manojsir-science blog: METALS - III : LONG ANSWERS

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Write a note on minerals in India.

ANS : [table to be posted]

Explain : Froth Floatation Process OR The process to concentrate sulphide ores.

ANS : This method is used for the concentration of sulphide ores of copper, zinc, lead, etc.

PRINCIPLE: This method is based on the principle of difference in the wetting properties of the ore and the gangue particles with water and oil.

PROCESS:

The powdered ore is mixed with water, containing small quantity of oil (pine oil or turpentine oil) in a large tank.
The sulphide particles in the ore get wet with oil while dust and sand particles do not get wet with oil.
The water containing ore is agitated violently by blowing air to form froth.
Sulphide particles being lighter float along with the froth at the surface which is removed from the top of the tank and collected.
Impurities being heavier collect at the bottom. The froth is then washed, filtered and dried.

Write a note on Magnetic Separation.

ANS :

PRINCIPLE:This method uses the principle of difference in the magnetic properties of the ore and the gangue (impurities).

EQUIPMENT:

A magnetic separator consists of a leather conveyer belt moving over two rollers. One of the rollers has a strong magnet in it.

PROCESS: Powdered ore is dropped on the moving belt at one end through a hopper. When it reaches the other end , nonmagnetic impurities fall down first whereas the magnetic particles fall later forming a separate heap.

This method is generally used for the concentration of iron ores.

Explain : Calcination with proper examples.

ANS : Calcination is a process in which concentrated ore is heated strongly in absence of air. Calcination is generally used
for conversion of metal carbonates and hydroxides in to their corresponding oxides and to remove volatile impurities.

Examples:

Write a note on 'Chemical Reduction' method.

ANS : The conversion of metal oxide into metal is called REDUCTION. When chemicals like carbon, carbon monoxide, aluminium, etc. are used as reducing agents, the process is called CHEMICAL REDUCTION.
In the carbon-reduction process, the metal oxide is mixed with coke (a form of carbon) and heated in a furnace. Carbon reduces metal oxide to free metal. Oxides of zinc, iron, copper, nickel, tin and lead are reduced by this method.

    ZnO_(s) + C_(s) → Zn_(s) + CO_(g)

    Fe₂O_3(s) + 3C → 2Fe_(s) + 3CO_(g)

    MnO_2(s) + 2C_(s) → Mn_(s) + 2CO_(g)

Carbon monoxide is also used as reducing agent.

    Fe₂O_3(s) + 3CO_(g) → 2Fe_(s) + 3CO_2(g)

Aluminium is used as reducing agent for the reduction of certain oxides of metals like chromium, manganese and iron. A large amount of heat is produced during the process and the metal is obtained in liquid (molten) state. The process is known as THERMIT PROCESS.

    Cr₂O_3(s) + 2Al_(s) → 2Cr_(l) +Al₂O_3(s)

    3MnO_2(s) + 4Al_(s) → 3Mn_(l) + 2Al₂O_3(s)

    Fe₂O_3(s) + 2Al_(s) → 2Fe_(l) + Al₂O_3(s)

Chemical reduction method cannot be used for highly reactive metals like sodium, potassium, aluminium, etc. Oxides of such metals are reduced by electrolytic reduction method.

Explain Electrochemical Reduction giving example.

ANS : Oxides of highly reactive metals like sodium, potassium and aluminium cannot be reduced by using chemical reduction ( by carbon or aluminium). If carbon is used large amount of heat is required and metal carbide is formed. These metals are obtained by electrolysis of their fused (molten) salts. The cathode acts as the reducing agent as it supplies electrons to metal ions.

ELECTROLYSIS OF SODIUM CHLORIDE
    In this process molten sodium chloride is taken as electrolyte in an electrolytic cell. The reactions taking place at the two electrodes are :
NaCl_(l) → Na⁺ + Cl^-

Na⁺ + e^- → Na_(s)
(at cathode)

Cl^- → Cl + e^-
(at anode)

Cl + Cl → Cl_2(g)

    If aqueous solution of sodium chloride is taken instead of molten sodium chloride, reduction of hydrogen ions (2H⁺) to hydrogen (H₂ gas) takes place instead of reduction of sodium ion(Na⁺) to sodium metal.

Explain : Reactivity Series of Metals.

ANS : The series of metals in the decreasing order of their reactivities is called reactivity series of metals. Some metals like sodium and potassium are highly reactive whereas metals like copper, silver and gold have very low reactivity.

The reactivities of metals can be compared by their reaction with dilute acid or by comparing their oxidation potential with the standard hydrogen potential. Based on their reaction with dilute acids the following series has been prepared :

K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au

The metals on the left of the hydrogen in the series are more reactive and they can displace hydrogen from acids. Metals on the right side of hydrogen are less reactive and they cannot liberate hydrogen from dilute aqueous acids.

similarly, a more reactive metal can displace a less reactive metal from its solution. Thus, a strip of more reactive zinc displaces less reactive copper from aqueous copper sulphate solution (Displacement reaction). The blue colour of copper sulphate solution disappears gradually and brown granules of copper are seen at the bottom.

