List of top Chemistry Questions asked in CBSE CLASS XII

Electrochemistry is the study of the relationship between chemical energy and electrical energy. Many spontaneously occurring chemical reactions liberate electrical energy. In electrolysis, electrical energy is converted directly into chemical energy. The product of an electrolytic reaction depends on the nature of the material being electrolysed and the type of electrode used. Oxidising and reducing species present in the electrolytic cell and their standard electrode potential too, affect the products of electrolysis. Electrolysis plays an important role in most people's daily lives, whether it is for the manufacturing of aluminium, electroplating of metals, or the synthesis of chemical compounds.

Michael Faraday was the first scientist who proposed two laws to explain the quantitative aspects of electrolysis, popularly known as Faraday's laws of electrolysis. Faraday's laws of electrolysis provide a basis for mathematical analysis of the mass deposited at electrodes and the amount of charge passed through them.

Faraday's laws are fundamental in various applications, including electroplating, metal extraction, battery technology and chemical synthesis. These laws also help in environmental monitoring and in various chemistry experiments.

Answer the following questions :

(b) State Faraday's second law of electrolysis.

Ethers are prepared by the dehydration of alcohols in presence of protic acids at 413 K. Symmetrical and unsymmetrical ethers can also be prepared by Williamson synthesis. This reaction involves \( \mathrm{S_N2} \) attack of an alkoxide ion on primary alkyl halide. If a tertiary alkyl halide is used, elimination reaction occurs and an alkene is formed, no ether is formed. C - O bond in ethers are cleaved under drastic conditions with excess of HI. When unsymmetrical ethers react with HI, the alkyl halide is formed from smaller alkyl group. If one of the alkyl group is tertiary, the alkyl halide is formed from the tertiary alkyl group because tertiary alkyl carbocation is more stable than the primary carbocation. Cleavage of alkyl aryl ethers takes place at the alkyl-oxygen bond due to more stable aryl-oxygen bond. The order of reactivity of hydrogen halides is HI > HBr > HCl. Aromatic ethers undergo electrophilic substitution reactions. The alkoxy group attached to the aromatic ring activates the ring towards electrophilic substitution and directs the incoming group to ortho- and para-positions.

Answer the following questions :

(a) Complete the following equations :

(i) \( \mathrm{C_6H_5{-}OCH_3 + CH_3Cl \xrightarrow[CS_2]{Anhyd.\ AlCl_3}} \)

(ii) \( \mathrm{C_6H_5{-}OCH_3 \xrightarrow{Conc.\ (HNO_3 + H_2SO_4)}} \)

Ethers are prepared by the dehydration of alcohols in presence of protic acids at 413 K. Symmetrical and unsymmetrical ethers can also be prepared by Williamson synthesis. This reaction involves \( \mathrm{S_N2} \) attack of an alkoxide ion on primary alkyl halide. If a tertiary alkyl halide is used, elimination reaction occurs and an alkene is formed, no ether is formed. C - O bond in ethers are cleaved under drastic conditions with excess of HI. When unsymmetrical ethers react with HI, the alkyl halide is formed from smaller alkyl group. If one of the alkyl group is tertiary, the alkyl halide is formed from the tertiary alkyl group because tertiary alkyl carbocation is more stable than the primary carbocation. Cleavage of alkyl aryl ethers takes place at the alkyl-oxygen bond due to more stable aryl-oxygen bond. The order of reactivity of hydrogen halides is HI > HBr > HCl. Aromatic ethers undergo electrophilic substitution reactions. The alkoxy group attached to the aromatic ring activates the ring towards electrophilic substitution and directs the incoming group to ortho- and para-positions.

Answer the following questions :

(b) Write the names of alkyl halide and sodium alkoxide used to prepare tert-butyl ethyl ether.

How do you convert the following ?

Benzoic acid to Benzene

Alcohols and Phenols are acidic in nature. Electron withdrawing groups in phenol increase its acidic strength and electron releasing groups decrease it. Alcohols undergo nucleophilic substitution with hydrogen halides to yield alkyl halides. Dehydration of alcohols gives alkenes. On oxidation, primary alcohols yield aldehydes with mild oxidising agents and carboxylic acids with strong oxidising agents, while secondary alcohols yield ketones. Tertiary alcohols are resistant to oxidation. The presence of \(-\mathrm{OH}\) group in phenols activates the aromatic ring towards electrophilic substitution and directs the incoming group to ortho and para positions due to resonance effect.

Write the structure of the major product expected from the dinitration of 3-methylphenol.

Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups - monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as D(+)-glucose. It was assigned the open structure on the basis of its reactions with HI, \(\mathrm{NH_2OH}\), \(\mathrm{Br_2}\) water, \((\mathrm{CH_3CO})_2\mathrm{O}\) and nitric acid.

Despite having the \(-\mathrm{CHO}\) group, glucose does not give Schiff's test and hydrogen sulphite addition product with \(\mathrm{NaHSO_3}\). It was found that glucose forms a six-membered ring in which one of the \(-\mathrm{OH}\) groups add to the \(-\mathrm{CHO}\) group and form a cyclic hemiacetal structure.

What type of carbohydrates are called reducing sugars ?

Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups - monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as D(+)-glucose. It was assigned the open structure on the basis of its reactions with HI, \(\mathrm{NH_2OH}\), \(\mathrm{Br_2}\) water, \((\mathrm{CH_3CO})_2\mathrm{O}\) and nitric acid.

Despite having the \(-\mathrm{CHO}\) group, glucose does not give Schiff's test and hydrogen sulphite addition product with \(\mathrm{NaHSO_3}\). It was found that glucose forms a six-membered ring in which one of the \(-\mathrm{OH}\) groups add to the \(-\mathrm{CHO}\) group and form a cyclic hemiacetal structure.

In D(+)-glucose, what do the letter 'D' and sign '(+)' represent ?

Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups - monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as D(+)-glucose. It was assigned the open structure on the basis of its reactions with HI, \(\mathrm{NH_2OH}\), \(\mathrm{Br_2}\) water, \((\mathrm{CH_3CO})_2\mathrm{O}\) and nitric acid.

Despite having the \(-\mathrm{CHO}\) group, glucose does not give Schiff's test and hydrogen sulphite addition product with \(\mathrm{NaHSO_3}\). It was found that glucose forms a six-membered ring in which one of the \(-\mathrm{OH}\) groups add to the \(-\mathrm{CHO}\) group and form a cyclic hemiacetal structure.

How do you explain the following ?

(i) Presence of a carbonyl group in glucose

(ii) Presence of five \(-\mathrm{OH}\) groups in glucose which are attached to different carbon atoms.

Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups - monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as D(+)-glucose. It was assigned the open structure on the basis of its reactions with HI, \(\mathrm{NH_2OH}\), \(\mathrm{Br_2}\) water, \((\mathrm{CH_3CO})_2\mathrm{O}\) and nitric acid.

Despite having the \(-\mathrm{CHO}\) group, glucose does not give Schiff's test and hydrogen sulphite addition product with \(\mathrm{NaHSO_3}\). It was found that glucose forms a six-membered ring in which one of the \(-\mathrm{OH}\) groups add to the \(-\mathrm{CHO}\) group and form a cyclic hemiacetal structure.

Draw the six-membered ring structure of \(\alpha\)-D(+)-glucose.

Draw the structures of major products in each of the following reactions :



Gives the structures of A, B and C in the following reactions :



What is the effect of denaturation on the structure of proteins ? What are the common types of secondary structures of proteins ?

Write IUPAC names of the following compounds :

(a) \( \mathrm{[Ag(NH_3)_2]\,[Ag(CN)_2]} \)

(b) \( \mathrm{[Co(en)_3]_2\,(SO_4)_3} \)

Assertion (A) : \( E^{\circ}_{\mathrm{Mn^{2+}/Mn}} \) value is highly negative.

Reason (R) : Because \( \mathrm{Mn^{2+}} \) is highly stable due to half-filled \( 3d^{5} \) configuration.

Assertion (A) : For complex reaction, order of reaction is given by the slowest step.

Reason (R) : Order of reaction is not applicable for the complex reaction.

Assertion (A) : \( \mathrm{(CH_3)_3C - O - CH_3} \) gives \( \mathrm{(CH_3)_3C - I} \) and \( \mathrm{CH_3OH} \) on reaction with HI.

Reason (R) : The reaction occurs by \( \mathrm{S_N1} \) mechanism.

Benzene diazonium chloride on reaction with phenol in weakly basic medium gives :

The coordination number of Co in \( \mathrm{[Co(en)_2Cl_2]^+} \) is :

The correct order of decreasing order of \( \mathrm{pK_b} \) values is :

Which of the following transition metals shows +1 and +2 oxidation states ?

Dehydration of tertiary alcohols with Cu at 573 K gives :

Which of the following is not correct for \( \mathrm{S_N1} \) reaction ?

Consider the decomposition of hydrogen peroxide (\( \mathrm{H_2O_2} \)) as per the equation given below :

\[ 2\mathrm{H_2O_2} \xrightarrow[\text{Alkaline medium}]{\mathrm{I^-}} 2\mathrm{H_2O} + \mathrm{O_2} \]

The mechanism for this reaction was found to be :

Step I : \( \mathrm{H_2O_2 + I^-} \rightarrow \mathrm{H_2O + IO^-} \) (slow)

Step II : \( \mathrm{H_2O_2 + IO^-} \rightarrow \mathrm{H_2O + I^- + O_2} \) (fast)

What is the molecularity of the reaction in Step II ?

Give the structures of A, B and C in the following reactions :





Consider the decomposition of hydrogen peroxide (\( \mathrm{H_2O_2} \)) as per the equation given below :

\[ 2\mathrm{H_2O_2} \xrightarrow[\text{Alkaline medium}]{\mathrm{I^-}} 2\mathrm{H_2O} + \mathrm{O_2} \]

The mechanism for this reaction was found to be :

Step I : \( \mathrm{H_2O_2 + I^-} \rightarrow \mathrm{H_2O + IO^-} \) (slow)

Step II : \( \mathrm{H_2O_2 + IO^-} \rightarrow \mathrm{H_2O + I^- + O_2} \) (fast)

Determine the order of reaction with respect to \( \mathrm{H_2O_2} \), \( \mathrm{I^-} \) and overall order of reaction.