Metallurgy

(i) A : Bauxite and C : Cryolite are ores of aluminium. E : Sodium hydroxide solution is used in the dressing of the ore.

(ii) Fill in the blanks :

(a) The ore from which aluminium is extracted must first be treated with sodium hydroxide solution so that pure Aluminium oxide can be obtained.

(b) Pure Aluminium oxide is dissolved in Cryolite to make a conducting solution

(iii) Na₃AlF₆ — formula of Cryolite

(i) Iron and Chromium are used to make stainless steel

(ii) Copper and Zinc are used to make brass.

(i) Mercury is liquid at room temperature.

(ii) Cryolite [Na₃AlF₆] is added to lower the fusion temp, of the electrolytic bath in the extraction of Al

(iii) Roasting is the process of heating an ore to a high temperature in the presence of air.

Silver Nitrate (AgNO₃)
Reason — Copper displaces silver from AgNO₃ soln. forming the blue copper [II] nitrate soln.

it loses electrons and is oxidized

Reason — metals readily give the valence electron during reactions and are hence oxidised.

(i) Sodium hydroxide is used to react with bauxite as a first step in obtaining pure aluminium oxide.

(ii) Conversion of Al(OH)₃ - to pure alumina by heating Al(OH)₃

$\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100^{\circ}\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}$

(iii) Element of Anode and Cathode are:

• Cathode — Carbon lining [gas carbon] of the cell.
• Anode — Thick carbon rods (graphite) attached to copper clamps dipping into fused electrolyte.

(iv) 2Al³⁺ + 6e^- ⟶ 2Al

(v) Al - 3e^- ⟶ Al³⁺ (Hoope's process - Electrorefining of Aluminium)

Copper and zinc

(i) Electrodes A and B are made of graphite (carbon).

(ii) Aluminium is formed at cathode (i.e., electrode A).

(iii) Fused alumina (Al₂O₃) and Cryolite (Na₃AlF₆) are the compounds in the electrolyte C.

(iv) The graphite [carbon] anodes (i.e electrode B) are continuously replaced during the electrolysis because :

1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. 2C + O₂ ⟶ 2CO [2CO + O₂ ⟶ 2CO₂]
3. The carbon anode is hence consumed and renewed periodically after a certain period of usage.

Aluminium oxide

Reason — Amphoteric oxides like aluminium oxide react with acids as well as alkalis giving salt and water.

Haematite is the chief ore of iron.

Or

Bauxite is the chief ore of aluminium.

(i) A is cathode and B is anode.

(ii) Mixture of cryolite, BaF₂, AlF, CaF₂ serves as the electrolyte

(iii) Pure molten aluminium with carbon electrodes serves as the cathode.

Aluminium is a strong reducing agent and has a strong affinity for oxygen. It reduces Fe₂O₃, to Fe which fills the gap. Hence it is used for thermite (Fe₂O₃) welding.

(i) Aluminium

(ii) Copper

(iii) Iron

Non-polar covalent compounds are formed from metals.

Reason — A non-polar covalent compound is formed when electrons are shared equally between two atoms and metals generally donate electrons and form bonds. Hence, they do not form non-polar covalent compounds

Bronze is an alloy of zinc, copper and tin.

1. Aluminium is found abundantly in the earth's crust.

2. Difference between calcination and roasting is as follows:

3. Froth floatation process

4. Chemical formula of ores are :
   Haematite ⟶ Fe₂O₃ : Ore of iron
   Bauxite ⟶ Al₂O₃.2H₂O : Ore of aluminium

5. Constituents of electrolyte are :
   Fused Alumina (Al₂O₃) — 1 part by weight.
   Cryolite (Na₃AlF₆) — 3 part by weight.
   Fluorspar (CaF₂) —1 part by weight.

Calcium

Reason — It belongs to the group II A of the periodic table, hence it is an alkaline earth metal.

