Standard Grade Chemistry
Summary Notes
Topic 11. Metals
General
Learning Outcomes
 Metals conduct electricity when solid or liquid.
 You must be able to relate the specific properties of metals e.g density,
electrical and thermal conductivity, malleability and strength to their uses.
 Explain the need for recycling metals in terms of the finite nature of metal
resources.
 Describe the reactions of different metals with oxygen, water and dilute
acid.
 These reactions give an indication of the reactivity of the metal.
 Ores are naturally occurring compounds of metals.
 The less reactive metals are found uncombined in the earth’s crust and the
more reactive metals have to be extracted from ores.
 Give examples of social and industrial factors which resulted in large scale
extraction of more reactive metals.
 Some metals can be obtained from metal oxides by heat alone; some by
heating with carbon.
 Iron is produced from iron ore in the blast furnace.
 The two key reactions that take place in the blast furnace are :
 production of carbon monoxide
 reduction of iron oxide
 an alloy is a mixture of metals or of metals with non-metals
 give examples of the important uses of alloys e.g. brass, solder and stainless
steel
Credit
Learning Outcomes
 The extract of a metal from its ore is an example of reduction.
 Explain in terms of the reactivity of the metal why some metals can be
obtained from metals by heat alone; and why some metal oxides need to be
heated with other substances e.g. carbon or carbon monoxide; and why some
metals cannot be obtained by these methods.
 Work out empirical formula (or molecular formulae) from masses or
percentage composition.
 Work out percentage masses of elements in compounds from their names or
formulae.
Metals today
General
Metals are often recognised because they have special physical properties which
distinguish them from other materials. Some of these properties are :
1. Metals tend to be more dense than non-metals (Reminder : the density of a
substance is the mass of a given volume)
density =
2.
3.
4.
5.
mass
volume
Metals tend to be good conductors of heat
Metals tend to be good conductors of electricity
Most metals can be beaten into different shapes – they are said to be malleable
Many metals are very strong (that is why they are used in bridges and buildings)
When choosing a metal for a particular task it is important to take the specific
properties of the metal into account. It is also important to consider the availability
and cost of the metal. The following list shows how the specific properties of a metal
can relate to its use.
property
Melting point
Density
Conduction of heat
All metals are solids at room
temperature except mercury
which has a very low melting point
The densities of metals vary
greatly e.g. potassium 0.86g/cm3
to platinum 21.5g/cm3
Aluminium has a density of
2.70g/cm3
All metals are good conductors of
heat
Conduction of
electricity
Strength
All metals are excellent
conductors of electricity
Some metals are strong under
heavy loads
Malleability
Metals can be bent and flattened
into many different shapes
use
Mercury is used in
thermometers
Aluminium is used in
aircraft manufacture
Aluminium, copper and
stainless steel are all used
for making cooking pots
Copper is used extensively
for electrical wiring
Steel is used to makes
girders and reinforce
concrete
Many everyday objects are
made from metals pressed
into flat sheets
We get all our metals from the Earth’s crust. Just like our fossil fuels – coal, crude oil
and natural gas – our supply of metals is limited and there will come a time when they
will run out. Metals are a finite resource.
We live in a ‘throw away’ society. When we have finished drinking our coke or eating our
chips we tend to throw the cans and paper into the bin. Some of this waste can be
valuable. A lot of metal objects we throw away can be collected and used again. This is
called recycling.
Reaction of metals and oxygen
When metals are heated in oxygen, they usually combine to form a metal oxide. A
metal oxide is a compound of a metal and oxygen.
metal + oxygen  metal oxide
magnesium + oxygen  magnesium oxide
2Mg + O2  2MgO
Reaction of metals and water
Metals which lie above hydrogen in the electrochemical series react with water,
releasing hydrogen gas and forming the metal hydroxide.
metal + water  metal hydroxide + hydrogen
calcium + water  calcium hydroxide + hydrogen
Ca + 2H2O  Ca(OH)2 + H2
The further up the electrochemical series the more vigorous is the reaction with
water.
