Get notes, summary, questions and answers, MCQs, extras, competency-based questions and PDFs of Metallurgy: ICSE Class 10 Chemistry (Concise/Selina). However, the notes should only be treated as references, and changes should be made according to the needs of the students.
Summary
Elements are found in nature. Some are metals, and others are non-metals. Metals are usually shiny and can be shaped. They are found in the Earth’s crust. Sometimes metals are found pure, like gold. Often, they are mixed with other things in rocks called minerals. If we can get a metal easily and at a low cost from a mineral, that mineral is called an ore. Ores also contain unwanted earthy materials called gangue, which are like impurities mixed with the useful part. Common metals like iron, aluminium, and zinc have their own special ores.
To get pure metals from ores, several steps are followed. First, the ore is crushed into a powder. Then, the gangue is removed. This step is called dressing the ore. One way is washing with water, where heavier ore particles settle down. Another method uses magnets if the ore or the impurity is magnetic. For some ores, a method called froth flotation is used. In this method, powdered ore is mixed with oil and water, and air is blown through. The ore particles attach to oily bubbles and float to the surface, separating from the waste materials that sink. Sometimes, chemicals are used to dissolve the ore and leave impurities behind.
After cleaning, the ore is often turned into a metal oxide. If the ore is a sulphide, it is heated strongly in air; this is called roasting. If it is a carbonate ore, it is heated without much air, or with a limited supply; this is called calcination. These heating methods also help remove other unwanted substances.
Next, the metal oxide is changed into metal. This step is called reduction. How this is done depends on how reactive the metal is. Very reactive metals like sodium or aluminium need electricity to separate them from their oxides when the oxides are in a molten state. This is electrolytic reduction. Less reactive metals like iron or copper can have their oxides heated with carbon. Carbon takes the oxygen away from the metal oxide, leaving the metal.
Even after reduction, the metal might not be pure enough. So, it is refined. One way is distillation, where metals that boil easily are heated, turn into vapor, and are then cooled to get pure metal. Another way is liquation, for metals that melt easily. The impure metal is heated on a sloping surface, and the pure metal melts and flows away, leaving impurities behind. Electro-refining uses electricity. Impure metal is made one electrode, and a thin strip of pure metal is the other. They are placed in a special liquid containing a salt of the metal. Pure metal moves from the impure block to the pure strip when electricity passes.
Aluminium is a common metal. Its main ore is bauxite. Bauxite is first purified using a sodium hydroxide solution. This solution dissolves the aluminium compounds in the bauxite, leaving behind impurities like red mud, which are filtered off. The solution is then treated to get pure aluminium hydroxide. This aluminium hydroxide is then heated strongly to get pure aluminium oxide, also called alumina. Alumina has a very high melting point. To extract aluminium, alumina is dissolved in molten cryolite and fluorspar. This mixture melts at a lower temperature and conducts electricity better than molten alumina alone. Electricity is passed through this molten mixture. Aluminium metal collects at the negative electrode (cathode). Oxygen gas is formed at the positive electrode (anode), which is made of carbon. This oxygen reacts with the carbon anode, so the anode gets slowly used up and needs to be replaced.
Sometimes, metals are mixed with other metals or non-metals to make alloys. Alloys have different properties from the original pure metals. For example, steel is an alloy of iron and carbon, and it is much stronger than iron. Brass is an alloy of copper and zinc. Bronze is made from copper and tin. Duralumin, often used in aircraft parts, is mostly aluminium mixed with copper and small amounts of other metals, making it light yet strong. Solder is an alloy of lead and tin; it is used for joining electrical wires and components because it melts at a relatively low temperature. When mercury is mixed with other metals, the alloy is called an amalgam. Alloys are made to improve qualities like strength, change appearance, resist rusting, or alter melting points.
Workbook solutions (Concise/Selina)
Intext Questions and Answers I
1. (a) Name the three classes in which elements are classified. Which was the first metal used by man?
Answer: Elements have been classified into metals and non-metals based on their properties. Certain elements which show properties of both metals and non-metals are called metalloids or semi-metals. So, the three classes are metals, non-metals, and metalloids.
Copper was the first metal to be used by man for making utensils, weapons and for other purposes.
(b) Name the metal and non-metal present in abundance in the earth crust
Answer: The metal present in abundance in the earth’s crust is Aluminium (8%). The non-metal present in abundance in the earth’s crust is Oxygen (46.6%).
2. Name the metal which is a constituent of:
(a) blood pigment,
(b) plant pigment.
Answer:
(a) The metal which is a constituent of blood pigment (haemoglobin) is iron.
(b) The metal which is a constituent of plant pigment (chlorophyll) is magnesium.
3. Give the importance of the following for living beings :
(a) Nitrogen,
(b) Hydrogen,
(c) Carbon.
Answer: (a) Nitrogen is the most abundant element present in the atmosphere. Its presence in air reduces the rate of combustion. Due to its inertness, it is also used to preserve food. Life would not have been possible in the absence of non-metals like carbon, oxygen, nitrogen and hydrogen.
(b) Hydrogen, the lightest element known, is used in the hydrogenation of vegetable oils to make ghee, as a fuel and in the manufacture of compounds. It is also the essential part of organic compounds. Life would not have been possible in the absence of non-metals like carbon, oxygen, nitrogen and hydrogen.
(c) Proteins, fats, carbohydrates, enzymes, vitamins, etc., are all compounds of carbon, and are essential for the growth and development of living organisms. Life would not have been possible in the absence of non-metals like carbon, oxygen, nitrogen and hydrogen.
4. State the position of the following in the Periodic Table :
(a) Alkali metals,
(b) Alkaline earth metals,
(c) Halogens,
(d) Aluminium.
Answer: (a) Alkali metals are placed in Group IA of the Periodic Table, on the left side.
(b) Alkaline earth metals are placed in Group IIA of the Periodic Table, on the left side.
(c) Halogens are placed in Group VIIA (or 17) of the Periodic Table, on the right side.
(d) Aluminium is in Period 3, Group IIIA (13) of the Periodic Table.
5. Name:
(a) a liquid non-metal,
(b) two metalloids
(c) a metal which does not corrode easily
(d) two metals which react with cold water,
(e) a non-metal which can form a positive ion,
(f) a non-metal which shows reducing property,
Answer: (a) A liquid non-metal is bromine.
(b) Two metalloids are Boron (B) and Silicon (Si).
(c) A metal which does not corrode easily is gold. Gold being least reactive, does not react with atmospheric reagents.
(d) Two metals which react with cold water are sodium and potassium.
(e) A non-metal which can form a positive ion is hydrogen.
(f) A non-metal which shows reducing property is carbon.
6. From the list of characteristics given below, select the five which are relevant to non-metals and their compounds :
A. Ductile,
B. Conduct electricity,
C. Brittle,
D. Acidic oxides,
E. Basic oxides,
F. Discharged at anode,
G. Discharged at cathode, H. Ionic chlorides,
I. Covalent chlorides,
J. Reaction with dilute sulphuric acid yields hydrogen,
K. 1, 2 or 3 valence electrons,
L. 5, 6, 7 valence electrons.
(Write the five letters corresponding to the correct characteristics).
