Ch15Sec 2.ppt

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15.2 Homogeneous Aqueous Systems >

Chapter 15

Water and Aqueous Systems

15.1 Water and Its Properties

15.2 Homogeneous Aqueous Systems

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Although it sounds

absurd, an

ordinary dill pickle

from the deli can

be a source of

light when

connected to an

electric current!

CHEMISTRY & YOU

CHEMISTRY & YOU

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15.2 Homogeneous Aqueous Systems > SolutionsSolutions

Solutions

What types of substances

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Solutions

What types of substances

dissolve most readily in water?

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Solvents and Solutes

• In a solution, the dissolving medium

is the

solvent

.

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Solvents and Solutes

• A solvent dissolves the solute.

• The solute becomes dispersed in the

solvent.

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Solvents and Solutes

• Solutions are homogeneous mixtures.

• Solute particles can be atoms, ions, or

molecules.

• If you filter a solution through filter

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Substances that dissolve most

readily in water include ionic

compounds and polar covalent

compounds.

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Substances that dissolve most

readily in water include ionic

compounds and polar covalent

compounds.

• Nonpolar covalent compounds, such as

methane, and compounds found in oil, grease, and gasoline, do not dissolve in water.

• However, oil and grease will dissolve in gasoline.

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The Solution Process

• A water molecule is polar, with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms.

• As individual solute ions break away from the crystal, the negatively and positively charged ions become surrounded by

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The process by which the positive and

negative ions of an ionic solid become surrounded by

solvent molecules is called solvation.

Solvated ions

Surface of ionic solid

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The Solution Process

• Polar solvents such as water dissolve ionic

compounds and polar compounds.

• Nonpolar solvents such as gasoline dissolve

nonpolar compounds.

• This relationship can be summed up in the

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Which of these compounds should not

dissolve in water?

A. HCl

B. C

₄

H

₁₀

C. KI

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Which of these compounds should not

dissolve in water?

A. HCl

B. C

₄

H

₁₀

C. KI

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15.2 Homogeneous Aqueous Systems > Electrolytes and Electrolytes and

Nonelectrolytes

Electrolytes and Nonelectrolytes

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Nonelectrolytes

Electrolytes and Nonelectrolytes

Why are all ionic compounds

electrolytes?

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Nonelectrolytes

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Nonelectrolytes

In order for the bulb to light, an electric current must flow

between the two electrodes that are immersed in the solution.

• Sodium chloride, a strong electrolyte, is nearly 100%

dissociated into ions in water.

To (+) electrode

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Nonelectrolytes

• Mercury(II) chloride, a weak electrolyte, is

only partially

dissociated in water.

To (+) electrode

To (–) electrode

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Nonelectrolytes

• Glucose, a nonelectrolyte, does not dissociate in

water.

• A nonelectrolyte is a compound that does not conduct an electric current in either an aqueous

solution or the molten state. To (+) electrode

To (–) electrode

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Nonelectrolytes

Some polar molecular compounds are

nonelectrolytes in the pure state but

become electrolytes when they dissolve

in water.

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Nonelectrolytes

Some polar molecular compounds are

nonelectrolytes in the pure state but

become electrolytes when they dissolve

in water.

• For example, ammonia (NH₃(g)) is not an electrolyte in the pure state.

• Yet an aqueous solution of ammonia conducts an electric current because ammonium ions (NH₄+)

and hydroxide ions (OH–) form when ammonia

dissolves in water.

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Nonelectrolytes

Not all electrolytes conduct electric

current to the same degree.

• In a solution that contains a strong

electrolyte, all or nearly all of the solute exists as ions.

• A weak electrolyte conducts an electric

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Nonelectrolytes

Your cells use electrolytes, such as sodium and potassium ions, to carry electrical

impulses across themselves and to other cells.

• An electrolyte imbalance can occur if you become dehydrated.

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15.2 Homogeneous Aqueous Systems > CHEMISTRYCHEMISTRY && YOU YOU

Pickles contain table salt. Why can

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Electrolytes conduct an electric current

when they are in an aqueous solution.

Table salt, or NaCl, is a strong

electrolyte. The water and salt in the

pickle form a solution that conducts an

electric current. The electric current

causes the pickle to glow.

CHEMISTRY & YOU

CHEMISTRY & YOU

Pickles contain table salt. Why can

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Explain why you must be extremely

careful when using electricity near a

swimming pool.

The chlorinated water in a swimming

pool is a solution that can conduct an

electric current. If a current is

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15.2 Homogeneous Aqueous Systems > HydratesHydrates

Hydrates

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The water contained in a crystal is called

the

water of hydration

or water of

crystallization.

Hydrates

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The forces holding the water

molecules in hydrates are not very

strong, so the water is easily lost

and regained.

Hydrates

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The forces holding the water

molecules in hydrates are not very

strong, so the water is easily lost

and regained.

Hydrates

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Heating of a sample of blue

CuSO₄5H₂O begins.

After a time, much of the blue hydrate has been converted to white

anhydrous CuSO₄.

CuSO₄5H₂O(s) CuSO₄(s) + 5H₂O(g)

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• A piece of filter paper that has been dipped in an

aqueous solution of cobalt(II) chloride and then dried is

blue in color (anhydrous CoCl₂).