Magnesium oxide

Reason — They are most electropositive and have strong affinity for oxygen. Hence, they cannot be reduced by normal reducing agents

1. The alloy contains Cu and Zn, is hard and is used in decorative articles. — (B) Brass
2. It is stronger than Aluminium, light and is used in making light tools. — (A) Duralumin
3. It is lustrous, hard, corrosion resistant and used in surgical instruments. — (D) Stainless steel
4. Tin lowers the melting point of the alloy and is used for soldering purpose. — (E) Solder
5. The alloy is hard, brittle, take up polish and is used for making statues. — (C) Bronze

Aluminium

(i) Bauxite is reacted with a conc. solution of NaOH (caustic alkali) under pressure for 2 hrs as a first step in obtaining Al₂O₃.
The impurities present in bauxite mainly Fe₂O₃ and SiO₂ remain unaffected with conc. NaOH as impurities are not amphoteric.
Bauxite, being amphoteric reacts with the base forming salt [sodium aluminate] and water. Hence, impurities are separated out. Therefore, caustic alkali is added to bauxite ore during purification of bauxite.

(ii) Balanced chemical equation for the conversion of impure bauxite to sodium aluminate is given below:

$\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 ^{\circ}\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}$

(iii) Fluorspar is added to the electrolyte mixture as it lowers the fusion point of the mixture i.e., the mixture fuses around 950°C instead of 2050°C.

Zinc oxide is an amphoteric oxide. It reacts with acids and alkalis giving salt and water.

Bronze

(i) Y is the metallic element.

(ii) Metal atoms tend to have a maximum of three electrons in the outermost energy level.

(iii) Non-metallic elements tend to form acidic oxides while metals tend to form basic oxides.

(iv) Non-metallic elements tend to be bad conductors of heat and electricity.

(v) Metals tend to lose electrons and act as reducing agents in their reactions with elements and compounds.

(i) The compound is cryolite [Na₃AlF₆]. It —

1. lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(ii) 2Al³⁺ + 6e^- ⟶ 2Al

(iii) The anodes are continuously replaced during the electrolysis because :

1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. 2C + O₂ ⟶ 2CO [2CO + O₂ ⟶ 2CO₂]
3. The carbon anode is hence consumed and renewed periodically after a certain period of usage.

Calcination

it is strong reducing agent.

Reason — Aluminium is a strong reducing agent, it reduces Fe₂O₃, to Fe which fills the gap. Hence it is used for thermite (Fe₂O₃) welding.

(i) Brass — Cu [60-80%], Zn [40-20%]

(ii) Duralumin — Al [95%], Mg [0.5%], Mn [0.5%], Cu [4%]

(iii) Bronze — Cu [80%], Zn [1%], Sn [19%]

(i) Malleability

(ii) Cryolite (Na₃AlF₆)

(iii) Zinc Blende (ZnS).

Solder

Reason — Solder is an alloy of lead and tin.

Metals like Zinc are less electropositive and have less affinity for oxygen, hence their oxides like ZnO can be reduced by reducing agents like CO. Metal like Aluminium on the other hand are most electropositive and have strong affinity for oxygen. Hence, their oxides like aluminium oxide cannot be reduced by a reducing agent.

(i) Na₂O

(ii) Al₂O₃

3X - 6e^- ⟶ 3X²⁺

2Y + 6e^- ⟶ 2Y^3-

1. Addition of Cryolite :
   (i) lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
   (ii) enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
   (iii) enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.
2. Sodium hydroxide is added to bauxite ore during purification of bauxite. Bauxite is reacted with a conc. solution of NaOH under pressure for 2 hrs as a first step in obtaining Al₂O₃. The impurities present in bauxite mainly Fe₂O₃ and SiO₂ remain unaffected with conc. NaOH as impurities are not amphoteric. Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.
3. Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode.

The layer of powdered coke is sprinkled over the electrolytic mixture as :

1. it prevents burning of carbon electrodes in air at the emergence point from the bath.
2. it minimizes or prevents heat loss by radiation.

Metals are good reducing agents because they are electron donors.

