List of top Chemistry Questions asked in CBSE CLASS XII

(a) Why is third ionization enthalpy of manganese very high ?

(b) Why Zn, Cd and Hg are not regarded as transition elements ?

(a) Write the major product in the following reaction (the reactant is shown in the figure below) :

\( \text{Reactant} + \mathrm{Br_2} \xrightarrow{\text{heat}} {?} \)

(b) Define Chirality.

(c) Why racemisation occurs in \( \mathrm{S_N1} \) reaction ?



(a) Define didentate ligand. Give an example.

(b) Indicate the type of isomerism exhibited by the given complex :

\( \mathrm{[Pt(NH_3)(H_2O)Cl_2]} \)

(a) Define Collision frequency.

(b) Write the units of

(i) first order reaction and (ii) zero order reaction

(a) Resistance of a conductivity cell filled with 0.1 M KCl solution is 100 \(\Omega\). If the resistance of the same cell when filled with 0.01 mol L\(^{-1}\) KCl solution is 300 \(\Omega\), calculate the conductivity and molar conductivity of 0.01 mol L\(^{-1}\) KCl solution. The conductivity of 0.1 M KCl solution is \(1.29 \times 10^{-2}\) S cm\(^{-1}\).

(b) (i) Write any two advantages of \( \mathrm{H_2 - O_2} \) fuel cell.

(ii) Why does the cell potential of mercury cell remain constant throughout the life ?

(a) Write the reagents which are used in the given conversions :

(i) Phenol to 2, 4, 6 - tribromophenol

(ii) Propene to propan-1-ol

(iii) Butan-2-one to butan-2-ol

(b) Explain the mechanism of acid catalyzed hydration of alkene to form corresponding alcohol.

(a) How will you convert the following:

(i) Propanone to Propene

(ii) Benzoic acid to Benzaldehyde

(iii) Benzene to m-Nitroacetophenone

(b) (i) Arrange the following compounds in increasing order of their reactivity towards HCN:

\( \mathrm{(CH_3)_2C=O} \), \( \mathrm{(CH_3)_3C{-}CO{-}C(CH_3)_3} \), \( \mathrm{CH_3CHO} \)

(ii) Identify the compounds, which would undergo Aldol condensation:

\( \mathrm{HCHO} \), \( \mathrm{C_6H_5CHO} \), \( \mathrm{CH_3CHO} \), and the cyclic ketone (a six-membered carbon ring with one C=O) shown in the figure below



Like \( \mathrm{NH_3} \), nitrogen atom of amine is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore \( sp^3 \) hybridised and the geometry of amines is pyramidal. Lower aliphatic amines are soluble in water due to the formation of hydrogen bond with water molecules. The solubility decreases as the molar mass of amines increases due to increase in size of hydrophobic part. Higher amines are insoluble in water. However amines are less soluble in water than alcohols because of low electronegativity of nitrogen as compared to oxygen. Boiling points of isomeric amines follow the order \( 1^{\circ} > 2^{\circ} > 3^{\circ} \). It is due to the fact the amines are held together due to hydrogen bonding. Extent of hydrogen bonding is more in primary amines than in secondary amines as two hydrogen atoms are available for hydrogen bond formation. Tertiary amines do not show hydrogen bonding because of the absence of hydrogen atom attached to nitrogen. Amines can be prepared from, nitro compounds, nitriles, amides etc.

Complete the following equations:

(i) \( \mathrm{CH_3CONH_2} \xrightarrow[\mathrm{(ii)\ H_2O}]{\mathrm{(i)\ LiAlH_4}} \)

(ii) The compound shown in the figure below (a benzene ring carrying \( \mathrm{NO_2} \) and \( \mathrm{CH_3} \) at opposite positions) \( \xrightarrow{\mathrm{Fe + HCl}} \)



Like \( \mathrm{NH_3} \), nitrogen atom of amine is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore \( sp^3 \) hybridised and the geometry of amines is pyramidal. Lower aliphatic amines are soluble in water due to the formation of hydrogen bond with water molecules. The solubility decreases as the molar mass of amines increases due to increase in size of hydrophobic part. Higher amines are insoluble in water. However amines are less soluble in water than alcohols because of low electronegativity of nitrogen as compared to oxygen. Boiling points of isomeric amines follow the order \( 1^{\circ} > 2^{\circ} > 3^{\circ} \). It is due to the fact the amines are held together due to hydrogen bonding. Extent of hydrogen bonding is more in primary amines than in secondary amines as two hydrogen atoms are available for hydrogen bond formation. Tertiary amines do not show hydrogen bonding because of the absence of hydrogen atom attached to nitrogen. Amines can be prepared from, nitro compounds, nitriles, amides etc.

