CBSE Class 12 Chemistry Important Questions 2027 (Repeated in Board Exams)

For an electrolyte consisting of a monovalent cation and a monovalent anion undergoing dissociation in water, the van't Hoff factor (i) is : (A) less than 1 (B) equal to 1 (C) greater than 1 (D) greater than 2

Calculate the molarity and molality of a KI solution if the density of 20% (mass/mass) aqueous solution of KI is 1.2 g mL^-1. [Given : Molar mass of KI = 166 g mol^-1]

(i) The cell in which the following reaction occurs : 2Fe³⁺ (aq) + 2I^- (aq) -> 2Fe²⁺ (aq) + I₂ (s) has E^o_(cell) = 0.24 V at 298 K. Calculate delta_rG^o and log K_C of the cell reaction. [Given : 1 F = 96500 C mol^-1] (ii) State the following laws : (I) Faraday's first law of electrolysis (II) Kohlrausch's law of independent migration of ions

(i) Write the Nernst equation and calculate the emf of the following cell at 298 K : Mg (s) | Mg²⁺ (0.001 M) || Cu²⁺ (0.0001 M) | Cu (s) Given : E^o_Mg²⁺/Mg = - 2.36 V, E^o_Cu²⁺/Cu = + 0.34 V [log 10 = 1] (ii) Define fuel cell and write its two advantages.

[case context: Conductivity of Aqueous Solutions - conductivity meter readings for two solutions A (Red LED bright, Green LED dim) and B (Red LED dim, Green LED off).] Refer to the table given below and draw the molar conductivity vs. concentration curve for solution A and B.

Write the Nernst equation for the following: I. Ni (s) + Cu²⁺ (aq) -> Ni²⁺ (aq) + Cu (s) II. Al (s) + FeSO₄ (aq) -> Al₂(SO₄)₃ (aq) + Fe (s) III. Mg (s)/Mg²⁺ (aq)//Ag^+ (aq)/Ag(s)

(a) Write the anode and cathode reactions and the overall cell reaction occurring in a lead storage battery during its use. (b) Calculate the potential for half-cell containing 0.01 M K₂Cr₂O₇(aq), 0.01 M Cr³⁺ (aq) and 1.0 × 10^-4 M H^+(aq). The half cell reaction is Cr₂O₇^2-(aq) + 14H^+(aq) + 6e^- -> 2Cr³⁺(aq) + 7H₂O(l) and the standard electrode potential is given as E° = 1.33 V. [Given: log 10 = 1]

When a certain conductivity cell was filled with 0.05 M KCl solution, it has a resistance of 100 ohm at 25 degC. When the same cell was filled with 0.02 M AgNO₃ solution, the resistance was 90 ohm. Calculate the conductivity and molar conductivity of AgNO₃ solution. (Given: Conductivity of 0.05 M KCl solution = 1.35 x 10^-2 ohm^-1 cm^-1)

Attempt either A or B. A. I. The rate of a reaction triples when the temperature changes from 298 K to 318 K. Calculate the energy of activation of the reaction assuming that it does not change with temperature. (Given R = 8.314 JK^-1mol^-1, log 3 = 0.4771) II. Identify the order of reaction and write its integrated rate equation mentioning what each term in the equation represents. (graph of concentration of R vs time, a straight line with k = -slope)

[Case: passage on rate of reaction, rate law, rate constant and order.] Write two factors that affect the rate of reaction.

The elements of 3d transition series are given as: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn. Answer the following: (a) Copper has exceptionally positive E°_M²⁺/M value, why? (b) Which element is a strong reducing agent in +2 oxidation state and why? (c) Zn²⁺ salts are colourless. Why?

