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Water and theProperties of Liquids

Chapter 13

Water and theProperties of Liquids

Chapter 13

Hein * Best * Pattison * Arena

Eugene PasserChemistry DepartmentBronx Community College

© John Wiley and Sons, Inc.

Version 1.0

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Chapter Outline13.1 What is a Liquid

13.2 Evaporation

13.3 Vapor Pressure 13.10 Structure of the Water Molecule

13.11 The Hydrogen Bond

13.4 Surface Tension

13.5 Boiling Point

13.9 Physical Properties of Water

13.12 Formation and Chemical Properties of Water

13.13 Hydrates

13.6 Freezing Point or Melting Point

13.7 Changes of State

13.8 Occurrence of Water

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What is a Liquid?What is a Liquid?

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The properties of liquids are intermediate between the extremes of gases and liquids.

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Properties of a Liquid

• Particles are close together

• Incompressible

• Definite volume

• Takes the shape of its container

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EvaporationEvaporation

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Evaporation or vaporization is the escape of molecules from the liquid state to the gas or vapor state.

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When molecules of higher than average kinetic energy escape from a liquid, the remaining liquid is cooler than it was before they escaped.

liquid vaporevaporation

Evaporation or vaporization is the escape of molecules from the liquid state to the gas or vapor state.

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H2O(l) → H2O(g)

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Sublimation occurs when a solid changes directly to a gas bypassing the liquid state.

I2(s) → I2(g)

solid vaporsublimation

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Vapor PressureVapor Pressure

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Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid.

liquid vaporevaporation

condensation

In a closed system, after a liquid evaporates, some of the vapor molecules strike the surface of the container and return to the liquid by condensation.

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13.1

Molecules in an open beaker evaporate from the liquid and disperse into the atmosphere. Evaporation will continue until all of the liquid is gone.

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13.1

Molecules leaving the liquid are confined to a limited space. With time, the concentration in the vapor phase will increase until an equilibrium between liquid and vapor is established.

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Measurement of Vapor Pressure at 20oC and

30oC

Measurement of Vapor Pressure at 20oC and

30oC

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• Vapor pressure is independent of the amount of liquid and vapor present.

• It increases with temperature.

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• The system is evacuated.• The mercury manometer

attached to the flask shows equal pressure in both legs.

T = 200C13.2

Vapor Pressure Measurement

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• Water has been added to the flask.

T = 200C

• The water begins to evaporate.

• This water exerts a vapor pressure which is indicated by the manometer.

13.2

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• When equilibrium is established, the vapor pressure of the water inside the flask remains constant at 17.5 torr.

T = 200C13.2

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• The temperature is changed to 30oC.

T = 300C

• Equilibrium is reestablished.

• The vapor pressure of the water has increased to 31.8 torr.

13.2

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Rate of evaporation: ethyl ether > ethyl alcohol > water

Vapor pressure at any temperature: ethyl ether > ethyl alcohol > water

The rate of evaporation of a liquid is proportional to its vapor pressure.

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Volatility

T = 20oC

vapor pressure ethyl ether = 442.2 torr

vapor pressure water = 17.5 torr

vapor pressure mercury = 0.0012 torr

moderately volatile

nonvolatile

volatile

Substances that evaporate readily are said to be volatile.

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Surface TensionSurface Tension

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Surface tension is the resistance of a liquid to an increase in its surface area.

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• Surface tension is the result of the molecules at the surface of a liquid being pulled inward by molecules that lie below the surface layer.

• This pulls the surface of a liquid into a sphere.

• Substances with large attractions between molecules have high surface tensions.

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Capillary action is the spontaneousrising of a liquid in a narrow tube.

Capillary Action

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• Capillary action results from the cohesive forces within the liquid and the adhesive forces between the liquid and the walls.

• If the adhesive forces between the liquid and the walls of its container exceed the cohesive forces between the molecules of the liquid, the liquid will climb the walls of the container.

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Meniscus

• When a liquid is placed in a glass cylinder, the surface of the liquid shows a curve called the meniscus.

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meniscus concaveshape

concave shape occurs because adhesive forces between water glass > cohesive forces between water molecules

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Boiling PointBoiling Point

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• Boiling point is the temperature at which the vapor pressure of a liquid is equal to the external pressure above the liquid.

• The normal boiling point is the temperature at which a liquid boils at one atmosphere pressure.

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water boils when its vapor pressure = 1 atm

or 760 torr

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The normal boiling points for different substances can be plotted on a vapor pressure versus temperature curve.

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13.4

Vapor pressure-temperature curves for ethyl-ether, ethyl alcohol and water.

