Full Marks: 35Pass Marks: 14

Very SAQ [any 10] 10×2=20
1) Differentiate between molarity and molality for a solution. How does a change in temperature influence their values?
2) Non-ideal solutions exhibit either positive or negative deviations from Raoult’s law. What are these deviations and why are they caused? Explain with one example for each type.
3) Define the term osmotic pressure.
4) What is meant by reverse osmosis?
5) State Kohlrausch’s law of independent migration of ions. Why does the conductivity of a solution decreases with dilution?
6) Give an example of a fuel cell and write the cathode and anode reactions for it. 
7) Write two differences between ‘order of reaction’ and ‘molecularity of reaction.'.
8) What are pseudo first order reactions? Give one example of such reactions.
9) Define the following terms. (i) Rate constant  (ii) Activation energy
10) Distinguish between physisorption and chemisorption.
11) Define each of the following terms. (i) Micelles (ii) Calcination
12) What is the difference between lyophobic sol and lyophilic sol?
13) Describe the principles of Zone refining.
14) Describe the principles of Vapour Phase Refining.
15) What is 'Copper matte'?
16) What is the role of dil. NaCN in extraction of gold?

SAQ [any 3] 3×3=9
1) Name the sulphide ore of Copper. Describe how pure copper is extracted from this ore. 
2) Define the following terms with suitable examples : (i) Peptization (ii) Electrophoresis (iii) Dialysis 
3) Describe the role of the following : (i) Cryolite in the extraction of aluminium from pure alumina. (ii) NaCN in the extraction of silver from a silver ore. (iii) Coke in the extraction of iron from its oxides
4) Name an important ore of silver. How is silver extracted from its sulphide ore? Give balanced chemical equations involved in the extraction of pure silver. 

One marks question. 6×1=6
1) Describe the role of NaCN in froth floatation process
2) Name the method of refining of metals such as Germanium
3) What are Crystalline Solids
4) Define Frenkel defects.
5) Define Schottky defects.
6) Define coordination number.

VSQ
1) Ans:
Molality is defined as the number of moles of the solute per kilogram of the solvent. It is represented by $m$.
Molality($m$) $= \frac{(no. \, of \, moles \, of \, solute) \times 1000}{mass \, of \, solvent \,  (in \, gram)}$
It does not change with change in temperature.
Molarity is defined as the number of moles of solute dissolved in one litre or one cubic decimetre of the solution.
Molarity($M$) $= \frac{no. \, of \, moles \, solute \times 1000}{volume \, of \, the \, solution \, (in \, mL}$
It decreases with increasing in temperature.

2) Ans:
Positive deviation means A—B interactions are weaker than A—A and B—B interactions while, opposite is true in case of negative deviation. For non-ideal solutions, vapour pressure is either higher or lower than that predicted by Raoult’s law. If it is higher, the solution exhibits positive deviation and if it is lower, it exhibits negative deviation from Raoult’s law. In case of positive deviation, A—B interactions are weaker than A—A and B—B interactions. Due to this, vapour pressure increases which results in positive deviation. e.g. Ethanol+Acetone and $CS_2$+Acetone show positive deviation. For positive deviation, $ \Delta H $ (mixing) = Positive In case of negative deviation, A—B interactions are stronger than A—A and B—B interactions. Due to this, vapour pressure decreases which results in negative deviation. e.g. Phenol + Aniline and Chloroform+ Acetone show negative deviation. For negative deviation, $ \Delta H $ (mixing) = Negative

3) Ans: Osmotic pressure is the extra pressure which is applied on the solution just to prevent the flow of solvent into the solution through a semipermeable membrane.

4) Ans:
If a pressure higher than the osmotic pressure is applied in the solution, the solvent will flow from the solution into the pure solvent through the semi-permeable membrane and the process is called reverse-osmosis.

5) Ans: Kohlrausch’s law of independent migration of ions states that the limiting molar conductivity of an electrolyte can be expressed as the sum of the individual contributions of the anion and the cation of the electrolyte, e.g. $ \lambda ^o_{m(CH_3COOH)}= \lambda^o _{CH_3COO^-} \, + \, \lambda ^o _{H^+}$ The conductivity of a solution is related with the number of ions present per unit volume of the solution. When the solution is diluted, the number of - ions per unit volume decreases. Hence, conductivity or specific conductance of the solution decreases.

6) Ans: Fuel cells :Galvanic cells that are designed to convert the chemical energy from combustion of fuels like hydrogen, methane, methanol directly into electrical energy are called fuel cells. The efficiency of fuel cell is higher than that of galvanic cell, e.g. hydrogen-oxygen fuel cell, in which hydrogen and oxygen are bubbled through porous carbon electrodes into conc. KOH solution. Finely divided Pt or Pd metals are incorporated into the electrodes for increasing the rate of electrode reactions. At cathode $O_2(g) \, + \, 2H_2O(l) \, + \, 4e^{-} \rightarrow 4OH^-(aq) $ At anode $ 2H_2(g) \, + \, 4OH^-(aq) \rightarrow 4H_2O(l) \, + \, 4e^{-} $ Overall reaction $ 2H_2(g) \, + \, O_2 (g) \rightarrow 2H_2O(l) $

7) Ans:

Molecularity of a reaction Order of reaction
The number of reacting species which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction. The sum of powers of the concentrations of the reactants in the rate law expression is called the order of that chemical reaction.
Molecularity is. always a whole number value. Order may have zero, whole number, negative or fractional, values.
8) Ans:
Pseudo first order reaction: The reaction which is bimolecular but has order one, is called pseudo first order reaction, e.g. acidic hydrolysis of ester.