Which of the following elements will: I. exhibit similar magnetic behaviour and why? Magnesium (Atomic No. 12), Chromium (Atomic No. 24), Iron (Atomic No. 26) and Molybdenum (Atomic No. 42). II. form white salts and why? Zinc (Atomic No. 30), Scandium (Atomic No. 21), Nickel (Atomic No. 28) and Vanadium (Atomic No. 23)

The name inner transition metals is often used to refer to the A. lanthanoids B. actinoids C. both lanthanoids and actinoids D. d block elements

Answer the following questions: I. Are the enthalpies of atomisation of Zinc and Copper matched correctly? Justify your answer. (Enthalpy of atomisation / kJmol^-1: Zinc 339, Copper 130) II. Out of sulphuric acid and hydrochloric acid, which acid will you prefer for permanganate titrations and why? III. 5NO₂^- + 2MnO₄^- + 6H^+ -> (complete the equation)

(i) What is lanthanoid contraction ? What are the consequences (any two) of lanthanoid contraction ? (ii) (I) Why does E^o_Cu²⁺/Cu have positive value (+ 0.34 V) ? (II) Why do actinoids show a wide range of oxidation states ?

Complete and balance the following chemical equations: (a) 2MnO₄^-(aq) + 10I^-(aq) + 16H^+(aq) -> (b) Cr₂O₇^2-(aq) + 6Fe²⁺(aq) + 14H^+(aq) ->

Write the formula of the following coordination complex: I. diaquasilver(I) dichloridoargentate(I) II. dihydroxidobis(triphenylphosphine)nickel(II)

(i) Write the hybridisation and magnetic behaviour of the following complexes : (I) [Ni(CN)₄]^2- (II) [FeF₆]^3- (ii) What type of isomerism is shown by [Co(NH₃)₅(NO₂)]²⁺ and [Co(NH₃)₅(ONO)]²⁺ ? [Given : Atomic number : Ni = 28, Fe = 26]

Assertion (A): [Mn(CN)₆]^3- has a magnetic moment of two unpaired electrons while [MnCl₆]^3- has a paramagnetic moment of four unpaired electrons. Reason (R): [Mn(CN)₆]^3- is inner orbital complexes involving d²sp³ hybridisation, on the other hand, [MnCl₆]^3- is outer orbital complexes involving sp³d² hybridisation. Select the most appropriate answer from the options given below: a. Both A and R are true and R is the correct explanation of A b. Both A and R are true but R is not the correct explanation of A. c. A is true but R is false. d. A is false but R is true.

Define the following terms : (a) Ambidentate ligand (b) Crystal field splitting energy

Formulation of Cobalt(III) Chloride-Ammonia Complexes: Colour / Formula / Solution conductivity corresponds to Yellow / [Co(NH₃)₆]³⁺ 3Cl^- / Y Purple / [CoCl(NH₃)₅]²⁺ 2Cl^- / 1:2 electrolyte Green / X / 1:1 electrolyte 'X' and 'Y' in the above table are: a. X=[Co(NH₃)₆]²⁺ 3Cl^-, Y= 1:3 b. X= [Co(NH₃)₄Cl₂]^+ Cl^-, Y= 1:3 c. X=[Co(NH₃)₄Cl₂]^+ Cl^-, Y= 1:1 d. X=[Co(NH₃)₄Cl₂]³⁺ 3Cl^-, Y= 1:1

The counter ion in the coordination compound [Co(NH₃)₅(NO₂)]Cl₂ is A. Ammine B. Cobalt C. Chloride D. Nitro

Give reasons : (i) Why are low spin tetrahedral complexes not formed ? (ii) Why is [Co(en)₃]³⁺ complex more stable than [Co(NH₃)₆]³⁺ ? (iii) Why are geometrical isomers not possible for tetrahedral complexes having two different types of unidentate ligands coordinated to the central metal ion ?

The secondary valency of Co in [Co(en)₃]₂ (SO₄)₃ is : (A) 12 (B) 2 (C) 3 (D) 6

Arrange the following compounds in increasing order of their boiling points: (i) (CH₃)₂CH-CH₂Br (ii) CH₃CH₂CH₂CH₂Br (iii) (CH₃)₃C-Br The correct order is (A) (ii) < (i) < (iii) (B) (i) < (ii) < (iii) (C) (iii) < (i) < (ii) (D) (iii) < (ii) < (i)

Write two differences between S_N1 and S_N2 reactions. Out of chlorobenzene and 2,4,6-trinitrochlorobenzene, which one is more reactive towards nucleophilic substitution reactions ?

