12th chemistry Important questions
1. What is the difference between minerals and ores?
- Limestone (CaCO3) is used as a basic flux in the extraction of iron from its oxide Fe2O3. Limestone decomposes to form CaO
CaCO3 🠖CaO + CO2
- Impurity silica (SiO2)reacts with CaO to form fusible slag calcium silicate.
CaO(s) + SiO2(s) 🠖 CaSiO3(s)
Flux Gangue Slag
4. Which type of ores can be concentrated by the froth flotation method? Give two examples for ores.
- Sulphide ores can be concentrated by the froth flotation method.
Ex: Galena (PbS), Zinc blende (ZnS)
5. Describe a method for refining nickel. (or) Write a note about Mond’s Process.
- Impure nickel is heated in a stream of carbon monoxide at around 350K. Nickel reacts with CO to form a highly volatile nickel tetracarbonyl. The solid impurities are left behind.
Ni(s) +4CO(g)🠖 350 K Ni[CO]4(g)
- On heating nickel tetra carbonyl around 460K, decomposes to give pure nickel.
Ni[CO]4(g) 460 K Ni(s)+ 4CO(g)
6. Explain zone refining process with an example
Zone refining process:
Principle :
- When an impure metal is melted and allowed to solidify, the impurities will prefer to remain in the molten region. Impurities are more soluble in the melt than in the solid state metal.
Process:
- The impure metal is taken in the form of a rod. One end of the rod is heated using a mobile induction heater, melting the metal on that portion of the rod.
- When the heater is slowly moved to the other end pure metal crystallises while impurities will move on to the adjacent molten zone formed due to the movement of the heater.
- As the heater moves further away, the molten zone containing impurities also moves along with it.
- This process is repeated several times by moving the heater in the same direction again and again to achieve the desired purity level.
- This process is carried out in an inert gas atmosphere to prevent the oxidation of metals.
- Germanium, Silicon and Gallium which are used as semiconductor are refined by this process.
7. Using the Ellingham diagram given below.
(A) Predict the conditions under which
- Aluminium might be expected to reduce magnesia.
- Ellingham diagram for the formation of Al2O3 and MgO intersects around 1600K. Above this temperature aluminium line lies below the magnesium line. Hence we can use aluminium to reduce magnesia above 1600K.
2. Magnesium could reduce alumina.
- In the Ellingham diagram below the 1600K magnesium line lies below the aluminum line. Hence below 1600K magnesium can reduce alumina.
B)It is possible to reduce Fe2O3 by coke at a temperature around 1200K
- In the Ellingham diagram above the 1000K carbon line lies below the iron line. Hence it is possible to reduce Fe2O3 by coke at a temperature around 1200K.
8. Give uses of Zinc.
- Metallic zinc is used in galvanisation to protect iron and steel structures from rusting and corrosion.
- Zinc is used to produce die - castings in the automobile, electrical and hardware industries.
- Zinc oxide is used in the manufacture of paints, rubber, cosmetics, pharmaceuticals, plastics, inks, batteries, textiles and electrical equipment.
- Zinc sulphide is used in making luminous paints, fluorescent lights and x - ray screens.
- Brass an alloy of zinc is used in water valves and communication equipment as it is highly resistant to corrosion
9. Explain the electrometallurgy of Aluminium.
Hall – Herold Process:
Cathode : Iron tanked lined with carbon
Anode : Carbon blocks
Electrolyte : 20% solution of alumina obtained from bauxite + Molten Cryolite + 10% calcium chloride (lowers the melting point of the mixture)
Temperature: Above 1270K
- REACTION :
Ionisation of Alumina : Al2O3 →2Al3+ + 3O2-
Reaction at cathode: 2Al3+ (melt) + 6e →2Al(l)
Reaction at anode: 6O2- (melt)→ 3O2 + 12e−
Since carbon acts as anode the following reaction also takes place on it.
C(s) + O2−(melt)→ CO + 2e−
C(s) + 2O2−(melt) →CO2 + 4e−
During electrolysis anodes are slowly consumed due to the above two reactions. Pure aluminium is formed at the cathode and settles at the bottom.
