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ISC Class 12 Chemistry Solved Question Paper 2005

Short Answer Type

11.

Correct the following statements :

Graphite has a two-dimensional sheet-like structure in which each carbon atom is sp³ hybridized.

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12.

Correct the following statements :

For water-system, the number of phases that can exist in equilibrium is two.

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13. Calculate the mole fraction of water in a sodium hydroxide solution which has 80 g of NaOH and 54 g of H₂O. [Relative atomic masses : Na = 23, O = 16, H = 1].

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14. The bond angles in CH₄ (109.5°), NH₃ (107°) and H₂O (104.5°) are different although these molecules have the same hybridization, sp³. Give a suitable reason for this.

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15.

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.

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16. State the main characteristics of an ionic and a network type of crystal citing a suitable example for each.

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17.

For the cell

Zn | Zn²⁺ (a = 1) | | Cu²⁺ (a = 1) | Cu

Given that E_Zn . Zn²⁺ = +0.761 volt; E_cu²⁺, 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}].

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18.
(i) What is the physical significance of entropy ?

(ii) How is the entropy of the universe changing ?

(i) Physical significance of entropy : Entropy is a measure of randomness or disorder of a system. Greater the randomness, higher will be the entropy the system for a given substance, the entropy of the system is maximum for the gaseous state and minimum for the crystalline solid state. That is why the entropy of a substance in the liquid state is in between the entropies of the substance in crystalline solid and gaseous state.

The process such as melting of ice, spreading of ink in water etc. are spontaneous and involve an increase in the randomness of the system. Hence all such processes cause an increase in the entropy of the system.

(ii) Entropy of Universe : Since all the spontaneous processes are irreversible and hence they are accompanied by an increase in the entropy of the system. Hence, the entropy of the universe is tending towards a maximum.

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