Zn_(s) + CuSO_4(aq) → ZnSO_4(aq) + Cu_(s)

If a strip of copper is placed in the solution of ZnSO₄, no chemical reaction occurs as copper is less reactive than zinc.

Write two equations each for obtaining metal oxides and hydroxides.

ANS :
OXIDES

When sodium reacts with oxygen it forms sodium oxide.
4Na_(s) + O_2(g) → 2Na₂O_(s)

Iron reacts with steam and forms iron oxide and hydrogen gas.
3Fe_(s) + 4H₂O_(g) → Fe₃O_4(s) + 4H_2(g)

HYDROXIDES

Sodium oxide reacts with water and forms sodium hydroxide.
Na₂O_(s) + H₂O_(l) → 2NaOH_(aq)

Magnesium reacts slowly with cold water and forms magnesium hydroxide and hydrogen gas.
Mg_(s) + 2H₂O_(l) → Mg(OH)_2(aq) + H_2(g)

Give names of different steps of metallurgy and write a brief note on each step.

ANS : Different steps involved in metallurgy are :

Crushing and grinding of ore
Concentration or enrichment of ore
Roasting, calcination and smelting
Reduction
Refining and purification of metals

CRUSHING AND GRINDING OF ORE : The ore in the form of big rocks is broken into small pieces with the help of crushers. These small pieces are then powdered with the help of a ball mill or stamp mill.

CONCENTRATION OR ENRICHMENT OF ORE : The process of removal of impurities from an ore is called ' Ore Concentration'. Different methods are employed for concentration of ore depending on the nature of ore and the impurities in it.

Froth Floatation Process is used for the concentration of sulphide ores.

Magnetic Separation method is used for removal impurities present in an ore with magnetic properties or for concentration of a nonmagnetic ore containing magnetic impurities.

Centrifugal method is used when there is a large difference in the densities of ore and impurities.

Ores of highly reactive metals are concentrated by chemical method.

ROASTING, CALCINATION,SMELTING :
Roasting is used to convert sulphide ores into oxides by heating for a longer period.

Calcination is the process of heating the ore strongly in absence of air to remove (i) volatile impurities (ii)water [from hydrated ore] or (iii) to convert carbonate ore into metal oxide.

Smelting is used to get an ore reduced to metal by heating whereby the metal is obtained in molten state.

REDUCTION : The conversion of metal oxide into metal is called 'Reduction'. For chemical reduction, reducing agents like carbon, carbon monoxide and aluminium are used. Metals like zinc, iron, copper, nickel, tin and lead are obtained by chemical reduction of their oxides.
Fe₂O_3(s) + 3C_(s) → 2Fe_(s) + 3CO_(g)

3MnO₂ + 4Al → 3Mn + 2Al₂O₃

The oxides of highly reactive metals cannot be reduced by chemical reduction using carbon or aluminium. Such metals are extracted by electrolytic reduction of their molten salts. During electrolysis the cathode acts as reducing agent by supplying electrons to metal ions. Sodium, potassium and aluminium are obtained by this method.

REFINING OF METALS : The metals obtained from their ores after several metallurgical processes still contain some impurities. The removal of these impurities is called refining of metals. Refining is done mainly by two processes.

LIQUATION (Liquefaction) method is used to remove impurities from metals like tin, lead and bismuth whose melting points are low as compared to those of impurities.
ELECTROLYTIC REFINING is used to refine metals like zinc, lead, aluminium, copper, silver, gold, etc. The impure metal works as anode, a strip of pure metal is taken as cathode and a suitable salt of the metal to be purified in aqueous solution is taken as an electrolyte. The impure metal enters the solution from anode and then gets deposited on cathode. The impurities remain in the solution.

In addition, the metals (in fact, metalloids) like silicon, germanium, etc. which are needed in ultrapure state for certain applications are obtained by zone refining method.

Write a note in detail : Refining of Metals by Electrochemical Method.

ANS : Metals like zinc, lead, aluminium, copper, silver, gold, etc. are refined by this process.

A rod of impure metal is taken as anode and a strip of pure metal is taken as cathode. an aqueous solution of a simple or complex salt of the metal to be purified is taken as electrolyte. On passing electric current at suitable voltage, pure metal is deposited at the cathode. The impurities either remain in the solution or collect at the bottom of the anode as 'Anode Mud'.

For example, to obtain pure copper, impure copper rod is taken as anode, a strip of pure copper is taken as cathode and aqueous solution of copper sulphate is taken as the electrolyte.