Answer: The five characteristics relevant to non-metals and their compounds are:
C. Brittle
D. Acidic oxides
F. Discharged at anode
I. Covalent chlorides
L. 5, 6, 7 valence electrons
7. (a) Why are alkali metals kept in kerosene oil ?
Answer: Alkali metals are very reactive, they react with atmospheric reagents like oxygen, carbon dioxide and water vapour and form compounds, so they are kept in inert solvent.
(b) Why is hydrogen kept in the metal activity series ?
Answer: Hydrogen can also form positive ion H – e⁻ → H⁺ similar to the metals.
(c) Why do gold ornaments look new even after several years of use ?
Answer: Gold being least reactive, does not react with atmospheric reagents.
8. 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) arrange the above metals in the decreasing order of reactivity.
Answer: (a) Copper does not react with dilute hydrochloric acid because metals below hydrogen cannot displace hydrogen from dilute acid.
(b) Iron can form 2+ and 3+ ions (e.g., Fe²⁺ in FeO and Fe³⁺ in Fe₂O₃).
(c) The above metals in decreasing order of reactivity are: Sodium > Magnesium > Zinc > Iron > Copper.
9. Which metal occurs as:
(a) a sulphide,
(b) a halide,
(c) a carbonate,
(d) an oxide.
Also give the names of their respective ores.
Answer: (a) Zinc occurs as a sulphide. Its respective ore is Zinc blende (ZnS).
(b) Sodium occurs as a halide. Its respective ore is Rock salt (NaCl).
(c) Calcium occurs as a carbonate. Its respective ore is Limestone (CaCO₃).
(d) Aluminium occurs as an oxide. Its respective ore is Bauxite (Al₂O₃.2H₂O).
10. Distinguish between :
(a) a mineral and an ore,
(b) an ore and a metallic compound.
Answer: (a) Minerals are the naturally occuring compounds of metals which are generally mixed with other substances such as soil, mud, sand, silica (SiO₂), limestone, rocks, etc. Ores are those minerals from which metals are extracted commercially at a comparatively lower cost and with minimum effort. All minerals are not ores, but all ores are minerals.
(b) An ore is a mineral from which a metal is extracted commercially at a comparatively lower cost and with minimum effort. A metallic compound is a substance formed from the chemical combination of a metal with one or more other elements. An ore is thus a specific type of naturally occurring metallic compound (or mineral) that is economically viable for metal extraction, whereas the term “metallic compound” is more general.
11. Which metal can be extracted from each one of the following ores:
(a) bauxite (b) calamine (c) haematite.
Answer: (a) Aluminium can be extracted from bauxite (Al₂O₃.2H₂O).
(b) Zinc can be extracted from calamine (ZnCO₃).
(c) Iron can be extracted from haematite (Fe₂O₃).
12. Explain the following terms:
(a) ore,
(b) gangue.
Answer: (a) Ores are those minerals from which metals are extracted commercially at a comparatively lower cost and with minimum effort.
(b) Gangue or matrix refers to the earthly impurities such as soil, mud, sand, silica (SiO₂), limestone, rocks, etc., which are generally mixed with minerals.
Intext Questions and Answers II
1. Give the principles of:
(a) hydrolytic method,
(b) froth floatation process,
(c) electromagnetic separation.
Answer: (a) The principle of the hydrolytic method, also known as Gravity separation or Levigation, is that the difference in the densities of the ore and the gangue is the main criterion.
(b) The principle of the froth floatation process is that this process depends on the preferential wettability of the ore with oil (pine oil) and the gangue particles by water.
(c) The principle of electromagnetic separation, also known as Magnetic Separation, is based on the magnetic properties of the ores.
2. (a) Name the methods by which concentrated ore is converted to metallic oxide.
(b) State three objectives achieved during the roasting of ores.
Answer: (a) Depending upon the nature of the ores, two methods are used to convert the concentrated ore into its oxide: (i) roasting and (ii) calcination.
(b) Three objectives achieved during the roasting of ores are:
(i) Moisture is removed.
(ii) Organic matter is oxidised and removed.
(iii) Impurities of sulphur, phosphorus and arsenic are oxidised and removed as volatile impurities.
3. Name:
(a) the processes involved in
(i) dressing of the ores
(ii) refining of ores.
(b) two metallic oxides which cannot be reduced by carbon, carbon monoxide or hydrogen.
Answer: (a) (i) The processes involved in dressing of the ores, also known as concentration of an ore, include the hydrolytic method (gravity separation), magnetic separation, and the froth flotation method. Chemical method or leaching is also a process.
(ii) The processes involved in refining of ores, which is the process by which crude metal is purified, include distillation, liquation, and electro refining.
(b) Two metallic oxides which cannot be reduced by common reducing agents like carbon, carbon monoxide or hydrogen are Aluminium oxide (Al₂O₃) and Magnesium oxide (MgO). Oxides of highly active metals like potassium, sodium, calcium, magnesium and aluminium have great affinity towards oxygen and so cannot be reduced by common reducing agents.
4. Why does iron or zinc not occur free in nature?
Answer: Iron and zinc do not occur free in nature because most of the metals are reactive so they occur in a combined state in the form of their oxides, carbonates, halides, sulphides, sulphates, etc. Metals which are less reactive and do not react under normal conditions with oxygen, water, carbon dioxide and other common reagents, occur in native state. Iron and zinc are moderately reactive metals.
5. What do you observe when hydrogen is passed over heated copper oxide?
Answer: When hydrogen is passed over heated copper oxide (which is black), the black copper oxide is reduced to copper (which is pink/brown) and water is formed. The equation is CuO + H₂ → Cu + H₂O.
6. Compare roasting and calcination.
Answer: The comparison of roasting and calcination is as follows:
e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂ (at 800–900 °C)
e.g. ZnCO₃ → ZnO + CO₂
7. (a) Name an ore of zinc.
(b) Which process is applied to concentrate it?
(c) How is concentrated ore changed to oxide?
Answer: (a) An ore of zinc is zinc blende (ZnS). Other ores include zincite (ZnO) and calamine (ZnCO₃). The main ore of zinc is zinc blende.
(b) For zinc blende (ZnS), which is a sulphide ore, the froth floatation process is applied to concentrate it.
(c) If the concentrated ore is zinc blende (a sulphide ore), it is changed to oxide by roasting, which involves heating the ore in excess of air:
2ZnS + 3O₂ → 2ZnO + 2SO₂
If the concentrated ore is calamine (a carbonate ore), it is changed to oxide by calcination, which involves heating the ore in the absence of air:
ZnCO₃ → ZnO + CO₂
8. (a) Some metallic oxides can be reduced by hydrogen, carbon and carbon monoxide and some cannot. Explain.
(b) Write balanced equation for the reduction of copper (II) oxide by hydrogen. (2019)
Answer: (a) The ability of hydrogen, carbon, or carbon monoxide to reduce a metallic oxide depends on the reactivity of the metal, which is indicated by its position in the activity series.
