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“Water and Aqueous Systems”. “Aqua” Latin = water. Liquid Water and it’s Properties. OBJECTIVES: Describe the hydrogen bonding that occurs in water. Explain the high surface tension and low vapor pressure of water in terms of hydrogen bonding. The Water Molecule.

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water and aqueous systems

“Water and Aqueous Systems”

“Aqua” Latin = water

liquid water and it s properties
Liquid Water and it’s Properties
  • OBJECTIVES:
    • Describe the hydrogen bonding that occurs in water.
    • Explain the high surface tension and low vapor pressure of water in terms of hydrogen bonding.
the water molecule
The Water Molecule
  • Water is a simple tri-atomic molecule.
  • Each O-H bond is highly polar, because of the high electronegativity of the oxygen
  • bond angle = 104,5 o
  • due to the angular (bent) shape, the O-H bond polarity vectors do not cancel out. This means water as a whole is polar.
the water molecule1
The Water Molecule
  • Water’s bent shape and ability tohydrogen 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
high surface tension
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
surface tension

O

H

H

O

H

H

Surface Tension

d-

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

d+

d+

d-

d+

d+

surface tension1
Surface Tension
  • A water molecule in the middle of solution is pulled in all directions.
surface tension2
Surface Tension
  • Not true at the surface.
  • Only pulled down and to each side.
  • Holds the molecules together.
  • Causes surface tension.
surface tension3
Surface Tension
  • Water drops are round, because all molecules on the edge are pulled to the middle- not to the air.
surface tension4
Surface Tension
  • Glass has polar molecules.
  • Glass can hydrogen bond.
  • Attracts the water molecules.
  • Some of the pull is up a cylinder.
meniscus
Meniscus
  • Water curves up along the side of glass.
  • This makes the meniscus, as in a graduated cylinder
  • Plastics are non-wetting; no attraction
meniscus1
Meniscus

In Plastic

In Glass

surface tension5
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
low vapour pressure
Low vapour pressure
  • Hydrogen bonding also explains water’s unusually low vapor pressure.
    • Holds water molecules together, so they do not escape
    • good thing- lakes and oceans would evaporate very quickly
specific heat capacity
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: SHC = heat energy 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!
water vapour and ice
Water Vapour and Ice
  • OBJECTIVES:
    • Account for the high heat of vaporization and the high boiling point of water, in terms of hydrogen bonding.
water vapor and ice
Water Vapor and Ice
  • OBJECTIVES:
    • Explain why ice floats in water.
evaporation and condensation
Evaporation and Condensation
  • Because of the strong hydrogen bonds, it takes a large amount of energy to change water from a liquid to a vapour.
  • 2260 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.
slide19
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?
slide20
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.
slide21

H

H

H

O

O

H

H

H

H

H

O

O

H

O

H

O

H

H

H

H

O

H

O

O

H

H

H

H

H

H

O

O

O

H

H

H

Liquid

Solid

slide22
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.
aqueous solutions
Aqueous Solutions
  • OBJECTIVES:
    • Explain the significance of the statement “like dissolves like”.
aqueous solutions1
Aqueous Solutions
  • OBJECTIVES:
    • Distinguish among strong electrolytes, weak electrolytes, and non-electrolytes, giving examples of each.
solvents and solutes
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
aqueous solutions2
Aqueous Solutions
  • Water dissolves ionic compounds and polar covalent molecules best.
  • The rule is: “like dissolves like”
  • Polar dissolves polar.
  • Non-polar dissolves non-polar.
  • Oil is non-polar.
    • Oil and water don’t mix.
  • Salt is ionic- makes salt water.
how ionic solids dissolve
How Ionic solids dissolve
  • Called solvation.
  • Water breaks the + and - charged pieces apart and surrounds them.
  • In some ionic compounds, the attraction between ions is greater than the attraction exerted by water
    • Barium sulphate and calcium carbonate
how ionic solids dissolve1

H

H

H

H

O

O

O

H

H

H

H

O

O

H

H

O

O

H

H

H

H

H

H

O

H

O

H

How Ionic solids dissolve
slide29
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 doesn’t dissolve non-polar molecules because the water molecules can’t hold onto them.
  • The water molecules hold onto each other, and separate from the non-polar molecules.
  • Non-polars? No repulsion between them
electrolytes and non electrolytes
Electrolytes and Non-electrolytes
  • 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 sulphate- will conduct when molten, but is insoluble in water!
electrolytes and non electrolytes1
Electrolytes and Non-electrolytes
  • Do not conduct? Non-electrolytes.
    • 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
electrolyte summary
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 ® Na+and Cl-

  • These ions can conduct electricity.
slide33
Non-electrolytes do not conduct electricity when dissolved in water or molten
  • Polar covalent molecules such as methanol (CH3OH) don’t fall apart into ions when they dissolve.
  • Weak electrolytes don’t fall completely apart into ions.
  • Strong electrolytes do ionise completely.
water of hydration or water of crystallization

+ heat

- heat

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)sulphate pentahydrate.
hydrates
Hydrates
  • Since heat can drive off the water, the forces holding it are weak
  • If a hydrate has a vapour pressure higher than that of water vapor in air, the hydrate will effloresce by losing the water of hydration
hydrates1
Hydrates
  • Some hydrates that have a low vapour pressure remove water from the air to form higher hydrates- called hygroscopic
    • used as drying agents, or dessicants
    • packaged with products to absorb moisture
hydrates2
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
heterogeneous aqueous systems
Heterogeneous Aqueous Systems
  • OBJECTIVES:
    • Explain how colloids and suspensions differ from solutions.
heterogeneous aqueous systems1
Heterogeneous Aqueous Systems
  • OBJECTIVES:
    • Describe the Tyndall effect.
mixtures that are not solutions
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
  • Colloids: heterogeneous mixtures with particles between size of suspensions and true solutions (1-100 nm)
mixtures that are not solutions1
Mixtures that are NOT Solutions
  • The small particles are the dispersedphase, and are spread throughout the dispersion medium
  • The first colloids were glues. Others include mixtures such as gelatin, paint, aerosol sprays, and smoke
mixtures that are not solutions2
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
mixtures that are not solutions3
Mixtures that are NOT Solutions
  • Flashes of light are seen when colloids are studied under a microscope- light is reflecting- called Brownian motiontodescribe the chaotic movement of the particles
mixtures that are not solutions4
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?