The reactions taking place at both the electrodes are :

Crude copper contains very small amounts of iron, silver and gold. Iron dissolves in the solution whereas silver and gold collect at anode as anode mud and are obtained in the native state.

Describe Extraction of aluminium from bauxite.

ANS :
CONCENTRATION OF BAUXITE [BAYER'S PROCESS] :

bauxite is concentrated by Bayer's process to obtain pure aluminium oxide (alumina).
Bauxite is first roasted by heating to convert ferrous oxide into ferric oxide. The ore is then dried, powdered and treated with 45% aqueous solution of caustic soda (sodium hydroxide) and is heated to about160°C in a closed vessel for 6 to 8 hours at 5 to 6 atmospheric pressure. Sodium aluminate is formed.

The insoluble impurities are filtered. Sodium aluminate is soluble in water and is obtained as filtrate. Sodium aluminate is then hydrolysed by adding excess water and constantly stirring it whereby gelatinous precipitates of aluminium hydroxide are obtained. To enhance the formation of Al(OH)₃, some Al(OH)₃ is added from outside as seedling.

The precipitates of Al(OH)₃ are then washed repeatedly with water, dried and heated up to 1200°C.

Thus, 99.5% pure alumina (Al₂O₃) is obtained.

ELECTROLYSIS OF ALUMINIUM OXIDE [HALL-HAROULT PROCESS] :

A special electrolytic cell invented by 'Hall' and 'Haroult' is used to reduce alumina to aluminium metal.

Pure aluminium oxide is mixed with cryolite (Na₃AlF₆). The mixture is then melted in iron tank lined with carbon from inside. The carbon lining acts as cathode. The anode consists of a number of carbon rods which are dipped into the molten mass of Al₂O₃ and cryolite.

When electric current is passed, aluminium metal is obtained at the cathode and oxygen gas gets liberated at anode. This oxygen gas reacts with carbon anode to form carbon dioxide. Due to this the carbon anode is gradually consumed.

Molten aluminium metal being heavier than the electrolyte, is collected at the bottom of the tank.
At Cathode

2Al⁺³_(aq) + 6e^- → 2Al_(l)

At Anode

C_(s) + O^-2 → CO_(g) + 2e^-

C_(s) + 2O^-2 → CO_2(g) + 4e^-

Describe extraction of iron using Blast Furnace.

ANS : Blast furnace is used to obtain iron metal from haematite (Fe₂O₃). A proper mixture of haematite, coke and calcium carbonate is added to the blast furnace from the top and hot air is blown from the bottom of the blast furnace.

[Diagram of blast furnace is only for information]

At the bottom of the furnace coke burns in air to form carbon dioxide and large amount of heat is produced.
C_(s) + O_2(g) → CO_2(g) + Heat

In the center of the furnace carbon dioxide reacts with hot coke and forms carbon monoxide.
CO_2(g) + C_(s) → 2CO_(g)

In the upper part of the furnace the temperature is 400°C to 700°C. Here the reaction between iron oxide and carbon monoxide takes place in three stages and iron oxide is reduced to iron.
3Fe₂O_3(s) + CO_(g) → 2Fe₃O_4(s) + CO_2(g)

Fe₃O_4(s) + CO_(g) → 3FeO_(s) + CO_2(g)

FeO_(s) + CO_(g) → Fe_(s) + CO_2(g)

Due to large amount of heat produced in the furnace, limestone (CaCO₃) decomposes to calcium oxide. Calcium oxide reacts with sand (SiO₂) present as impurity in the ore and forms calcium silicate known as SLAG.
CaO_(s) + SiO_2(s) → CaSiO_3(l)[slag]

Both slag and iron ore are in molten state due to very high temperature in the lower part of the furnace. Both are collected separately from the bottom of the furnace. Once started, the blast furnace works for many months.

Mention physical properties of metals.

ANS :

Metals have lustrous surface which can be polished.
Metals are solid (exception : gallium and mercury are in liquid state).
Metals are heavy (exception : sodium, potassium, magnesium and aluminium are comparatively lighter).
Metals are hard and cannot be cut with knife (exception : sodium, potassium and lead are soft and can be cut with knife).
Metals are good conductors of heat and electricity (exception : lead and mercury are bad conductors of heat).
Metals are ductile, i.e., they can be drawn in to wires.
Metals are malleable, i.e., they can be hammered and sheets can be prepared from them.
Metals have high melting points and boiling points. The melting point of iron is 1539°C.
Metals possess high tensile strength.
Most of the metals are sonorous, i.e., they produce sounding noise on collision.
Metals can be mixed with other metals and nonmetals to form alloys.

Write a note on chemical properties of metals OR Explain how metals react with oxygen, water, dilute acids, chlorine and hydrogen giving examples.

ANS : Metals are electropositive elements since they have a tendency to lose electrons and form positively charged ions.