Classify the following amines as primary, secondary or tertiary:

(i) A naphthalene ring carrying the group \( \mathrm{N(CH_3)_2} \) (shown in the figure below)

(ii) The same naphthalene ring carrying the group \( \mathrm{NH_2} \) in place of \( \mathrm{N(CH_3)_2} \)



(a) Give chemical tests to distinguish between the following pairs of compounds:

(i) Acetophenone and Benzophenone

(ii) Propanal and Propanone

(iii) Pentan-2-one and Pentan-3-one

(b) (i) Which of the following acids is stronger and why?

\( \mathrm{CH_2FCOOH} \), \( \mathrm{CH_3COOH} \)

(ii) Arrange the following compounds in the increasing order of their boiling points:

\( \mathrm{CH_3OCH_3} \), \( \mathrm{CH_3CH_2CH_3} \), \( \mathrm{CH_3CH_2OH} \), \( \mathrm{CH_3CHO} \)

(B) Write the reaction of glucose with the following:

(a) \( \mathrm{H_2N-OH} \)

(b) \( \mathrm{(CH_3CO)_2O} \)

(c) conc. \( \mathrm{HNO_3} \)

(a) Write the major product in the following reaction (the reactant is the six-membered ring with one C=C double bond shown in the figure below): \( \text{ring} + \mathrm{Br_2} \xrightarrow{\text{heat}} \; ? \)

(b) Define Racemic mixture.

(c) p-dichlorobenzene has a higher melting point as compared to its ortho and meta isomers. Why?



(a) Complete and balance the following equations:

(i) \( 2\mathrm{KMnO_4} \xrightarrow{513\ \mathrm{K}} \)

(ii) \( \mathrm{Na_2Cr_2O_7 + 2KCl} \longrightarrow \)

(b) Why is it difficult to separate lanthanoid elements in pure state?

(a) Draw the structure of the given compound :

4-Bromo-3-methylpent-2-ene

(b) What happens when chloroethane is treated with aqueous potassium hydroxide ?

(a) First ionization enthalpy of Cr is lower than that of Zn. Why ?

(b) Mention any one property of the transition elements which makes them good catalysts.

(a) \( \mathrm{[Co(NH_3)_6]^{3+}} \) is diamagnetic whereas \( \mathrm{[CoF_6]^{3-}} \) is paramagnetic. Justify the statement. [Atomic number of Co = 27]

(b) Write the electronic configuration for \( d^4 \) ion if \( \Delta_0 \gt P \) on the basis of crystal field theory.

(A) Define the following term:

(a) Reducing sugar

(b) Differentiate between the following:

(i) Fibrous proteins and globular proteins

(ii) Nucleotide and nucleoside

When 1.5 g of a non-volatile solute was dissolved in 90 g of benzene, the boiling point of benzene got raised from 353.23 K to 353.93 K. Calculate the molar mass of the solute. (Given \( K_b \) for benzene = 2.52 K kg \( \mathrm{mol^{-1}} \))

(a) Define activation energy.

(b) Write the unit of rate constant for

(i) zero order reaction, (ii) second order reaction.

Differentiate between the following :

(i) Fibrous protein and Globular protein

(ii) Peptide linkage and Phosphodiester linkage

How do you explain the presence of following in open structure of glucose ?

(a) all the six carbon atoms are in a straight chain.

(b) five \( \mathrm{-OH} \) groups which are attached to different carbon atoms.

(c) an aldehyde group.

Assertion (A) : The presence of -OH group in phenols directs the incoming group at ortho- and para- positions.

Reason (R) : -OH group in phenols deactivates the aromatic ring.

What type of deviation is shown by a mixture of chloroform and acetone from Raoult's law ? Give reason. What will happen to the boiling point of the solution on mixing chloroform and acetone ?

Assertion (A) : It is not possible to separate the components of an azeotrope by fractional distillation.

Reason (R) : Minimum boiling azeotrope is formed by the solutions showing large positive deviation from Raoult's law.

Assertion (A) : Actinoids show irregularities in their electronic configurations.

Reason (R) : Due to varying stabilities of \( f^0 \), \( f^7 \) and \( f^{14} \) occupancies of the 5f orbitals.