Principles & Isolation of Elements Test - 1

The correct answer is MgCO3 → MgO + CO2.

• Calcination
  • It is the process in which the ore of the metal is heated to high temperature in the absence or limited supply of air or oxygen.
  • Example - The limestone is brought to a temperature high enough to expel the carbon dioxide and the lime is produced.
• In option 3, the thermal decomposition of magnesium carbonate produces magnesium oxide and carbon dioxide.
• This reaction takes place at a temperature of over 650°C.

• Since we know that the absence of oxygen is essential for calcination, only the third option satisfies this condition.  

Concept:

Bauxite: Al(OH)₃ Aluminium Hydroxide

Calamine: ZnCO₃ Zinc Carbonate

Siderite: FeCO₃ Ferrous carbonate

Malachite: CuCO₃.Cu(OH)₂ Cupric Carbonate

A carbonate is a salt of carbonic acid (H₂CO₃) characterized by the presence of the carbonate ion, a polyatomic ion with the formula of CO²⁻.

The name may also refer to a carbonate ester, an organic compound containing the carbonate group

C(= O)(O–)₂.

Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium. Bauxite consists mostly of the aluminium minerals gibbsite.

Usually, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution at a temperature of 150 to 200°C. At these temperatures, the aluminium is dissolved as sodium aluminate (the Bayer process). The aluminium compounds in the bauxite may be present as gibbsite (Al(OH)₃), boehmite (AlOOH) or diaspore (AlOOH); the different forms of the aluminium component will dictate the extraction conditions.

The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and the most important ore of aluminium, contains only 30–60% aluminium oxide (Al₂O₃), the rest being a mixture of silica, various iron oxides, and titanium dioxide.

Concept:

• Aluminium is extracted from its ore, bauxite. The ore is dug out from the open-cast mines and contains aluminium oxide.
• The bauxite ore is purified to yield a white powder from which aluminium is extracted.
• The process of its extraction is done by electrolysis.

• Usually, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution at a temperature of 150 to 200°C.

• At these temperatures, the aluminium is dissolved as sodium aluminate (the Bayer process).

• The aluminium compounds in the bauxite may be present as gibbsite (Al(OH)₃), boehmite; the different forms of the aluminium component will dictate the extraction conditions.
• The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and the most important ore of aluminium, contains only 30–60% aluminium oxide (Al₂O₃), the rest being a mixture of silica, various iron oxides, and titanium dioxide.

Explanation:

• Leaching of Aluminium ore: In the process, the ore is treated with specific reagents to dissolve it whereas the impurities remain undissolved.
• The ore is then recovered from the solution by chemical processes.
• Bauxite contains many impurities such as titanium oxide, silica, and ferric oxides. As aluminium oxide is amphoteric it can be extracted.
• For its extraction, it is digested with caustic soda for several hours under pressure at temperature 150-170 °C.
• Sodium aluminate NaAlO₂ is formed which is soluble and gets dissolved.
• The impurities remain insoluble as red mud and are filtered out.
• Aluminium hydroxide Al(OH)3 is precipitated from the filtrate which is filtered out and calcified to get pure alumina.

​

• The flow diagram of the process is given below:

Hence, Bauxite ore is concentrated by Leaching.

Additional Information

Gold, silver are also extracted from their native ores by Leaching. 

• Silver containing lead as an impurity is removed by cupellation.
• Cupellation is a refining process in which ores or alloyed metals are treated under very high temperatures to separate noble metals present in the ore.

Additional Information 

Following are the refining process of metals:

The correct answer is option 1, i.e. Bauxite, Magnetite, Malachite and Calamine respectively.

Table: Methods of Extraction and Chief Ores of Metals:

Concept:

In Hall-Heroult’s process, aluminium is formed at the cathode. The cathode is made out of carbon. In this method, Al₂ O₃ is melted with crylite, Na₃ [AlF₆] and electrolyzed in a graphite lined steel tank, which serves as the cathode. The anode is also made of graphite.

The cell runs continuously and at intervals, molten aluminium is drained from the bottom of the cell and more bauxite is added. The electrolytic reaction is as follows:

At cathode: Al³⁺ + 3e^- → Al

At anode:

C (s) + O^2- (melt) → CO (g) + 2e^-

The correct answer is option 1 i.e Al + Mg.

• Magnalium is an alloy of Aluminium and Magnesium.
• It is used in making metal mirrors, the frame of aeroplanes and scientific instruments.

Some important alloys to remember are: 

Explanation:

An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulfides.

From the Ellingham diagram, we can say that any oxide with the lower value of ΔG° is more stable than oxide with a higher value of ΔG°. We can also predict that the oxide placed higher in the diagram can be reduced by the element involved in the formation of its oxide placed lower at that temperature in the diagram.

We know that, ΔGis Gibbs free energy. ΔG° is standard Gibbs energy change.

It is happening in case of ZnO for its reduction by Al at 1400℃.

Example:

Concept -

Van Arkel Method: 

• This method is used for the refinement of metals. 
• Through this method, ultra-pure metal is obtained. 
• The impure metals are turned or converted into volatile compounds. 
• Then the metal is decomposed (thermal breakdown) to obtain the pure form of metal. 
• Metals like titanium (Ti), Zirconium (Zr), Hafnium (Hf), Silicon (Si)  are purified by using this method. 

Purification of Titanium by this method: 

• ​Volatile titanium tetra-iodide ( TiI4 ) is generated when impure titanium metal is heated with iodine at a temperature of 250⁰C. 
• Because contaminants do not react with iodine, they are left behind.
• At 1400 degrees Celsius, the titanium tetra-iodide vapour is passed across a heated tungsten filament.
• Pure titanium is placed on the filament and removed when the vapour decomposes.
• Iodine is recycled i.e. can be reused. 
• Reaction - 

Ti + 2I₂ → TiI₄ (at 250°C)

TiI₄ → Ti + 2I₂ (at 1400°C)

Explanation -

• Van Arkel method is a method used to obtain pure metals. 
• The impure metals are first converted to their volatile form and then decomposed. 

Metals like titanium (Ti), Zirconium (Zr), Hafnium (Hf), Silicon (Si) are purified using this method.

Hence the correct answer to this question is - Titanium(Ti).

Smelting is a process of extracting metal from its ore by reducing metal oxide by heating with coke

ZnO + C → Zn + CO

Carbon monoxide formed also acts as a reducing agent.

ZnO + CO → Zn + CO₂

Other processes are defined as:-

• Refining is a process of removing impurities or unwanted elements from a substance, typically as part of an industrial process
• Roasting is a process of heating of sulphide ore to a high temperature in the presence of air; The sulphide ores are converted into oxides by heating strongly in the presence of excess air
• The carbonate ores are changed into oxides by heating strongly in limited air. This process is known as calcination