Copper and tin

Froth flotation process

(i) Sodium hydroxide

(ii) Conversion of Al(OH)₃ - to pure alumina - heat on Al(OH)₃

$\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100^{\circ}\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}$

(iii) Cryolite (Na₃AlF₆)

(iv) 2Al³⁺ + 6e^- ⟶ 2Al

(v) The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. [2C + O₂ ⟶ 2CO; 2CO + O₂ ⟶ 2CO₂] The carbon anode is hence consumed and renewed periodically after a certain period of usage. It is therefore preferable to use a number of graphite electrodes as anode instead of a single electrode.

(i) Pulverisation

(ii) Calcination

(i) Solder [fuse metal]

(ii) Zinc blende [ZnS]

(iii) Copper oxide (CuO)

(i) Cryolite and fluorspar are components of the electrolyte. Addition of cryolite enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity. Addition of both cryolite & fluorspar lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.

(ii) The layer of powdered coke is sprinkled over the electrolytic mixture as :

1. it prevents burning of carbon electrodes in air at the emergence point from the bath.
2. it minimizes or prevents heat loss by radiation.

(i) The metal whose oxide, which is amphoteric, is reduced to metal by carbon reduction — Pb.

(ii) The divalent metal whose oxide is reduced to metal by electrolysis of it's fused salt is — Mg

Alkali metals have one electron in the outermost shell and hence during reactions they readily give the valence electron and get themselves oxidised. Hence, they are good reducing agents.

Froth flotation process

Reason — Sulphide ores like ZnS and PbS are lighter than the impurities present. They are concentrated by froth flotation process.

The most common ore of iron is haematite

(i) Brass — Copper & Zinc

(ii) Duralumin — Aluminium & Copper.

Bauxite [Al₂O₃.2H₂O]

Baeyer's Process

(i) At cathode : 2Al³⁺ + 6e^- ⟶ 2Al

(ii) Conversion of Al(OH)₃ - to pure alumina - heat on Al(OH)₃

$\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100^{\circ}\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}$

Aluminium brings lightness

$\text{CuO} + \text{H}_2 \xrightarrow{\Delta} \text{Cu} + \text{H}_2\text{O} [\text{g}]$

Calcination

Pb < Zn < Ca < K

(i) Al₂O₃.2H₂O — Bauxite

(ii) Na₃AlF₆ — Cryolite

(iii) NaAlO₂— Sodium aluminate

(i) Addition of cryolite —

1. lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(ii) The layer of powdered coke is sprinkled over the electrolytic mixture as :

1. it prevents burning of carbon electrodes in air at the emergence point from the bath.
2. it minimizes or prevents heat loss by radiation.

(iii) Cathode

Oxygen gas

The ore is wetted by the oil and gets separated from the gangue in the form of froth. The impurities gets wetted by water and remain behind in the sink.
Since the ore is lighter, it comes on the surface with the froth and impurities are left behind.

Alloy

An alloy used to make statues is bronze.

Conversion of impure bauxite to sodium aluminate:

$\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 ^{\circ}\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}$

Conversion of sodium aluminate to aluminium hydroxide:

$\underset{\text{[sodium aluminate]}}{\text{NaAlO}_2} + 2\text{H}_2\text{O} \xrightarrow{50 - 60 ^{\circ}\text{C}} \underset{\text{[sodium hydroxide]}}{\text{NaOH}} + \underset{\text{[aluminium hydroxide]}}{\text{Al(OH)}_3}$

Conversion of Al(OH)₃ - to pure alumina - heat on Al(OH)₃:

$\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100^{\circ}\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}$

Precipitated Al(OH)₃ is ignited to give pure alumina [Al₂O₃]

(i) Aluminium

(ii) Fluorine

Physical and chemical property differences between metals and non-metals:
Metal atoms have larger atomic size so their valence electrons are held less tightly by the nucleus. This allows the valence electrons to move freely making metals good conductors of heat and electricity. Metals have 1, 2 or 3 valence electrons and tend to lose one or more electrons when supplied with energy and form cations. Thus, they are electropositive in nature and strong reducing agents.

