Chp 13: Liquids and Water: Sections 1 – 3, 7, 8
Review: Bonding
Ionic Bonding:
Attraction between positive and negative ions.
Metal and non-metal.
Electrons lost or gained to form ions.
Covalent Bonding:
Sharing of electrons.
Non-metal and non-metal.
Polar Molecules
In a covalent molecule, the electrons are not always distributed evenly.
Because of:
Unequal sharing of electrons.
Shape of molecule.
Result:
One side of the molecule is more negative (electrons spend more time there).
One side is more positive.
Polar molecule.
Polar Bonds
Covalent Bond.
Unequal sharing of electrons.
The result of 2 atoms with different electronegativities bonded together.
The bigger the difference the more polar the bond.
Is sometimes indicated by an arrow pointed in the direction of the more electronegative atom.
Shapes of Molecules
The shape of a molecule decides whether it is a polar molecule or not.
Example: water.
Shape: bent.
O
H H
Non-Polar Molecules
If a molecule has no polar bonds, it is non-polar.
O2
Some molecules have polar bonds that cancel out because of the shape of the molecule.
These molecules are also non-polar.
CH4
BF3
Polar vs. Non-Polar
Polar molecules attract each other.
High boiling points.
Mix with other polar molecules.
Don’t mix well with non-polar molecules.
Oil and water.
Non-polar molecules don’t attract other molecules well.
Low boiling points.
Mix well with other non-polar molecules.
Like dissolves like.
Attraction of Polar Molecules
Polar molecules have one end that’s more negative and one that’s more positive.
Not as large of a difference as two ions, but still significant.
This is called a "dipole"
The negative part of one molecule attracts the positive part of another.
“dipole-dipole" interaction.
 Not as strong as a covalent bond.
Attraction of Non-Polar Molecules
Non-Polar molecules do not have dipoles.
However, a temporary dipole may be induce in a non-polar molecule.
electrons in one molecule may briefly be on one side more than the other as they move around.
this may cause electrons in a nearby molecule to briefly move away due to repulsion.
a temporary dipole is induced.
temporary dipoles in nearby molecules attract each other.
These intermolecular interactions are called “dispersion forces or London forces"
 Not as strong as a dipole-dipole interactions.
More about Water
Shape: bent.
Polar molecule.
Water molecules attract each other.
The H from one water is attracted to the electrons on the oxygen of another water.
Hydrogen bonding.
Hydrogen Bonding
 A special type of dipole-dipole interaction
 A type of intermolecular force.
 Attraction between one molecule and another.
 To H-bond, a molecule needs a H that is covalently bonded to an O or N (or F).
 That H is attracted to a lone pair of electrons on another molecule.
 Not as strong as a covalent bond.
More Hydrogen Bonds
Hydrogen bonding is present in other molecules besides water:
Helps to hold the strands of DNA together.
Helps straight hair to curl when heated or dried.
But wetting hair can break these temporary bonds.
Helps to keep enzymes and other proteins in their correct shape.
Properties of Liquids Depend on Intermolecular Forces
Boiling Point
The greater the intermolecular force, the higher the BP.
The more attracted the liquid molecules are to each other, the more energy it takes to get them
into the gas phase.
H-bonds > dipole > dispersion
 H2O > H2S
Surface Tension (see more later)
Polar and Ionic Compounds
Water ( polar compound) is good at dissolving ionic compounds (salts).
The negative end of the water pulls the positive ions (cations).
The positive end of the water molecule pulls the negative ions (anions).
The ions in the salt separate and are surrounded by water molecules.
They dissolve.
Polar and Non-Polar Substances
Water (polar compound) is not good at dissolving or mixing with non-polar substances.
Example: oil, fat.
The water molecules are more attracted to each other than the oil.
The oil can’t get between the water molecules.
The substances don’t mix.
More about Liquids: Surface Tension
The attraction of water molecules for each other causes water to “bead up”.
This is due to the attraction between the water molecules at the surface and the ones below.
Called “surface tension”.
Also occurs in other liquids, but in water more than others.
The greater the intermolecular force, the greater the surface tension.
H-bonds > dipole > dispersion
Ice
When ice forms, the polar water molecules stack on each other.
Positive oxygen lines up with negative hydrogens on other molecules.
But water molecules have a bent shape so there’s space between the molecules.
Result: water expands when it freezes.
Result: ice is less dense than liquid water.
Result: ice floats.
Soap
Dirt and grease have many non-polar components.
Difficult for water to dissolve.
Soap molecules have a polar end and a long non-polar tail:
Soap and Dirt
The non-polar end of the soap surrounds the non-polar dirt and grease.
“Like dissolves like”.
A micelle is formed.
The polar end of the soap is attracted to water molecules.
The soap-dirt micelle is surrounded by water molecules and washed away.
Soap breaks the surface tension of water.
Making Soap
Soap making has been around for centuries.
Animal fat was treated with lye.
Lye is sodium hydroxide, NaOH.
Drain opener, oven cleaner.
Originally, the lye was obtained from ashes.
The lye breaks the fat into “fatty acid salts” (soap molecules).
Detergents
Detergents are synthetic soap molecules.
They are similar to soap molecules with a non-polar tail and a polar end.
Detergents are found in toothpaste, laundry detergent, dish soap, shampoo, etc.
Homework: Chapter 13 (Starts on p. 364)
Exercises: 3, 5, 6, 8, 10, 45, 46, 49 – 56