Date: 07/05/2022

SET No. 3

Question Paper Code

56/5/3

Time: 2 hrs.

Class-XII

Max. Marks: 35

CHEMISTRY (Theory)

Term-II

(CBSE-2022)

GENERAL INSTRUCTIONS

Read the following instructions very carefully and strictly follow them:

1. This question paper contains 12 questions. All questions are compulsory.
2. This question paper comprises of three sections - Section A, B and C.
3. Section A - Q. No. 1 to 3 are very short-answer type questions carrying 2 marks each.
4. Section B - Q. No. 4 to 11 are short-answer type questions carrying 3 marks each.
5. Section C - Q. No. 12 is case based question carrying 5 marks.
6. Use of log tables and calculators is not allowed.

SECTION-A

1. Explain the following reactions:

<math>[1 \times 2 = 2]</math>

1. Wolff-Kishner reduction
2. Cannizzaro reaction

Sol. (a) Wolff-Kishner reduction:

[1] The carbonyl group of aldehydes or ketones are reduced to CH₂ group on treatment with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol.

<math>C = O \xrightarrow{NH_2NH_2} C = NNH_2 \xrightarrow{KOH/ethylene glycol} CH_2 + N_2</math>

1. Cannizzaro reaction: [1]

Aldehydes which do not have an <math>\alpha</math>-hydrogen atom, undergo self reduction and oxidation (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol and another is oxidised to carboxylic acid salt.

<math display="block">H \supset C = O + H \supset C = O \xrightarrow{Conc. KOH} H - C - O K^{+} + CH_{3}OH</math>

Formaldehyde Potassium formate Methanol

<math>[1 \times 2 = 2]</math>

1. Arrange the following in the increasing order of the property mentioned:
2. CH₃COOH, CICH₂COOH, FCH₂COOH (Acid strength)
3. CH₃CHO, CH₃CH₂OH, CH₃CH₂CH₃ (Boiling Points)

Sol. (a) Electron withdrawing group increases the acidic strength as it stabilizes the conjugate base. [1]

Order of acidic strength: CH₃COOH < CICH₂COOH < FCH₂COOH

1. Stronger the intermolecular forces of attraction, higher will be the boiling point. [1] Order of boiling point: CH₃CH₂CH₃ < CH₃CHO < CH₃CH₂OH

<math>[1 \times 2 = 2]</math>

1. Answer the following questions (Do any two):
2. Identify the order of reaction from the following unit for its rate constant: Lmol-1s-1
3. The conversion of molecules A to B follow second order kinetics. If concentration of A is increased to three times, how will it affect the rate of formation of B?
4. Write the expression of integrated rate equation for zero order reaction

Sol. (a) Unit of rate constant for any nth order reaction could be written as following Unit = <math>(\text{mol } L^{-1})^{1-n} s^{-1}</math> <math>[\frac{1}{2}]</math>

Comparing this with given unit <math>L \text{ mol}^{-1} \text{ s}^{-1} = (\text{mol } L^{-1})^{1-n} \text{ s}^{-1}</math> <math>\Rightarrow</math> <math>(\text{mol } L^{-1})^{-1} \text{ s}^{-1} = (\text{mol } L^{-1})^{1-n} \text{ s}^{-1}</math>

<math>1 - n = -1</math>

n = 2

<math>[\frac{1}{2}]</math>