XII – ISC Chemistry Paper 2018

ISC Paper 2018

Chemistry

Maximum Marks: 70

Time allowed: 3 hours

• Answer all questions in Part I and six questions from Part II, choosing two questions from Section A, two from Section B and two from Section C.

• All working, including rough work, should be done on the same sheet as, and adjacent to, the rest of the answer.

• The intended marks for questions or parts of questions are given in brackets [ ].
• Balanced equations must be given wherever possible and diagrams where they are helpful.

• When solving numerical problems, all essential working must be shown.
• In working out problems use the following data:

Gas constant R = 1.987 cal deg^-1 mol^-1 = 8.314 JK^-1 mol^-1 = 0.0821 dm³ atm K^-1 mol^-1 . 1L atm = 1 dm³ atm = 101.3 J. 1 Faraday = 96500 Coulombs.

Avogadro’s number = 6.023 × 10²³

Part – I (20 Marks)

Answer all questions.

Question 1.

(a) Fill in the blanks by choosing the appropriate word/words from those given in the brackets:

[4×1]

(square pyramidal, electrical, 74; 26, sp³d² , sp³d, chemical, 68, 32, tetrahedral, yellow, white, iodoform, Lucas)

(i) A Galvanic cell converts _______ energy into _______ energy.

(ii) The percentage of unoccupied spaces in bcc and fcc arrangements are _______ and _____respectively.

(iii) Propan-2-ol on reaction with iodine and sodium hydroxide gives ________ precipitate and the reaction is called ___________ test.

(iv) The geometry of XeOF₄ molecule is ______ and the hybridisation of Xenon atom in the molecule is ________.

Solution

(b) Complete the following statements by selecting the correct alternative from the choices given: [4×1]

(i) During the course of an S_N1 reaction, the intermediate species formed is:

(1) a carbocation

(2) a free radical

(3) a carbanion

(4) an intermediate complex

Solution

(ii) Purification of aluminium by electrolytic refining is called:

(1) Serpeck’s process

(2) Hoope’s process

(3) Hall’s process

(4) Baeyer’s process

Solution

(iii) An aqueous solution of urea freezes at – 0.186°C, K_f for water = 1.86 K kg mol^–1, K_b for water = 0.512 K kg mol^–1. The boiling point of urea solution will be:

(1) 373.065 K

(2) 373.186 K

(3) 373.512 K

(4) 373.0512 K

Solution

(iv) In the dehydration of alcohols to alkenes by heating with concentrated sulphuric acid, the initiation step is:

(1) formation of carbocation

(2) formation of an ester

(3) protonation of alcohol molecule

(4) elimination of water

Solution

(c) Match the following: [4×1]

(i) Rate constant                                                              (a) Dialysis

(ii) Biodegradable polymer                                            (b) Glycine

(iii) Zwitter ion                                                                  (c) Arrhenius equation

(iv) Purificaiton of colloids                                              (d) PHBV

Solution

(d) Answer the following questions: [4×2]

(i)  (1) Why does the density of transition elements increase from Titanium to Copper? (at. no. Ti = 22, Cu = 29)

(2) Why is zinc not regarded as a transition element? (at. no. Zn = 30)

Solution

(ii) Identify the compounds A, B, C and D.

{CH_3CN{\overset{H_2O/H^+}{\longrightarrow}} A {\overset{NH_3}{\longrightarrow}} B {\overset{heat}{\longrightarrow}} C {\overset{Br_2/KOH}{\longrightarrow}} D}

Solution

(iii) Calculate the osmotic pressure of a solution prepared by dissolving 0.025 g of K₂SO₄ in 2.0 litres of water at 25°C assuming that K₂SO₄ is completely dissociated. (mol. wt. of K₂SO₄ = 174 g mol^–1)

Solution

(iv) What type of isomerism is shown by the following coordination compounds:

[PtCl₂(NH₃)₄] Br₂ and [Pt Br₂ (NH₃)₄ ] Cl₂

Write their IUPAC names.

Solution

Question 2

(a)

(i) Write the rate law expression for the reaction A + B + C → D + E , if the order of reaction is first, second and zero with respect to A, B and C, respectively.

(ii) How many times the rate of reaction will increase if the concentration of A, B and C are doubled in the equation given in (i) above?

Solution

OR

(b) The rate of reaction becomes four times when the temperature changes from 293 K to 313 K. Calculate the energy of activation (E_a ) of the reaction assuming that it does not change with temperature.

(R = 8.314 J K^–1 mol^–1)

Solution

Question 3

(a) How do antiseritics differ from disinfectants?

(b) State the role of the following chemicals in the food industry:

(i) Sodium benzoate

(ii) Aspartame

Solution

Question 4

An aromatic organic compound [A] on heating with NH₃ and Cu₂O at high pressure gives [B]. The compound [B] on treatment with ice cold solution of NaNO₂ and HCL gives [C], which on heating with Cu/HCL gives compound [A] again. Identify the compounds [A], [B] and [C]. Write the name of the reaction for the conversion of [B] to [C].

Solution

Question 5

Write the names of the monomers for each of the following polymers:

(a) Bakelite

(b) Nylon–2–nylon–6

Solution

Question 6

Name the purine bases and pyrimidine bases present in RNA and DNA.

Solution

Question 7

(a) How will you obtain the following? (Give balanced equation.)

(i) Picric acid from phenol

(ii) Ethyl chloride from diethyl ether.

Solution

OR

(b) How will you obtain the following? (Give balanced equation.)

