Water and Aqueous Systems

Ch. 17

Liquid Water and its Properties

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The Water Molecule, in review…

• The water molecule, H – O – H, is made up of covalent bonds that are highly polar.

• It has a bent shape with 105° bond angles.

• Oxygen has a slightly negative charge (more electronegative), while Hydrogen has a slightly positive charge.

• Water molecules attracted to each other through hydrogen bonds.

DRAW!

• Many unique and important properties of water are due to it’s hydrogen bonds! Including its…

1. High surface tension

2. Low vapor pressure

3. High specific heat capacity

4. High heat of vaporization

5. High boiling point

6. Ice is less dense than water

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Surface Properties1. Surface tension = inward force, or pull, that tends

to minimize the surface area of a liquid.– Due to the hydrogen bonding (attraction)

between water molecules.– Water has HIGH surface tension, holds drops

in a spherical shape.– Surfactant = wetting agent, soap or detergent,

that decreases surface tension (interferes with H-bonding)

2. LOW vapor pressure; hydrogen bonds keep water in liquid form from escaping into gaseous form.

Specific Heat Capacity

3. Specific Heat Capacity (C) = it takes 4.18 J (1 cal) of heat energy to raise the temperature of 1g of water 1°C.– Due to hydrogen bonding,

water has a HIGH specific heat capacity

– Cwater = 4.18 J/(g°C)– C = Q = heat (J or cal)

m x ΔT mass (g) x (°C)

Water Vapor and Ice

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Evaporation + Condensation4. Heat of Vaporization = amount of E needed to

convert 1g of a substance from liquid to gas at the boiling point.

4. Water has a HIGH heat of vaporization due to H-bonding

5. Heat of Vaporization = Heat of Condensation

6. Hvap of water = 2.26 kJ/g or 2260 J/g

7. Hfus of water = 334 J/g

8. Phase change use: H = m x Hvap/fus

Temp. change use: H = m x ΔT x C

Remember…

• When to use which H equation:

0°C

100°C

H = m x Hfus

H = m x Hvap

H = m x ΔT x Cice

H = m x ΔT x Csteam

H = m x ΔT x Cwater

Heat Energy

Tem

per

atu

re (

°C)

Temp. Change

Phase Change

Exothermic

Endothermic

Boiling Point

• Usually molecular compounds with a low molar mass have a low boiling point.

5. Water has a HIGH boiling point due to hydrogen bonding!

Ice

• Typically the density of a solid is greater than it’s liquid.

6. Ice is less dense than water!

• Hydrogen bonding holds water molecules in a rigid framework, crystallize (spaced far apart – larger volume, less dense)

• As it melts the framework collapses (closer together, more dense).

Aqueous Solutions

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Solvents + Solutes• Aqueous solution = water containing

dissolved substance.

– Solute = dissolved particle.

– Solvent = dissolving medium; always water in an aqueous solution.

• Ionic compounds + polar covalent molecules dissolve easily.

• Nonpolar covalent molecules do not dissolve easily.

The Solution Process• Solvation = the process that occurs when a solute

dissolves.– Ionic + Polar = negative and positive ions or poles

are attracted to different regions of water molecule and are pulled apart.

• “Like dissolves like” – – Ionic + polar substances can dissolve together.

• Ex: salt water (NaCl is ionic; water is polar).– Nonpolar and nonpolar substances can dissolve

together.• Ex: gasoline + oil dissolve together, both are

nonpolar!

Why don’t oil + water mix??

Electrolytes + Nonelectrolytes• Electrolytes = compounds that conduct electric

current in aqueous solutions or in a molten state.– ALL ionic compounds and most polar

molecules are electrolytes.• Nonelectrolytes = compounds that do not

conduct electric current in either aqueous or molten state.– Many molecular (nonpolar) compounds are

nonelectrolytes since they are not made up of ions

• Weak electrolyte = only a fraction of solute exists as ions in solution (not dissolve completely, usually polar).– Ex: HgCl2, NH3, sugar

• Strong electrolyte = almost all of solute exists as separate ions in solution (dissolves completely, usually ionic).– Ex: NaCl, HCl, KCl

Water of Hydration• Water of Hydration = the water in a crystal.

– Hydrate = a compound that contains

water of hydration.

• Ex: CuSO4(aq) CuSO4•5H2O(s)

blue liquid blue crystals (hydrate)

CuSO4•5H2O (s) CuSO4(s) + 5H2O(g)

white anhydrous powder

evaporates

heated

CuSO4•5H2O(s)Copper (II) Suflate

Pentahydrate

CuSO4(s) – Anhydrous Copper (II) Sulfate

•Methane hydrate… a solid form of water that contains a large amount of methane within its crystal structure [that] occur both in deep sedimentary structures, and as outcrops on the ocean floor.

“Fire Ice”Energy Source + Global Warming Pollutant

Hydrate Formula• Writing a hydrate formula:

– Name of compound,Prefix (# of water) “-Hydrate”– Ex: MgSO4•7H2OMagnesium sulfate heptahydrate– Ex: Ba(OH)2•8H2OBarium hydroxide octahydrate– Ex: Sodium sulfate decahydrateNa2SO4•10H2O– Ex: Calcium chloride dihydrateCaCl2•2H2O

• Writing an anhydrous formula:– Ex: MgSO4•7H2O(s)MgSO4(s) + 7H2O(g)

Hydrates• Hydrates lose or gain water easily because

forces keeping them together are weak.• A hydrate which evaporates easily will

effloresce – lose water of hydration– Hydrate Anhydrous

• Hydrates that remove water from air easily (gain water) are called hygroscopic– Anhydrous Hydrate– Used as drying agents –dessicants.

•Deliquescent compounds remove sufficient water from the air to dissolve completely and form solutions!

% Mass

• % Mass = grams part x 100% grams whole

• Ex: Calculate the % mass of water in CuSO4•5H2O.

% mass of water = 5H2O x 100%

CuSO4•5H2O = 90g x 100% = 36.0% water 249.6g

Heterogeneous Aqueous Systems

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NO CLASS IS DOING THIS

SECTION!!

Suspensions

• This chapter has focused on homogeneous solutions, now we will look at heterogeneous mixtures.

• Suspensions = mixture from which particles settle out upon standing.

– Particles are fairly large– Can be separated through filtration.

Colloids• Colloids = heterogeneous mixture containing

particles of intermediate size.– Can be in solid, liquid, or gas phase– Ex: glue, gelatin, paint, aerosol sprays, smoke– Do not settle out with time and cannot be

separated by filtering.– Exhibit Tyndall effect, scattering of visible light in

all directions.– Brownian motion = chaotic movement of colloidal

particles; caused by collisions of particles with water molecules – prevents particles from settling!

Tyndall Effect

Brownian Motion