REACTION WITH OXYGEN : Metals form oxides with oxygen. These oxides are basic in nature because when dissolved in water they form alkaline solutions. For example, sodium reacts with oxygen to give sodium oxide.
4Na_(s) + O_2(g) → 2Na₂O_(s)

Sodium oxide reacts with water to form sodium hydroxide ( an alkali).
Na₂O_(s) + H₂O_(l) → 2NaOH_(aq)

Oxides of metals like aluminium and zinc are called AMPHOTERIC OXIDES as they exhibit both acidic and basic behaviour.

REACTION WITH WATER : Metals form either metal oxides or hydroxides when they react with water. Hydrogen gas is evolved in both cases.
Highly reactive metals like sodium and potassium react violently with cold water.
2Na_(s) + 2H₂O_(l) → 2NaOH_(aq) + H_2(g)

Magnesium reacts slowly with cold water but rapidly with hot boiling water.
Mg_(s) + 2H₂O_(l) → Mg(OH)_2(aq) + H_2(g)

Zinc and iron react with steam only (i.e., they do not react with cold or warm water).
Zn_(s) + H₂O_(g) → ZnO_(s) + H_2(g)

3Fe_(s) + 4H₂O_(g) → Fe₃O_4(s) + 4H_2(g)

Less reactive metals like lead, copper, silver and gold do not react with water at all.

REACTION WITH DILUTE ACIDS : Depending on their reactivity, metals react with dilute acids at different rates and liberate hydrogen gas.
Mg_(s) + 2HCl_(aq) → MgCl_2(aq) + H_2(g)

2Al_(s) + 6HCl_(aq) → 2AlCl_3(aq) + 3H_2(g)

Copper does not react with dilute acids.

REACTION WITH CHLORINE : Metals react with chlorine to form metal chlorides which are ionic compounds.
2Na_(s) + Cl_2(g) → 2NaCl_(s)

REACTION WITH HYDROGEN : All metals do not combine with hydrogen. Only reactive metals like sodium, potassium and calcium form their hydrides with hydrogen.
2Na_(s) + H_2(g) → 2NaH_(s)

Ca_(s) + H_2(g) → CaH_2(s)

2K_(s) + H_2(g) → 2KH_(s)

Write a note on Corrosion.

ANS :

Corrosion is a process in which rust is formed on metal surface when it is exposed to air, water or moisture. Oxygen of air, carbon dioxide and moisture(water) are mainly responsible for corrosion.
Some of the metals undergo corrosion. For example : (i) Green coloured salt is formed on copper and brass vessels.
(ii) Iron becomes rusted because of its reaction with oxygen of air. Iron rust is mainly iron oxide (Fe₂O₃.xH₂O) which is red in colour.
Corrosion results in wastage of billions of rupees. Therefore, it is better to prevent corrosion.

PREVENTION OF CORROSION :

Rusting of iron can be prevented by covering its surface with a paint. This method is less effective because rusting starts again if the paint is removed from some part.
Iron is galvanised by applying a thin layer of more active zinc on its surface. Zinc prevents rusting effectively. Even if a small amount of zinc is removed from the surface, rusting does not start again because zinc spreads to cover the surface and prevents corrosion.
To prevent corrosion of iron plates of steamers, blocks of more reactive metals like magnesium or zinc are combined with iron plates and then allowed to come in contact with seawater. Here iron plates act as cathode and magnesium(or zinc) act as anode because the oxidation potential of iron is less. The magnesium/zinc blocks get corroded in seawater so they are replaced periodically.
Corrosion can also be prevented by using suitable chemical inhibitors which form a chemical bond with the metal surface and prevent corrosion.
A layer of enamel applied on the metal can prevent corrosion.

Write a note on Alloys.

ANS : An alloy is a homogeneous solid solution of two or more metals or a metal and a nonmetal. An alloy is generally prepared by heating the mixture of its constituents to molten state and then cooling it at room temperature. An alloy is prepared to obtain material with desired properties because the properties of constituents of an alloy are modified to some extent. Sometimes the melting point of an alloy is lower than that of the constituents. For example, solder is an alloy of lead and tin. Its melting point is lower than that of lead and tin and it is used in soldering of electrical wires. [Table to be posted]

Pure gold cannot be used to make jewellery as it is very soft. To make it hard, it is generally alloyed with copper or silver. The purity of gold is expressed in CARATS. Pure gold is 24 carats. 18 carat gold means it contains 18 parts of gold in 24 parts by weight of the alloy.

Distinguish between metals and nonmetals based on physical properties.
ANS :

Distinguish between metal and nonmetal based on chemical properties.

ANS:

manojsir-science blog

Thursday, March 22, 2007

METALS - III : LONG ANSWERS

GIVE ANSWER IN DETAIL

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