CBSE Class 12 PCM Chemistry Board Paper 2026 — Set 3

The intermediate formed during the slowest step involved in the dehydration of alcohol is :

Which of the following will be the least reactive towards nucleophilic substitution reaction ?

According to Werner’s theory, the primary valencies of the central metal atom :

Which of the following reagent is used to distinguish between (C₂H₅)₂NH and (C₂H₅)₃N ?

Among the following, which is the strongest base ?

In aqueous solution, Cr₂O₇²⁻ ion converts to which of the following in alkaline medium ?

The mole fraction of a solute in 2·0 molal aqueous solution is :

On electrolysis of very dilute aqueous solution of NaCl using platinum electrodes :

For a reaction 3A → 2B, the rate of reaction +d[B]/ dt is equal to :

On hydrolysis, which of the following carbohydrates gives only glucose ?

Assertion (A) : Separation of Zr and Hf is difficult. Reason (R) : Because Zr and Hf lie in the same group of the periodic table.

Assertion (A) : Phenol is less acidic than 4-methylphenol. Reason (R) : The presence of an electron releasing group in phenol makes it less acidic.

Assertion (A) : Aromatic primary amines cannot be prepared by Gabriel phthalimide synthesis. Reason (R) : Gabriel phthalimide synthesis is used for the preparation of primary aliphatic amines.

Assertion (A) : Order of reaction is applicable to elementary as well as complex reactions. Reason (R) : Order of a reaction is an experimental quantity.

For the compound having molecular formula C₄H₉Br, write : (a) the isomer which is most reactive towards S_N1 displacement. (b) the isomer which, on reacting with Na metal in the presence of dry ether, gives 2,5-Dimethylhexane.

For a reaction A + B → Products, the rate law is :

Rate = k[A][B]³ᐟ²

Write the overall order of the reaction. Can this reaction be an elementary reaction ? Give reason in support of your answer.

Explain why, on addition of 1 mol of KCl to 1 litre of water, the boiling point of water increases, while the addition of 1 mol of methyl alcohol to 1 litre of water decreases the boiling point.

Write IUPAC names of the following compounds :

(i) [PtCl₂(en)₂]SO₄

(ii) (NH₄)₂[CoF₄]

Define the following terms with a suitable example in each case :

(i) Ambidentate ligand

(ii) Double salt

How do you explain the following ?

(a) Presence of a straight chain in glucose

(b) Presence of five − OH groups in glucose which are attached to different carbon atoms

The half-life period of a radioactive element is 1·5 × 10¹⁰ years. Calculate the time in which the activity of the element is reduced to 75% of its original value.

[Given : log 2 = 0·30, log 3 = 0·48, log 4 = 0·60]

State Kohlrausch’s law of independent migration of ions. With the help of a curve, explain why it is not easy to determine Λ°ₘ for weak electrolytes by extrapolating the concentration − molar conductivity curve, as it is for strong electrolytes.

An antifreeze solution is prepared by dissolving 31 g of ethylene glycol (Molar mass = 62 g mol⁻¹) in 600 g of water. Calculate the freezing point of the solution. (K_f for water = 1·86 K kg mol⁻¹

Answer the following questions about the complexes

[NiCl₄]²⁻ and [Ni(CN)₄]²⁻

(i) Write the hybridization involved in each case.

(ii) Which of them is the inner orbital complex and which one is the outer orbital complex ?

(iii) Compare their magnetic behaviour. [Atomic number : Ni = 28]

(i) Name two coordination compounds which are important in biological systems.

(ii) What is meant by chelate effect ? Give an example.

(iii) Why are low spin tetrahedral complexes rarely formed ?

Give reasons for the following :

(a) Although chlorine is an electron-withdrawing group, yet it is ortho-, para-directing in electrophilic aromatic substitution reactions.

(b) Aryl halides cannot be prepared by reacting phenol with concentrated halogen acids or phosphorus halides.

(c) Chloroform is stored in closed dark coloured bottles.

How will you obtain the following from benzenediazonium chloride ? Give chemical equations involved :

(a) Chlorobenzene

(b) Benzene

(c) Benzonitrile

An organic compound (A) with molecular formula C₃H₅N on reaction with C₆H₅MgBr followed by hydrolysis, gives a compound (B). Compound (B) forms an orange-red precipitate with 2,4-DNP reagent and does not give iodoform test. It neither reduces Tollens’ or Fehling’s reagent nor does it decolourise bromine water. On drastic oxidation with chromic acid it gives a carboxylic acid (C) having molecular formula C₇H₆O₂. Identify the compounds (A), (B) and (C). Write the reactions of compound (A) with C₆H₅MgBr followed by hydrolysis to give compound (B).

(i) Describe giving reason which one of the following pairs has the property indicated :

(I) Fe or Cu − higher melting point

(II) Ti³⁺ or Sc³⁺ − coloured in aqueous solution

(III) Cr or Zn⁻ higher third ionisation enthalpy

(ii) Write the ionic equations for the oxidizing action of – MnO⁻₄ in acidic medium with :

(I) Fe²⁺ ion

(II) I⁻ ion

(i) A black-brown coloured solid (A) when fused with KOH in the presence of air, produces a dark green coloured compound (B) which on electrolytic oxidation in alkaline medium gives a dark purple coloured compound (C). Identify (A), (B) and (C). Write the reactions involved.

(ii) What happens when an acidic solution of the green compound (B) is allowed to stand for some time ? Also write the equation involved. What is this type of reaction called ?

Write the product(s) when :

(I) One mol of ethanal is treated with 1 mol of CH3OH in the presence of dry HCl gas. (II) Benzaldehyde is treated with conc. NaOH. (III) Ethanoic acid is heated in the presence of P₂O₅.

(ii) Write a simple chemical test to distinguish between Ethanal and Propanal.

(iii) Write the name of the reagent to transform Allyl alcohol to Propenal.

(i) Draw the structure of the semi carbazone of acetone

(ii) Why areα-hydrogen atoms of aldehydes and ketones acidic in nature ?

(iii) Arrange the following compounds in increasing order of their reactivity towards HCN

CH₃COCH₃, CH₃CHO, (CH₃)₃C − C =O − CH₃

(iv) Write the reaction involved in Etard reaction

Calculate the electrode potential of a half-cell for zinc electrode dipping in 0·01 M ZnSO₄ solution at 25°C. Given : E°_ Zn²⁺ /Zn = − 0·76 V [log 10 = 1]

(ii) Write anode, cathode and overall reaction involved in dry cell

(iii) Equilibrium constant (K_c) is related to E°_cell but not to E_cell. Why ?

(i) The conductivity of 0·001 M solution of acetic acid is 3·905 ˣ 10⁻⁵ S cm⁻¹. Calculate its molar conductivity and degree of dissociation (α). Given : λ°_ CH₃COO⁻ = 40·9 S cm²mol⁻¹.

λ° _H⁺ =349.6S cm²mol⁻¹.

(ii) Give reasons for the following : (I) Why does a mercury cell deliver a constant voltage for its entire life ? (II) Why is it necessary to use salt bridge in a galvanic cell ?