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Metals

Chapter: Metals

Definition

Any element that has tendency to lose electron and become more stable is called a metal, because of this property of loosing electrons, metals are said to be electropositive in nature.

All the metals except mercury are solids at room temperature.

The metals lie to the left of the zigzag line in the periodic table.

Physical properties of metals:

Metals have the following physical properties:-

  1. They are strong. They won’t break if you press, drop or try to tear them. It is hard to cut them.
  2. They are malleable. The property of any metal being beaten to thin sheets without breaking is called malleability.
  3. They are ductile. The ability of metals to be drawn into thin wires is called ductility.
  4. They are sonorous
  5. They are shiny when polished.
  6. They are good conductors of electricity
  7. They have high melting and boiling points
  8. They are heavy and have high density.

Chemical properties of metals

  1. They react with oxygen to form oxides. Magnesium burns in air to form magnesium oxide.
  2. Metal oxides are bases. They neutralise acids forming salts and water.
  3. Metals form positive ions when they react. Magnesium forms magnesium ions (Mg 2+) when it reacts with oxygen.
  4. For the metals in the numbered groups, the charge on the ion is the same as the group number. But the transition elements have variable valency. They can form ions with different charges. For example, Cu+ and Cu2+.

All metals are different

All metals are different. They do not share all of those properties. For example, all conduct electricity and their oxides act as bases.

A reactive element has a strong drive to become a compound so that its atoms gain stable outer shells. So the metal reacts readily with other elements and compounds.

Reaction of metals with water:

Sodium reacts violently with cold water whizzing over the surface, forming a clear solution of sodium hydroxide and hydrogen gas.

 

 

Reaction of metals with hydrochloric acid:

It is not safe to add sodium or potassium to acid in the lab, as the reactions are explosively fast.

Potassium, sodium, lithium and calcium react violently with dilute HCl, forming the metal salt (either sulphate or chloride) and hydrogen gas. The reaction is similar to the reaction with water.

 

 

Hydrogen is displaced

When a metal reacts with water or hydrochloric acid, it drives out and takes its place. This shows that the metal is more reactive than hydrogen. It has a stronger drive to form a compound. But silver and copper do not react with water or acid. So they are less reactive than hydrogen.

It is a redox reaction

The displacement of hydrogen is a redox reaction. When magnesium reacts with hydrochloric acid its atoms lose electrons. The hydrogen ions from the acid gain electrons.

Metals in competition

1. Competing with carbon

Carbon is more reactive than some metals. It will reduce their oxides to form the metal.

2. Competing with other metals, for oxygen

A metal will reduce the oxide of a less reactive metal. The reduction always gives out heat and it is exothermic reaction.

3. Competing to form ions in solution

A metal displaces a less reactive metal from solutions of its compounds.

The reactivity series

 

 

Things to remember about the reactivity series

  1. The reactivity series is actually a list of the metals in order of their drive to form positive ions with stable outer shells. The more easily its atoms can give up electrons the more reactive the metal will be.
  2. So a metal will react with a compound of a less reactive metal by pushing the less reactive metal out of the compound and taking its place.
  3. The more reactive the metal, the more stable its compounds are. They do not break down easily.
  4. The more reactive the metal, the more difficult it is to extract from its ores, since these are stable compounds.
  5. The less it likes to form compounds, the less reactive the metal.

Thermal decomposition and its general rules

Thermal decomposition is the easy break down of compounds on heating. The following are the general rules for thermal decomposition:

  1. The lower a metal is in the reactivity series, the more readily its compounds decompose when heated.
  2. Carbonates, except those of sodium and potassium decompose to the oxide and carbon dioxide.
  3. Hydroxides except those of sodium and potassium decompose to the oxide and water.
  4. Nitrates except those of sodium and potassium decompose to the oxide, nitrogen dioxide and oxygen. The nitrates of sodium and potassium form nitrates and oxygen.

