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Chapter 15Water and Aqueous Systems

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Section 15.1 Water and It’s Properties

• Objectives – Explain the high surface tension and low

vapor pressure of water in terms of the structure of the water molecule and hydrogen bonding

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Water in Liquid State

The Water Molecule • The Bonds– Each O-H bond is a highly polar covalent bond

because of the high electronegativity of the oxygen

• The Molecule– Due to the bent shape, the O-H bond polarities do

not cancel. This means water as a whole is polar molecule

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The Water Molecule

δ-

δ+ δ+H H

O

δ- means a partial negative charge

δ+ means a partial positive charge

Thus, water has a partial negative end (Oxygen) and a partial positive end (Hydrogen) – and it is called “polar” because of these areas of difference

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The Water Molecule

• Water’s bent shape and ability to hydrogen bond gives it many special properties – Water molecules are attracted to one another by

intermolecular forces called hydrogen bonds

• This hydrogen bonding gives water:– High surface tension – Low vapor pressure

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High Surface Tension

Surface Tension The inward force or pull that tends to minimize the surface area of a liquid – Liquid water acts like it has a

“skin” or a tight film stretched across it• In a glass water bulges over the

top– Water can support a small

amount of weight – All because water has ability to

form hydrogen bonds

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Surface Tension• One water molecule can hydrogen

bond to another because of this electrostatic attraction between the oxygen of one molecule and the hydrogen of another molecule.– (electrostatic is just another way of

saying + attracts to -)

• Hydrogen bonding can also occur with other charged molecules

surrounding water on all sides.

H HO

d+

d+

d-

H HO

d+

d-

d+

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Surface Tension

• Surface tension explains why water drops are round– All molecules on the edge

are pulled to the middle - not outward to the air!

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Low Vapor Pressure • Hydrogen bonding also explains water’s

unusually low vapor pressure – Remember vapor pressure– Hydrogen bonds hold water

molecules together, so they do not escape easily and evaporation is slow• This is a good thing, because lakes and

oceans would evaporate very quickly due to their large surface area

– Remember Reference Table H

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Section 15.2 – Homogeneous Aqueous Solutions

• Objectives:– Distinguish between a solvent and a solute – Describe what happens in the solution process– Explain why all ionic compounds are electrolytes – Demonstrate how the formula for a hydrate is

written

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Solvents and Solutes

• Solution – A homogenous mixture

• Solvent – The dissolving medium

• Solute – The dissolved particles

• Aqueous solution – A solution with water as the solvent

• Particle size less than 1 nm – cannot be separated by filtration

Uniform throughout

Not chemically combined

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Practice Problem

• Which formula represents a homogeneous mixture?

1. H2O (l)

2. HCl (aq)

3. H2S (g)

4. NaH (s)

Compound – H & O are chemically combined

Compound – H & S are chemically combined

Compound – Na & H are chemically bonded

(aq) indicates aqueous solution

Remember (aq) stands for aqueous solution – always a homogenous mixture

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Parts of an Aqueous Solution

• Solute – A solute is the dissolved substance in a solution

• Salt in salt water• Sugar in soda drinks

• Solvent – A solvent is the dissolving medium in a solution

• Water in salt water • Water in soda

Notice: in an aqueous solution – the solvent is always water

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Aqueous Solutions • Water dissolves ionic compounds and polar

covalent molecules best – The rule is: likes dissolve likes • Polar dissolves polar • Polar dissolves ionic • Nonpolar dissolves nonpolar

– Examples • Oil and water do not mix (water is polar, so • Salt dissolves in water

oil must be nonpolar)(salt is an ionic cmpd)

See REFERENCE TABLE F for compounds that will dissolve in Aqueous Solutions

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Table F

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The Solution Process

H HOH

H OH

HO

H HO

HHO

HH

O

HH

OH

H

O

HH

O

These ions have been surrounded by water, and are now dissolved!

These ions have been pulled away from the main crystal structure by water’s polarity.

How ionic solids dissolve in water

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Practice Problem• Which compounds will dissolve in water?

1. MgCO3

2. BaSO4

3. Na2S

4. CaCrO4

X

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Practice Problem• Which compounds will dissolve in water?

1. MgCO3

2. BaSO4

3. Na2S

4. CaCrO4

X

X

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Practice Problem• Which compounds will dissolve in water?

1. MgCO3

2. BaSO4

3. Na2S

4. CaCrO4

X

X

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Practice Problem• Which compounds will dissolve in water?

1. MgCO3

2. BaSO4

3. Na2S

4. CaCrO4

X

X

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Practice Problem

• Which of the following ions when combined with Cl- forms an insoluble substance in water?

1. Fe2+

2. Mg2+

3. Pb2+

4. Zn2+

Remember if not on Table F – is it ionic or polar covalent?

Use your reference table!!

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Practice Problem

• Based on Reference Table F, which of these solutions has the lowest saturated concentration of dissolved ions?

1. NaCl(aq)

2. MgCl2 (aq)

3. NiCl2(aq)

4. AgCl(aq)

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Why do some ionic compounds dissolve and others do not?

• In some ionic compounds, the attraction between ions is greater than the attraction exerted by water – Solids will dissolve ONLY if the attractive force of water

molecules is stronger than the attractive force of the ionic compound

– If not, the solids are insoluble

• Water does not dissolve nonpolar molecules (like oil)– Water has no positive or negative site on a Nonpolar

molecule to which it would be attracted.

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Electrolytes and Nonelectrolytes • Electrolytes

– Compounds that conduct an electric current in aqueous solution OR in the molten state • Conduction of electricity requires ions that are mobile and thus able

to carry an electrical current • All ionic compounds are electrolytes because they dissociate into

ions (also called salts)» Barium sulfate – will conduct when molten, but is insoluble in

water

• Nonelectrolytes – Do not conduct electricity

• Molecular compounds because they do not have ions • Polar covalent molecules such as alcohols - methanol (CH3OH) do

not fall apart into ions when they dissolve

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The ions are free to move when they are molten or in aqueous solution, and thus they are able to conduct the electric current.