Please Limited time Offer!

Chemistry Notes on Metals: Copper, Zinc and lead.

a)Natural occurrence

Copper is found as uncombined element/metal on the earths crust in Zambia, Tanzania, USA and Canada .The chief ores of copper are:

          (i)Copper pyrites(CuFeS2)

          (ii)Malachite(CuCO3.Cu(OH)2)

          (iii)Cuprite(Cu2O)

b)Extraction of Copper from Copper Pyrites. Chemistry Notes

Copper pyrites are first crushed into fine powder. The powdered ore is the added water and oil. The purpose of water is to dissolve hydrophilic substances/particle. The purpose of oil is to make cover copper ore particle so as to make it hydrophobic

Air is blown through the mixture. Air creates bubbles that stick around hydrophobic copper ore. The air bubbles raise through buoyancy small hydrophobic copper ore particles to the surface. A concentrated ore floats at the top as froth. This is called froth flotation. The concentrated ore is then skimmed off.The ore is then roasted in air to form copper(I)sulphide ,sulphur(IV)oxide and iron (II) oxide.

2CuFeS2(s) + 4O2(g)                        Cu2S(s) + 3SO2(g)  + 2FeO(s)

Limestone (CaCO3) and silicon(IV)oxide (SiO2) are added and the mixture heated in absence of air.Silicon(IV)oxide (SiO2) reacts with iron (II) oxide to form Iron silicate which constitutes the slag and is removed.

     FeO(s)    +    SiO2(s)                               FeSiO3(s)

The slag separates off from the copper(I)sulphide. Copper(I)sulphide is then heated in a regulated supply of air where some of it is converted to copper (I) oxide.

2Cu2S (s)  +  3O2(g)                          2Cu2S(s)  +   2SO2(g)

The mixture then undergo self reduction in which copper(I)oxide is reduced by copper(I)sulphide to copper metal.

Cu2S (s)  +  2Cu2O (s)              6Cu (s)  +   SO2(g)

The copper obtained has Iron, sulphur and traces of silver and gold as impurities.It is therefore about 97.5% pure. It is refined by electrolysis/electrolytic method.

During the electrolysis of refining copper, the impure copper is made the anode and a small pure strip is made the cathode.

Electrode ionization takes place where:

At the anode;

                   Cu(s)                    Cu2+ (aq)   +    2e 

Note: Impure copper anode dissolves/erodes into solution and decreases in size.

At the Cathode;

                   Cu2+ (aq)   +    2e                   Cu(s) 

Note: The copper ions in the electrolyte(CuSO4) are reduced and deposited as copper metal at the cathode. The copper obtained is 99.98% pure.

Valuable traces of silver and gold collect at the bottom of the electrolytic cell as sludge. It is used to finance the extraction of copper pyrites.

d) Uses of copper

Copper is mainly used in:

(i)making low voltage electric cables,contact switches, cockets and plugs because it is a good conductor of electricity.

(ii)Making solder  because it is a good thermal conductor.

(iii)Making useful alloys e.g.

          -Brass is an alloy of copper and Zinc(Cu/Zn)

          -Bronze is an alloy of copper and Tin(Cu/Sn)

          -German silver is an alloy of copper ,Zinc and Nickel(Cu/Zn/Ni)

(iv)Making coins and ornaments.

e) Environmental effects of extracting copper from Copper Pyrites– Chemistry notes

(i)Sulphur(IV)oxide is a gas that has a pungent poisonous smell that causes head ache to human in high concentration.

(ii)Sulphur(IV)oxide  gas if allowed to escape dissolves in water /rivers/rain to form weak sulphuric(IV)acid lowering the pH of the water leading to marine pollution, accelerated corrosion/rusting of metals/roofs and breathing problems to human beings.

(iii)Copper is extracted by open caste mining leading to land /environmental /soil degradation.

f) Test for the presence of copper in an ore

Copper naturally exist in its compound as Cu2+ /Cu+

Copper (I) / Cu+ is readily oxidized to copper(II)/ Cu2+

If an ore is suspected to contain Cu2+ /Cu+ it is;

          (i)added hot concentrated sulphuric(VI)/Nitric(V)acid to free the ions present.

