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15.2 Homogeneous Aqueous Systems >15.2 Homogeneous Aqueous Systems >

Chapter 15Water and Aqueous Systems

15.1 Water and Its Properties

15.2 Homogeneous Aqueous Systems

15.3 Heterogeneous Aqueous Systems

Although it sounds absurd, an ordinary dill pickle from the deli can be a source of light when connected to an electric current!

CHEMISTRY & YOUCHEMISTRY & YOU

How can you make a pickle glow?

SolutionsSolutions

Solutions

What types of substances dissolve most readily in water?

SolutionsSolutions

Solutions

What types of substances dissolve most readily in water?

• An aqueous solution is water that contains dissolved substances.

SolutionsSolutions

Solvents and Solutes

• In a solution, the dissolving medium is the solvent.

• The dissolved particles in a solution are the solute.

SolutionsSolutions

• A solvent dissolves the solute.

• The solute becomes dispersed in the solvent.

• Solvents and solutes may be gases, liquids, or solids.

SolutionsSolutions

• Solutions are homogeneous mixtures.

• Solute particles can be atoms, ions, or molecules.

• If you filter a solution through filter paper, both the solute and solvent pass through the filter.

SolutionsSolutions

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

SolutionsSolutions

• 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.

SolutionsSolutions

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 solvent molecules and the ionic crystal dissolves.

SolutionsSolutions

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

The Solution Process

SolutionsSolutions

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 expression “like dissolves like.”

Which of these compounds should not dissolve in water?

B.C4H10

Which of these compounds should not dissolve in water?

B.C4H10

Electrolytes and Electrolytes and NonelectrolytesNonelectrolytes

Electrolytes and Nonelectrolytes

Why are all ionic compounds electrolytes?

Electrolytes and Nonelectrolytes

Why are all ionic compounds electrolytes?

• An electrolyte is a compound that conducts an electric current when it is in an aqueous solution or in the molten state.

All ionic compounds are electrolytes because they dissociate into ions.

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

To (–) electrode

• Mercury(II) chloride, a weak electrolyte, is only partially dissociated in water.

To (+) electrode

To (–) electrode

• 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 (+)

electrodeTo (–)

electrode

Some polar molecular compounds are nonelectrolytes in the pure state but become electrolytes when they dissolve in water.

• This change occurs because such compounds ionize in solution.

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.

NH3(g) + H2O(l) NH4+(aq) + OH–(aq)

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 current poorly because only a fraction of the solute in the solution exists as ions.

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.

• When you exercise, you can lose water and electrolytes from your body through perspiration.

Pickles contain table salt. Why can electric current flow through a pickle, causing it to glow?

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.

Pickles contain table salt. Why can electric current flow through a pickle, causing it to glow?

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 introduced into the water, any swimmers could be shocked.

HydratesHydrates

Hydrates

Why do hydrates easily lose and regain water?

The water contained in a crystal is called the water of hydration or water of crystallization.

HydratesHydrates

• A compound that contains water of hydration is called a hydrate.

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

HydratesHydrates

• A substance that is anhydrous does not contain water.

HydratesHydrates

Heating of a sample of blue CuSO45H2O begins.

After a time, much of the blue hydrate has been converted to white anhydrous CuSO4.

CuSO45H2O(s) CuSO4(s) + 5H2O(g)– heat

+ heat

HydratesHydrates

• 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) chloride hexahydrate (CoCl26H2O).

Each hydrate contains a fixed quantity of water and has a definite composition.

HydratesHydrates

Some Common Hydrates

Formula Chemical name Common name

MgSO47H2O Magnesium sulfate heptahydrate Epsom salt

Ba(OH)28H2O Barium hydroxide octahydrate

CaCl22H2O Calcium chloride dihydrate

CuSO45H2O Copper(II) sulfate pentahydrate Blue vitriol

Na2SO410H2O Sodium sulfate decahydrate Glauber’s salt

KAl(SO4)212H2OPotassium aluminum sulfate dodecahydrate

Na2B4O710H2O Sodium tetraborate decahydrate Borax

FeSO47H2O Iron(II) sulfate heptahydrate Green vitriol

H2SO4H2O Sulfuric acid hydrate (mp 8.6oC)

Percent by mass H2O = 100%mass of water

mass of hydrate

HydratesHydrates

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

using the following equation:

HydratesHydrates

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.

The water molecules in hydrates are held by weak forces, so hydrates often have an appreciable vapor pressure.

HydratesHydrates

Hygroscopic Hydrates

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

Hydrated ionic compounds that have low vapor pressure remove water from moist air to form higher hydrates.

HydratesHydrates

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 spontaneously absorbs a second molecule of water when exposed to moist air.

Calculate the percent by mass of water in washing soda, sodium carbonate decahydrate (Na2CO310H2O).

Sample Problem 15.1Sample Problem 15.1

Finding the Percent by Mass of Water in a Hydrate

formula of hydrate = Na2CO310H2O

UNKNOWN

percent H2O = ? %

Analyze List the known and the unknown.1

To determine the percent by mass, divide the mass of water in one mole of the hydrate by the molar mass of the hydrate and multiply by 100%.

Determine the mass of 10 mol of water.

Calculate Solve for the unknown.2

For every 1 mol of Na2CO310H2O, there are 10 mol of H2O.

mass of 10 mol H2O = 10[(2 1.0 g) + 16.0 g] = 180.0 g

Determine the mass of 1 mol of the hydrated compound.

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

= 286.0 g

molar mass of Na2CO310H2O

Calculate the percent by mass of water in the hydrate.

mass of watermass of hydratepercent by mass H2O = 100%

= 62.94%

180.0 g286.0 g = 100%

• 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

HydratesHydrates

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 normally moist air.

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.

HydratesHydrates

Calculate the percent by mass of water in epsom salt, magnesium sulfate heptahydrate (MgSO47H2O).

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

= 246.4 g

molar mass of MgSO47H2O

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

126.0 g246.4 g percent by mass H2O = 100% = 51.14%

Key Concepts and Key Concepts and Key EquationKey Equation

All ionic compounds are electrolytes because they dissociate into ions.

mass of watermass of hydratepercent by mass H2O =

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 molecules surround the positive and negative ions

• 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 electricity poorly because only a fraction of the solute exists as ions

• 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 the air

• 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 vapor pressure than that of the water in the air

• Ionic compounds and polar covalent compounds dissolve most readily in water to form aqueous solutions.

• Ionic compounds dissolve in water when the polar water molecules attract the ions of the solute, causing the individual solute ions to break away from the ionic crystal.

BIG IDEABIG IDEA

Bonding and Interactions

END OF 15.2END OF 15.2

15.2 Homogeneous Aqueous Systems > 1 Copyright © Pearson Education, Inc., or its affiliates. All...

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