Oxides of highly active metals like potassium, sodium, calcium, magnesium, and aluminium have a great affinity towards oxygen and are very stable. Therefore, they cannot be reduced by common reducing agents like carbon, carbon monoxide, or hydrogen. These metals are typically extracted by electrolysis of their fused salts.
Metals in the middle of the activity series, such as iron, zinc, lead, and copper, form oxides that are less stable. These metallic oxides can be reduced by heating with suitable reducing agents like carbon, carbon monoxide, or hydrogen. For example, copper oxide, lead (II) oxide, and iron (III) oxide can be reduced by these agents. Zinc oxide can be reduced by carbon.
(b) The balanced equation for the reduction of copper (II) oxide by hydrogen is:
CuO + H₂ → Cu + H₂O
9. How are the following metallic oxides reduced. Write equations:
(a) Iron (II) oxide,
(b) Zinc oxide.
Answer: (a) Iron (II) oxide can be reduced by carbon, carbon monoxide, or hydrogen. The equations are:
FeO + C → Fe + CO
FeO + CO → Fe + CO₂
FeO + H₂ → Fe + H₂O
(b) Zinc oxide is reduced by heating with carbon (coke). Zinc oxide cannot be reduced by carbon monoxide or hydrogen as they are unable to overcome the affinity of zinc towards oxygen.
The equation is: ZnO(s) + C(s) → Zn(s) + CO(g) (at 1400°C, to get zinc spelter)
10. State why aluminium is extracted from its oxide by electrolysis while copper, lead, iron by reducing agents and mercury and silver by thermal decomposition.
Answer: Aluminium is extracted from its oxide by electrolysis because aluminium oxide, due to its great affinity for oxygen, is a very stable compound. It is not reduced easily by common reducing agents like carbon, carbon monoxide or hydrogen. Aluminium is a highly active metal.
Copper, lead, and iron are moderately reactive metals, and their oxides are less stable. These metals are in the middle of the activity series. Their oxides can be reduced by heating with suitable reducing agents like carbon, carbon monoxide, or hydrogen. For example, iron, lead and copper are obtained by the reduction of their oxides with carbon, carbon monoxide and hydrogen.
Mercury and silver are low in the activity series and are very less reactive. The oxides of these metals are unstable and can be reduced to metals by heating alone (thermal decomposition). For example, oxides and nitrates of Hg, Ag, Au decompose to give the metals on being strongly heated.
2HgO → 2Hg + O₂ (above 300°C)
2Ag₂O → 4Ag + O₂ (above 300°C)
11. An ore on being heated in air forms sulphurous anhydride. Write the process used for the concentration of this ore.
Answer: If an ore on being heated in air forms sulphurous anhydride (SO₂), it indicates that the ore is a sulphide ore. The process generally used for the concentration of sulphide ores, like zinc blende (ZnS) and galena (PbS), is the froth floatation process.
12. (a) Define roasting. Name an ore on which roasting is done. Give balanced equation.
(b) Define calcination. Give an example and equation for calcination.
Answer: (a) Roasting is a process of heating the concentrated ore to a high temperature below its melting point in the presence of excess air.
Sulphide ores are generally subjected to roasting. An ore on which roasting is done is zinc blende (ZnS).
The balanced equation for roasting of zinc blende is:
2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g) (at 800-900°C)
(b) Calcination is a process where if an ore is a carbonate or a hydrated oxide, it is heated in the absence of air (or air might be supplied in a limited quantity) to a temperature that is high but insufficient to melt the ore.
An example of an ore subjected to calcination is calamine (ZnCO₃), a carbonate ore.
The equation for calcination of calamine is:
ZnCO₃(s) → ZnO(s) + CO₂(g) (on heating)
13. How are the metals like sodium, potassium and calcium obtained. Give equations.
Answer: Metals like sodium, potassium, and calcium are highly active metals. Their oxides or other compounds cannot be reduced by common reducing agents. These metals are obtained by electrolytic reduction of their fused metallic salts (halides or oxides) using inert electrodes.
Potassium:
Electrolyte: Fused potassium bromide (KBr)
Reaction: KBr ⇌ K⁺ + Br⁻
Reaction at cathode: K⁺ + e⁻ → K
Reaction at anode: Br⁻ – e⁻ → Br; Br + Br → Br₂
Sodium:
Electrolyte: Fused sodium chloride (NaCl)
Reaction: NaCl ⇌ Na⁺ + Cl⁻
Reaction at cathode: Na⁺ + e⁻ → Na
Reaction at anode: Cl⁻ – e⁻ → Cl; Cl + Cl → Cl₂
Calcium:
Electrolyte: Fused calcium chloride (CaCl₂)
Reaction: CaCl₂ ⇌ Ca²⁺ + 2Cl⁻
Reaction at cathode: Ca²⁺ + 2e⁻ → Ca
Reaction at anode: Cl⁻ – e⁻ → Cl; Cl + Cl → Cl₂ (or 2Cl⁻ → Cl₂ + 2e⁻)
14. Give equations for the reduction of
(a) Iron (II) oxide
(b) Iron (III) oxide
(c) Lead (II) oxide
(d) Zinc oxide
Answer: (a) Reduction of Iron (II) oxide:
FeO + C → Fe + CO
FeO + CO → Fe + CO₂
FeO + H₂ → Fe + H₂O
(b) Reduction of Iron (III) oxide (e.g., by aluminothermy):
Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + Heat
Iron (III) oxide can also be reduced by carbon or carbon monoxide. For example: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
(c) Reduction of Lead (II) oxide:
PbO + C → Pb + CO
PbO + CO → Pb + CO₂
PbO + H₂ → Pb + H₂O
(d) Reduction of Zinc oxide (by carbon):
ZnO(s) + C(s) → Zn(s) + CO(g) (at 1400°C)
15. (a) On which factors does purification of metals depend?
(b) Name the methods used for purification.
(c) With a labelled diagram explain electro-refining of a particular metal.
Answer: (a) The method of purification used depends upon the:
(i) nature of the metal,
(ii) nature of the impurities present in the metal and
(iii) purpose for which the metal is to be used.
(b) The methods used for purification (refining) of metals include:
(i) Distillation
(ii) Liquation
(iii) Electro refining
(c) Electro-refining: In this method, an electrolytic cell is used which utilizes electrical energy to drive a chemical reaction. This electrolytic cell has three components: an electrolyte and two conducting electrodes (a cathode and an anode) contained in a non-conducting vessel. The impure metal is made the anode, while a thin sheet of pure metal is made the cathode. The electrolyte used is a salt solution of the metal which is to be refined. Little H₂SO₄ is often added to increase the conductance. The electrodes are suspended in the solution of electrolyte. On passing electric current, the metal ions from the electrolyte get reduced into metal at the cathode and deposit there. An equivalent mass of metal dissolves from the anode and goes into solution as metal ions.
At anode: M – ne⁻ → Mⁿ⁺ (where M is the impure metal)
At cathode: Mⁿ⁺ + ne⁻ → M (pure metal deposits)
Pure metal deposits at the cathode and impurities from the anode settle down below it, forming anode mud. Metals like copper, silver, gold, nickel, aluminium and zinc are refined by this process.