Non-metal atoms have relatively smaller atomic size. So, their valence electrons are held tightly by the nucleus, they are not free to move. This makes non-metals bad conductors of heat and electricity. Non-metals have 5, 6 or 7 electrons in the valence shells and tend to gain electrons to form anions. Thus, non-metals are electronegative in nature and strong oxidizing agents.

Definition of Metal:
A Metal is an element which ionizes by losing electrons when energy is supplied to it and is hence an electron donor. Metals have 1, 2 or 3 valence electrons (positive valency of 1, 2 or 3) and tend to form positive ions (cations) on ionization. Thus, Metals form electrovalent compounds. Metals form at least one basic oxide. For example — K₂O, Na₂O, CaO, MgO, CuO, etc.

Position [group] in the periodic table:

(i) Na — alkali metal — Group I (IA)

(ii) Mg — alkaline earth metal — Group 2 (IIA)

(iii) Fe and Zn transition elements — Group 8 (VIII), Group 12 (IIB)

(iv) Inner transition elements — Group 3(IIIB) but are shown separately as f-block of the periodic table.

(v) Al — post transition element — Group 13 (III A)

Gold and Platinum occur in the free metallic form.

The metallic compounds in combined form are :

(i) halides — Cryolite[Na₃AlF₆], Fluorspar[CaF₂]

(ii) oxides — Bauxite[AlO₃.2H₂O]

(iii) sulphide — Iron Pyrite[FeS₂], Zinc blende[ZnS]

(i) Difference between mineral & ore

(i) Difference between matrix and flux

Dressing of the ore converts the impure ore to pure concentrated ore. Hence, it is an essential process in the extraction of metal from it's ore.

Magnetic separation process is used to separate magnetic ore from non-magnetic gangue or magnetic gangue from non-magnetic ore. Electromagnetic wheel is used in magnetic separation so that magnetic particles get attracted to the magnetic wheel and separate from the non-magnetic particles.

As the ore is lighter, it comes on the surface with the froth. Gangue is heavier so it settles down.

In bauxite ore, the gangue minerals are fine grained and intimately locked with the ore making magnetic separation extremely difficult. On the other hand, Aluminium is highly reactive metal. Dressing of bauxite ore through chemical leaching by Baeyer's process is much easier. Hence, magnetic separation is not used during the dressing of bauxite ore in the extraction of aluminium.

The step is not necessary as the main ore of Aluminium is Bauxite (Al₂O₃.2H₂O) and it is already an oxide.

Calcination is carried out on carbonates and hydrated ores, where water of hydration and other volatile impurities get removed. Calcination of carbonate ores gives out carbon dioxide. Hence, the ore is heated in absence of air. On the other hand, sulphide ores are generally roasted so that sulphur present as impurity in the ore is given out as sulphur dioxide. Hence, roasting is carried out in the presence of excess air.

Generally, sulphide ores are roasted, so SO₂ is given off. Calcination is carried on carbonates and hydrated ores , so CO₂ or water vapours is given off.

Activity series is a series of metals arranged according to their decreasing reactivity.

The metal at the top of the series

1. is most easily oxidized
2. is most electropositive and most reactive
3. can displace the metal below from it's salt soln.

Due to these reasons, the extraction of metals from the ores is based on the position of the metal in the activity series.

Metals from K to Al, are most electropositive, have strong affinity for oxygen hence, they cannot be reduced by reducing agents like coke. They can be reduced by electrolysis only.

Mercury is least electropositive and have least affinity for oxygen hence, its oxide can be reduced by heat alone whereas Zinc is more electropositive than Mercury and have more affinity for oxygen as compared to mercury hence, reduction of the oxide of zinc to it's metal can be done by coke.

