Tin and Copper

Tin

Tin does not occur naturally as a free element. The main source is the mineral cassiterite or tin stone, SnO_2. In Nigeria was mined in 1930 at Jos Plateau.

Extraction from its Ores

Tin ore  is crushed and washed with water. This is called the concentration of the ore. The tin ore is roasted in air to remove impurities, such as arsenic and antimony as volatile oxides. The product is mixed with powdered charcoal and heated to 1300^oC. to reduce the oxide. Molten tin is tapped off. Iron compounds, which might be present as impurities are removed by electromagnetic separation.

The metal is extracted from its ore by carbon reduction. The concentrated ore is mixed with coke and heated in a furnace.

SnO_2(s) + 2C_(s) → Sn_(l) + 2CO_(g)

The tin obtained is purified. It is separated from copper, iron and any other element present as impurities by either thermal -heating beyond it melting point of 232 K, and running off the molten tin, leaving behind any less fusible impurities – or by electrolytic means.

Much purer tin is obtained by the electrolysis of aqueous solution of tin(II) chloride, SnCl₂ – the impure tin is made anode, while the cathode is pure tin.

Test for Tin ions

Hydrogen Sulphide – Pass hydrogen sulphide into a solution of the unknown salt acidified with dilute hydrochloric acid. Tin(ii) ions are present if a brown precipitate which dissolves in yellow ammonium sulphide and in hot concentrated hydrochloric acid is obtained

Mercury (ii) chloride- The formation of a white precipitate of mercury(i) chloride would indicate the presence of tin(ii) ions.

Physical Properties of Tin

1. Tin is solid, with silvery white appearance with lustre.
2. It’s melting point is 232^oC.
3. It is malleable and soft (enough to be cut with a knife).
4. It is a good conductor of heat and electricity.
5. It exists in three different forms. These are grey tin of density 5.76 g/cm³; white tin of density 7.28 g/cm³; and rhombic tin of density 6.6 g/cm³. These allotropes (the different forms) can be converted from one to another by changes in temperature.

grey white rhombic

1. Tin is not ductile enough to be drawn into wires.

Chemical properties of Tin

1. Reaction with Oxygen – it does not react with oxygen, except at temperature above 1300^oC.

Sn_(s) + O_2(g) → SnO_2(s)  at temperatures greater than 1300^oC

Hence, It does not corrode or explode in air.

1. With nitrogen and carbon – no reaction occurs.
2. With non metals, example, chlorine – it reacts when heated with chlorine to form tin(IV) chloride.

Sn_(s) + 2Cl_2(g) → SnCl_4(s)

1. With acids – it reacts with acids to different degrees at different concentrations and temperatures. With dilute HCl – no reaction occurs.

With Concentrated HCl the reaction is rapid, producing tin(II) chloride.

Sn_(s) + 2HCl_(aq) → SnCl_2(s) + H_2(g)

With dilute H₂SO₄ – no reaction occurs. With hot concentrated H₂SO₄ a reaction occurs to release SO₂.

Sn_(s) + 2H₂SO₂ → SnSO_4(aq) + SO_2(g) + 2H₂O_(g)

With dilute HNO₃ – tin reacts with dilute HNO₃, forming Sn(NO₃)₂ and hydrogen.

With conc. HNO₃ – tin reacts with conc. HNO₃ , forming SnO₂.

1. With alkalis – it forms trioxostannate(IV) salts and hydrogen with conc. solutions of alkalis.

Sn_(s) + 2NaOH_(aq) + H₂O_(l) → Na₂SnO_3(aq) + 2H_2(g)

Uses of Tin

There are a number of ways tin can be used. These include:

1. For coating steel – the coating is done by electrolytic method. Tin prevents corrosion in steel, such may be used to can food and drinks.
2. For making alloys together with lead, antimony and copper. Alloys of tin are important, such as soft solder, pewter, bronze and phosphor bronze. A niobium-tin alloy is used for superconducting magnets.
3. Due to its resistance to atmospheric corrosion and low melting point, it can be used to make sheet glass.
4. The most important tin salt used is tin(II) chloride, which is used as a reducing agent and as a mordant for dyeing calico and silk. Tin(IV) oxide is used for ceramics and gas sensors. Zinc stannate (Zn₂SnO₄) is a fire-retardant used in plastics.
5. Some tin compounds have been used as anti-fouling paint for ships and boats, to prevent barnacles. However, even at low levels these compounds are deadly to marine life, especially oysters. Its use has now been banned in most countries.
6. A very important application of tin is tin-plating. Tin-plating is the process by which a thin coat of tin is placed on the surface of steel, iron, or another metal. Tin is not affected by air, oxygen, water, acids, and bases to the extent that steel, iron, and other metals are. So the tin coating acts as a protective layer.
7. Another tin alloy is Babbitt metal. Babbitt metal is a soft alloy made of any number of metals, including arsenic, cadmium, lead, or tin. Babbitt metal is used to make ball bearings for large industrial machinery. The Babbitt metal is laid down as a thin coating on heavier metal, such as iron or steel. The Babbitt metal retains a thin layer of lubricating oil more efficiently than iron or steel.

