Chemistry Question Paper 2019 (Set-3)

General Instructions:

1. All questions are compulsory
2. Section A : Q. no. 1 to 5 are very short-answer questions and carry 1 mark each.
3. Section B : Q. no. 6 to 12 are short-answer questions and carry 2 marks each.
4. Section C: Q. no. 13 to 24 are also short-answer questions and carry 3 marks each.
5. Section D : Q. no. 25 to 27 are long-answer questions and carry 5 marks each.

1. There is no overall choice. However, an internal choice has been provided in two questions

of one mark, two questions of two marks, four questions of three marks and all the three questions of five marks weightage. You have to attempt only one of the choices in such questions.

1. Use log tables if necessary. Use of calculators is not allowed.

Question 1

Arrange the following in decreasing order of solubility in water:

<math>(CH_3)_3N</math>, <math>(CH_3)_2NH</math>, <math>CH_3NH_2</math>

SOLUTION:

Higher the value of hydration enthalpy, more is the solubility. Also, larger the hydrocarbon part attached to the nitrogen of amine, lower is the tendency to form hydrogen bonds and lower is the solubility.

Thus, the decreasing order of solubility in water is <math>CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N</math>

Question 2

What type of stoichiometric defect is shown by ZnS and why?

SOLUTION:

The stoichiometric defect shown by ZnS is Frenkel defect which is due the large difference in the size of the ions, i.e. due to small size of Zn²⁺.

Question 3

Write one stereochemical difference between <math>S_N1</math> and <math>S_N2</math> reactions.

SOLUTION:

S_N1 reactions lead to the formation of racemic mixture whereas S_N2 reactions lead to inversion in configuration of product.

Question 4

Why are medicines more effective in colloidal state?

OR

What is difference between an emulsion and a gel?

SOLUTION:

Some medicines are more effective in colloidal form because medicine in colloidal form is easily absorbed by the body tissues.

For example, Antibiotics like streptomycin in the form of the colloidal sol is injected into the body for the more effective result.

OR

Emulsion: It is a colloidal mixture in which both the dispersed phase and the dispersion medium are liquids. The two or more liquids present in the mixture are generally immiscible in nature. Example: Milk.

Gel: It is a colloid in which the dispersed phase i.e. liquid has combined with the dispersion medium i.e. solid to produce a semisolid material. Example: jellies.

Question 6

When MnO₂ is fused with KOH in the presence of KNO₃ as an oxidizing agent, it gives a dark green compound (A). Compound (A) disproportionates in acidic solution to give purple compound (B). An alkaline solution of compound (B) oxidises KI to compound

1. whereas an acidified solution of compound (B) oxidises KI to (D). Identify (A), (B), (C), and (D).

SOLUTION:

Potassium permanganate is prepared by fusion of MnO₂ with an alkali metal hydroxide and an oxidising agent like KNO₃. This produces the dark green K₂MnO₄ which disproportionates in a neutral or acidic solution to give permanganate.

<math>2 \operatorname{MnO}_2 + 4 \operatorname{KOH} + \operatorname{O}_2 \rightarrow 2 \operatorname{K}_2 \operatorname{MnO}_4 + 2 \operatorname{H}_2 \operatorname{O}_4</math>

(A) Dark Green

<math>3 \, \text{MnO}_4^{2-} + 4 \text{H}^+ \rightarrow 2 \, \text{MnO}_4^- + \text{MnO}_2 + 2 \text{H}_2 \text{O}</math>

(B) purple

In acid solutions:

lodine is liberated from potassium iodide :

<math>10I^{-} + 2 \,\mathrm{MnO_{4}^{-}} + 16H^{+} \rightarrow 2 \,\mathrm{Mn^{2+}} + 8H_{2}O + 5I_{2}</math>

(B)

(D)