Making use of the reactivity series

1. The thermite process

This is used to repair rail and tram lines. Powdered aluminium and iron oxide are put in a container over the damaged rail. When the mixture is lit, the aluminium reduces the iron oxide to molten iron in a very vigorous reaction. The iron runs into the cracks and gaps in the rail and hardens.

2. Making simple cells

A simple cell consists of two different metals in an electrolyte. Electrons flow from the more reactive metal so it is called the negative pole. The other metal is the positive pole.

3. The sacrificial protection of iron

Iron is used in big structures such as oil rigs and ships. But it has one drawback: it reacts with oxygen and water forming iron oxide or rust. To prevent this, the iron can be teamed up with a more reactive metal like zinc or magnesium. Zinc gets oxidised instead of iron. This is called sacrificial protection. The zinc block must be replaced before it all dissolves away.

4. Galvanising

This is another way of using zinc to protect iron. It is used for the steel in car bodies and the corrugated iron for roofing.

In galvanising, the iron or steel is coated with zinc. For roofing, the iron is dipped in a bath of molten zinc. Galvanising is carried out by electrolysis for car bodies. The zinc coating keeps air and moisture away. But if the coating gets damaged, the zinc will still protect the iron by sacrificial protection.

Metals in the earth's crust

The composition of the Earth’s crust

We get some metals from the sea, but most from the Earth’s crust- the Earth’s hard outer layer. The crust is mostly made of compounds. It also contains some elements such as copper, silver, mercury, platinum and gold. These occur native or uncombined because they are unreactive.

Extraction of metals:

Mining is the process of extracting metals from the earth. It depends on the metal’s reactivity.

  1. The most unreactive metals- such as silver and gold- occur in their ores as elements. We need to separate the metal from sand and other impurities. It does not involve chemical reactions.
  2. The ores of all the other metals contain the metals as compounds. These have to be reduced to give the metal.
  3. The compounds of the more reactive metals are very stable and need to be electrolysed to reduce them. Though this is a powerful method, it costs a lot because it uses a lot of electricity.
  4. The compounds of the less reactive metals are less stable and can be reduced using a suitable reducing agent.

Extracting iron

The blast furnace

This diagram shows the blast furnace used for extracting iron from its ore. It is an oven shaped like a chimney at least 30 meters tall.

 

A mixture called the charge containing the iron ore is added through the top of the furnace. Hot air is blasted in through the bottom. After a series of reactions, liquid iron collects at the bottom of the furnace.

What’s in the charge?

The charge contains three things:

1. Iron ore

The chief ore of iron is called hematite. It is mainly iron oxide, Fe2O3, mixed with sand and some other compounds.

2. Limestone

This common rock is mainly calcium carbonate, CaCO3

3. Coke

This is made from coal and is almost pure carbon.

The reactions in the blast furnace

Stage 1:

The coke burns and gives heat. Carbon reacts with oxygen in the air giving carbon dioxide.

 

 

Stage 2: Carbon monoxide is made

The carbon dioxide reacts with more coke like this

 

 

Stage 3: the iron oxide is reduced

This is where the actual extraction occurs. Carbon monoxide reacts with the iron ore, giving liquid iron:

 

 

What is limestone for?

The limestone breaks down in the heat of furnace:

 

 

The calcium oxide that forms reacts with the sand which is called silica.

 

 

The calcium silicate forms a slag which runs down the furnace and floats on the iron. The iron down the blast furnace is called pig iron. It is impure. Carbon and sand are the main impurities. Some is run into moulds to make cast iron. This is hard but brittle because of its high carbon content. So it is used only for things like canisters for bottled gas and drain covers.

But most of the iron is turned into steels.

Extracting aluminium

Aluminium is the most abundant metal in the Earth’s crust. It main ore is bauxite which is aluminium oxide mixed with impurities such as sand and iron oxide.

