CBSE Half Yearly Sample Paper 2020-21 Chemistry for class 12

CBSE Half Yearly Sample Paper 2020-21 Chemistry for class 12

Sub- Chemistry

Time: 3:00 hour/ M.M.-70

General Instructions:

1. There are 33 questions in this question paper. All questions are compulsory.
2. Section A:-Q. No. 1 to 20 are objective type questions and carry 1 mark each.
3. Q.No. 21 and 22 are passage based questions carrying 5 marks.
4. Section B:-Q. No. 23 to 27 are short answer questions and carry 2 marks each
5. Section C: Q. No. 28 to 32 are short answer questions and carry 3 marks each.
6. Section D: Q. No. 33 to 35 are long answer questions carrying 5 marks each.

SECTION-A

1- Which of the following options are correct for [Fe(CN)₆]^3-complex?

a) d²sp³hybridisation

b) sp³d hybridization

c) paramagnetic

d) diamagnetic

2- Transition metals, despite high E° oxidation, are poor reducing agents. The incorrect reason is-

a) high heat of vaporization.

b) high ionization energies.

c) low heats of hydration.

d) complex forming nature.

3- Helium is preferred to be used in balloons instead of hydrogen because it is

a) incombustible

b) lighter than hydrogen

c) more abundant than hydrogen

d) non polarizable

4- Freundlich adsorption isotherm is given by the expression  = k p^1/n which of the m following conclusions can be drawn from this expression.

a) When  = 0, the adsorption is independent of pressure.

b) When  = 0, the adsorption is directly proportional to pressure.

c) When n = 0,  vs p graph is a line parallel to x-axis.

d) When n = 0, plot of  vs p is a curve.

5- Which of the following statements about the catalyst is true?

a) A catalyst accelerates the rate of reaction by bringing down the activation energy.

b) A catalyst does not participate in reaction mechanism.

c) A catalyst makes the reaction feasible by making ∆G more negative.

d) A catalyst makes equilibrium constant more favourable for forward reaction.

6- Which of the following pairs of ions are isoelectronic and isostructural?

a) CO^2-₃ , NO^–₃

b) ClO^–₃, CO^2-₃

c) SO^2-₃, NO^–₃

d) Clo^–₃, SO^2-₃

7- The property which is not characteristic of transistion metals is

a) variable oxidation states.

b) tendency to form complexes.

c) formation of coloured compounds.

d) natural radioactivity.

8- The correct ascending order of adsorption of the following gases on the same mass of charcoal at same temperature and pressure is-

a) CH₄< H₂< SO₂

b) H₂ < CH₄ < SO₂

c) SO₂< CH₄< H₂

d) H₂ < SO₂ < CH₄

9- A new galvanic cell of E^u more than E°dl of Daniel cell is connected to Daniel cell in a manner that new cell gives electrons to cathode, what will happen-

a) E_cell will increase

b) E_cell will decrease

c) No change will take place

d) Daniel cell will work as electrolytic cell where Zn will be deposite on zinc rod and copper will dissolve from copper rod.

10- According to adsorption theory of catalysis, the speed of the reaction increases because-

a) The concentration of the reactant molecules at the active centres of the catalyst becomes high due to adsorption.

b) In the process of adsoption, the activation energy of the molecules becomes large.

c) Adsorption produces heat which increases the speed of the reaction.

d) Adsorption lowers the activation energy of the reaction.

11- Which of the following are d-block elements but not regarded as transistion elements?

a) Cu, Ag, Au

b) Zn, Cd, Hg

c) Fe, Co, Ni

d) Ru, Rh, Pd

12- Primary and secondary valency of Pt in [Pt(en)₂Cl₂] are

a) 4, 4

b) 4, 6

c) 6, 4

d) 2, 6

13. In which of the pair of ions, both species contain S—S bond?\

a) S₄O^2-₆,  S₂O^2-₃       

b) S₂O^2-₇, S₂O^2-₈      

c) S₄O^2-₆ , S₂O^2-₇   

d) S₂O^2-_7, S₂O^2-₃

14- When chlorine reacts with cold and dilute solution of sodium hydroxide, it forms

a) Cl^–and ClO^–

b) Cl^– and ClO₂^–

c) Cl^–and ClO₃^–

d) Cl^– and ClO₄^–

15- EDTA is used for the estimation of

a) Na⁺and K⁺ions

b) Cl^– and Br^– ions

c) Cu²⁺and Cs⁺ions

d) Ca²⁺ and Mg²⁺ ions

In the following questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.

i) Assertion and reason both are correct and the reason is correct explanation of assertion.

ii) Assertion and reason both are correct but reason does not explain assertion.

iii) Assertion is correct but reason is incorrect.

iv) Both assertion and reason are incorrect.

v) Assertion is incorrect but reason is correct.

16- Assertion: HI cannot be prepared by the reaction of KI with concentrated H₂SO₄.
Reason: HI has lowest H-X bond strength among halogen acids.

17- Assertion: An ordinary filter paper impregnated with collodion solution stops the flow of colloidal particles.
Reason: Pore size of the filter paper becomes more than the size of colloidal particle.

