chemistry notes

The majority of other metals are found chemically combined with other elements in
the form of an ore. An ore is a rock that contains a sufficient quantity of the
metal to make it worth extracting the metal from it.

Ores are often mixtures of different minerals. Usually a combination of physical

and chemical processes is needed to remove the metal from the other elements in the
ore. Many ores contain a metal oxide or metal carbonate.

Very reactive metals, like sodium, are extracted from their ores using electricity
– this process is known as electrolysis. Less reactive metals, like iron, can be
obtained by smelting (the ore is roasted with carbon). The ores of very unreactive
metals, like copper, are heated to decompose the ore first, before smelting is
carried out.

The reactivity of the different metals can be compared by observing their reactions
– the more vigorous the reaction, the more reactive the metal.

Some metal carbonates, like limestone (calcium carbonate), decompose on heating – a

process known as thermal decomposition. Thermal decomposition of a metal carbonate
can be the first stage in the extraction of a metal.


An example of such a displacement reaction is when iron is added to a blue copper

sulfate solution. Iron is more reactive than copper. A chemical change occurs –
iron displaces the copper, bonding with the sulfate to make iron sulfate, which is
a pale green solution.

The word equation for the reaction is:

iron + copper sulfate → iron sulfate + copper

When reactive metals react with acids, a displacement reaction occurs and hydrogen
is displaced. As long as a metal is above hydrogen in the reactivity series, they
will react to displace hydrogen and form bubbles of the gas:
zinc + hydrochloric acid → zinc chloride + hydrogen

Non-metals also undergo displacement reactions. Chlorine is more reactive than

iodine. When chlorine gas is passed through sodium iodide solution, the chlorine
displaces the iodine as follows:
chlorine + sodium iodide → sodium chloride + iodine


Coal is a richer source of carbon than wood. When it is burned slowly to

temperatures over 1000 °C, in the absence of oxygen, coke is made. This was
discovered in the 18th century. Coke is used in industry today to extract some
metals from their ores.

When oxygen is removed from a metal, we call this reduction. It is the opposite of
adding oxygen, which is known as oxidation. The carbon acts as a reducing agent.
Iron ore occurs in the form of iron oxide (Fe2O3) – it is called haematite.

To extract the iron, powdered coke is added to the crushed iron ore along with
limestone, which removes impurities. All the ingredients are roasted in a large
furnace about 30 metres in height, known as a blast furnace. Air is injected into
the bottom of the furnace to improve the reducing properties of the carbon. A blast
furnace operates at temperatures up to 1650 °C.

Malachite is copper carbonate, a form of copper ore. You can see its striking green
colour in Figure 3.3.6a. In order to extract the copper, the carbonate first has to
be heated to over 200 °C. This causes thermal decomposition and carbon dioxide is
released. Copper oxide is made. The reaction is:
CuCO3 → CuO + CO2

The copper oxide is then heated with carbon in the form of coke. Because carbon is
more reactive than copper, the copper from copper oxide is displaced:
2CuO + C → 2Cu + CO2

One of the ores of lead is galena, which is lead sulfide. To extract the metal, the
ore is first crushed into small particles. These are added to a mixture of oil and
water. Air is blown upwards to push the lead ore particles to the top, separating
them from other impurities – this is called ‘froth flotation’.
Oil is needed because lead sulfide is very dense and this helps it to rise to the
top. The lead sulfide is then heated in air at high temperatures, converting it to
lead oxide. The word equation for the reaction is:
lead sulfide + oxygen → lead oxide + sulfur dioxide

Roasting lead oxide with carbon reduces it to lead. The word equation for the
reaction is:
lead oxide + carbon → lead + carbon dioxide

Zinc ore (sphalerite) is zinc sulfide. Before it can be displaced with carbon, the
zinc sulfide needs to be changed into zinc oxide. This is achieved by blowing hot
air through a concentrated mixture of zinc sulfide and water, by froth flotation.
The concentrated zinc sulfide is then roasted in air to make zinc oxide. This is
reduced by roasting it with carbon.

The symbol equations for the production of lead and zinc from their oxides are:
2PbO + C → 2Pb + CO2
2ZnO + C → 2Zn + CO2
In reality, processes for extracting metals from their ores are inefficient because
not all the metal is removed from ores. The yield of the process is less than 100
per cent, where the yield is defined by:


Once a mine ceases to operate, companies have to plant the area with trees and
other plants to restore the landscape – this is known as reclamation. The hole
caused by mining can often be made into a lake, adding recreational value.

Deep mining can lead to subsidence. The ground becomes unstable, causing it to cave
in.

Waste liquids from the mining process are called leachates. They have to be stored
in specially lined pools to avoid contaminating the ground or water systems.

Recycling avoids the need to mine and process new material, saving metal reserves
and energy. One tonne of new aluminium cans requires five tonnes of aluminium ore
and produces five tonnes of waste that is hazardous to the environment.

These are exothermic reactions – exothermic means ‘to give out heat.

If the energy transferred to the surroundings during the bond-making process is

higher than the energy absorbed during the bond-breaking process, the reaction is
exothermic

energy of reactants – energy of products = energy transferred



In an endothermic reaction more energy is absorbed than is given out. Endothermic

means ‘to take in heat’. This results in a reaction in which the temperature is
seen to fall as the reaction proceeds.

endothermic is negative
exothermic is positive