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Chemistry Unit 5
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UNIT 5C H E M
PROPERTIES AND USES OF SUBSTANCES
I S T R Y
RELATING PROPERTIES TO USES AND PRODUCTION OF
SUBSTANCES
Scientists need to know the different chemical properties of substances. They can use this
knowledge to make new, useful materials.
Metal Oxides and Metal Hydroxides
Alumina is the common name for aluminium oxide. One property of alumina is that it is
amphoteric. This means that it will act as an acid or a base depending on the conditions. If an
acid is present it will react as a base. If a base is present it will act as an acid.
When aluminium oxide reacts with an acid it reacts in the same way as other metal oxides,
such as magnesium oxide, that are bases.
Aluminium oxide reacts with hot hydrochloric acid to give aluminium chloride and water. the
aluminium chloride is soluble, so you get aluminium chloride solution.
Al2O3 + 6HCl 2AlCl3 + 3H2O
Aluminium oxide will react with bases such as sodium hydroxide to form aluminates. For
example:
Al2O3 + 6NaOH + 3H2O 2Na3Al (OH)6 (sodium aluminate)
Other aluminates can form depending on the conditions. Hot concentrated sodium hydroxide
will give sodium tetrahydroxoaluminate, 2NaAl (OH) 4. You will not be expected to know all
aluminates that they form.
In general, aluminium oxide is chemically inert except under certain conditions, e.g., when a
hot acid or base is present. This means it has a lot of uses including filler, paint, sunscreens,
and glass. The amphoteric nature of alumina means it can be used as a medium for
chromatography as a basic, acidic or neutral medium.
It is also an effective desiccant and can be used at a range of basic and acidic pHs. It is stable
over pH range 2-13. This also makes it suitable for environmental clean-up and separation
applications.
Group 1 and group 2 metal oxides are basic. If they dissolve in water, then they will react
with the water to form metal hydroxides. This forms an alkali solution. For example:
Na2O + H2O 2NaOH
CaO + H2O Ca (OH)2
The oxide ion is a very strongly basic anion due to its very small size and high (2-) charge.
The oxide ion in the metal oxide reacts with water to produce hydroxide ions, because a
hydroxide ion is the strongest base that can persist in water.
Calcium oxide (quicklime) reacts with water to form calcium hydroxide (lime). This can then
be used by farmers to raise the pH of acidic soil.
Magnesium oxide can be used as a desiccant when preserving books in a library. Some
papers produce acidic sulphur oxides when oxidised in air. Although magnesium oxide is not
a very good desiccant, it is used because due to its basic nature it will neutralise the acidic
sulphur oxides produced and so help to preserve the book.
,Sodium hydroxide, which is produced when sodium oxide reacts with water, is an important
compound in the chemical industry as well as I everyday life. For example, it is used in some
processes to make plastics and soaps. It is used in food processing in many ways. Some
examples are to peel fruits and to process coca and chocolate. It is found in drain cleaner or in
over cleaner. These uses all rely on the basic properties of the compound.
Magnesium hydroxide, Mg (OH)2 is used in the treatment of acidic effluent. Factories that
produce or use sulphuric acid will also produce acidic liquid effluent which may have to be
treated before being released to the environment, especially if it has high acid content.
Impurities are precipitated and removed. The acidic liquid can be neutralised using calcium
hydroxide, lime.
This reaction occurs.
H2SO4 + Ca (OH)2 CaSO4 + 2H2O
The calcium hydroxide is a base, and a neutral calcium sulphate is formed. Calcium sulphate
is a salt. Although other bases are available, calcium hydroxide is cheap and simple to use.
Electrolysis
Electrolysis breaks down compounds into simpler substances. It is used in some industrial
processes. Ionic substances are decomposed by an electrical charge being passed through the
during electrolysis. The ions must be free to move for this to work so the compound must
either be molten (reduced to liquid form by heating) or in solution.
Figure 5.1 shows the electrolysis of molten sodium chloride. Sodium is produced at the
cathode (positive electrode) and chlorine is produced at the anode (negative electrode).
The only ions present are sodium and chloride ions and so only sodium and chlorine can be
produced by this reaction.
Figure 5.2 shows the electrolysis of sodium chloride solution.