          (ii)the free ions are then added a precipitating reagent; 2M sodium hydroxide /2M aqueous ammonia which forms;

                   I) an insoluble blue precipitate in excess of 2M sodium hydroxide if Cu2+ ions are present.

                   I) an insoluble blue precipitate in 2M aqueous ammonia that dissolve to royal/deep blue solution in excess if Cu2+ ions are present.

ObservationInference
blue precipitate in excess 2M NaOH(aq)Cu2+
blue precipitate,dissolve to royal/deep blue solution in excess 2M NH3(aq)Cu2+

Sample questions on Chemistry Notes: Extraction pf Metals

Copper is extracted from copper pyrites as in the flow chart outlined below. Study it and answer the questions that follow

ZINC and LEAD

a)Natural occurrence

Zinc occurs mainly as:

(i)Calamine-Zinc carbonate(ZnCO3)

          (ii)Zinc blende-Zinc sulphide(ZnS)

Lead occurs mainly as Galena-Lead(II)Sulphide mixed with Zinc blende:

b)Extraction of Zinc/Lead  from Calamine ,Zinc blende and Galena.

During extraction of Zinc , the ore is first roasted in air:

For Calamine Zinc carbonate decompose to Zinc oxide and carbon(IV) oxide gas.

          ZnCO3(s)                       ZnO(s)        +       CO2(g)

Zinc blende does not decompose but reacts with air to form Zinc oxide and sulphur(IV) oxide gas.

 Galena as a useful impurity also reacts with air to form Lead(II) oxide and sulphur(IV) oxide gas.

               2ZnS(s)  + 3O2(g)                       2ZnO(s)      +       2SO2(g)

  (Zinc blende)

    2PbS(s)  + 3O2(g)                         2PbO(s)      +       2SO2(g)

    (Galena)

The oxides are mixed with coke and limestone/Iron(II)oxide/ Aluminium (III) oxide and heated in a blast furnace.

At the furnace temperatures limestone decomposes to quicklime/CaO and produce Carbon(IV)oxide gas.

          CaCO3(s)             CaO(s) + CO2 (g)

Carbon(IV)oxide gas reacts with more coke to form the Carbon(II)oxide gas.

          C(s)  +  CO2 (g)                               2CO (g)

Both Carbon(II)oxide and carbon/coke/carbon are reducing agents.

 The oxides are reduced to the metals by either coke or carbon (II)oxide.

          ZnO(s)        +       C(s)                     Zn(g)  + CO (g)

 PbO(s)       +       C(s)                     Pb(l)  + CO (g)

PbO(s)        +       CO(s)                  Pb(l)  + CO2 (g)

PbO(s)        +       CO(s)                  Pb(g)  + CO2 (g)

At the furnace temperature:

(i)Zinc is a gas/vapour and is collected at the top of the furnace. It is condensed in a spray of molten lead to prevent reoxidation to Zinc oxide. On further cooling , Zinc collects on the surface from where it can be tapped off

(ii)Lead is a liquid and is ale to trickle to the bottom of the furnace from where it is tapped off.

Quicklime/CaO, Iron(II)Oxide, Aluminium(III)oxide are used to remove silica/silicon(IV)oxide as silicates which float above Lead preventing its reoxidation back to Lead(II)Oxide.

          CaO(s) + SiO2(s)           CaSiO3(s/l)

                                                          (Slag-Calcium silicate)

          FeO(s) + SiO2(s)           FeSiO3(s/l)

                                                          (Slag-Iron silicate)

          Al2O3(s) + SiO2(s)                  Al2SiO4(s/l)

                                                          (Slag-Aluminium silicate)

e) Uses of Lead

 Lead is used in:

          (i)making gun-burettes.

 (ii)making protective clothes against nuclear (alpha rays/particle)     radiation in a nuclear reactor.