18- Assertion: An ordinary filter paper impregnated with collodion solution stops the flow of colloidal particles.
Reason: Pore size of the filter paper becomes more than the size of colloidal particle.

19- Assertion: Order of the reaction can be zero or fractional.
Reason: We cannot determine order from balanced chemical equation

20- Assertion: ECe|, should have a positive value for the cell to function.
Reason: E_cathode, < E_anode

SECTION-B

21- The d-block of the periodic table contains the elements of the groups 3-12 and are known as transition elements. In general, the electronic configuration of these elements is (n – 1) d1–10 ns1–2.The d-orbitals of the penultimate energy level in their atoms receive electrons giving rise to the three rows of the transition metals i.e., 3d, 4d and 5d series. However, Zn, Cd and Hg are not regarded as transition elements. Transition elements exhibit certain characteristic properties like variable oxidation states, complex formation, formation of coloured ions and alloys, catalytic activity, etc. Transition metals are hard (except Zn, Cd and Hg) and have a high melting point.

i) Why are Zn, Cd and Hg non-transition elements?

ii) Which transition metal of 3d series does not show variable oxidation states?

iii) Why do transition metals and their compounds show catalytic activity?

iv) Why are melting points of transition metals high?

v) Why is Cu²⁺ ion coloured while Zn²⁺ ion is colourless in aqueous solution?

22- The halogens have the smallest atomic radii in their respective periods. The atomic radius of fluorine is extremely small. All halogens exhibit – 1 oxidation state. They are strong oxidising agents and have maximum negative electron gain enthalpy. Among halogens, fluorine shows anomalous behaviour in many properties. For example electro negativity and ionisation enthalpy are higher for fluorine than expected whereas bond dissociation enthalpy, m.p. and b.p. and electron gain enthalpy are quite lower than expected. Halogens react with hydrogen to give hydrogen halides (HX) and combine amongst themselves to form a number of compounds of the type XX’,XX’₃,XX’₅ and XX’₇ called interhalogens.

i) Why halogens have maximum negative electron gain enthalpy ?

ii) Why fluorine shows anomalous behaviour as compared to other halogens ?

iii) Arrange the hydrogen halides (HF to HI) in the decreasing order of their reducing character.

iv) Why fluorine is a stronger oxidizing agent than chlorine? In the interhalogen compounds?¢

v) What are the sizes of X and X¢ in the interhalogen compounds ?

23- In a close packing of N spheres, how many (i) tetrahedral and (ii) octahedral sites are present?

24- (i) Explain why ortho-nitrophenol is more acidic than ortho-methoxyphenol Briefly.

24- (ii)  Why is CO a stronger ligand than NH₃ in complexes ?

25- The boiling point of benzene is 353.23 K when 1.80 g of a non-volatile, non-electrolytic solute was dissolved in 90 g of benzene the boiling point was raised to 354.11 K. Calculate the molar mass of the solute. (Kb for benzene = 2.53 K.Kg mol-1).

26- Write the mechanism of the following reaction:-

CH₃CH₂OH      ^{———-HBr———-}     CH₃CH₂BR +H₂O

27- What are enantiomers? Draw the structures of the possible enantiomers of 3-methylpent-1-ene.

28- Define first order reaction. Calculate the half-life of a first order reaction from their rate constants given below:

a) 200 s^-1

b) 2 min^-1

c) 4 year^-1

29- Give reasons for the following:

i) Transition metals form alloys.

ii) Mn₂O₃ is basic whereas Mn₂O₇ is acidic.

iii) EU²⁺is a strong reducing agent.

30- How the following conversions can be carried out ?

i) Choloroethane to butane

ii) 2-Choloropropane ti 1-propanol

iii) Isopropyl alcohol to iodoform

31- Give reasons for the following:-

i) First ionization energies of 5d elements are higher than those of 3d and 4d elements.

ii) Actinoid contraction is greater from element to element than lanthanoid contraction.

32- Account for the following:

i) Zn is not considered as a transition element.

ii) Transition metals form a large number of complexes.

iii) The E0value for the Mn³⁺/Mn²⁺ couple is much more positive than that form Cr³⁺/Cr²⁺

33- (a) Write the structures of main products when benzene diazonium chloride reacts with the following reagents.

i) CuCN

ii) CH₃CH₂OH

iii) KI

33- (b) Out of [CoFe₆]^3-and [Co(C₂O₄)]^3-, which one complex is:

i) diamagnetic

ii)more stable

iii) Outer orbital complex and

iv) low spin complex?
(Atomic no. of (Co = 27)

34- (a) Using IUPAC norms write the formulae for the following:

i) Pentamminenitrito-O-cobalt(III) chloride

ii) Potassium tetracyanidonickelate (II)

34- (b) How do you convert the following:

i) Phenol to Anisole

ii) Ethanol to Propan-2-ol

34- (c) Why phenol undergoes electrophilic substitution more easily than benzene?

35- An aromatic compound ‘A’ on treatment with CHCl₃ / KOH gives two compounds ‘B’ and ‘C’. Both B and C give the same product ‘D’ when distilled with zinc dust. Oxidation of D gives E having molecular formula C₇H₆O₂. The sodium salt of E on heating with sodalime gives F which may also be obtained by distilling A with zinc dust. Identity A to F.