16. Choose the correct option :
(a) The metal other than aluminium, which has a strong affinity for oxygen is :
(A) Copper (B) Magnesium (C) Silver (D) Gold
(b) A metallic oxide which cannot be reduced by normal reducing agents :
(A) Zinc oxide (B) Magnesium oxide (C) Copper (II) oxide (D) Iron (III) oxide
Answer: (a) (B) Magnesium. Magnesium is a highly reactive metal, high in the activity series, and has a strong affinity for oxygen.
(b) (B) Magnesium oxide. Magnesium oxide (MgO) is an oxide of a highly reactive metal and cannot be reduced by normal reducing agents like carbon or carbon monoxide.
17. Fill in the blanks :
(a) Usually ………… (sulphide/carbonate) ores are subjected to ………… (calcination/roasting) which is done in the absence of air.
(b) Zinc blende is converted to oxide by ………… (roasting/calcination) process.
(c) Froth floatation process is generally used to concentrate ………… ores (sulphide/carbonate).
Answer: (a) Usually carbonate ores are subjected to calcination which is done in the absence of air.
(b) Zinc blende is converted to oxide by roasting process.
(c) Froth floatation process is generally used to concentrate sulphide ores.
Intext Questions and Answers III
1. State the position of aluminium in the Periodic Table.
Answer: Aluminium is in Period 3, Group IIIA (13) of the Periodic Table.
2. (a) Give the chemical names and formulae of main ores of (i) aluminium (ii) iron (iii) zinc.
Answer: The chemical names and formulae of the main ores are:
(i) Aluminium: The main ore is bauxite, which is hydrated aluminium oxide, Al₂O₃.2H₂O. Another ore is cryolite, which is sodium aluminium fluoride, Na₃AlF₆.
(ii) Iron: The chief ore is haematite, which is anhydrous ferric oxide, Fe₂O₃. Other ores include brown haematite (hydrated ferric oxide, 2Fe₂O₃.3H₂O), magnetite (triferric tetraoxide, Fe₃O₄), iron pyrites (iron disulphide, FeS₂), and siderite (ferrous carbonate, FeCO₃).
(iii) Zinc: The main ore is zinc blende, which is zinc sulphide, ZnS. Other ores include calamine (zinc carbonate, ZnCO₃) and zincite (zinc oxide, ZnO).
(b) Which impurities are present in bauxite ?
Answer: Bauxite contains impurities such as sand, ferric oxide, and titanium oxide.
(c) What is red mud, how is it removed ?
Answer: Red mud consists of insoluble impurities like ferric oxide, sand, etc., left behind when bauxite dissolves in concentrated caustic soda solution during the Bayer’s process. It is removed by filtration.
3. In order to obtain 1 tonne of aluminium, the following inputs are required. 4 tonnes of bauxite, 150 kg of sodium hydroxide and 600 kg of graphite. The aluminium compound in bauxite is aluminium oxide and the main impurity is iron (III) oxide. Aluminium is obtained by the electrolysis of aluminium oxide dissolved in cryolite.
(a) When bauxite is treated with sodium hydroxide solution, what happens to :
(i) the aluminium oxide,
(ii) the iron (III) oxide ?
Answer: (i) When bauxite is treated with sodium hydroxide solution, the aluminium oxide (Al₂O₃.2H₂O) dissolves and forms sodium meta aluminate (NaAlO₂) because of the amphoteric nature of aluminium. The reaction is: Al₂O₃.2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O.
(ii) The iron (III) oxide, being an impurity, does not dissolve and remains as part of the insoluble red mud.
(b) (i) Name the process used for the purification of bauxite.
Answer: The process used for the purification of bauxite is Bayer’s process.
(ii) Write the equation for the action of heat on aluminium hydroxide.
Answer: The equation for the action of heat on aluminium hydroxide is:
2Al(OH)₃ –(heat, 1000°C)–> Al₂O₃ + 3H₂O
(c) (i) Write the formulae of cryolite.
Answer: The formula of cryolite is Na₃AlF₆.
(ii) Write down the word which correctly completes the following sentence.
By dissolving aluminium oxide in cryolite a …….. (conducting/non conducting) solution is produced.
Answer: By dissolving aluminium oxide in cryolite a conducting solution is produced.
(iii) Why is so much graphite required for the electrolytic process ?
Answer: So much graphite is required for the electrolytic process because the graphite anode gets oxidised by the oxygen evolved at the anode, forming carbon monoxide and carbon dioxide. Therefore, the anode has got to be replaced from time to time.
(iv) Write the equation for the reaction which takes place at cathode.
Answer: The equation for the reaction which takes place at the cathode during the electrolytic reduction of alumina is:
4Al³⁺ + 12e⁻ → 4Al
(v) What is the cathode made up of ?
Answer: The cathode is made up of the inner carbon lining of the electrolytic cell, which is an iron tank lined with gas carbon.
4. Aluminium is extracted from its chief ore, bauxite. The ore is first purified and then the metal is extracted from it by electrolytic reduction.
(a) Write three balanced equations for the purification of bauxite.
Answer: The three balanced equations for the purification of bauxite (Bayer’s process) are:
- Al₂O₃.2H₂O + 2NaOH –(140°C-150°C)–> 2NaAlO₂ + 3H₂O (Leaching of bauxite with caustic soda)
- NaAlO₂ + 2H₂O –(cool to 50°C)–> NaOH + Al(OH)₃↓ (Hydrolysis of sodium meta aluminate)
- 2Al(OH)₃ –(heat, 1000°C)–> Al₂O₃ + 3H₂O (Ignition of aluminium hydroxide to get alumina)
(b) Name a chemical used for dissolving aluminium oxide. In which state is the chemical used ?
Answer: A chemical used for dissolving aluminium oxide (in bauxite) is sodium hydroxide (NaOH). It is used as a concentrated caustic soda solution.
(c) Write an equation for the reaction which takes place at the anode during the extraction of aluminium by the electrolytic process.
Answer: The reactions at the anode during the extraction of aluminium by the electrolytic process are:
6O²⁻ – 12e⁻ → 6[O]
3O + 3O → 3O₂
The oxygen evolved then reacts with the carbon anode:
2C + O₂ → 2CO
2CO + O₂ → 2CO₂
5. (a) A to F below relate to the source and extraction of either zinc or aluminium:
A. Bauxite,
B. Coke,
C. Cryolite,
D. Froth floatation,
E. Sodium hydroxide solution,
F. Zinc blende.
(i) Write down the three letters each from the above list which are relevant to:
1. Zinc,
2. Aluminium.
Answer:
- Zinc: F (Zinc blende), B (Coke), D (Froth floatation)
- Aluminium: A (Bauxite), C (Cryolite), E (Sodium hydroxide solution)
(ii) Fill in the blanks using the most appropriate words from A to F.
1. The ore from which aluminium is extracted must first be treated with E so that pure aluminium oxide can be obtained.
2. Pure aluminium oxide is dissolved in C to make a conducting solution.
Answer:
- The ore from which aluminium is extracted must first be treated with E (Sodium hydroxide solution) so that pure aluminium oxide can be obtained.
