chapter 17 “water and aqueous systems” milbank high school

Chapter 17“Water and Aqueous

Systems”

Milbank High School

Section 17.1Liquid Water and it’s

Properties OBJECTIVES:

–Describe the hydrogen bonding that occurs in water.

Section 17.1Liquid Water and it’s

Properties OBJECTIVES:

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

The Water 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

The Water Molecule Water’s bent shape and ability to

hydrogen bond gives water many special properties!

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.Fig. 17.4, p.476

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.

Meniscus

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

Low vapor pressure Fig. 17.6, p.477 Hydrogen bonding also explains

water’s unusually low vapor pressure.

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 T

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.

Section 17.2Water Vapor and Ice

OBJECTIVES:

–Explain why ice floats in water.

Evaporation and Condensation 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.

Ice Solid metals sink in liquid metal.

– But, ice floats in water. Why? Water becomes more dense as it cools

until it reaches 4ºC. As the molecules slow down, they arrange

themselves into honeycomb shaped crystals.

(Fig. 17.9, p.481)

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.

Section 17.3Aqueous Solutions

OBJECTIVES:

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

Section 17.3Aqueous Solutions

OBJECTIVES:

–Distinguish among strong electrolytes, weak electrolytes, and nonelectrolytes, 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. Nonpolar dissolves nonpolar. Oil is nonpolar.

–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. 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

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 nonpolar

molecules because the water molecules can’t hold onto them.

The water molecules hold onto each other, and separate from the nonpolar molecules.

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)

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

Electrolytes and NonelectrolytesTable 17.3, p.485 lists some

common electrolytes and nonelectrolytes

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.

Nonelectrolytes 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 ionize 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)sulfate pentahydrate.- heat

+ heat

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

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

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

Section 17.4Heterogeneous Aqueous

Systems OBJECTIVES:

–Explain how colloids and suspensions differ from solutions.

Section 17.4Heterogeneous Aqueous

Systems OBJECTIVES:

–Describe the Tyndall effect.

Mixtures that are NOT Solutions

Suspensions: mixtures that slowly settle upon standing.

Colloids: heterogeneous mixtures with particles between size of suspensions and true solutions (1-100 nm)

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

Table 17.6, p.492 summarizes the properties of solutions, colloids, and suspensions

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.