Various isomeric haloalkanes with the general formula C₄H₉Cl undergo hydrolysis reaction. Among them, compound "A" is the most reactive through S_N1 mechanism. Identify "A" citing the reason for your choice. Write the mechanism for the reaction.

Complete the following reactions by writing the major and minor product in each case (any 2) a. CH₃CH₂Br + KCN -> b. CH₃CH₂CH = CH₂ + HBr -> c. (CH₃)₂CHCHClCH₃ + alc KOH ->

What happens when : (a) n-butyl chloride is treated with alcoholic KOH ? (b) Chlorobenzene is treated with CH₃Cl in the presence of anhydrous AlCl₃ ?

Draw the structure of the major monohalo product for each of the following reaction: (a) 4-chloro (ethyl)benzene + Br₂, Heat -> ? (b) 1-methylcyclohex-1-ene + HBr -> ? (c) 4-(hydroxymethyl)phenol + HCl, Heat -> ?

Assertion (A) : p-nitrophenol is less acidic than phenol. Reason (R) : Nitro group is an electron-withdrawing group, which stabilises the phenoxide ion.

[Case context: passage on boiling points of alcohols and phenols increasing with number of carbon atoms and intermolecular hydrogen bonding.] Arrange the following compounds in order of their increasing boiling points : Propan-1-ol, Ethanol, Butan-2-ol, Butan-1-ol

[Case context: The boiling points of alcohols and phenols increase with increase in the number of carbon atoms due to increase in van der Waals forces. The -OH group in alcohols and phenols is involved in intermolecular hydrogen bonding, due to which their boiling points are higher than those of ethers, haloalkanes and haloarenes of comparable molecular masses. Alcohols and phenols are acidic in nature. Electron-withdrawing groups in phenol increase its acidic strength, while electron-releasing groups decrease it. The presence of -OH group in phenols activates the aromatic ring towards electrophilic substitution and directs the incoming group to the ortho and para positions due to resonance effect. The Reimer-Tiemann reaction of phenol yields salicylaldehyde, whereas Kolbe's reaction of phenol yields salicylic acid. Aspirin is formed by the acetylation of salicylic acid.] (i) Write the name of the major product(s) formed in : (I) Reimer-Tiemann reaction (II) Kolbe's reaction (ii) Name the method by which aspirin is formed from salicylic acid.

Propan-2-ol on oxidation with CrO₃ gives : (A) Propanal (B) Propanoic acid (C) Propene (D) Propanone

(a) Write the mechanism of the following reaction: CH₃CH₂OH --H^+, 443 K--> CH₂=CH₂. (b) Write the main product in each of the following reactions: (i) CH_3-CH_2-CH=CH₂ --a) B₂H₆ b) 3H₂O₂/OH^- --> (ii) phenol (C₆H₅OH) --a) aq. NaOH b) CO₂, H^+ -->

Arrange the products obtained in the following cases in the increasing order of their pKa values: A. Oxidation of ethanol in presence of acidified potassium dichromate B. Reaction of propanoic acid with Br₂ in the presence of red Phosphorus C. Reaction of isopropyl magnesium bromide with carbon dioxide, followed by hydrolysis. D. Reaction of propanoic acid with Cl₂ in the presence of red Phosphorous.

Which of the following will give a yellow or orange ppt. with 2,4 DNP? (i) Porpanal (ii) Propanone (iii) Propanoic acid A. (i) and (ii) B. (ii) and (iii) C. (i) and (iii) D. (i), (ii) and (iii)

Which of the following compound gives an oxime with hydroxylamine: a. CH₃COCH₃ b. CH₃COOH c. (CH₃CO)₂O d. CH₃COCl

OR An organic compound 'X', does not undergo aldol condensation. However 'X' with compound 'Y' in the presence of a strong base react to give the compound 1,3-diphenylprop-2-en-1-one. a. Identify 'X' and 'Y' b. Write the chemical reaction involved. c. Give one chemical test to distinguish between X and Y.

