[3 Mark Question Answer]

Question 13 Marks

Answer the following question :
What is the difference between calcination and roasting.

Answer

Calcination	Roasting
1. The ore is heated in the absence of air	1. The ore is heated in excess of air.
2. Moisture and organic impurities are removed, and the ore becomes porous and more reactive.	2. Volatile impurities are removed as oxides ($SO_2, P_2O_5, As_2O_3$), and the ore becomes porous and more reactive.
3. Carbonate and hydrated ores are calcined, and $CO _2$ or water vapour is given off. $ ZnCO _3 \rightarrow ZnO + CO _2 $	3. Sulphide ores are roasted, so $SO_2$ is given off. $(2 ZnS +3 O _2 \xrightarrow{80^{-}-90^{\circ}} 2 ZnO +2 S$

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Question 23 Marks

The sketch below illustrates the refin ing of aluminium by Hoopes process

(a) Which of A and B is th e cathode and which one is the anode?
(b) What is the electroly te in the tank?
( c) What material is used for th e cathode?

Answer

(a) A is cathode and B is anode.
(b) Molten fluorides of Al, Na and Ba.
(c) Graphite rods.

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Question 33 Marks

Aluminium is extracted from its chief ore bauxite. The ore is first purified and then the metal is extracted from it by electrolytic reduction.
Name a chemicals used for dissolving aluminium oxide. In which state of sub-division is the chemical used?

Answer

Fluorspar and cryolite act as solvent. The percentage by weight composition is as follows:
Alumina - 20% by mass
Cryolite - 60% by mass
Fluorspar - 20% by mass

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Question 43 Marks

Aluminium is extracted from its chief ore bauxite. The ore is first purified and then the metal is extracted from it by electrolytic reduction.
Write three balanced equations for the purification of bauxite by Halls process.

Answer

$Na _3 AlF _6 \rightleftharpoons 3 Na ^{+}+ Al _3^{+}+6 F ^{-}$
$CaF _2 \rightleftharpoons Ca _2{ }^{+}+2 F ^{-}$
$Al _2 O _3 \rightleftharpoons 2 Al _3{ }^{+}+3 O _2^{-}$

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Question 53 Marks

From the metals copper, iron, magnesium, sodium and zinc, select a different metal in each case which:
(a) Does not react with dilute hydrochloric acid
(b) Can form $2^+$ and $3^+$ ions
(c) Has a hydroxide that reacts with both acid and alkalis.
(d) Does not react with cold water, but reacts with steam when heated.

Answer

(a) Copper
(b) Iron
(c) Zinc
(d) Magnesium

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Question 63 Marks

The following question refer to the extraction of aluminium and iron from their ores:
Iron and aluminum ores both, contain impurities. Explain briefly how these impurities are removed in each case.

Answer

Aluminium	Iron
1. Aluminium ore contains impurities of $FeO$ and $SiO_2$	1. Iron ore contains impurities of silica and sand. These are removed by magnetic separation .
2. Bauxite containing $FeO$ is calcinated at high temperature	2. $FeO$ is oxidized to $Fe_2O_3$
3. Calcinated ore is then treated with $NaOH $when $Al_2O_3$ is converted into soluble $NaAIO_2$(sodium meta - aluminate)	3 . The insoluble $Fe_2O_3$ and silica can thus be filtered off.
4. The filtrate is hydrolysed to get Aluminum hydroxide which on ignition gives pure alumina$ ( Al_2O_3).$

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Question 73 Marks

The following question refer to the extraction of aluminium and iron from their ores:
What is the most important chemical process in the extraction of any metal? State how this essential step is carried out in the extraction of: i. iron, and ii. Aluminium.

Answer

Reduction of the oxide is the most important chemical process in the extraction of any metal.
(i) In case of iron :
$
Fe _2 O _3+3 CO \longrightarrow 2 Fe +3 CO _2
$
(ii) In case of aluminium:
$Al _2 O _3$ cannot be easily reduced; hence it is subjected to electrolysis. Aluminium is collected at the cathode.

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Question 83 Marks

An aqueous solution of sodium chloride is not used for electrolytic reduction of sodium metal. Why?

Answer

An aqueous solution of sodium chloride is not used for electrolytic reduction of sodium metal because if we electrolyse an aqueous solution of sodium chloride, then as soon as sodium metal is produced at the cathode, it will react with water to form sodium hydroxide. So electrolysis of aqueous sodium chloride solution will produce sodium hydroxide and not sodium metal.

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Question 93 Marks

In the Halls process for extraction of aluminium, Sate the location of cathode and anode and explain what occurs at each electrode.