(iii)Mixed with tin(Sn) to make solder alloy

f) Uses of Zinc

  Zinc is used in:

          (i)Galvanization-when iron sheet is dipped in molten Zinc ,a thin layer of Zinc is formed on the surface.Since Zinc is more reactive than iron ,it reacts with elements of air(CO2/ O2 / H2O) to form basic Zinc carbonate(ZnCO3.Zn(OH)2).This sacrificial method protects iron from corrosion/rusting.

          (ii)As negative terminal and casing in dry/Laclanche cells.

          (iii)Making brass alloy with copper(Cu/Zn)

g) Chemistry: Environmental effects of extracting Zinc and Lead.

(i) Lead and Lead salts are carcinogenic/causes cancer

(ii)Carbon(IV)oxide is a green house gas that causes/accelerate global warming.

(iii)Carbon(II)oxide is a colourless odourless poisonous /toxic gas that combines with haemoglobin in the blood to form stable carboxyhaemoglobin reducing free haemoglobin leading to death.

(iv) Sulphur(IV)oxide is a gas that has a pungent poisonous smell that causes headache to human if in high concentration.

(v)Any leakages in Sulphur(IV)oxide gas escapes to the water bodies to  form weak sulphuric(VI)acid lowering the pH of the water. This causes marine pollution /death of aquatic life, accelerated rusting/corrosion of metals/roofs and breathing problems to human beings.

h) Test for presence of Zinc/ Lead.

If an ore is suspected to contain Zinc/Lead it is:

I.added hot concentrated Nitric(V)acid to free the ions present.

Note:

 Concentrated Sulphuric(VI)acid forms insoluble PbSO4 thus cannot be used to free the ions in Lead salts.

II.the free ions are then added a precipitating reagent mostly 2M sodium hydroxide or 2M aqueous ammonia with the formation of;

          (i)a soluble precipitate in excess of 2M sodium hydroxide if Zn2+, Pb2+, Al3+ions are present.

          (ii)a white precipitate that dissolves to form a colorless solution in excess 2M aqueous ammonia if Zn2+ions are present.

          (iii)an insoluble white precipitate in excess 2M aqueous ammonia if Pb2+,  Al3+ions are present.

(iv) Pb2+ ions form a white precipitate when any soluble SO42-, SO32-, CO32-, Cl, is added while Al3+ ions do not form a white precipitate

          (v) Pb2+ ions form a yellow precipitate when any soluble I(e.g. Potassium/sodium Iodide) is added while Al3+ ions do not form a yellow precipitate.

        (vi) Pb2+ ions form a black precipitate when any soluble S(e.g. Potassium/sodium sulphide) is added while Al3+ ions do not form a black precipitate.i.e;

ObservationInference
White precipitate in excess 2M NaOH (aq)Zn2+, Pb2+, Al3+ ions
White precipitate that dissolves to form a colourless solution in excess 2M NH3(aq)Zn2+ ions
White precipitate in excess 2M NH3(aq)Pb2+, Al3+ ions
White precipitate on adding about 4 drops of either  Na2CO3(aq), Na2SO3(aq), Na2SO4(aq), H2SO4(aq), HCl(aq), NaCl(aq)Pb2+ions
Yellow precipitate on adding about 4 drops of of KI(aq).NaI (aq)Pb2+ ions
Black precipitate on adding aout 4 drops of Na2S(aq)/K2S(aq)Pb2+ ions

GENERAL SUMMARY OF METALS– Chemistry Notes

a) Summary methods of extracting metal from their ore

Add oil, water, and blow air to form froth to concentrate the ore if it is a low grade
If near the surface use open cast mining / quarrying
Position on the earth’s crust

The main criteria used in extraction of metals is based on its position in the electrochemical/reactivity series and its occurrence on the earth’s crust.

GENERAL SUMMARY OF METALS

a) Summary methods of extracting metal from their ore

Add oil, water, and blow air to form froth to concentrate the ore if it is a low grade
If near the surface use open cast mining / quarrying
Position on the earth’s crust

The main criteria used in extraction of metals is based on its position in the electrochemical/reactivity series and its occurrence on the earth’s crust.