- Pure aluminium oxide is dissolved in C (Cryolite) to make a conducting solution.
(iii) Write the formula of cryolite.
Answer: The formula of cryolite is Na₃AlF₆.
6. Explain with reasons:
(a) In the electrolytic reduction of alumina, the graphite anode is gradually consumed. OR Why the anode has to be replaced from time to time in this process?
Answer: During the electrolytic reduction of alumina, oxygen is evolved at the anode. This oxygen then reacts with the carbon (graphite) anode. The anode is oxidised to carbon monoxide, which further forms carbon dioxide. The reactions are:
2C + O₂ → 2CO
2CO + O₂ → 2CO₂
Because the anode gets oxidised by the oxygen evolved at it, it is gradually consumed and has got to be replaced from time to time.
(b) Roasting is carried out on sulphide ores and not on carbonate ores.
Answer: Roasting is carried out on sulphide ores and not on carbonate ores because their chemical nature and the changes required to convert them to oxides are different.
Sulphide ores are roasted, meaning they are heated in the presence of excess air, to convert them into metal oxides. During roasting, metal sulphides are oxidised to metal oxides, and sulphur dioxide gas is given off. For example: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
Carbonate ores, on the other hand, are converted to metal oxides by calcination. Calcination is a process where the ore is heated in the absence of air or in a limited supply of air. During calcination, metal carbonates decompose to give metal oxide and carbon dioxide gas. For example: ZnCO₃ → ZnO + CO₂.
Thus, roasting is suitable for sulphide ores as it involves oxidation, while calcination is suitable for carbonate ores as it involves decomposition.
(c) Carbon can reduce lead oxide but not aluminium oxide.
Answer: Carbon can reduce lead oxide but not aluminium oxide because of the difference in the reactivity of lead and aluminium and the stability of their oxides. Aluminium oxide, due to aluminium’s great affinity for oxygen, is a very stable compound. It is not reduced easily by common reducing agents like carbon, carbon monoxide or hydrogen. Lead, being less reactive than aluminium, forms lead oxide which can be reduced by carbon. For example, PbO + C → Pb + CO. Oxides of highly active metals like aluminium cannot be reduced by common reducing agents like coke (carbon).
(d) Electrolytic reduction is done to obtain aluminium?
Answer: Electrolytic reduction is done to obtain aluminium because aluminium oxide is a very stable compound due to aluminium’s great affinity for oxygen. It is not reduced easily by common reducing agents like carbon, carbon monoxide or hydrogen. Hence, electrolytic reduction was chosen as the method for reducing alumina.
(e) Why ‘the food containing iron salts’ should not be cooked in aluminium utensils?
Answer: Aluminium is a reactive metal and is placed above iron in the metal activity series. According to the activity series, any metal higher up in the series displaces any metal below it from the salt solution of the latter. If food containing iron salts is cooked in aluminium utensils, the more reactive aluminium from the utensil may react with the iron salts. This reaction could lead to the displacement of iron by aluminium, with aluminium ions going into the food.
(f) A neutral gas other than oxygen is formed at the anode during electrolysis of fused alumina.
Answer: During the electrolysis of fused alumina using carbon anodes, oxygen is initially evolved at the anode. This oxygen then reacts with the carbon of the anode at high temperatures. The anode is oxidised to carbon monoxide (CO), which is a neutral gas, and this can further oxidise to carbon dioxide (CO₂). The reactions are:
2C + O₂ → 2CO (carbon monoxide – neutral gas)
2CO + O₂ → 2CO₂
Thus, carbon monoxide, a neutral gas other than oxygen, is formed at the anode.
(g) Why is powdered coke sprinkled on top of the electrolyte?
Answer: Powdered coke is sprinkled over the surface of the electrolytic mixture during the extraction of aluminium because it serves two main purposes:
(i) It reduces heat loss by radiation from the surface of the molten electrolyte.
(ii) It prevents the burning of the carbon anode by atmospheric oxygen.
(g) Why is powdered coke sprinkled on top of the electrolyte?
Answer: Powdered coke is sprinkled over the surface of the electrolytic mixture during the extraction of aluminium because it serves two main purposes:
(i) It reduces heat loss by radiation from the surface of the molten electrolyte.
(ii) It prevents the burning of the carbon anode by atmospheric oxygen.
7. For each substance listed below, explain its significance in the extraction of aluminium :
(a) Bauxite
Answer: Bauxite (Al₂O₃.2H₂O) is significant in the extraction of aluminium because it is the main ore from which aluminium is extracted. Bauxite typically contains about 60% Al₂O₃.
(b) Sodium hydroxide
Answer: Sodium hydroxide (conc. caustic soda solution) is significant in the extraction of aluminium because it is used in the Bayer’s process for the purification of bauxite ore. Bauxite is heated with concentrated sodium hydroxide solution, which dissolves the aluminium oxide (due to its amphoteric nature) forming soluble sodium meta aluminate (NaAlO₂), while impurities like ferric oxide and sand remain insoluble and are removed by filtration. The reaction is: Al₂O₃. 2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O. The filtrate containing sodium hydroxide is also reused.
(c) Cryolite
Answer: Cryolite (Na₃AlF₆) is significant in the electrolytic reduction of alumina for several reasons:
- It acts as a solvent for alumina (Al₂O₃), which has a very high melting point (approx. 2050°C). Cryolite lowers the fusion temperature of the electrolytic mixture to about 950°C, making electrolysis feasible at a lower temperature and reducing energy consumption.
- It increases the electrical conductivity of the molten mixture, as pure alumina is almost a non-conductor of electricity.
(d) Graphite
Answer: Graphite is significant in the extraction of aluminium as it is used for making the electrodes in the electrolytic cell (Hall-Héroult’s process). Thick rods of graphite are suspended in the fused electrolyte and serve as the anode. The inner lining of the electrolytic cell, made of gas carbon (a form of graphite), acts as the cathode. The graphite anode is consumed during the process as it reacts with the oxygen evolved.
8. Distinguish between electrolytic methods of reduction and refining.
Answer: Electrolytic reduction and electrolytic refining are two different processes used in metallurgy.
- Purpose: Electrolytic reduction is a method used for extracting highly reactive metals (like sodium, potassium, calcium, aluminium) from their molten compounds (ores like oxides or halides). Electrolytic refining is a method used for purifying metals that have already been extracted, to obtain them in a very pure state.
- Materials Involved: In electrolytic reduction, the molten ore or a fused salt of the metal is electrolyzed. In electrolytic refining, an impure block of the metal to be purified is made the anode, a thin sheet of the pure metal is made the cathode, and the electrolyte is a salt solution of the same metal.
- Outcome: In electrolytic reduction, the metal is obtained at the cathode from its ions in the molten ore. In electrolytic refining, pure metal from the anode dissolves and deposits onto the cathode, while impurities either settle down as anode mud or remain dissolved in the electrolyte.
9. Give three ways in which the metal zinc differs from the non-metal carbon. At least one of the differences must be a chemical difference.