• When the paper is exposed to moist air, it turns pink

because of the formation of the hydrate cobalt(II)

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Each hydrate contains a fixed quantity of

water and has a definite composition.

Hydrates

Some Common Hydrates

Formula Chemical name Common name

MgSO₄7H₂O Magnesium sulfate heptahydrate Epsom salt Ba(OH)₂8H₂O Barium hydroxide octahydrate

CaCl₂2H₂O Calcium chloride dihydrate

CuSO₄5H₂O _{Copper(II) sulfate pentahydrate} _{Blue vitriol} Na₂SO₄10H₂O _{Sodium sulfate decahydrate} _{Glauber’s salt}

KAl(SO₄)₂12H₂O Potassium aluminum sulfate

dodecahydrate Alum

Na₂B₄O₇10H₂O Sodium tetraborate decahydrate Borax

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Percent by mass H₂O = _{mass of hydrate}mass of water  100%

Hydrates

To determine what percent by mass of a

hydrate is water:

• First determine the mass of water in one mole of hydrate.

• Then determine the molar mass of the hydrate. • The percent by mass of water can be calculated

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Efflorescent Hydrates

• If a hydrate has a vapor pressure higher than the pressure of water vapor in the air, the

hydrate will lose its water of hydration, or

effloresce.

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Hygroscopic Hydrates

• These hydrates and other compounds that remove moisture from air are called

hygroscopic.

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Hygroscopic Hydrates

• Calcium chloride is used as a desiccant in the laboratory. • A desiccant is a substance

used to absorb moisture

from the air and create a dry atmosphere.

Calcium chloride monohydrate

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Calculate the percent by mass

of water in washing soda,

sodium carbonate

decahydrate (Na

₂

CO

₃



10H

₂

O).

Sample Problem 15.1

Sample Problem 15.1

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KNOWN

formula of hydrate = Na₂CO₃10H₂O

UNKNOWN

percent H₂O = ? %

Analyze

List the known and the unknown.

1

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Determine the mass of 10 mol of water.

Calculate

Solve for the unknown.

2

For every 1 mol of

Na₂CO₃10H₂O, there are 10 mol of H₂O.

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Determine the mass of 1 mol of the

hydrated compound.

Calculate

2

= (2  23.0 g) + 12.0 g + (3  16.0 g) + 180.0 g

= 286.0 g molar mass of

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Calculate the percent by mass of water in

the hydrate.

Calculate

2

mass of water mass of hydrate

percent by mass H₂O =  100%

= 62.94% 180.0 g 286.0 g

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• The mass of the water accounts for

more than half the molar mass of the

compound.

• So, a percentage greater than 50%

is expected.

Evaluate

Does the result make sense?

3

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Deliquescent Compounds

• These compounds are deliquescent, which means that they remove sufficient water from the air to dissolve completely and form

solutions.

Some compounds are so hygroscopic that

they become wet when exposed to

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Pellets of sodium hydroxide are deliquescent.

For this reason,

containers of NaOH should always be tightly stoppered.

The solution formed by a deliquescent substance has a lower vapor pressure than that of the water in the air.

Hydrates

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Calculate the percent by mass of water

in epsom salt, magnesium sulfate

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Calculate the percent by mass of water

in epsom salt, magnesium sulfate

heptahydrate (MgSO

₄



7H

₂

O).

= 24.3 g + 32.1 g + (4  16.0 g) + 126.0 g

= 246.4 g molar mass of

MgSO₄7H₂O

mass of H₂O = 7  ( 2  1.0 g + 16.0 g) = 126.0 g

126.0 g 246.4 g

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15.2 Homogeneous Aqueous Systems > Key Concepts and Key Concepts and

Key Equation

Substances that dissolve most readily in water include ionic compounds and polar covalent compounds.

All ionic compounds are electrolytes because they dissociate into ions.

The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained.

mass of water mass of hydrate

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15.2 Homogeneous Aqueous Systems > Glossary TermsGlossary Terms

• aqueous solution: water that contains dissolved substances

• solvent: the dissolving medium in a solution • solute: dissolved particles in a solution

• solvation: a process that occurs when an ionic solute dissolves; in solution, solvent

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• electrolyte: a compound that conducts an

electric current when it is in an aqueous solution or in the molten state; all ionic

compounds are electrolytes, but most covalent compounds are not

• nonelectrolyte: a compound that does not

conduct an electric current in aqueous solution or in the molten state

• strong electrolyte: a solution in which a large

portion of the solute exists as ions

• weak electrolyte: a solution that conducts

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• water of hydration: water molecules that are an integral part of a crystal structure

• hydrate: a compound that has a specific number of water molecules bound to each formula unit

• anhydrous: a substance that does not contain water

• effloresce: to lose water of hydration; the

process occurs when the hydrate has a vapor pressure higher than that of water vapor in

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• hygroscopic: a term describing salts and other compounds that remove moisture from the air

• desiccant: a hygroscopic substance used as a drying agent

• deliquescent: describes a substance that

removes sufficient water from the air to form a solution; the solution formed has a lower