List of top Chemistry Questions asked in CBSE CLASS XII

Among the following, which one has the lowest value of pK$_b$ ?

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

The oxidation number of Co in [Co(NH$_3$)$_5$(ONO)]SO$_4$ is :

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

Which of the following compounds is most reactive towards nucleophilic substitution reaction with aqueous NaOH ?

The mole fraction of a solute in 2.0 molal aqueous solution is :

Answer the following: \[ (a)(i)\;\text{Calculate the electrode potential of a half-cell for zinc electrode dipping in }0.01M\;ZnSO_4\text{ solution at }25^\circ C. \] \[ E^\circ_{Zn^{2+}/Zn}=-0.76V,\quad \log 10=1 \] \[ (a)(ii)\;\text{Write anode, cathode and overall reaction involved in dry cell.} \] \[ (a)(iii)\;\text{Equilibrium constant }(K_c)\text{ is related to }E^\circ_{\text{cell}},\text{ but not to }E_{\text{cell}}.\text{ Why?} \]

Answer the following: \[ (a)(i)\;\text{Write the product(s) when:} \] \[ (I)\;\text{One mol of ethanal is treated with }1\text{ mol of }CH_3OH\text{ in the presence of dry }HCl\text{ gas.} \] \[ (II)\;\text{Benzaldehyde is treated with conc. }NaOH. \] \[ (III)\;\text{Ethanoic acid is heated in the presence of }P_2O_5. \] \[ (a)(ii)\;\text{Write a simple chemical test to distinguish between ethanal and propanal.} \] \[ (a)(iii)\;\text{Write the name of the reagent to transform allyl alcohol to propenal.} \]

Answer the following: \[ (a)(i)\;\text{Describe giving reason which one of the following pairs has the property indicated:} \] \[ (I)\;Fe\text{ or }Cu-\text{higher melting point} \] \[ (II)\;Ti^{3+}\text{ or }Sc^{3+}-\text{coloured in aqueous solution} \] \[ (III)\;Cr\text{ or }Zn-\text{higher third ionisation enthalpy} \] \[ (a)(ii)\;\text{Write the ionic equations for the oxidizing action of }MnO_4^-\text{ in acidic medium with:} \] \[ (I)\;Fe^{2+}\text{ ion} \] \[ (II)\;I^-\text{ ion} \]

Answer the following case-based questions on biomolecules: \[ (a)\;\text{Write the name of basic building units of proteins and nucleic acids. How can you differentiate between fibrous and globular proteins on the basis of their structures?} \] \[ (b)(i)\;\text{What products would be formed when a nucleotide from DNA containing thymine is hydrolyzed?} \] OR \[ (b)(ii)\;\text{Write one structural difference between DNA and RNA.} \] \[ (c)\;\text{Give one example each of a fat-soluble vitamin and a water-soluble vitamin.} \]

How will you obtain the following from benzenediazonium chloride? Give chemical equations involved: \[ (a)\;\text{Chlorobenzene} \] \[ (b)\;\text{Benzene} \] \[ (c)\;\text{Benzonitrile} \]

An organic compound \(A\) with molecular formula \(C_3H_5N\) on reaction with \(C_6H_5MgBr\) followed by hydrolysis gives compound \(B\). Compound \(B\) forms orange-red precipitate with 2,4-DNP reagent and does not give iodoform test. It neither reduces Tollens' or Fehling's reagent nor decolourises bromine water. On drastic oxidation with chromic acid it gives carboxylic acid \(C\) having molecular formula \(C_7H_6O_2\). Identify \(A\), \(B\), and \(C\). Write the reactions.

Answer the following case-based questions on alcohols, phenols and ethers: \[ (a)\;\text{Name the reagents used in the following reactions:} \] \[ (i)\;\text{Oxidation of a primary alcohol to aldehyde} \] \[ (ii)\;\text{Oxidation of a primary alcohol to carboxylic acid} \] \[ (b)\;\text{Write the reaction involved in Kolbe's reaction.} \] \[ (c)(i)\;\text{Why are tertiary alcohols resistant to oxidation?} \] OR \[ (c)(ii)\;\text{Write the products of the following reaction:} \] \[ (CH_3)_3C-O-C_2H_5 \xrightarrow{HI} \]

Give reasons for the following: \[ (a)\;\text{Although chlorine is an electron-withdrawing group, yet it is ortho-, para-directing in electrophilic aromatic substitution reactions.} \] \[ (b)\;\text{Aryl halides cannot be prepared by reacting phenol with concentrated halogen acids or phosphorus halides.} \] \[ (c)\;\text{Chloroform is stored in closed dark coloured bottles.} \]

An antifreeze solution is prepared by dissolving \(31g\) of ethylene glycol \((M=62g\,mol^{-1})\) in \(600g\) of water. Calculate the freezing point of the solution. \[ K_f\text{ for water}=1.86K\,kg\,mol^{-1} \]

State Kohlrausch's law of independent migration of ions. With the help of a curve, explain why it is not easy to determine \(\Lambda_m^\circ\) for weak electrolytes by extrapolating the concentration--molar conductivity curve, as it is for strong electrolytes.

The half-life period of a radioactive element is \(1.5\times 10^{10}\) years. Calculate the time in which the activity of the element is reduced to \(75\%\) of its original value. \[ \text{Given: }\log 2=0.30,\quad \log 3=0.48,\quad \log 4=0.60 \]

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

For a reaction \(A+B\rightarrow\) products, the rate law is: \[ Rate=k[A][B]^{\frac{3}{2}} \] Write the overall order of the reaction. Can this reaction be an elementary reaction? Give reason.

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 \(C_4H_9Br\), write:
(a) the isomer which is most reactive towards S_N1 displacement.
(b) the isomer which, on reacting with Na metal in dry ether, gives 2,5-dimethylhexane.

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

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.