Subject

Chemistry

Class

CBSE Class 12

Pre Boards

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Sample Papers

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 Multiple Choice QuestionsShort Answer Type

1.

Aluminum crystallizes in an fcc structure. Atomic radius of the metal is 125 pm. What is the length of the side of the unit cell of the metal? 


Aluminum crystallizes in FCC structure atomic radius (r) = 125 pm

Length of the side of the unit cell in FCC structure,  straight a equals fraction numerator 4 over denominator square root of 2 end fraction straight r space equals space 2 square root of 2 straight r end root

 Therefore            a= 2 x 1.414 x 125

a = 353.5 pm

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2.

How are interhalogen compounds formed? What general compositions can be assigned to them?


An interhalogen compound is a molecule which contains two or more different halogen atoms. A most interhalogen compound known are binary. Their formula is generally XYn Where n=1, 3, 5, or 7 and X is the less electronegative of two halogens. On hydrolysis, they ionise to give rise to poly -halogen ion. For example

Br2 (l) + F2 (g) -->  2BrF (g)

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3.

Of physisorption or chemisorption, which has a higher enthalpy of adsorption? 


Chemisorption has a higher enthalpy of adsorption as compared to physisorption.

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4.

(a) For a reaction A + B -->  P, the rate law is given by,

r = k [A]1/2 [B]2.

What is the order of this reaction?

(b) A first order reaction is found to have a rate constant k = 5·5 x 10-14 s-1. Find the half-life of the reaction. 


(a)

 For A + B-->  P

r = k [A]1/2 [B]2

The order of the reaction = 2 plus 1 half space equals 5 over 2 space equals 2.5

(b) For first order reaction

k = 5.5 x 10-14 s-1

Half-life period (straight t subscript bevelled 1 half end subscript) for the first order reaction

 straight t subscript bevelled 1 half space equals end subscript fraction numerator 0.693 over denominator straight k end fraction space equals fraction numerator 0.693 space over denominator 5.5 space straight x space 10 to the power of negative 14 end exponent end fraction

straight t subscript bevelled 1 half end subscript space equals 1.29 space straight x space 10 to the power of 13 space straight s

 

 

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5.

Name the method used for refining of copper metal.


The electrolytic method is used for refining copper metal.

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6.

The standard electrode potential (E°) for Daniell cell is +1·1 V. Calculate the   G° for the reaction

Zn(s) + Cu2+ (aq)  ----> Zn+ (aq) + Cu (s)

(1 F = 96500 C mol-1).


E° for Daniel cell = 1.1 V

Zn(s) + Cu2+ (aq)--->  Zn2+ (aq) + Cu(s)

1F = 96500 C mol-1

G° =?

n = 2 (no. of e-s exchanged)

Since,  G° = -nFE°

Therefore,  G° = -2 x 96500 x 1.1

G° = -212300 J mol-1

G° = -212.3 kJ mol-1

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7.

(a) Why does the presence of an excess of lithium make LiCl crystals pink?

(b) A solid with cubic crystal is made of two elements P and Q. Atoms of Q are at the corners of the cube and P at the body-center. What is the formula of the compound? 


(a) When crystals of LiCl is heated in presence of excess of lithium, Cl- ions from crystal diffuse on the surface and combine with ionized Li to form LiCl. The released unpaired electrons occupy the anionic sites known as F-centers. The pink colour results by excitation of these electrons when they absorb energy from visible light falling on them.

(b) It is given that the atoms of Q are present at the corners of the cube. Therefore, number of atoms of Q in one unit cell =

Also, it is also given that the atoms of P are present at the body-center.

Therefore, number of atoms of P in one unit cellequals 1 over 8 straight x 8 = 1

This means that the ratio of the number of P atoms to the number of Q atoms, P: Q = 1:1

Hence, the formula of the compound is PQ.

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8.

Define the following terms giving an example of each:

(i) Associated colloids

(ii) Lyophilic sol

(iii) Adsorption


(i) Associated colloids: Associated colloids are those colloids which behave as normal strong electrolytes at low concentrations but exhibit colloidal properties at higher concentrations due to the formation of aggregated particles thus formed are micelles. For e.g. grease droplet surrounded by stearate ions.

(ii) Lyophilic sol: Lyophilic means liquid-loving colloidal sols directly formed by mixing substances like gum, gelatine etc. with a suitable liquid (the dispersion medium) are called lyophilic sols. If the dispersion medium is separated from dispersed phase (say by evaporation), the sol can be reconstituted by simply remising with the dispersion medium that is why these sols are also known as reversible sols.

(iii) Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is known as adsorption. The molecular species which accumulates on the surface is termed as adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.

For the e.g. aqueous solution of raw sugar, when passed over beds of animal charcoal, becomes colourless as the colouring substances are adsorbed by the charcoal.

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9.

Draw the structures of the following molecules:

(i) XeF6

(ii) H2S2O7


(i) Structure of XeF6 :

 

(ii) H2S2O7

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10.

Outline the principles of refining of metals by the following methods: 

(i) Zone refining

(ii) Vapour phase refining


Principles of refining of metals by:

(i) Zone refining: The impurities are more soluble in the melt than in the solid state of the metal. Hence, the pure metal crystallises out of the melt and impurities pass on into the adjacent molten zone and like this at one end impurities get concentrated. Silicon, indium, boron, gallium etc. can be purified by this process.

(ii) Vapour phase refining: Vapour phase refining is the process of refining metal by converting it into its volatile compound and then, then decomposing it to obtain a pure to carry out this process

(a) The metal should form a volatile compound with an available reagent.

(b) The volatile compound should be easily decomposable so that the metal can be easily recovered.

Nickel, Zirconium, and titanium are using this method.
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