filtered out
• Colloids exhibit the Tyndall effect- the
  scattering of visible light in all directions.
• suspensions also show Tyndall effect

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Mixtures that are NOT Solutions

• Flashes of light are seen when colloids are
  studied under a microscope- light is reflecting-
  called Brownian motion to describe the chaotic
  movement of the particles
• Table 17.6, p.492 summarizes the properties of
  solutions, colloids, and suspensions

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Mixtures that are NOT Solutions

• Emulsions- colloids dispersions of liquids in
  liquids
• an emulsifying agent is essential for maintaining
  stability
• oil and water not soluble but with soap or
  detergent, they will be.
• Oil and vinegar dressing?
• Mayonnaise? Margarine?