“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 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 Molecule Water’s 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

High Surface Tension liquid water acts like it has a skin

–glass of water bulges over the topWater forms round drops

–spray water on greasy surfaceAll because water hydrogen bonds

Surface Tension One water

molecule hydrogen bonds to another.

Also, hydrogen bonding occurs to other molecules all around.

H HO

+

+

-

H HO

+

-

+

Surface TensionA water

molecule in the middle of solution is pulled in all directions.

Surface Tension Not true at the

surface. Only pulled down

and to each side. Holds the

molecules together. Causes surface

tension.

Surface TensionWater drops

are round, because all molecules on the edge are pulled to the middle- not to the air.

Surface Tension Glass has polar

molecules. Glass can

hydrogen bond. Attracts the

water molecules. Some of the pull

is up a cylinder.

Meniscus Water curves up

along the side of glass.

This makes the meniscus, as in a graduated cylinder

Plastics are non-wetting; no attraction

MeniscusIn Glass

In Plastic

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

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 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 T 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

OBJECTIVES:

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

Water Vapor and Ice

OBJECTIVES:

–Explain why ice floats in water.

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.

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?

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.

HHO

H

HO

H

H

O

HH

O

H

H O HH

O

H

HO

HHOH

H

O

H

HO H H

O

HH

O

Liquid Solid

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

OBJECTIVES:

–Explain the significance of the statement “like dissolves like”.

Aqueous Solutions

OBJECTIVES:

–Distinguish among strong electrolytes, weak electrolytes, and non-electrolytes, giving examples of each.

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 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 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 dissolve

H HOH

H OH

HO

H HO

HHO

HH

O

HH

OH

H

O

HH

O

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- 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-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 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.

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)

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.- heat

+ heat

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

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

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

OBJECTIVES:

–Explain how colloids and suspensions differ from solutions.

Heterogeneous Aqueous Systems

OBJECTIVES:

–Describe the Tyndall effect.

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 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

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 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

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?