(i) Anisole from phenol

(ii) Ethyl acetate from ethanol.

Solution

Question 8

40% of a first order reaction is completed in 50 minutes. How much time will it take for the completion of 80% of this reaction?

Solution

Question 9

(a) The freezing point of a solution containing 5.85 g of NaCL in 100 g of water is –3.348°C. Calculate van’t Hoff factor for this solution. What will be the experimental molecular weight of NaCL?

(K_f for water = 1.86 K kg mol^–1, at wt, Na = 23, Cl = 35.5)

Solution

OR

(b) An aqueous solution containing 12.48 g of barium chloride (BaCl₂ ) in 1000g of water, boils at 100.0832°C. Calculate the degree of dissociation of barium chloride.

(K_b for water = 0.52 K kg mol^–1, at. wt. Ba = 137, Cl = 35.5)

Solution

Question 10

Examine the defective crystal given below and answer the question that follows :

A+	B–	A+	B–	A+
B–		B–	A+	B–
A+	B–	A+		A+
B–	A+	B–	A+	B–

State if the above defect is stoichiometric or non-stoichiometric. How does this defect affect the density of the crystal ? Also, write the term used for this type of defect.

Solution

Question 11

Give reason for each of the following :

(a) For ferric hydroxide sol the coagulating power of phosphate ion is more than chloride ion.

(b) Medicines are more effective in their colloidal form.

(c) Gelatin is added to ice creams.

Solution

Question 12

(a) For the complex ion [ Fe (CN)₆]^3- , state :

(i) the type of hybridisation

(ii) the magnetic behaviour

(iii) the oxidation number of the central metal atom

Solution

(b) Write the IUPAC [ Co(en)₂  Cl₂ ]⁺ ion and draw the structures of its geometrical isomers.

Solution

Question 13

(a) Explain why :

(i) Mn²⁺ is more stable than Fe²⁺ towards oxidation to +3 state.

(At. no. of Mn = 25, Fe = 26)

(ii) Transition elements usually form coloured ions

(iii) Zr and Hf exhibit similar properties

(At. no. of Zr = 40, Hf = 72)

Solution

OR

Complete and balance the following chemical equations :

(i) KMnO₄ + KI + H₂SO₄ →_____ +  _____  +  _____ +  _____

(ii) K₂Cr₂O₇ + H₂SO₄ + H₂S →_____ +  _____  +  _____ +  _____

(iii) KMnO₄ + H₂SO₄ + FeSO₄ →_____ +  _____  +  _____ +  _____

Solution

Question 14

(a) Arrange the following in the increasing order of their basic strength :

C₂H₅NH₂, C₆H₅NH₂, (C₂H₅)₂ NH

(b) Give a balanced chemical equation of convert methyl cyanide to ethyl alcohol.

(c) What happens when benzene diazonium chloride reacts with phenol in weak alkaline medium ?

(Give balanced equation).

Solution

Question 15

Name the sulphide ore of Copper. Describe how pure copper is extracted from this ore.

Solution

Question 16

(a)  (i)  Calculate the emf and ΔG° for the cell reaction at 25°C :

Zn(s) | Zn²⁺_(aq) || Cd²⁺_(aq) | Cd_(s)

.            (0.1M)       (0.01M)

Given E° Zn²⁺ / Zn = -0.763 and E° Cd²⁺ / Cd = -0.403V

(ii) Define the following terms :

(1) Equivalent conductivity

(2) Corrosion of metals

Solution

(b)  (i)  The specific conductivity of a solution containing 5 g of anhydrous BaCl₂ (mol. wt = 208) in 1000 cm³ of a solution is found to be 0.0058 ohm^–1cm–1. Calculate the molar and equivalent conductivity of the solution.

(ii) What is an electrochemical series ? How is it useful in predicting whether a metal can liberate hydrogen from acid or not ?

Solution

Question 17

(a) (i) Explain why :

(1) Nitrogen does not form pentahalides

(2) Helium is used for filling weather balloons

(3) ICl is more reactive than I₂

Solution

(ii) Draw the structures of the following :

(1) HClO₄

(2) H₃PO₃

Solution

OR

(b) (i) Explain why :

(1) Mercury loses its meniscus in contact with ozone.

(2) Halogens are coloured and the colour deepens on moving down in the group from fluorine to iodine.

(3) Hydride of sulphur is a gas while hydride of oxygen is a liquid.

Solution

(ii) Complete and balance the following reactions :

(1) NaCL + MnO₂ + H₂SO₄ →_____ +  _____  +  _____ +  _____

(2) KMnO₄ + SO₂ + H₂O →_____ +  _____  +  _____ +  _____

Solution

Question 18

(a)   (i)   Give balanced equations for the following reactions:

(1) Benzaldehyde reacts with hydrazine.

(2) Acetic acid reacts with phosphorous pentachloride.

(3) Acetone reacts with sodium bisnlphite.

Solution

(ii) Give one chemical test each to distinguish between the following pairs of compounds:

(1) Ethanol and acetic acid

(2) Acetaldehyde and benzaldehyde

Solution

OR

(b) (i) Write chemical equations to illustrate the following name reactions:

(1) Clemmensen’s reduction

(2) Rosenmund’s reduction

(3) HVZ reaction

Solution

(ii) Explain why :

(1) Acetaldehyde undergoes aldol condensation, but formaldehyde does not.

(2) Acetic acid is weaker acid as compared to formic acid.

Solution