When the sodium chloride is dissolved in water, hydrogen and hydroxide ions are present as
well as the sodium and chloride ions. In this case, the chlorine is still produced at the anode
by hydrogen gas is produced at the cathode rather than sodium. It is only possible for one
type of ion to discharge at each electrode. The ion which is select to discharge at an electrode
depends on a number of factors.
The position of the ion in the electrochemical series affects its ease of discharge at an
electrode. When metals react, they lose electrons to become positive ions. If a metal is placed
in water, then the metal atoms will tend to lose electrons to the water and become positive.
This in turn attracts the negative electrons back to the metal. This is an example of an
equilibrium process.
For example, magnesium is water will have the following equilibrium reaction.
Mg2+ (aq) + 2e- ⇌ Mg (s)
A piece of copper in water will behave in the same way.
Cu2+ (aq) + 2e- ⇌ Cu (s)
, As copper is less reactive than magnesium, it will form ions less easily so the equilibrium for
copper in water is further to the left than that for magnesium in water.
For both the metals, there is a potential difference between the negative charge on the metal
and the positive charge of the solution around it. The potential difference is bigger for
magnesium than it is for copper. This potential difference can be measured as a standard
electrode potential and then the metals can be placed in order of the standard electrode
potentials (E°) in the electrochemical series.
Hydrogen is also included in the electrochemical series (see Table 5.1).
Equilibrium half E° (volts) The lower the metal is in the electrochemical
equation series, the more likely it is to be discharged.
So, in a solution of copper sulphate, the
Li+ (aq) + e- ⇌ Li (s) -3. 03
copper ions be discharged to form copper
K+ (aq) + e- ⇌ K (s) -2. 92
atoms rather than the hydrogen ions. In a
Na+ (aq) + e- ⇌ Na (s) -2. 71 solution of sodium chloride, hydrogen gas is
Mg2+ (aq) + 2e- ⇌ Mg (s) -2. 37 given off at the cathode rather than sodium
Al3+ (aq) + 3e- ⇌ Al (s) -1. 66 metal.
Zn2+ (aq) + 2e- ⇌ Zn (s) -0. 76 The concentration of the ions in solution will
+ -
2H (aq) + 2e ⇌ H2 (g) 0 also have an effect on which ion is
Cu2+ (aq) + 2e- ⇌ Cu (s) +0. 34 discharged. The most concentrated ion tends
- -
½ Cl2 (aq) + e ⇌ Cl (aq) +1. 36 to be discharged, no matter where the ions are
Table 5.1 – Part of the electrochemical series.
solution containing the chloride ion, the chloride ion will be discharged rather than the
hydroxide ion, even though the hydroxide ion is more easily oxidised.
Transition Metals
Transition metals are the d block elements found between group 2 and 3 on the periodic table.
They are used in a variety of industrial processes and transition metals and their compounds
have a wider number of uses. Transition metals have incomplete d-sub-shells as a stable ion.
If we look at the transition elements in period 4, across the period, from scandium to zinc, the
3d-orbitals are being filled. The pattern is regular. (Expect for chromium and copper, which
do not follow the principle of completely filling the lowest energy level first.) the sub-shell
energy levels is the third and fourth energy levels overlap. The 4s-sub-shell fills before the
3d-sub-shell.
When they react, transition element atoms lose electrons to form positive ions. Transition
metals lose their 4s electrons before their 3d electrons. They form ions with more than one
stable oxidation state. They can all form compounds with metal ions in the +2 oxidation state.
In solution, transition metal compounds form complex ions. A complex ion consists of a
central metal ion surrounded by ligands. A ligand is a molecule or ion that donates a pair of
electrons to the central transition metals ion to form a dative covalent bond.
Figure 5.3 shows the complex ion [Fe (H 2O)6 ]2+. Each of the six H 2O ligands forms one
dative covalent bond with the central metal ion.
Other complex ions include [Cu (H2O)6 ]2+,
[Al (H2O)6]3+ and [CuCl4]2-.
Transition metals, oxides of transition metals
and transition metal complexes are all used as
industrial catalysts.
Platinum and rhodium are used catalytic
converters in car exhausts. They convert
carbon
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