Answer: The metal zinc differs from the non-metal carbon in the following ways:
- Electrical Conductivity: Zinc is a good conductor of heat and electricity. Carbon, in most of its allotropic forms (like diamond), is a poor conductor of electricity (though graphite is an exception and conducts electricity).
- Reaction with Dilute Acids (Chemical difference): Zinc reacts with dilute acids like hydrochloric acid or sulphuric acid to liberate hydrogen gas. For example, Zn + 2HCl → ZnCl₂ + H₂. Carbon does not react with dilute non-oxidizing acids to liberate hydrogen gas.
- Nature of Oxides (Chemical difference): Zinc forms zinc oxide (ZnO), which is an amphoteric oxide (behaving as both acidic and basic, though primarily basic). Carbon forms acidic oxides like carbon dioxide (CO₂) or a neutral oxide like carbon monoxide (CO).
10. (a) Aluminium is a more active metal than iron, but suffers less corrosion. Why?
Answer: Aluminium forms a thin, protective layer of aluminium oxide (Al₂O₃) on its surface, which prevents further oxidation and corrosion. This oxide layer acts as a barrier, protecting the underlying metal from exposure to air and moisture, thereby minimizing corrosion.
(b) Explain and give reasons why aluminium vessels should not be cleaned with powders containing alkalis.
Answer: Aluminium vessels should not be cleaned with powders containing alkalis because aluminium is an amphoteric metal. This means it reacts with both acids and strong alkalis. When aluminium comes into contact with alkalis (like sodium hydroxide present in some cleaning powders), it reacts to form soluble aluminates and hydrogen gas. This reaction causes the aluminium metal of the vessel to corrode or dissolve, damaging the vessel. The amphoteric nature of aluminium is evident as its oxide, alumina, dissolves in caustic soda solution to form sodium meta aluminate.
11. Given below in column A is a schematic diagram of the electrolytic reduction of alumina. Identify the parts labelled as A, B and C with the correct options from the Column B.
Answer: Based on the typical diagram for the electrolytic reduction of alumina (Hall-Héroult process):
- A (the lining of the tank) is the Cathode (Option 3).
- B (the rods dipping into the electrolyte) is the Anode (Option 2).
- C (the molten substance in the tank) is the Electrolyte mixture (Option 4).
12. Fill in the blanks:
(a) During the concentration of bauxite ore, aluminium goes in …………….. (soluble/insoluble) part because of its …………….. (acidic/basic/amphoteric) nature.
Answer: During the concentration of bauxite ore, aluminium goes in soluble part because of its amphoteric nature.
(b) In Hoope’s process, pure aluminium is collected at the ………………. (top/bottom) of the electrolytic cell.
Answer: In Hoope’s process, pure aluminium is collected at the top of the electrolytic cell.
Intext Questions and Answers III
1. Explain the following:
(a) Zinc is used to cover iron so as to prevent rusting of iron. Why?
Answer: As zinc is more reactive than iron, it forms a dense and impermeable layer of zinc oxide on the surface. This protective layer prevents air and moisture from reaching the iron beneath, thereby protecting it from rusting.
(b) In construction work, why is the alloy of aluminium-duralumin used rather than pure aluminium?
Answer: In construction work, the alloy of aluminium, duralumin, is used rather than pure aluminium because duralumin has a strength up to six times greater than pure aluminium.
2. What is an alloy? How do the properties of an alloy differ from its constituents?
Answer: An alloy is a homogeneous mixture of two or more metals or of one or more metals with certain non-metallic elements.
The properties of alloys are often greatly different from those of the components. For example, gold is too soft to be used without a small percentage of copper. The corrosion and oxidation resistance of steel is markedly increased by incorporation of 15 to 18% of chromium and often a few per cent of nickel. The presence of carbon up to 1.5% profoundly affects the properties of steel. Similarly, a low percentage of molybdenum improves the toughness and wear resistance of steel.
3. Both brass and bronze contain copper as major constituents. Name other elements in these alloys.
Answer: In brass, which contains copper as a major constituent, the other element is zinc.
In bronze, which also contains copper as a major constituent, the other elements are tin and zinc.
4. Name an alloy of:
(a) aluminium used in aircraft construction.
Answer: An alloy of aluminium used in aircraft construction is Duralumin.
(b) lead used in electrical wiring or electrical work in joining metals.
Answer: An alloy of lead used in electrical wiring or electrical work in joining metals is Solder (or Fuse metal).
(c) copper in electrical appliances or household vessels.
Answer: An alloy of copper used in electrical appliances or household vessels is Brass.
(d) zinc used in naval ships.
Answer: An alloy containing zinc used in naval ships (for marine engines) is Brass.
5. What is an amalgam? State its use with an example.
Answer: An amalgam is a mixture or an alloy of mercury with a number of metals or alloys such as sodium, zinc, gold and silver as well as with some non-metals.
An example of its use is dental amalgam, which is a mixture of mercury and a silver-tin alloy, used in dentistry.
6. (a) State two properties of brass that render it more useful for some purposes than its components.
Answer: Two properties of brass that render it more useful for some purposes than its components (copper and zinc) are:
(i) Brass is stronger than its components – copper and zinc.
(ii) Brass resists corrosion.
(b) Name a metal which forms a liquid alloy at ordinary temperature.
Answer: A metal which forms a liquid alloy (amalgam) at ordinary temperature is mercury. Sodium amalgam is an example of such an alloy which is liquid at room temperature.
7. Name the constituents of:
(a) Duralumin
Answer: The constituents of Duralumin are Aluminium (95%), Copper (4%), Magnesium (0.5%), and Manganese (0.5%).
(b) Solder
Answer: The constituents of Solder (or Fuse metal) are Lead (50%) and Tin (50%).
(c) Bronze
Answer: The constituents of Bronze are Copper (80%), Tin (18%), and Zinc (2%).
(d) Brass
Answer: The constituents of Brass are Copper (60-70%) and Zinc (40-30%).
8. Name the following:
(a) A metal which is liquid at room temperature.
Answer: A metal which is liquid at room temperature is mercury.
(b) A metal which is always present in amalgam.
Answer: A metal which is always present in amalgam is mercury.
(c) The process of heating an ore to a high temperature in the presence of air.
Answer: The process of heating an ore to a high temperature in the presence of air is Roasting.
(d) The compound formed by the reaction between calcium oxide and silica.
Answer: The compound formed by the reaction between calcium oxide (which can be formed from CaCO₃ flux) and silica (gangue) is slag (Calcium silicate, CaSiO₃).
(e) A compound which is added to lower the fusion temperature of the electrolytic bath in the extraction of aluminium.
Answer: A compound which is added to lower the fusion temperature of the electrolytic bath in the extraction of aluminium is Cryolite.
(f) Name an allotrope of a non-metal that allows electricity to pass through it.
Answer: An allotrope of a non-metal (carbon) that allows electricity to pass through it is Graphite.
9. Name the alloy used for the following purposes.
(a) Making electric circuits
Answer: The alloy used for making electrical connections (soldering purposes) in electric circuits is Solder.
