List of top Chemistry Questions asked in CBSE CLASS XII

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





(i) Write the products formed when benzaldehyde reacts with the following reagents :

(I) 2,4-Dinitrophenylhydrazine

(II) Acetophenone in the presence of dilute NaOH followed by heating

(ii) Give reasons for the following :

(I) Benzoic acid does not undergo Friedel-Crafts reaction.

(II) Carboxylic acids have higher boiling point than alcohols of comparable molecular mass.

(iii) Write simple chemical test to distinguish between propanal and propanone.

(i) Conductivity of 0.0024 M acetic acid is \( 7.2 \times 10^{-5}\ \mathrm{S\ cm^{-1}} \). If \( \Lambda^\circ_m \) for acetic acid is \( 390.5\ \mathrm{S\ cm^2\ mol^{-1}} \), then calculate the degree of dissociation (\( \alpha \)) of acetic acid.

(ii) Calculate the cell potential for the following half cell reaction at \( 25^\circ \mathrm{C} \) :

\( \mathrm{Ag^+(aq) + 1\,e^- \longrightarrow Ag(s)} \)

Given that : \( [\mathrm{Ag^+}] = 0.01\ \mathrm{M} \) and \( E^\circ_{\mathrm{Ag^+/Ag}} = +0.80\ \mathrm{V} \)

[log 10 = 1]

(iii) Write the name of the electrolyte used in :

(I) Dry cell

(II) Fuel cell (\( \mathrm{H_2 - O_2} \))

(i) Calculate \( E^\circ_{cell} \) for the following reaction which is at equilibrium :

\( \mathrm{Cu(s) + 2Ag^+(aq) \rightleftharpoons Cu^{2+}(aq) + 2Ag(s)} \)

Equilibrium constant \( (K_c) \) for the cell is \( 10^{15} \).

[Given : log 10 = 1]

(ii) Write anode, cathode and overall reaction of lead storage battery when it is in use.

(iii) How much electricity is required in coulombs for the oxidation of 1 mol of FeO to \( \mathrm{Fe_2O_3} \) ?

(i) How will you bring about the following conversions :

(I) Bromobenzene to 1-phenylethanol

(II) Benzene to m-nitroacetophenone

(ii) An organic compound with the molecular formula \( \mathrm{C_8H_8O} \) forms 2,4-DNP derivative, reduces Tollens' reagent and undergoes Cannizzaro reaction. On vigorous oxidation with acidic or alkaline \( \mathrm{KMnO_4} \) it gives Benzene-1,2-dicarboxylic acid. Identify the compound and write the products when it undergoes Cannizzaro reaction.

(iii) Arrange the following compounds in increasing order of their acid strength :

\( \mathrm{C_6H_5COOH,\ O_2N - CH_2 - COOH,\ CF_3 - COOH,\ HCOOH} \)

Answer the following questions :

(i) Why is chemistry of actinoids more complicated as compared to lanthanoids ?

(ii) \(E^\circ_{M^{2+}/M}\) values are not regular for first row transition elements (3d series). Why ?

(iii) Identify the following :

(I) Oxoanion of chromium which is stable in acidic medium.

(II) The lanthanoid element that exhibits +4 oxidation state.

(iv) Write the products obtained on heating \(\mathrm{KMnO_4}\).

(v) Predict which of the following ions will be coloured in aqueous solution :

\(\mathrm{Cu^{+}}\), \(\mathrm{Sc^{3+}}\), \(\mathrm{Fe^{2+}}\), \(\mathrm{Ti^{3+}}\), \(\mathrm{Zn^{2+}}\)

(i) Account for the following :

(I) \(\mathrm{MnO}\) is basic while \(\mathrm{Mn_2O_7}\) is acidic.

(II) Iron has higher enthalpy of atomization than that of copper.

(III) \(\mathrm{Mn^{3+}}\) is a stronger oxidising agent than \(\mathrm{Cr^{3+}}\).

(ii) How do you prepare potassium dichromate from sodium chromate ? Write balanced chemical equation for each step.

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, \(\mathrm{(CH_3CO)_2O}\) and nitric acid.

Despite having the - 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 - OH groups add to the - CHO group and form a cyclic hemiacetal structure.

Answer the following questions :

(i) 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, \(\mathrm{(CH_3CO)_2O}\) and nitric acid.

Despite having the - 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 - OH groups add to the - CHO group and form a cyclic hemiacetal structure.

Answer the following questions :

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

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, \(\mathrm{(CH_3CO)_2O}\) and nitric acid.

Despite having the - 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 - OH groups add to the - CHO group and form a cyclic hemiacetal structure.

Answer the following questions :

How do you explain the following ?