1 atmosphere pressure

ethyl alcohol boiling point

78.4oC

Each point on the curve represents a vapor-liquid equilibrium at a particular temperature and pressure.

water boiling point

78.4oC

ethyl ether boiling point

34.6oC

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The boiling point at any temperature can be found.

36At 500 torr the boiling point of ethyl alcohol = 68oCAt 700 torr the boiling point of ethyl alcohol = 77oC

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Freezing PointFreezing Pointor Melting Pointor Melting Point

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• A liquid changing into a solid is said to be freezing or solidifying.

• A solid that is changing into a liquid is said to be melting.

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The temperature at which the solid phase of a substance is in equilibrium with its liquid phase is know as the freezing point or melting point of the substance.

solid liquidmelting

freezing

40H2O(s) H2O(l)melting

freezing

Pressure = 1 atmosphere

Temperature = 0oC

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Changes of StateChanges of State

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13.5

Heating curve for a pure substance.

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Heat of FusionHeat of Fusion

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The energy required to change one gram of a solid at its melting point into a liquid is called the heat of fusion.

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The heat of fusion of ice at 0oC is 335 J/g.

How many joules of energy are needed to change 10.0 g of ice at 0.00oC to water 20.0oC?

Determine the joules necessary to melt 10.0 g of ice.

33.35 x 10 J 10.0 g335 J

=1 g

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How many joules of energy are needed to change 10.0 g of ice at 0.00oC to water 20.0oC?

Determine the joules necessary to heat 10.0 g of water from 0.00oC to 20.0oC.

The total heat absorbed by the system is the heat required to melt the ice plus the heat required to raise the water temperature from 0.00oC to 20oC.

3350 J + 837 J = 4.19 x 103 J

The specific heat of water is 4.184 J/goC.

837 J 10.0 g o

4.184 J1 g C

o20.0 C =

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The energy required to change one gram of a liquid at its boiling point into a vapor is called the heat of vaporization.

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How many kilojoules of energy are needed to change 20.0 g of water at 20.0oC to steam at 100.0oC?

Determine the kilojoules necessary to heat 20.0 g from 20oC to 100oC.

6.71 kJ 20.0 g o

4.184 Jg C

The specific heat of water is 4.184 J/goC.

(mass) (sp.ht) (Δt) = energy

1 kJ1000 J

o o100. C - 20.0 C

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How many kilojoules of energy are needed to change 20.0 g of water at 20.0oC to steam at 100.0oC?

Determine the kilojoules necessary to change 20.0 g of water at 100.oC to steam at 100oC.

45.2 kJ 20.0 g2.26 kJ

1 g

The total heat absorbed by the system is the heat required to raise the water temperature from 20oC to 100oC plus the heat required to change the water to steam.

3350 J + 837 J = 4.19 x 103 J

The heat of vaporization of water at 100oC is 2.26 kJ/g.

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Occurrence of WaterOccurrence of Water

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• Water covers about 75% of the earth’s surface.

• About 97% of the earth’s water is in the oceans.

• About 3% of the earth’s water is in the fresh water and two-thirds of this is locked up in polar ice caps and glaciers.

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• 70 elements have been detected in seawater.– Chlorine, sodium, magnesium and

bromine are commercially extracted from sea water.

• Water constitutes about 70% of human body mass.– 92% of blood plasma is water.

– 80% of muscle tissue is water.

– 60% of a red blood cell is water.

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Physical Properties of WaterPhysical Properties of Water

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Ice and water exist together in equilibrium at 0oC.

55H2O(s) H2O(l)melting

freezing

Pressure = 1 atmosphere

Temperature = 0oC

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Density of WaterDensity of Water

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• The density of water reaches a maximum at 4oC.

• As it cools to 4oC it contracts in volume and rises in density.

• When it is cooled from 4oC to 0oC it expands in volume and decreases in density.

• When water changes to ice, its volume expands by 9%. The density of ice at 0oC is less than the density of water at 0oC. This is why ice floats in water.

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Structure of theStructure of theWater MoleculeWater Molecule

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• O-H bond length = 0.096 nm

• H-O-H bond angle = 105o

• Shape: nonlinear, bent

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• Each OH bond is polar.– Electronegativity hydrogen = 2.1

– Electronegativity oxygen = 3.5

– Hydrogen has a partial positive charge and oxygen has a partial negative charge.

• The nonlinear structure of water accounts for the polarity of the molecule.

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(a) Lewis structure of water showing electron distribution.

unpaired electrons

(b) Bond angle and bond length.

(c) Molecular orbital structure.

(c) Dipole representation.

