Water, Polar molecules, and an Intro to Solutions (p264 -280)

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Transcript Water, Polar molecules, and an Intro to Solutions (p264 -280)

Water, Polar molecules, and an
Intro to Solutions (p264 -280)
Lesson 1
December 5th, 2011
Review of bond types
Intermolecular
Force
Bonding Model
Ionic
- Involves electron transfer, forms cations
and anions
- Cations and anions are attracted to each
other
Review of bond types
Intermolecular Force Bonding Model
Polar Covalent
-
Involves unequal sharing of pairs of electrons
by atoms of two different atoms
Bonds can involve 1, 2, or 3 pairs of electrons.
Review of bond types
Intermolecular Force Bonding Model
Covalent
-
Involves equal sharing of pairs of electrons
Bonds can involve 1, 2, or 3 pairs of electrons.
Polar Molecules
If a molecule contains polar covalent bonds,
the entire molecule may have a positive end
and a negative end.
 This would then be classified as a polar
molecule.

Polar molecules
Not all molecules that contain polar covalent
bonds are polar molecules.
 This can be due to the shape of the
molecule.

Guidelines for predicting polar
and nonpolar molecules
Type
Polar AB
HAx
Description
Examples
Diatomic compounds
CO(g)
Any molecule with a single H
HCl(g)
AxOH Any molecule with a single OH at C2H5OH(l)
one end
OxAy Any molecule with an O at one end H2O(l), OCl2(g)
NxAy
Any molecule with an N at one end NH3(g), NF3(g)
Guidelines for predicting polar
and nonpolar molecules
Type
Non Ax
polar
CxAy
Description
Examples
All elements
Cl2(g), N2(g)
Most carbon compounds
(including organic solvents,
fats and oils)
CO2(g),
CH4(g)
Intermolecular Forces

Van der Waals forces

Dipole dipole – attractive force between polar
molecules

London dispersion force – attractive force
between all molecules
Hydrogen bonds
 A strong dipole-dipole force between a
positive hydrogen atom of one molecule, and
a highly electronegative atom (N, O, F) in
another molecule.

Water: Essential for Life
70% of the earth is covered in water yet only
3 % of it is fresh water and only 1 % of this is
water is in a liquid state.
 Canada has the most abundant supply of
fresh water in the world but this doesn’t
mean that we shouldn’t learn how to
conserve water.


The water cycle is one method of natural
purification. Energy from the sun causes
water to evaporate, dissolved contaminates
are left behind as it rises. When the water
condenses it returns to earth as
precipitation.
Physical properties of
water
Pure water is colourless, odourless, and
tasteless.
 Whether or not an object sinks or floats
relates to its density compared to water.


Pure water density = 1.0 g / mL
Water is unique in the fact that its density
decreases when it becomes a solid, this is
what causes ice to float.
 This in turn acts a blanket which prevents
the water underneath from freezing and
killing all life.

Heat capacity is a measure of how much
heat it takes to heat one gram of a
substance 1 ° C.
 Water requires 4.18 J of heat to increase the
temperature of 1 gram 1 ° C.

Chemical Properties of
Water
Water is a polar molecule that consists of
oxygen and hydrogen molecules. The
oxygen has a larger electronegativity and so
it creates a partial – charge as it draws
hydrogen’s electrons closer to it.
 Draw this

The large difference in charge allows for
Hydrogen bonding to occur between
molecules.
 This provides water with its unique surface
tension, high boiling point and ice that is less
dense. (see page 87 fig 7 for diagram)


The polarity of water also allows water to
dissolve other polar molecules.
dissolves Like – polar dissolves polar and nonpolar dissolves non-polar
 Like

Ionic substances disassociate into charged ions
when dissolved in water form electrolytic solutions
– conduct electricity

Draw this


Substances that stay electronically neutral when
dissolved in water are called nonelectrolytes.
Ex glucose

Due to waters great ability to be a solvent it
can become contaminated easily.
What’s in Polluted Water?

As the amount of human activity increases
the amount of contaminants found in nearby
water also increase. Contaminants are
classified into three types.
1. Physical Contaminants

Objects that do not dissolve in water.

Ex – oil, plastic, tree branches, leaves, peat,
silt.
2. Biological Contaminants

Biological contaminants include bacteria
and viruses which may make the water
unsafe to consume.
3. Chemical Contaminants

Chemicals that are soluble in water.


Ex – metal ions, pesticides, fertilizers.
These are not visible but can lead to visible
effects such as an algae bloom
Solutions
Most substances contain water because it is
so good at dissolving other substances.
 AKA the “Universal Solvent”


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Technically it is not universal because water
does not dissolve non-polar substances. Only
non-polar can dissolve non-polar.
All aqueous solutions use water as the
solvent and are transparent.

A solution is a homogeneous mixture of
substances composed of at least one solute
and one solvent that are uniform throughout
the sample. This can be in a liquid or gas
state.
Solute – A substance that is dissolved in
a solvent
 Solvent – The medium in which a solute is
dissolved.

Properties of Aqueous
Solutions
Compounds can be classified as either
electrolytes or non-electrolytes.
 Electrolytes – solutes that form aqueous
solutions that conduct electricity.


Non-electrolytes – solutes that form
aqueous solutions that do not conduct
electricity.
Most molecular compounds are
nonelectrolytes, except for acids.
 Solutions can also be classified as acids,
bases, or neutral. These will be covered
more in the next unit.

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Questions – P 276 figure 6- predict
solubility, p 269 # 8, p 271 # 4, 5, 6, p 277 #
4,5, p 279 # 9-12, p280 # 2-4