Correct the following statements :
Graphite has a two-dimensional sheet-like structure in which each carbon atom is sp3 hybridized.
Correct the following statements :
For water-system, the number of phases that can exist in equilibrium is two.
Radium has a half-life of 1580 years. How long would it take for each of the samples of radium given below to disintegrate ?
(i) 20% of a 5 g sample.
(ii) 20% of a 50 g sample.
For the cell
Zn | Zn2+ (a = 1) | | Cu2+ (a = 1) | Cu
Given that EZn . Zn2+ = +0.761 volt; Ecu2+, Cu = +0.339 volt
(i) Write the cell reaction
(ii) Calculate the emf and free energy change at 298 K involved in the cell. [Faraday’s constant F = 96500 coulomb equivalent– 1].
(i) What is the physical significance of entropy ?
(ii) How is the entropy of the universe changing ?
For the extraction of aluminium, write
(i) The most important ore.
(ii) One method for the purification of the mentioned ore.
(iii) The role of cryolite and calcium fluoride in electrolysis.
i) Bauxite (A12O3 . 2H2O) is the most important ore of aluminium.
(ii) Hall’s Process of purification of Bauxite : Bauxite ore is first fused with sodium carbonate. The Al2O3 forms sodium meta aluminate with sodium carbonate. The fused mass is now extracted with water where Fe2O3 and SiO2remain insoluble in the residue.
Al2O3 +Na2CO3 --> 2NaAlO3 +CO2
This solution is warmed to 50 - 60°C and then CO2 is passed through it. Al(OH)3 is separated out as precipitate.
2NaAlO2 +CO2 +3H2O --> 2Al(OH)3 +Na2CO3
Now the precipitate of Al(OH)3 is filtered, washed and dried up.
(iii) Role of cryolite and calcium fluoride : The pure alumina is a bad conductor of electricity and has a fusion temperature of about 2000°C which is more than the boiling point of aluminium (1800°C). So, to overcome these difficulties a mixture of alumina, cryolite and calcium fluoride in the ratio of 1 : 2 : 1 is used for electrolysis. This mixture is a good conductor of electricity and has a fusion temperature of 900°C.