VCE Chemistry
Unit 2.1
Water as a solvent
http://highered.mcgrawhill.com/sites/0072410159/student_view0/chapter5
/figures_alive_.html
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.
Physical Properties
 Colourless,odourless liquid at 25oC
 Relatively high melting(0oC) and boiling
temperature(100oC)
 High heat capacity - Requires a lot of energy to
heat it up
 High latent heat
 Water expands when frozen
 Density of ice less than water
 Non-conductor
 Good solvent for polar compounds
Bonding
Bonding Between Water Molecules (intermolecular)
Most properties can be explained because of the
hydrogen bonding between the 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 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 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
1. A) 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
B) 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
This process is known as IONISATION.
The ions are said to be hydrated.
HCl(g) + H20(l) → H30+(aq) + Cl –(aq)
Hydronium
ion
2. 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.
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 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 animation
http://www.wwnorton.com/chemistry/tutor
ials/ch5.htm
Demo
http://genchem.chem.wisc.edu/demonstrations/Ge
n_Chem_Pages/11solutionspage/crystallization_fro
m_super.htm
Solubility Curves
Solubility Rules
References
Solubility
http://www.mhhe.com/physsci/chemistry/chang7/e
sp/folder_structure/cl/m2/s3/
Great site
http://fc.hwcdsb.edu.on.ca/~shawr/solutions.htm