The following are the steps in obtaining aluminium:

  1. First geologists test rocks and analyse the results, to find out how much bauxite is there. The mining begins if the tests are satisfactory.
  2. Bauxite is red brown in colour. It is usually near the surface so is easy to dig up.
  3. The ore is taken to a bauxite plant, where impurities are removed. The result is white aluminium oxide or alumina.
  4. The alumina is taken to another plant for electrolysis. It may even be sent to another country where electricity is cheaper.
  5. The extracted metal is made into sheets and blocks and sold to other industries.

Aluminium has many uses like making drink cans, food cartons, cooking foil, bikes, TV aerials, electricity cables, ships, trains and space rockets.

The electrolysis

 

The electrolysis is carried out in a large steel tank. This is lined with carbon which acts as the cathode. Huge blocks of carbon hang in the middle of the tank and act as the anode. Aluminium melts at 2045oC. It would be impossible to keep the tank that hot. Instead, the alumina is dissolved in molten cryolite or sodium aluminium fluoride which has much lower melting point.

The reactions at the electrodes

Once the alumina dissolves, its aluminium and oxide ions are free to move. They move to the electrode of opposite charge.

At the cathode:

The aluminium ions electrons:

 4Al3+ + 12e-     →       4Al    (reduction)

The aluminium drops to the bottom of the cell as molten metal. This is run off at intervals. Some will be mixed with other metals to make alloys. Some is run into moulds to harden into blocks.

At the anode:

The oxygen ions lose electrons:

            6O2-       →          3O2 + 12e-    (oxidation)

The oxygen gas bubbles off and reacts with the anode:

   C(s) +    O2 (g)       →       CO2 (g)   (oxidation of carbon)

So the carbon blocks get eaten away and need to be replaced.

The overall reaction

The alumina is broken down giving aluminium:

Aluminium oxide    →      aluminium + oxygen.

      2Al2O3 (l)     →     4Al (l)   +     3O2 (g)

Some properties of aluminium

  1. It is a bluish silver, shiny metal.
  2. It has a low density- it is light. Iron is the three times heavier.
  3. It is a good conductor of heat and electricity.
  4. It is malleable and ductile.
  5. It resists corrosion. This is because a fine coat of aluminium oxide forms on its surface and acts as a barrier
  6. It is not very strong when pure but it can be made much stronger by mixing it with other metals to form alloys.
  7. It is generally non-toxic. But it can harm humans if consumed in large quantities.

Making use of metals and alloys

  1. Aluminium is used for overhead electricity cables, cooking foil and food cartons, drink cans, coating CDs and DVDs.
  2. Copper is used for electrical wiring, saucepans and saucepan bases.
  3. Zinc is used for protecting steel for rusting, coating or galvanising iron and steel and for torch batteries.

Alloys

Metal becomes more useful when mixed with another substance. The mixture is called an alloy. Alloys have different properties. Also alloy may be more resistant to corrosion than the original metal was.

Brass is an alloy of copper and zinc. It is harder than copper and does not corrode. It also shines. It is used for door locks, keys, knobs, and musical instruments such as trumpets.

Steels and steel-making

Iron is very widely used metal. But pure form of iron is very soft and stretches quite easily. So mild steel and stainless steel which are alloys of iron are used for heavy purposes. Mild steel is formed by mixing a small amount of carbon with iron. This is hard and strong. It is used for buildings, ships, car bodies and machinery.

When nickel and chromium are mixed with iron, the result is stainless steel. This is hard and rustproof. It is used for cutlery, and equipment in chemical factories.

Making steels

Steels are made using the molten iron from the blast furnace.

Firstly unwanted impurities are removed from the iron.

  1. The molten iron from the blast furnace is poured into an oxygen furnace and a jet of oxygen is turned on.
  2. The oxygen reacts with the carbon forming carbon monoxide and carbon dioxide gases which are carried off. It reacts with other impurities to form acidic oxides.
  3. Then calcium oxide is added. It is a basic oxide. It reacts with silicon dioxide and the other acidic oxides to give a slag that is skimmed off.

Then other elements may be added. These are measured out carefully to give steels such as stainless with the required properties.