(b) Making medals
Answer: The alloy used for making medals is Bronze.
(c) Making parts of watches
Answer: The alloy used for making parts of watches is Brass.
(d) Surgical instruments
Answer: The alloy used for making surgical instruments is Stainless steel.
(e) Aircraft
Answer: An alloy used for making bodies of aircraft is Duralumin. (Magnalium is also used for aircraft).
10. Using the information in the table below, complete the following:
(a) ………… is the metallic element.
Answer: Y is the metallic element.
(b) Metal atoms tend to have a maximum of ………… electrons in the outermost energy level.
Answer: Metal atoms tend to have a maximum of 3 electrons in the outermost energy level.
(c) Non-metallic elements tend to form ………… oxides while metals tend to form ………… oxides.
Answer: Non-metallic elements tend to form acidic oxides while metals tend to form basic oxides.
(d) Non-metallic elements tend to be ………… conductors of heat and electricity.
Answer: Non-metallic elements tend to be very poor conductors of heat and electricity.
(e) Metals tend to ………… electrons and act as ………… agents in their reactions with elements and compounds.
Answer: Metals tend to lose electrons and act as reducing agents in their reactions with elements and compounds.
Exercise
MCQs
1. The main ore used for the extraction of iron is:
(a) Haematite
(b) Calamine
(c) Bauxite
(d) Cryolite
Answer: (a) Haematite
2. Heating an ore in a limited supply of air or in the absence of air at a temperature just below its melting point is known as:
(a) Smelting
(b) Ore dressing
(c) Calcination
(d) Bessemerisation
Answer: (c) Calcination
3. The compound that is not an ore of aluminium is :
(a) Cryolite
(b) Corundum
(c) Fluorspar
(d) Bauxite
Answer: (c) Fluorspar
4. Fused alumina is reduced to aluminium by electrolytic reduction, since :
(a) alumina is highly stable.
(b) alumina is least stable.
(c) alumina is not reduced by drying agents.
(d) alumina is not reduced by reducing agents.
Answer: (d) alumina is not reduced by reducing agents.
5. The chemical name of the principal ore of aluminium is:
(a) sodium aluminium fluoride
(b) aluminium oxide
(c) hydrated aluminium fluoride
(d) hydrated aluminium oxide
Answer: (d) hydrated aluminium oxide
6. Brass is an alloy of :
(a) Copper and tin,
(b) Copper and zinc,
(c) Zinc and lead,
(d) Lead and tin.
Answer: (b) Copper and zinc,
7. Steel is an alloy of iron and:
(a) Nickel,
(b) Zinc,
(c) Carbon,
(d) Aluminium.
Answer: (c) Carbon,
8. The reason for using aluminium in the alloy duralumin is:
(a) Al is brittle
(b) Al gives strength
(c) Al brings lightness
(d) Al lowers melting point
Answer: (c) Al brings lightness
9. Duralumin is an alloy with composition :
P Al and Mg
R Al and Mn
Q Al, Cu and Mg
(a) Only P
(b) Only Q
(c) Both P and R
(d) Only R
Answer: (b) Only Q
10. Assertion (A): Hydrogen is a non–metal and can even form positive ions.
Reason (R): Metals are defined as the elements which form positive ions by the loss of electrons.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
11. Assertion (A): Sodium and potassium are reactive elements.
Reason (R): Sodium and potassium occur in free states.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
12. Assertion (A): Minerals are the substances from which metals can be extracted profitably.
Reason (R): Ores are those minerals from which metals are extracted commercially and profitably.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (d) (4)
13. Assertion (A): Aluminium is extracted from its ore bauxite.
Reason (R): Bauxite is the only ore of aluminium.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
14. Assertion (A): Metallurgy deals with the production and purification of metals.
Reason (R): Reduction is one of the methods of extraction of metals.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
15. Assertion (A): The concentration of aluminium is not done by leaching process.
Reason (R): In leaching process, the ore is dissolved in an acid or alkali.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (d) (4)
16. Assertion (A): Bayer’s process is used in the extraction of aluminium from its main ore.
Reason (R): Bayer’s process is done for the refinement of aluminium.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
17. Assertion (A): Reactive metals like calcium, magnesium cannot be obtained by reduction.
Reason (R): Reactive metals have a great affinity towards oxygen.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
18. Assertion (A): Aluminium is obtained from its oxide by electrolysis.
Reason (R): Aluminium has a great affinity towards oxygen so common reducing agents are not able to reduce it.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
19. Assertion (A): For electrical heating devices like electric iron and heater, alloys are used.
Reason (R): Alloys have a greater resistance and low melting point.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
20. Assertion (A): Brass is used in making medals, statues, etc.
Reason (R): Brass is hard and easily cast.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
Very Short Answer Type
1. Name the following:
(a) A metal which is found abundantly in the earth’s crust.
(b) The process used for the enrichment of sulphide ore.
(c) A metal present in cryolite other than sodium.
(d) A metal which is unaffected by dilute or concentrated acids.
(e) A metal present in period 3, group 1 of the periodic table.
(f) The property possessed by metals by which they can be beaten into sheets.
(g) A compound added to lower the fusion temperature of electrolytic bath in the extraction of aluminium.
(h) The ore of zinc containing its sulphide.
(i) The solution used to react with Bauxite as a first step in obtaining pure aluminium oxide, in the Baeyer’s process.
(j) The compound added to pure alumina to lower the fusion temperature during the electrolytic reduction of alumina.
Answer: (a) The metal which is found abundantly in the earth’s crust is Aluminium.
(b) The process used for the enrichment of sulphide ore is froth floatation.
(c) A metal present in cryolite (Na₃AlF₆) other than sodium is Aluminium.
(d) A metal which is unaffected by dilute or concentrated acids is Gold.
(e) A metal present in period 3, group 1 of the periodic table is Sodium (Na).
(f) The property possessed by metals by which they can be beaten into sheets is malleability.
(g) A compound added to lower the fusion temperature of the electrolytic bath in the extraction of aluminium is Cryolite.
(h) The ore of zinc containing its sulphide is Zinc blende.
(i) The solution used to react with Bauxite as a first step in obtaining pure aluminium oxide, in the Baeyer’s process is concentrated caustic soda solution (conc. NaOH solution).
(j) The compound added to pure alumina to lower the fusion temperature during the electrolytic reduction of alumina is Cryolite.
2. Choose the most appropriate answer from the following list of oxides which fits the description. Each answer may only be used once : [SO₂, SiO₂, Al₂O₃, MgO, CO, Na₂O]
Answer: (a) A basic oxide is Na₂O.
(b) An oxide which dissolves in water forming an acid is SO₂.
(c) An amphoteric oxide is Al₂O₃.
(d) A covalent oxide of a metalloid is SiO₂.
Short Answer Type
1. State the main components of the following alloys.
(a) Brass
(b) Duralumin
(c) Bronze
Answer: The main components of the alloys are:
(a) Brass: The main components are 60-70% Copper (Cu) and 40-30% Zinc (Zn).