EN = 2.1

EN = 3.5

13.7

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The Hydrogen BondThe Hydrogen Bond

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water has the highest

melting point

water has the highest

boiling point

water has the highest heat of vaporization

water has the highest heat of fusion

water has the lowest

molar mass

The melting point, boiling point, heat of fusion and heat of vaporization of water are extremely high and do not fit the trend of properties relative to molar mass within Group VIA.

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Water exhibits these unusual properties because of hydrogen bonding between water molecules.

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• A hydrogen bond is an intermolecular bond.

F—HO—HN—H

• A hydrogen bond is formed between polar molecules that contain hydrogen covalently bonded to a small, highly electronegative atom: F, O, N.

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• When hydrogen is attached to one of the small electronegative atoms: F, O, or N it will be attracted to another F, O, or N on another molecule.

hydrogen bond

covalent bond

covalent bond

• A dipole-dipole bond bond will be formed between the two molecules which is called a hydrogen bond.

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13.8

Water in the liquid and solid states exists as aggregates in which the water molecules are linked together by hydrogen bonds.

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Formation and Chemical Formation and Chemical Properties of WaterProperties of Water

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DecompositionDecomposition

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• Up to 2000oC, water decomposes no more than 1%.

2H2O(l) 2H2(g) + O2(g)sublimation

H2SO4 or NaOH

• If small amounts of H2SO4 or NaOH are added to water it will be decomposed by an electric current.

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FormationFormationFormationFormation

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1. Combination2H2 + O2 → 2H2O + 484 kJ

2. NeutralizationHCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

3. Combustion of hydrogen containing substances.CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + 803kJ

4. Metabolic oxidation in living cells.

2C6H12O6(aq) + 6O2(g) 6CO2(g) + 6H2O(l) + 2519 kJenzymes

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Reactions of Water with Reactions of Water with Metals and NonmetalsMetals and Nonmetals

Reactions of Water with Reactions of Water with Metals and NonmetalsMetals and Nonmetals

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1. Metals that react with cold water to produce hydrogen and a metal hydroxide

2Na(s) + 2H2O(l) → 2H2(g) + 2NaOH(aq)

2K(s) + 2H2O(l) → 2H2(g) + 2KOH(aq)

Ca(s) + 2H2O(l) → H2(g) + Ca(OH)2(aq)

2. Metals that react with high temperature steam to produce hydrogen and a metal oxide.

2Al(s) + 3H2O(g) → 2H2(g) + Al2O3(aq)

Zn(s) + 2H2O(g) → 2H2(g) + ZnO(aq)

3Fe(s) + 4H2O(g) → H2(g) + Ca(OH)2(aq)

reactivity

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3. Nonmetals that react with water.

2Cl2(g) + 2H2O(l) → 2HCl(aq) + HOCl(aq)

2F2(g) + 2H2O(l) → 4HF(aq) + O2(g)

Br2(l) + H2O(l) → HBr(aq) + HOBr(aq)

2C(s) + H2O(g) CO(g) + H2(g)1000oC

violent reaction

mild reaction

mild reaction

water gas forms

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Reactions of Water with Reactions of Water with Metal and Nonmetal Metal and Nonmetal

OxidesOxides

Reactions of Water with Reactions of Water with Metal and Nonmetal Metal and Nonmetal

OxidesOxides

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1. Basic anhydrides are metal oxides that react with water to form hydroxides

2Na2O(s) + H2O(l) → 2NaOH(aq)

CaO(s) + H2O(l) → Ca(OH)2(aq)

2. Insoluble metallic oxides do not react with water to form hydroxides.

Al2O3(s) + H2O(l) → no reaction

CuO(s) + H2O(l) → no reaction

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1. Acidic anhydrides are nonmetal oxides that react with water to form hydroxides

SO2(g) + H2O(l) → H2SO3(aq)

CO2(g) + H2O(l) → H2CO3(aq)

2. Insoluble metallic oxides do not react with water to form hydroxides.

Al2O3(s) + H2O(l) → no reaction

CuO(s) + H2O(l) → no reaction

N2O5(g) + H2O(l) → 2HNO3(aq)

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HydratesHydrates

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Solids that contain water as part of their crystalline structure are known as hydrates.

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Water in a hydrate is known as water of hydration or water of crystallization.

Solids that contain water as part of their crystalline structure are known as hydrates.

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Formulas of hydrates are written by first writing the formula for the anhydrous compound and then adding a dot followed by the number of water molecules present.

6H2OCoCl2

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• Water molecules in hydrates are bonded by electrostatic forces between polar water molecules and the positive or negative ions of the compound.

• These forces are weaker than covalent or ionic bonds.

• As a result, water of crystallization can be removed by moderate heating of the compound.

BaCl2 2H2O(s)

100oC BaCl2(s) + 2H2O(g)

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