Previous Year Paper Class XII

Class – XII
Subject – Chemistry

Time:3hrs                                                                                                                                               MM:70

General instructions:

All questions are compulsory.

Questions number 1 to 8 are very short –answer questions, carrying 1 mark each.

Questions number 9 to18 are short –answer questions, carrying 2 marks each.

Questions number19 to27 are short –answer questions, carrying 3 marks each.

Questions number28 to30 are long-answer questions of 5 marks each.

Q1. Define F-centres.

Q2. Why is enthalpy of chemisorption higher than that of physisorption?

Q3. An ore sample of PbS is contaminated with ZnS. Name one chemical which can be used to concentrate galena selectively by froth floatation method.

Q4. A and B liquids on mixing produce a warm solution. Which type deviation from  Raoult’s law is there?

Q5. Give the IUPAC name of the following compound: CH₃COCH₂CH₂CHO. 

Q6. Except for vitamin B₁₂, all other vitamins of group B, should be taken regularly in diet. Why?

Q7. The decomposition of ammonia gas on platinum surface has a rate constant k = 2.5 X 10^-4 mol/L/s. What is the order of a reaction?

Q8. What is the molecularity of the reaction: Cl → ½ Cl₂?

Q9. How does molar conductance vary with concentration in strong and weak electrolytes? Explain with the help of graph.

Q10. An element E crystallises in bcc structure. If the edge length of the cell is 1.469 X 10^-10 m and the density is 19.3 g/cm³, calculate the atomic mass of this element.

Q11. Define emulsions. Classify them giving an example of each.

Q12. Classify colloids on the basis of interaction between dispersed phase and dispersion medium. Write two differences between them.

Q13. (a) What is the covalence of nitrogen in N₂O₅?

(b) Explain why both N and Bi do not form pentahalides.

Q14. (a)Write the reaction involved in the preparation of tert-butyl ethyl ether by Williamson’s synthesis.

(b) Give reason: Propanol has higher boiling point than propane.

Q15. (a) How can you convert an amide into an amine having one carbon less than the starting compound?

(b) Give the IUPAC name and structure of the amine obtained by the above method if the amide is 3-Chlorobutanamide.

Q16. (a) Why does chlorine water lose its yellow colour on standing?

(b) What happens when Cl₂ reacts with cold and dilute solution of sodium hydroxide? Write the reaction involved.

Q17. How would you distinguish between 1⁰, 2⁰ and 3⁰ amines by a chemical test. Write the reactions to justify your answer.

Q18. Give the mechanism for the preparation of ethanol from ethene by acidic hydration.

Q19. Geeta and her sister Reeta called refrigeration mechanic for the repair of their refrigerator which was not lowering the temperature and food particles were getting destroyed. The mechanic came told the sisters that it requires filling of the gas. “But where has the earlier gas gone”, asked the sisters. The mechanic told that gas leaked out perhaps due to the poor repairs done by the earlier mechanic. They knew the ill effects of Freon gas and told the mechanic to fill and seal the gas carefully.

(a) What are the ill effects of Freon as discussed by Geeta and Reeta?

(b) How do you evaluate the consciousness of the sisters towards the environment?    

Q20. The decomposition of N₂O₅ (g) is a first order reaction with a rate constant of 5 X 10^-4 s^-1 at 45^oC, i.e. 2 N₂O₅ (g) → 4 NO₂ (g) + O₂ (g). If initial concentration of N₂O₅ is 0.25M, calculate its concentration after 2 min. Also calculate half life for decomposition of N₂O₅ (g).

Q21. (a) Name the method used for refining of (i) nickel, (ii) zirconium.

(b) The extraction of Au by leaching with NaCN involves both oxidation and reduction. Justify giving equations.

                                    

Q22. Write down the equations of hydrolysis of XeF₄ and XeF₆. Which of these two reactions is a redox reaction?

Q23. (a) Explain bondings in metal carbonyls with orital diagram.

(b) Write the configuration of Ni in the complex [NiCl₄]^2- in terms of t_2g and e_g.

Q24.

Q25. i) Explain the mechanism of acid catalysed dehydration of ethanol forming ethene.

ii) Convert methanal to Ethanol by using Grignard’s reagent.

iii) Write the reaction between phenol and Br₂ (aq).

Q26. Giving an example for each describe the following reaction:

i) HVZ reaction

ii) Reimer – Tiemer reaction

iii) Aldol condensation

Q27. i)Write the difference between antiseptic and disinfectants with one example in each.

ii) Give one example each of artificial sweetener and narrow spectrum antibiotic.

                Q28. (i) Account for the following : (i) Out of the ions Co²⁺, Sc³⁺ and Cr³⁺ ,which one would

 give coloured aqueous solutions  and why ?

(ii) Explain why chromium is a typical hard metal while mercury is a liquid.

(iii) Why do the transition elements form coloured compounds? Explain.

(iv)Why do the transition elements form interstitial compound?          

 (v) Complete the given reaction :                                                                                         

                  Cr₂O₇^2-  + H⁺  +Fe⁺²     →     ?

OR

 i) Transition metal compounds generally act as catalyst.(give reason)

ii) E⁰Mn³⁺/Mn²⁺ has higher positive value than E⁰ Cr³⁺/Cr^2+.(Atomic number  Cr=24,Mn=25)

iii) How does KMnO₄ can be prepared from pyrolusite ore?

iv) Why do the transition elements form coloured compounds? Explain.

Q29. (i) State Kohlrausch’s law.

(ii) Write down the reactions involved in the working of a H₂––O₂ fuel cell.

(iii) A solution of Ni(NO₃)₂ is electrolysed between platinum electrodes using a current of     5.0 amperes for 20.0 minutes. What mass of Ni is deposited at the cathode? [ Atomic mass of Ni = 58.7u ]

OR

i)Write the anode and cathode reaction of lead storage battery.

ii) Define the molar conductivity .

iii) Calculate the equilibrium constant for the reaction

2Cr(s) +3Cd⁺²→2Cr⁺³(s) +3Cd

[ E⁰Cr³⁺/Cr   =  – 0.74 V and   E⁰Cd²⁺/Cd  =  + 0.40V ]

           

Q30. i) An organic compound with the molecular formula C₉H₁₀O forms 2,4-DNP derivative, reduces Tollen’s reagent and undergoes Cannizzaro reaction .On vigorous oxidation it gives 1,2-benzenedicarboxylic acid. Identify the organic compound.

ii) Arrange the following acid in increasing order of acidity:

CH₃CH₂CH(Br)COOH, CH₃CHBrCH₂COOH, (CH₃)₂CHCOOH, CH₃CH₂CH₂COOH

(iii) Convert toluene  to benzaldehyde.

OR

Complete the following reaction.

i) Benzenamide to Chlorobenzene

ii)     Benzoyl chloride to benzaldehde                 

iii)Distinguish between the following by suitable chemical test and write chemical reaction:

a)Phenol and Benzoic acid

b)Benzylamine and Aniline.