(b) Duralumin: The main components are 95% Aluminium (Al), 4% Copper (Cu), 0.5% Magnesium (Mg), and 0.5% Manganese (Mn).
(c) Bronze: The main components are 80% Copper (Cu), 18% Tin (Sn), and 2% Zinc (Zn).
2. For each of the substances listed below, describe the role played in the extraction of aluminium.
(a) Cryolite
(b) Sodium hydroxide
(c) Graphite
Answer: The roles played by the substances in the extraction of aluminium are:
(a) Cryolite: Cryolite lowers the fusion temperature of alumina from 2050°C to 950°C and enhances its conductivity. It also acts as a solvent for the electrolytic mixture.
(b) Sodium hydroxide: In the Bayer’s process for the purification of bauxite ore, concentrated sodium hydroxide solution is used to dissolve bauxite (aluminium oxide) by heating under pressure. Bauxite dissolves and forms sodium meta aluminate (NaAlO₂) because of the amphoteric nature of aluminium, leaving behind insoluble impurities. The filtrate containing sodium hydroxide is again used to extract aluminium from bauxite.
(c) Graphite: In the electrolytic reduction of alumina (Hall-Héroult’s process), graphite is used for the electrodes. The inner carbon lining of the electrolytic cell acts as the cathode. Thick rods of graphite suspended in the fused electrolyte act as the anode. The anode (graphite) gets oxidised by the oxygen evolved at the anode, forming carbon monoxide and then carbon dioxide, and thus has to be replaced from time to time.
3. Write the chemical formulae of one main ore of iron and aluminium.
Answer: The chemical formula of one main ore of iron is Haematite, which is Fe₂O₃.
The chemical formula of the main ore of aluminium is Bauxite, which is Al₂O₃.2H₂O.
4. State the property of the metal being utilized in the following:
Answers:-
5. The following questions are relevant to the extraction of Aluminium. Answer in brief:
(a) State the reason for addition of caustic alkali to bauxite ore during purification of bauxite.
(b) Give a balanced chemical equation for the above reaction.
(c) Along with cryolite and alumina, another substance is added to the electrolyte mixture. Name the substance and give one reason for the addition.
Answer: (a) Caustic alkali (sodium hydroxide) is added to bauxite ore during its purification (Bayer’s process) because bauxite (Al₂O₃) dissolves in the concentrated caustic soda solution to form sodium meta aluminate due to the amphoteric nature of aluminium. This process leaves behind insoluble impurities like ferric oxide and sand (red mud), which can then be removed by filtration.
(b) The balanced chemical equation for the reaction of bauxite with caustic soda solution is:
Al₂O₃.2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O
(Bauxite) + (Caustic soda) → (Sodium meta aluminate) + (Water)
(c) Along with cryolite and alumina, fluorspar (Calcium fluoride, CaF₂) is added to the electrolyte mixture.
One reason for its addition is that, along with cryolite, it acts as a solvent for the electrolytic mixture and increases its conductivity, as pure alumina is almost a non-conductor of electricity. It also helps in lowering the fusion temperature of the mixture.
6. Write the constituents of electrolyte for the extraction of aluminium.
Answer: The constituents of the electrolyte for the extraction of aluminium (Hall-Héroult’s process) are a mixture of molten alumina (Al₂O₃) approximately 20%, cryolite (Na₃AlF₆) approximately 60%, and fluorspar (CaF₂) approximately 20%.
7. Write the equation for the reaction where the aluminum oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide.
Answer: The equation for the reaction where aluminium oxide (alumina) is obtained by heating aluminium hydroxide is:
2Al(OH)₃ –(heat, 1000°C)–> Al₂O₃ + 3H₂O
(Aluminium hydroxide) –(ignited at 1000°C)–> (Alumina) + (Water)
8. Write the equation for the reaction that occurs at the cathode during the extraction of aluminium by electrolysis.
Answer: The equation for the reaction that occurs at the cathode (inner carbon lining of the electrolytic cell) during the extraction of aluminium by electrolysis is:
4Al³⁺ + 12e⁻ → 4Al
9. Explain why it is preferable to use a number of graphite electrodes as anode instead of a single electrode, during the above electrolysis.
Answer: During the electrolytic extraction of aluminium, the anode is made of graphite. Oxygen is evolved at the anode, which then reacts with the carbon of the graphite anode, oxidizing it to carbon monoxide and carbon dioxide. As a result, the anode gets consumed and has to be replaced from time to time.
Using a number of graphite electrodes as anode is preferable because:
(i) It allows for the continuous replacement of individual consumed electrodes without completely stopping the electrolytic process.
(ii) It provides a large surface area for the anodic reaction to occur efficiently.
Long Answer Type
1. The following is an extract from ‘Metals in the Service of Man, Alexander and Street/Pelican 1976’: ‘Alumina (aluminium oxide) has a very high melting point of over 2000°C so that it cannot readily be liquefied. However, conversion of alumina to aluminium and oxygen, by electrolysis, can occur when it is dissolved in some other substance.’
(a) Which solution is used to react with bauxite as a first step in obtaining pure aluminium oxide ?
Answer: To obtain pure aluminium oxide as a first step, bauxite is heated under pressure with a concentrated caustic soda solution.
(b) The aluminium oxide for the electrolytic extraction of aluminium is obtained by heating aluminium hydroxide. Write the equation for this reaction.
Answer: The equation for obtaining aluminium oxide by heating aluminium hydroxide is:
2Al(OH)₃ –(heat, 1000°C)–> Al₂O₃ + 3H₂O
(c) Name the element which serves both as the anode and the cathode in the extraction of aluminium.
Answer: The element that serves as the cathode (as the gas carbon lining of the tank) and as the anode (as graphite rods) in the extraction of aluminium is carbon.
(d) Write the equation for the reaction that occurs at the cathode during the extraction of aluminium by electrolysis.
Answer: The equation for the reaction that occurs at the cathode during the extraction of aluminium by electrolysis is:
4Al³⁺ + 12e⁻ → 4Al
(e) Give the equation for the reaction which occurs at the anode when aluminium is purified by electrolysis.
Answer: The equation for the reaction that occurs at the anode when aluminium is purified by Hoope’s electrolytic process is:
Al – 3e⁻ → Al³⁺ (impure)
2. The following is a sketch of an electrolytic cell used in the extraction of aluminium :
(i) What is the substance of which the electrodes A and B are made ?
Answer: In the electrolytic cell for the extraction of aluminium, electrode A, which is the cathode, is made of the gas carbon lining of the tank. Electrode B, which is the anode, is made of graphite rods.
(ii) At which electrode (A or B) is the aluminium formed ?
Answer: Aluminium is formed at electrode A, the cathode.
(iii) What are the two aluminium compounds in the electrolyte C ?
Answer: The electrolyte C is a mixture of molten substances. The two aluminium compounds in this electrolyte are alumina (Al₂O₃) and cryolite (Na₃AlF₆).
(iv) Why is it necessary for electrode B to be continuously replaced ?
Answer: Electrode B, the anode, needs to be continuously replaced because it gets oxidised by the oxygen evolved at the anode, forming carbon monoxide and carbon dioxide.
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