- Net electrolysis reaction is
4Al3+ (melt) + 6O2- (melt) + 3C(s) 4Al(l) + 3CO2(g)
10. Explain the following terms with suitable examples. i) Gangue ii) Slag
i) Gangue:
- The non metallic impurities, rocky materials and siliceous matter present in the ores are called gangue.
- (eg): SiO2 is the gangue present in the iron ore Fe2O3 .
ii) Slag:
- Slag is a fusible chemical substance formed by the reaction of gangue with a flux.
- CaO(s) + SiO2(s) →CaSiO3(s)
- Flux gangue slag
11. Give the basic requirement for vapour phase refining.
- The metal is treated with a suitable reagent to form a volatile compound.
- Then the volatile compound is decomposed to give the pure metal at high temperature.
12. Describe the role of the following in the process mentioned.
i) Silica in the extraction of copper.
- In the extraction of copper, silica acts as an acidic flux to remove FeO as slag FeSiO3.
FeO(s) + SiO2(s) → FeSiO3(s)
Gangue Flux Slag
ii) Cryolite in the extraction of aluminium.
- As Al2O3 is a poor conductor, cryolite improves the electrical conductivity. In addition, cryolite serves as an added impurity and lowers the melting point of the electrolyte.
iii) Iodine in the refining of Zirconium.
- Iodine forms a Volatile tetraiodide with impure metal, which decomposes to give pure metal. Impure zirconium metal is heated in an evacuated vessel with iodine to form the volatile zirconium tetraiodide (ZrI4). The impurities are left behind, as they do not react with iodine.
Zr(s) + 2I2(s) → ZrI4(Vapour)
- On passing volatile zirconium tetraiodide vapour over a tungsten filament, it is decomposed to give pure zirconium.
ZrI4(Vapour) → Zr(s) + 2I2(s)
iv) Sodium cyanide in froth floatation.
- Sodium cyanide acts as a depressing agent in froth floatation process. When a sulphide ore of a metal contains other metal sulphides, the depressing agent sodium cyanide selectively prevent other metal sulphides coming to the froth.
- eg: NaCN depresses the floatation property ZnS present in Galena (PbS) by forming a layer of Zinc complex Na2 [Zn(CN)4]on the surface of Zinc sulphide.
13. Explain the principle of electrolytic refining with an example.
Principle: Electrolysis
Crude metal is refined by electrolysis carried out in an electrolytic cell.
Cathode : Thin strips of pure metal.
Anode : Impure metal to be refined.
Electrolyte : Aqueous solution of the salt of the metal with dilute acid. As current is passed,metal ions from the electrolytic solution will be deposited at the cathode. Less electro positive impurities in the anode settle down as anode mud.
Example : Electro refining of silver
Cathode : Pure silver
Anode : Impure silver rods.
Electrolyte: Acidified aqueous solution of silver nitrate.
When current passed, the following reactions will take place.
Reaction at anode: Ag(s) → Ag+(aq) + e−
Reaction at cathode: Ag+(aq) + e− → Ag(s)
At anode silver atoms lose electrons and enter the solution. From the solution, silver ions (Ag+)migrate towards the cathode. At cathode silver ions get deposited on the cathode.
15. Give the limitations of Ellingham diagram.
- It gives information about the thermodynamic feasibility of a reaction.
- It does not tell anything about the rate of the reaction.
- More over it does not give any idea about the possibility of other reactions that might be taking place.
- The interpretation of ∆G is based on the assumption that the reactants are in equilibrium with the product which is not always true.
CLASS 12 :
➤ All SUBJECTS unit wise Important Questions - CLICK HERE
CLASS 11 :
➤ All SUBJECTS unit wise Important Questions - CLICK HERE
CLASS 10 :
➤ All SUBJECTS unit wise Important Questions - CLICK HERE
******📣
Join *whatsapp group* 🔈
TN 12,11,10th IMPORTANT QUESTION and Centum Plan for board exams...
📣Join *whatsapp group* 🔈
*For Class 11,12th :* (only 11&12th should join here) https://chat.whatsapp.com/GxIjB2C6QsL2TVLDZoDgyR
*For Class 10th:*(only 9th & 10th should join here) https://chat.whatsapp.com/GUn6LRGKEd47j29yA3prF6
*You guys too join the group and share it with your school friends and make them to join this whatsapp group ...*