(i) Write the names of the product(s) formed when : (I) Benzoyl chloride undergoes Rosenmund reduction. (II) Ethanal is treated with CH₃MgBr, followed by hydrolysis. (III) Benzaldehyde undergoes Cannizzaro's reaction. (ii) Write a simple chemical test to distinguish between : (I) Propanal and Propanone (II) Phenol and Benzoic acid

How do you convert the following ? (a) Propanone to Propan-2-ol (b) Ethanal to Ethane (c) Sodium benzoate to Benzene

Would you expect benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Justify your answer.

Write the chemical equation when: (a) Butan-2-one is treated with Zn(Hg) and conc. HCl. (b) Two molecules of benzaldehyde are treated with conc. NaOH.

An organic compound 'A', having the molecular formula C₃H₈O on treatment with Cu at 573 K, gives 'B'. 'B' does not reduce Fehling's solution but gives a yellow precipitate of the compound 'C' with I₂/NaOH. Deduce the structures of A, B and C.

The organic compounds A, B and C are amines having equivalent molecular weight. A and B on reaction with benzene sulphonyl chloride give white precipitate, however white precipitate obtained from compound B remains insoluble in NaOH. The variation in the boiling point of A, B and C can be seen as : A. A> B>C B. B>A>C C. A=B > C D. C>B>A

Acetamide reacts with Br₂ and aqueous KOH to form : (A) Ethanamine (B) Ammonia (C) Methanamine (D) Ethanenitrile

Assertion (A): Aniline is a stronger base than ammonia. Reason (R): The unshared electron pair on nitrogen atom in aniline becomes less available for protonation due to resonance. Select the correct answer: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true, but R is not the correct explanation of A. (C) A is true, but R is false. (D) A is false, but R is true.

Answer any five of the following (32a-32g), 1 mark each. Write the chemical reaction involved in Gabriel phthalimide synthesis.

Identify A and B in each of the following reaction sequence: (a) CH₃CH₂Cl -NaCN-> A -H₂/Ni-> B (b) C₆H₅NH₂ -NaNO₂/HCl, 0-5 °C-> A -C₆H₅NH₂/H^+-> B

Answer any five of the following (32a-32g), 1 mark each. Aniline does not undergo Friedel-Crafts reaction. Why?

In the ring structure of fructose, the anomeric carbon is : (A) C2 (B) C3 (C) C4 (D) C1

[Case context: Carbohydrates are optically active polyhydroxy aldehydes or ketones or molecules that provide such units on hydrolysis. They are broadly classified into three groups : monosaccharides, oligosaccharides and polysaccharides. Monosaccharide units are held together by glycosidic linkages to form disaccharides or polysaccharides. Starch and cellulose are polysaccharides. Starch is a polymer of alpha-glucose and consists of two components : Amylose and Amylopectin. Amylose is water soluble, having 15-20% of starch, whereas Amylopectin is insoluble in water, having 80-85% starch. Chemically, amylose is a long unbranched chain of alpha-glucose units held together by C1-C4 glycosidic linkage. Amylopectin is a branched chain of alpha-glucose units held together by C1-C4 and C1-C6 glycosidic linkages.] Write two differences between Amylose and Amylopectin.

Attempt either A or B. A. Answer the following questions: I. Identify and give any one point of difference between the protein present in the hair and protein present in egg albumin. II. Both glucose and sucrose have aldehydic group, then why does only glucose reduces Fehling's Reagent and not sucrose? III. Why do amino acids behave as salts? IV. What chemical change takes place during curdling of milk? V. Doctor advised a 50 year old woman enough exposure to sunlight and addition of fish and egg yolk to her diet. What is the possible disease diagnosed by the doctor?

Write the reactions involved when D-glucose is treated with the following reagents: (a) HCN (b) Br₂ water

What are essential amino acids ? How do you explain the amphoteric behaviour of amino acids ?