Conversion of impure bauxite to sodium aluminate:

$\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 ^{\circ}\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}$

Conversion of sodium aluminate to aluminium hydroxide:

$\underset{\text{[sodium aluminate]}}{\text{NaAlO}_2} + 2\text{H}_2\text{O} \xrightarrow{50 - 60 ^{\circ}\text{C}} \underset{\text{[sodium hydroxide]}}{\text{NaOH}} + \underset{\text{[aluminium hydroxide]}}{\text{Al(OH)}_3}$

Conversion of Al(OH)₃ - to pure alumina - heat on Al(OH)₃:

$\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100^{\circ}\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}$

In the electrolytic reduction of pure alumina to pure aluminium by Hall Herault's process, the constitutents of electrolyte are cryolite (Na₃AlF₆) and fluorspar (CaF₂) and alumina (Al₂O₃)

The electrolytic reactions are as follows :

Cryolite : Na₃AlF₆ ⇌ 3Na¹⁺ + Al³⁺ + 6F^1-

Fluorspar : CaF₂ ⇌ Ca²⁺ + 2F^1-

Alumina : Al₂O₃ ⇌ 2Al³⁺ + 3O^2-

At cathode : 2Al³⁺ + 6e^- ⟶ 2Al

At anode : 3O^2- - 6e^- ⟶ 3[O] ⟶ 3O₂

Al³⁺ are discharged in preference to Na¹⁺ and Ca²⁺ ions due to it's lower position in the electrochemical series.

(i) Function of Sodium hydroxide — Sodium hydroxide is added to bauxite ore during purification of bauxite. Bauxite is reacted with a conc. solution of Sodium hydroxide under pressure for 2 hours as a first step in obtaining Al₂O₃.

The impurities present in bauxite mainly Fe₂O₃ and SiO₂ remain unaffected with conc. NaOH as impurities are not amphoteric.

Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.

$\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 ^{\circ}\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}$

(ii) Addition of Cryolite & Fluorspar

1. Lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. Enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. Addition of Cryolite enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(i) Fused alumina has a melting point around 2050°C. Such high temperatures, create the below difficulties:

1. A large amount of electrical energy is required to liberate Al₂O₃ and maintain the electrolyte in molten state at that temperature.
2. The liberated Al metal [m.p. 660°C] may also tend to volatilize out and get wasted.

Hence the fusion temperature of the electrolytic mixture has to be lowered.

(ii) The reasons for addition of mainly Cryolite [in a higher ratio] and Fluorspar to the electrolytic mixture are:

1. They lower the fusion point of the mixture i.e., the mixture fuses [melts] around 950°C instead of 2050°C.
2. They enhance the mobility of the fused mixture by acting as a solvent for the electrolytic mixture. Thus cryolite in the molten state of subdivision dissolves aluminium oxide.
3. Addition of cryolite enhances the conductivity of the mixture. Since pure alumina is almost a non-conductor of electricity.

(iii) The layer of powdered coke is sprinkled over the electrolytic mixture as:

1. It prevents burning of carbon electrodes in air at the emergence point from the bath.
2. It minimizes or prevents heat loss by radiation.

(iv) The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. [2C + O₂ ⟶ 2CO; 2CO + O₂ ⟶ 2CO₂] The carbon anode is hence consumed and renewed periodically after a certain period of usage. It is therefore preferable to use a number of graphite electrodes as anode instead of a single electrode.

(a) Alloy — Alloy is a substance prepared by adding other metals [or non-metals] to a base metal in appropriate proportions to obtain certain desirable properties. It is usually made by melting the components together and solidifying the mixture.

(b) Amalgam — Amalgam is an alloy in which the base metal is mercury. Examples are Na/Hg [liquid amalgam], Zn/Hg amalgam [used in voltaic cells].

1. X is a metal and will form electrovalent compounds only.

2. X³⁺ formed is a cation and element 'X' has a valency of +3.

3. If X³⁺ combines with oxygen the formula of the product is X₂O₃.

4. If the above product in the molten state is electrolysed the ion X³⁺ will get discharged at the cathode.

5. The ion X³⁺ accepts electrons and gets reduced to neutral atoms.

Al₂O₃

Magnetite

Hydrated aluminium oxide

Froth flotation method

HgO

1. A → Oxygen
2. B → Carbon
3. C → Hydrogen
4. D → Sodium hydroxide
5. E → H₂O

(i) Spathic Iron Ore (Iron [II] Carbonate, FeCO₃)

(ii) CuO

(iii) Aluminium hydroxide

(iv) Carbon (Graphite)

(v) Copper (Cu)