Copper

Copper was one of the first metals discovered and used by man. It is a stable metal readily obtained from its compounds. Copper ores are widely found around the world. The main ores are copper pyrites (CuFeS2), malachite (CuCO3.Cu (OH)2), chalcocite (CuS2) and cuprite (CuO).

Extracting copper from its ores

The method used to extract copper from its ores depends on the nature of the ore. Sulphide ores such as chalcopyrite (copper pyrites) are converted to copper by a different method from silicate, carbonate or sulphate ores.

The process:

The concentrated ore is heated strongly with silicon dioxide (silica) and air or oxygen in a furnace or series of furnaces.

• The copper(II) ions in the chalcopyrite are reduced to copper(I) sulphide (which is reduced further to copper metal in the final stage).
• The iron in the chalcopyrite ends up converted into an iron(II) silicate slag which is removed.
• Most of the sulphur in the chalcopyrite turns into sulphur dioxide gas. This is used to make sulphuric acid via the Contact Process.

An overall equation for this series of steps is:

The end product of this is called blister copper – a porous brittle form of copper, about 98 – 99.5% pure.

Purification of copper

When copper is made from sulphide ores by the first method above, it is impure. The blister copper is first treated to remove any remaining sulphur (trapped as bubbles of sulphur dioxide in the copper – hence “blister copper”) and then cast into anodes for refining using electrolysis.

Electrolytic refining

The purification uses an electrolyte of copper(II) sulphate solution, impure copper anodes, and strips of high purity copper for the cathodes.

The diagram shows a very simplified view of a cell.

For every copper ion that is deposited at the cathode, in principle another one goes into solution at the anode. The concentration of the solution should stay the same.

All that happens is that there is a transfer of copper from the anode to the cathode. The cathode gets bigger as more and more pure copper is deposited; the anode gradually disappears.

In practice, it isn’t quite as simple as that because of the impurities involved.

Physical properties of copper

1. Copper is a heavy, reddish-brown metal
2. It is very malleable and ductile
3. It has a density of 8.95 g cm^-1
4. It is a good conductor of heat and electricity
5. It has a high melting point of 1083^oC
6. It has a boiling point of 2300^oC
7. It also forms alloys very readily

Chemical properties of Copper

• Reaction with Air: It is resistant to pure dry air, but over a long period of time in a moist, impure atmosphere, it becomes coated with green, basic copper(II) tetraoxosulphate (VI) CuSO₄.3Cu(OH)₂ and trioxocarbonate (IV).

On heating in air or oxygen, copper is readily oxidized to give black copper (II)oxide

2Cu(s) + O₂(g) ——–> 2CuO(s)

• Effect of Acids: Copper is lower than hydrogen in the electrochemical series, hence, it is not capable of displacing hydrogen from dilute acids. It is however attacked by oxidizing acids like trioxonitrate (V) acid and tetraoxosulphate (VI) acids

3Cu(s) + 8HNO₃(aq) ——> 3Cu(NO₃)₂(s) + 4H₂O(l) 2NO(g)

• Displacement Reaction: Because of its low position in the activity and electrochemical series, copper is easily displaced from its compound

CuSO₄(aq) + Fe(s) ——-> FeSO₄(aq) + Cu(s)

• Hydrogen gas reduces copper oxides to the metal

CuO(s) + H₂(g) ——> Cu(s) + H₂O(l)

Test For Copper(II) Ions

With sodium hydroxide: Add a few drops of sodium hydroxide solution to a solution of copper salt. The formation of a blue precipitate which is insoluble in excess sodium hydroxide confirms the presence of copper(II) ions

Cu²⁺(aq) + 2NaOH(aq) ——-> Cu(OH)₂(s) + Na⁺(aq)

Uses of Copper

Amongst other things copper is used for:

1. Electrical wiring. It is a very good conductor of electricity and is easily drawn out into wires.
2. Domestic plumbing. It doesn’t react with water, and is easily bent into shape.
3. Boilers and heat exchangers. It is a good conductor of heat and doesn’t react with water.
4. Baking brass. Brass is a copper-zinc alloy. Alloying produces a metal harder than either copper or zinc individually. Bronze is another copper alloy – this time with tin.
5. Coinage. In the UK, as well as the more obvious copper-coloured coins, “silver” coins are also copper alloys – this time with nickel. These are known as cupronickel alloys. UK pound coins and the gold-coloured bits of euro coins are copper-zinc-nickel alloys.

ASSESSMENT (POST ANSWERS BELOW USING THE BOX)

1. Tin exists in 3 different forms. Mention them?
2. How can Tin be extracted?
3. The method of extraction of copper depends on what?
4. Mention 4 uses of copper