VCE ChemistryUnit 2.1

Water as a solvent

http://highered.mcgraw-hill.com/sites/0072410159/student_view0/chapter5/

figures_alive_.html

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Water

• Is essential to life.

• Covers 70% of earth’s crust, but only 1% is drinkable.

• In ocean, atmosphere and on land.

• Found naturally as gas, liquid or solid.

• 67% of our body.

What do you know already about

• Physical properties?

• Structure and bonding?

Physical Properties• Colourless,odourless liquidColourless,odourless liquid at 25 at 25ooCC• RelativelyRelatively high melting(0high melting(0ooC) and boilingC) and boiling

temperature(100temperature(100ooC)C)• High heat capacityHigh heat capacity -- Requires a lot of Requires a lot of

energy to heat it up energy to heat it up • High latent heatHigh latent heat• Water Water expands when frozenexpands when frozen• Density Density of ice less than waterof ice less than water• Non-conductorNon-conductor• Good solvent Good solvent for polar compoundsfor polar compounds

Bonding

bent

covalent intramolecular

bonding

PolarPolar molecule moleculeDiscrete Discrete molecularmolecular compound compound

Bonding Between Water Molecules

• Most properties can be explained because of the hydrogen bonding between the molecules. (Intermolecular)

Bonding Between Water Molecules

• Oxygen is much more electronegative than hydrogen.

• Water forms polar molecules because electrons are more strongly attracted to the oxygen atom.

Hydrogen Bonding

Melting and Boiling Temperatures

• Are higher than similar substances.

• This is because of the relatively high hydrogen bonding forces.

Specific Heat Capacity

• = energy needed to raise the temperature of 1g of the substance by 1oC.

• It takes 4.19 J to raise the temperature of 1g water by 1oC.

• Water requires a large amount of heat to increase its temperature because energy is needed to overcome the H-bonds.

Latent Heat

• Amount of heat needed to change the state of a substance.

• E.g. When heating ice the temperature remains constant until the ice is melted.

• Water has a high latent heat because of the hydrogen bonding holding the molecules together.

Latent Heat

Latent heat of fusion.

Energy needed to melt a substance.

Latent heat of vaporisation.

Energy needed to vaporize a substance.

Expansion on Freezing

• The regular arrangement of water molecules in ice takes up more space than the irregular arrangement in liquids.

• Density of ice < density of water.

Water Molecules

in Ice

Water As a Solvent

• Water will dissolve many substances

• Solutions in water are aqueous solutions (aq).

Solution Chemistry

In order for substance to dissolve:

• The particles of the solute are separated from each other.

• The particles of the solvent are separated from each other.

• The solvent and solute particles attract each other.

Substances That Dissolve

1. Molecular compounds that

A) form hydrogen bonds

B) ionise

2. Ionic compounds that dissociate

Molecular Substances 1

A.Those that can form hydrogen bonds with water.E.G. Ethanol

• Both water and ethanol are polar molecules. They are able to mix freely forming hydrogen bonding between the molecules

water

ethanol

hydrogen bonding

Animation (long): http://www.mpcfaculty.net/mark_bishop/ethanol_solution.htm

Molecular Substances 2

B.Those that ionise in water. E.g. hydrogen chloride

• HCl has a highly polarised bond which breaks in water.

• This is due to the force of attraction of the water molecules.

• Two ions are formed

Ionisation

• This process is known as IONISATION.

• The ions are said to be hydrated.

• HCl(g) + H20(l) H30+(aq) + Cl –(aq)

Hydronium

ion

Ionic Compounds

• E.g. NaCl

• The water molecules are attracted to the ions. They pull the sodium and chloride ions into the surrounding solution.

Sodium Chloride

Dissolving Salt

• H2O is a polar molecule.

• The charged regions are attracted to ions with the opposite charge.

• Hence, the positively charged regions of water molecules are attracted to Cl- ions, and the negatively charged regions of water molecules are attracted to Na+ ions.

• When several water molecules surround an ion in the crystal, the sum of the attractive forces between the water molecules and the ion may become strong enough to overcome the attractive forces between the cations and anions in the NaCl crystal.

Salt dissolving

http://ithacasciencezone.com/chemzone/lessons/03bonding/dissociate.htm

Good animations of salt dissolving

http://www.elmhurst.edu/~chm/vchembook/171solublesalts.html

http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/thermochem/solutionSalt.html

Hydration of Ions

• The ions are now hydrated.

• This is ion-dipole attraction.

• When positive and negative ions are separated from one another this called DISSOCIATION.

NaCl in Water

Hydration of Chloride ion

Hydration of Sodium Ion

Would you expect these molecules to be polar?

Solubility

• The maximum amount of a solute that will dissolve in a given quantity of solvent at a particular temperature.

• Usually measured as mass of solute in 100 g of water.

• A solution in which no more solute will dissolve is called a saturated solution.

Solubility Curves

• The relationship between solubility and temperature can be represented by a solubility curve.

• Each point on the curve represents as saturated solution. No more solute can be dissolved at that temperature.

• Saturated solution animationhttp://www.wwnorton.com/chemistry/tutorials/ch5.htm

Demohttp://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/11solutionspage/

crystallization_from_super.htm

Solubility Curves

Solubility Rules

References

• Solubility

http://www.mhhe.com/physsci/chemistry/chang7/esp/folder_structure/cl/m2/s3/

Great site

http://fc.hwcdsb.edu.on.ca/~shawr/solutions.htm