Answer

Cathode is the inner lining of gas-carbon of the electrolytic cell and anode is the thick carbon rods dipping into the fused electrolytes.
At cathode: Aluminium ions get reduced as:
$Al^{3+} + 3e^- → Al$
At anode: oxygen gas is liberated as:
$O^{2-} - 2e^- → [O]$
$[O] + [O] → O_2$
The oxygen formed at anode oxidizes the carbon of the anode to carbon dioxide.
$C + O_2 → CO_2$
As a result the anode gets oxidized and it has to be replaced periodically.

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Question 103 Marks

How are metals refined by the electrolytic methods?

Answer

Electrolytic method : This method is based upon the phenomenon of electrolysis and is widely used to refine a number of metals such as copper, silver, gold etc. In
this method, impure metal is made anode whereas a thin sheet of pure metal is used as cathode in an electrolytic tank. The electrolyte used in the tank is usually the acidified aqueous solution of a salt or complex salt of metal. On passing the electric current through electrodes the metal ions from anode go into the electrolyte solution. These cations gain electrons from the cathode and get deposited on it. The impurities either remain dissolved or get precipitated as anode mud.
At anode : $M → M^n + ne^-$
At cathode: $M^n + ne^-→ M$ where M is the metal to be refined .

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Question 113 Marks

What is meant by refining of metals? Name the three common methods used for refining.

Answer

Refining of metals: It is the further purification of metals obtained by reduction process to remove all the impurities.
Depending upon the nature of metal, nature of impurities and purpose for which metal is to be used.
The three methods used for refining are:

Liquation.
Distillation.
Electrolytic refining.

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Question 123 Marks

Comment on the statement that- "All ores are minerals but all minerals are not ores".

Answer

As we know that minerals are the naturally occurring compounds of metals which are generally mixed with earthy such as soil, sand, limestone and rocks while ores are those minerals from which a metal can be extracted profitably.
Hence "All ores are minerals but all minerals are not ores".

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Question 133 Marks

Give composition and uses of the following alloys. Mention the reason for its use.
Bronze

Answer

Alloys	Composition	Uses
Bronze	Cu = 80% Sn = 20% P, Pb, Zn etc. may be present in small quantities	Statues Coins Medals Utensils Decorative items

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Question 143 Marks

Give composition and uses of the following alloys. Mention the reason for its use.
Solder

Answer

Alloys	Composition	Uses
Solder	Pb = 50% Sn = 50%	Joining metal wires especially in electrical and electronic equipments

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Question 153 Marks

Give composition and uses of the following alloys. Mention the reason for its use.
Magnalium

Answer

Alloys	Composition	Uses
Magnalium	Al = 70 to 95% Mg = 5 to 30%	Machine parts Aircrafts Scientific instruments

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Question 163 Marks

Give composition and uses of the following alloys. Mention the reason for its use.
Brass

Answer

Alloys	Composition	Uses
Brass	Cu = 55 to 95% Zn = 5 to 45%	Hardware Electrical fixture Jewellery Decorative metal items Medals Musical Instruments

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Question 173 Marks

Give composition and uses of the following alloys. Mention the reason for its use.
Duralumin

Answer

Alloys	Composition	Uses
Duralumin	Al = 95% Cu = 4% Mg = 0.5% Mn = 0.5%	Aircrafts Automobiles Tools

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Question 183 Marks

Give composition and uses of the following alloys. Mention the reason for its use.
Stainless steel

Answer

Alloys	Composition	Uses
Stainless steel	Fe = 75% Ni = 8- 10% Cr = 15 - 18% C = 0.5 - 1%	Cutlery Utensils Surgical instruments Decorative articles Automobile bodies Furniture Scientific instruments

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Question 193 Marks

Describe the role played in the extraction of aluminum:
Sodium hydroxide

Answer

In the extraction of aluminium, the given compounds play the following roles:

Sodium hydroxide :
Two roles are played by sodium hydroxide in the extraction of aluminium.
First, finely grinded bauxite (ore of aluminium) is heated under pressure with conc. caustic soda solution (NaOH solution) for 2-8 hours at 140°C to 150°C to produce sodium aluminate. The chemical equation is as follows:
$Al_2O_3.2H_2O + 2NaOH → 2NaAlO_2+ 3H_2O$
Second, on diluting sodium aluminate with water and cooling to 50°C, sodium aluminate is hydrolysed to give aluminium hydroxide as a precipitate. Here, the impurities dissolve in sodium hydroxide.

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Chemistry [3 Mark Question Answer]

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