(i) Presence of a carbonyl group in glucose

(ii) Presence of five - 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, \(\mathrm{(CH_3CO)_2O}\) and nitric acid.

Despite having the - 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 - OH groups add to the - CHO group and form a cyclic hemiacetal structure.

Answer the following questions :

What type of carbohydrates are called reducing sugars ?

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 - 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.

Answer the following questions :

Why is ortho-nitrophenol more acidic than ortho-methoxyphenol ?

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 - 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.

Answer the following questions :

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

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 - 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.

Answer the following questions :

(i) Why are tertiary alcohols resistant to oxidation ?

(i) How many geometrical isomers are possible in each of the following complexes ?

(I) \( \mathrm{[Cr(C_2O_4)_3]^{3-}} \)

(II) \( \mathrm{[Co(NH_3)_3Cl_3]} \)

(ii) \( \mathrm{[Co(NH_3)_6]^{3+}} \) is an inner orbital complex whereas \( \mathrm{[Ni(NH_3)_6]^{2+}} \) is an outer orbital complex. Why ?

[Atomic number : Co = 27, Ni = 28]

(iii) Write the formula of Wilkinson catalyst and its use.

(i) Draw the structure of the semicarbazone of acetone.

(ii) Why are \( \alpha \)-hydrogen atoms of aldehydes and ketones acidic in nature ?

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

\( \mathrm{CH_3COCH_3,\ CH_3CHO,\ (CH_3)_3C-CO-CH_3} \)

(iv) Write the reaction involved in Etard reaction.

(v) Write the product when \( \mathrm{C_6H_{11}-CHO} \) reacts with Zn(Hg)/conc. HCl.



(i) Draw the structure of the semicarbazone of acetone.

(ii) Why are \( \alpha \)-hydrogen atoms of aldehydes and ketones acidic in nature ?

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

\( \mathrm{CH_3COCH_3,\ CH_3CHO,\ (CH_3)_3C-CO-CH_3} \)

(iv) Write the reaction involved in Etard reaction.

(v) Write the product when \( \mathrm{C_6H_{11}-CHO} \) reacts with Zn(Hg)/conc. HCl.



(b) (i) The conductivity of 0.001 M solution of acetic acid is \( 3.905 \times 10^{-5}\ \mathrm{S\ cm^{-1}} \). Calculate its molar conductivity and degree of dissociation (\( \alpha \)).

Given : \( \lambda^\circ_{\mathrm{CH_3COO^-}} = 40.9\ \mathrm{S\ cm^2\ mol^{-1}} \)

\( \lambda^\circ_{\mathrm{H^+}} = 349.6\ \mathrm{S\ cm^2\ mol^{-1}} \)

(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 ?

(i) Draw the structure of the semicarbazone of acetone.

(ii) Why are \(\alpha\)-hydrogen atoms of aldehydes and ketones acidic in nature ?

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

\( \mathrm{CH_3COCH_3} \), \( \mathrm{CH_3CHO} \), \( \mathrm{(CH_3)_3C-CO-CH_3} \)

(iv) Write the reaction involved in Etard reaction.

(v) Write the product when \( \mathrm{C_6H_{11}-CHO} \) reacts with Zn(Hg)/conc. HCl.

(i) The conductivity of 0.001 M solution of acetic acid is \( 3.905 \times 10^{-5} \) S cm\(^{-1}\). Calculate its molar conductivity and degree of dissociation (\(\alpha\)).

Given : \( \lambda^\circ_{\mathrm{CH_3COO^-}} = 40.9 \) S cm\(^2\) mol\(^{-1}\)

\( \lambda^\circ_{\mathrm{H^+}} = 349.6 \) S cm\(^2\) mol\(^{-1}\)

(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 ?

(a) Why is glycogen known as animal starch?

(b) Differentiate between essential amino acids and non-essential amino acids.

(c) What is the effect of denaturation on the structure of proteins?

(a) Define the following terms:

(i) Invert Sugar

(ii) Polysaccharides

(b) On the basis of structure differentiate between fibrous and globular proteins.

Write the major product in the following reaction:

\( \mathrm{C_6H_5Cl} \) (structure shown below) \( + \ \mathrm{CH_3-Cl} \ \xrightarrow[\text{dry ether}]{\mathrm{Na}} \ ? \)



(a) What products would be formed when a nucleotide from DNA containing thymine is hydrolysed ?

(b) How will you explain the presence of five -OH groups in glucose molecule which are attached to different carbon atoms ?

(c) Write the reaction of glucose with HCN.

(a) Define the following terms :

(i) Denaturation as related to proteins

(ii) Oligosaccharides

(b) On the basis of structure differentiate between Amylose and Amylopectin.