Reaction of metals with dilute acids
When a metal (which lies above hydrogen in the electrochemical series) is reacted with
a dilute acid, hydrogen gas is released and a salt is formed.
metal + acid  salt + hydrogen
zinc + hydrochloric acid  zinc (II) chloride + hydrogen
Zn + 2HCl  ZnCl2 + H2
Remember that the salt formed depends on the acid used.
Hydrochloric acid forms a chloride; sulphuric acid forms a sulphate.
The results of experiments also allow an order of reactivity of metals to be drawn up.
Order of reactivity of metals
Potassium
Sodium
react with water
Calcium
Magnesium
Aluminium
react with acid
Zinc
Iron
react with oxygen
Tin
Lead
Copper
Mercury
Silver
Gold
Metals And Their Ores
General / Credit
The metals we use in the world today are obtained from rocks in the Earth’s crust.
Most of the metals are in these rocks as compounds, usually metal oxides, metal
sulphides or metal carbonates.
Only a small number of metals, such as gold, silver and platinum are found in rock as
uncombined metals. These are the most unreactive metals. These metals were among
the first elements to be discovered because they did not have to be extracted from
their ores in some way.
Ores are naturally occurring compounds of metals found in rocks. Getting the metal
from its ore is not usually a simple process. It normally requires several stages.
1. The ore is crushed into smaller pieces so that the metal compound can be
separated from all the other compounds in the rock.
2. If the metal compound is not an oxide, it is usually roasted in air to change it
into an oxide.
3. The metal is obtained from the metal oxide. The method depends on the
reactivity of the metal.
Obtaining Metals From Their Ores
General / Credit
Some of the most unreactive metals are occasionally found as compounds, e.g. silver
is sometimes found as its oxide, silver (I) oxide. These metals can be relatively easily
obtained from their compounds by heating alone.
2AgO  2Ag + O2
Some metals that cannot be obtained by heat alone, can be obtained from their oxide
by heating with carbon.
Zinc, tin, iron and copper can all be extracted from their oxides in this way. When
these metal oxides are heated with carbon (or carbon monoxide) they break down to
give the metal and carbon dioxide.
metal oxide + carbon  metal + carbon dioxide
(or carbon monoxide)
iron (III) oxide + carbon  iron + carbon dioxide
2FeO + C  2 Fe + CO2
The most reactive metals cannot be obtained by heating their metal oxides with
carbon. E.g. aluminium oxide has to be broken down using electricity, by a process
known as electrolysis.
Reactivity And Reduction
Credit
We learned that metal oxides can be converted to metals in three ways. The conversion
of a metal oxide to a metal is sometimes called reduction, and the method selected
depends on the reactivity of the metal.
The three possible methods of reduction of metal oxides are :
1. Heating alone
2. Heating with carbon (or carbon monoxide)
3. Electrolysis
When metals combine with oxygen there is a release of energy and bonds are formed
between metals and the oxygen. To convert the metal oxide back into the metal the
same amount of energy must be supplied.
Energy is required to break the bonds between the metal and the oxygen, in the
metal oxide. The stronger the bond, the greater the energy needed.
Silver is not very reactive. It will not hold onto oxygen strongly. Heat is all that is
needed to release the oxygen.
Metals such as copper and iron are more reactive. Heat alone is not enough to make
them release their oxygen. Carbon, or carbon monoxide, is required to help ‘pull’ the
oxygen away.
The most reactive metals hold onto the oxygen so strongly that neither carbon, nor
carbon monoxide is able to pull the oxygen away. A supply of electricity is required to
obtain these metals.
Metal oxides are ionic compounds. To change the metal oxides into metals, the ionic
bonds must be broken. As part of this change the metal ions gain electrons to form
metal atoms.
Mg2+ + 2e  Mg
This is why the process is called reduction.
Alloys
General
Although there are only about 70 naturally occurring metal elements, there are
thousands of different metallic substances in common use.
Often the properties of a pure metal make it unsuitable for the purposes we wish to
use it. For example, it may rust easily or it is quite brittle.
The properties of metals can be altered by adding small amounts of other elements, to
make new substances called alloys. Alloys are mixtures of metals or of metals with nonmetals.