9) Ans:
(i) Rate of chemical reaction when concentration of each reactant is unity is called rate constant. It is also known as specific rate of reaction.
(ii) It is the extra energy contained by reactant molecules that results into effective collisions between them to form the products. It is denoted by $E_a $

10) Ans:
Physisorption Chemisorption
It arises because of van der Waals’ forces It is caused by chemical bond formation.
It is reversible in nature. It is irreversible.


11) Ans:
(i)Micelles :Some substances at higher concentrations show colloidal behaviour due to the formation of aggregates. These aggregated particles are called micelles or associated colloids
(ii)Calcination : The process of heating of metal ore in the absence of air is called calcination. 

12) And:
Lyophilic sol: The colloidal sols directly formed by mixing substances like gum, gelatin, starch with a suitable liquid (the dispersion medium) are Called lyophilic sols. If the dispersion medium is separated from dispersed phase, the sol can be reconstituted by simply mixing with the dispersion medium. That is why these sols are also called reversible sols. These sols are quite stable and cannot be easily coagulated. 
Lyophobic sols: The colloidal sols like gold do not formed directly by mixing with dispersion medium and can be prepared only by special methods. These are called lyophobic sols. These sols are readily coagulated on the addition of small amounts of electrolytes, or by heating or shaking and hence are not stable. Once coagulated, they do not give back the colloidal sol by simple addition of the dispersion medium. Hence, these sols are called irreversible sols.

13) Ans:
Zone-refining: It is based on the fact that the impurities are more soluble in the molten state than in the solid state of the metal. 

14) Ans:
Vapour phase refining :In this method, the metal is converted into its volatile compound and is collected elsewhere. It is then decomposed to give pure metal. 
So, the two requirements are 
(a) the metal should form a volatile compound with an available reagent. 
(b) the volatile compound should be easily decomposable for the recovery of metal

15) Ans:
Copper matte (containing $ Cu_2S$ and $ FeS$ ) is obtained by the fusion of roasted copper sulphide concentrated ores. It is done to separate the copper sulphide from the gangue and other metals that floats on the surface of the melt.

 16) Ans:
Role of NaCNn in the extraction of gold :In this process, leaching of gold ore with a dilute solution of NaCN occurs in the presence of air from which the gold is obtained later by displacement method. 
$4 \mathrm{Au}(s)+8 \mathrm{CN}^{-}(a q)+2 \mathrm{H}_2 \mathrm{O}(a q)+\mathrm{O}_2(g)$
$ \rightarrow 4\left[\mathrm{Au}(\mathrm{CN})_2\right]^{-}(\mathrm{aq})+4 \mathrm{OH}^{-}(a q) $
$2\left[\mathrm{Au}(\mathrm{CN})_2\right]^{-}(a q)+ \, \mathrm{Zn}(s) $
$\longrightarrow \, {\left[\mathrm{Zn}(\mathrm{CN})_4\right]^{2-}(a q)+2 \mathrm{Au}(s)(\boldsymbol{\imath}) }$

SAQ
1)
2)
3) Ans:
(i) Alumina ($Al_2O_3$) is subjected to electrolytic reduction to give pure aluminium. Alumina is mixed with Cryolite ($Na_3AIF_6$) which lowers its melting point and also increases the electrical conductivity of alumina, thereby facilitating the electrolytic reduction process. 
(ii) Role of $NaCN$ in the extraction of silver ($Ag$) is to do the leaching of Ag ore in the presence of air (for $O_2$) where silver ore gets dissolved and from which the silver is obtained later by replacement whereas impurities remain insoluble. 
$4Ag \, + \, 8CN^- \, + \, 2H_2O \, + O_2$
$ \rightarrow 4 \left[ Ag(CN)_2 \right]^- + 4 \left[ OH \right] ^- $
$2 \left[ Ag(CN)_2 \right] ^-  + Zn$
$  \rightarrow 2 Ag+ \left[  Zn(CN)_4 \right]^{-2}$
(iii) Coke is source of heat in blast furnace and acts as a reducing agent for iron ore. Coke is burnt to produce carbon monoxide which later reacts with the iron oxides to reduce them in steps to yield iron, further it reacts with silica impurities to remove them as soluble impurities. 

4) Ans:
Important ore mineral of silver is argentite ($Ag_2S$, silver sulphide). 
Extraction of silver involves leaching the metal with $CN^-$, this is an oxidation reaction.
$4Ag \, + \, 8CN^- \, + \, 2H_2O \, + O_2$
$ \rightarrow 4 \left[ Ag(CN)_2 \right]^- + 4 \left[ OH \right] ^- $
Later the pure metal is recovered by displacement method. 
$2 \left[ Ag(CN)_2 \right] ^-  + Zn$
$  \rightarrow 2 Ag+ \left[  Zn(CN)_4 \right]^{-2}$