TWEED RIVER HIGH SCHOOL
2006
PRELIMINARY CHEMISTRY
Unit 1
The Chemical Earth
Part 4
Energy is required to extract elements from their naturally
occurring sources.

Identify the differences between physical and chemical
change in terms of rearrangement of particles.
Matter can undergo two types of changes:
1. Physical change
Physical changes occur without a change in the composition of the
particular substance.
Physical changes include changes in volume, density and changes
of state, eg solid  liquid.
Physical changes involve relatively small amounts of energy. For
example in a change of state, the intermolecular forces
between the molecules need to be broken. These intermolecular
forces are weak and require a small amount of energy to break
them.
1
2. Chemical change
Chemical changes are those in which new substances with
different compositions and properties are formed.
When a chemical reaction occurs, chemical bonds between the
atoms in the molecule need to be broken and reformed to make
the new substance.
Bonds between atoms are much stronger than between molecules,
therefore, chemical changes require a larger amount of energy.
For example:
Process
Type of Change
Energy
Involved
(kJmol-1)
H2O (l)  H2O (g)
H2O (l)  H2 (g) + ½ O2(g)
Physical
+44
Chemical
+286
In the space below draw diagrams to show what happens when
water changes to water vapour and to hydrogen and oxygen gas.
(Fig 5.1, p68 Chemistry Contexts)
2

Identify light, heat and electricity as the common forms of
energy that may be released
or absorbed during the
decomposition or
substances
synthesis
of
and
identify
examples of these changes occurring in everyday life.

Elements cannot be (chemically) decomposed

Compounds under certain conditions can be decomposed into
their constituent elements or simpler substances.

Thermal decomposition:
Thermal decomposition is the process by which heat breaks
compounds down into simpler substances.
For example, sodium hydrogen carbonate (baking soda) is used
in cooking to produce carbon dioxide to make cakes etc rise.
The reaction for this is:
heat
2NaHCO3(s)  Na2CO3(s) + CO2(g) + H2O(g)

Decomposition by light.
Light energy can cause the decomposition of some compounds.
For example silver salts, such a silver bromide, decompose in
the presence of ultraviolet light. The process is used in black and
white photography. The reaction for this process is:
1. AgBr(s)
uv
 Ag(s) + Br(s)
3

Decomposition by Electricity
These are known as electrolysis reactions.
The most common of these reactions is the decomposition of
water as previously discussed. (Draw Fig 5.5, p75 Chemistry
Contexts).
Another example is the electrolytic reduction of alumina (Al2O3)
Products of electrolysis, Aluminium and carbon dioxide, are
obtained by using:
-
a carbon anode and iron container lined with carbon as the
cathode.
-
a high current density
-
a high temperature, 1000C
Electroplating, eg chromium is also an electrical decomposition
reaction.
Homework: Outline the process of electroplating chrome. Include
diagrams.
4

Plan and safely perform a first-hand investigation to show
the decomposition of a carbonate by heat, using appropriate
tests to identify carbon dioxide and the oxide as the
products of the reaction.
Obtain the practical sheets and using these sheets write up your
experiments including the following points:


Aim

Risk Assessment

Method

Results

Discussion

Have your write up approved before you do the practical.

You will then perform the experiment in the lab.
Gather information using first-hand or secondary sources to:
-
observe the effect of light on silver salts and identify an
application of the use of this reaction.
-
observe
the
electrolysis
of
water,
analyse
the
information provided as evidence that water is a
compound and identify an application of the use of this
reaction.
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Part A – Silver salts:
Obtain the practical sheets and using these sheets write up your
experiments including the following points:

Aim

Risk Assessment

Method

Results

Discussion

Have your write up approved before you do the practical.

You will then perform the experiment in the lab.
Part B – Electrolysis of Water.
Observe and record the set up of the Hofmann voltameter.
Note that in addition to water in the voltameter, dilute sulfuric acid is
added as an electrolyte.
This experiment will be performed as a demonstration.
Observe the results as demonstrated and write up a practical report.
In your report you are to identify and application of this reaction.
Note: Both reports are to include complete chemical and ionic
equations.

Analyse and present information to model the boiling of
water and the electrolysis of water tracing the movements of
and changes in arrangements of molecules.
From the unit notes and class discussions, you are to prepare a report
covering the above dot point.
6

Explain that the amount of energy needed to separate atoms
in a compound is an indication of the strength of attraction,
or bond between them.
Bond Energy
There are three general types of compounds:
-
ionic
-
covalent
-
covalent network
When these compounds form, they form chemical bonds between the
atoms.
To decompose a compound, large amounts of energy are required.
This is needed to break the chemical bond. The stronger the attraction
between atoms, a larger the amount of energy is required to break the
bond.
Examples of bond energies:
Bond energy from several covalent bonds:
Covalent Bond
Bond Energy (kJmol-1)
H-H
436
H-Cl
431
Cl-Cl
242
O-H
463
C-C
346
C-0
358
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The ionic lattice energies for several ionic compounds:
Ionic Compound
Ionic lattice Energy
(kJmol-1)
NaCl
788
KCl
718
MgCl2
2523
CaCl2
2255
MgO
3800
CaO
3419
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