Examples of common alloys include :
Alloy
Main Metal
Other
elements
mild steel
stainless steel
18 carat gold
brass
iron
iron
gold
copper
carbon
chromium, nickel
silver, copper
zinc
bronze
copper
tin
solder
lead
tin
Uses
girders, car bodies
cutlery, sink tops
jewellery
ornaments, taps,
door handles
gears, bearings,
statues, propellers
electrical
connections
The most important alloys are those based on steel. The iron needed to make steel is
extracted by heating its ores with carbons.
This is carried out industrially by using the blast furnace.
The blast furnace is a very tall steel tower, lined on the inside with fireproof bricks.
Iron ore is loaded from the top along with coke (almost pure carbon) and limestone.
Hot air is blown from the base of the furnace.
The coke initially reacts with the oxygen from the air, forming carbon dioxide. This
reaction heats the furnace.
carbon + oxygen  carbon dioxide
C + O2  CO2
As the carbon dioxide moves through more coke it reacts forming carbon monoxide.
carbon dioxide + carbon  carbon monoxide
CO2 + C  2CO
The carbon monoxide reduces the iron (III) oxide in the ore to iron.
iron (III) oxide + carbon monoxide  iron + carbon dioxide
Fe2O3 + 3CO  2Fe + 3CO2
Molten iron and slag (limestone combined with impurities from the ore) are collected
at the base of the furnace.
The molten iron is taken to an oxygen furnace. There oxygen is passed through the
molten iron and reacts with the unreacted carbon to give carbon dioxide. Other
elements are then added to make different steels.
Empirical Formulae And Percentage Composition
Credit
We know that the formula for a compound gives us information about the ratio of
atoms found in the compound.
For example, Fe2O3 contains 2 atoms of iron and 3 atoms of oxygen. The ratio of atoms
(Fe : O) is 2:3.
The gram formula mass of a compound can easily be found if the relative atomic mass
of each element is known.
For example the gram formula mass of Na2SO4 is (2 x 23) + (1 x 32) + (4 x 16) = 142g
This quantity of a substance is also called 1 mole of the substance. One mole of any
compound has a mass equal to the gram formula mass.
The formula of a compound not only gives us the ratio of atoms found in the compound
but also the ratio of moles. By calculating the mole ration we can work out the simplest
(or empirical) formula for a compound.
The following example shows how to calculate an empirical formula.
In an experiment a chemist found that a substance was made of 23g of sodium and 8g
of oxygen. Find the simplest (or empirical) formula of the substance.
Step 1
Step 2
Step 3
Write down the symbols of the elements present
Write down the mass (or percentage) of each
element present
Divide each by its RAM
Step 4
Divide each answer by the smallest
Na
O
23
8
23/23 8/16
=1
=0.5
1/0.5 0.5/0.5
=2
=1
Therefore Empirical formula = Na2O
In the above example, the formula Na2O is the correct formula for sodium oxide. This
does not always happen when working out empirical formula.
Using the same method to work out the empirical formula for ethane (C2H6) you would
get the answer CH3, because that is the simplest formula for the compound.
Unfortunately it is not the correct formula.
The only way to know if you have the correct formula is to know the formula mass.
Example : The simplest formula of a compound was calculated to be CH3. The formula
mass of the compound is known to be 30. What is the correct formula for this
compound?
The formula mass of CH3 is 12 + (3 x 1) = 15
Since the formula mass is 30, there must be twice as many atoms present.
The correct formula must be C2H6.
Percentage Composition
Sometimes a chemist is not given the mass of each element in the compound, but the
results are given as percentages. This is called percentage composition for the
compound and the figures can be used in exactly the same way to calculate the
empirical formula.
It is also important that you can calculate the percentage composition of a compound
from its formula.
Example
What is the percentage composition of sodium carbonate?
Step 1
Write the correct formula for the compound
Na2CO3
Step 2
Work out the formula mass
Step 3
Write down the mass of each element in the formula
Step 4
Calculate the percentage of each element
dividing each mass by the formula mass and
multiplying by 100
(2 x 23) + 12 + (3 x 16)
= 106
Na = 46
C = 12
O = 48
Na = 46/106 x 100 = 43.4%
C = 12/106 x 100 = 11.3%
O= 48/106 x 100 = 45.3%