List of top Questions asked in CBSE CLASS XII

Describe, giving the reason, which one of the following pairs has the property indicated :
(I) Fe or Cu – higher melting point
(II) $\mathrm{Ti^{3+}}$ or $\mathrm{Sc^{3+}}$ – coloured in aqueous solution
(III) Cr or Zn – higher third ionisation enthalpy
A black-brown coloured solid (A), when fused with KOH in the presence of air, produces a dark green compound (B), which on electrolytic oxidation in alkaline medium gives a dark purple compound (C). Identify (A), (B) and (C) and write the reactions involved.
What happens when an acidic solution of the green compound (B) is allowed to stand for some time ? Write the equation and name the type of reaction.
Calculate the electrode potential of a half-cell for a zinc electrode dipping in 0·01 M $\mathrm{ZnSO_4}$ solution at 25°C. [Given : $\mathrm{E^\circ_{Zn^{2+}/Zn} = -0.76\,V}$, log 10 = 1]
Equilibrium constant $\mathrm{(K_c)}$ is related to $\mathrm{E^\circ_{cell}}$, but not to $\mathrm{E_{cell}}$. Why ?
The conductivity of 0·001 M acetic acid is $\mathrm{3.905\times10^{-5}}$ S $\mathrm{cm^{-1}}$. Calculate its molar conductivity and degree of dissociation $\mathrm{(\alpha)}$. [Given : $\mathrm{\lambda^\circ_{CH_3COO^-} = 40.9}$, $\mathrm{\lambda^\circ_{H^+} = 349.6\ S\,cm^2\,mol^{-1}}$]
Give reasons : (I) Why does a mercury cell deliver a constant voltage for its entire life ? (II) Why is it necessary to use a salt bridge in a galvanic cell ?
Write the anode, cathode and overall reaction involved in a dry cell.
Arrange the following compounds in increasing order of their reactivity towards HCN : $\mathrm{CH_3COCH_3,\ CH_3CHO,\ (CH_3)_3C\text{-}CO\text{-}CH_3}$.
Write the reaction involved in the Etard reaction.
Write the product when
reacts with Zn(Hg)/conc. HCl.
Draw / write the structure of the semicarbazone of acetone.
Why are $\mathrm{\alpha}$-hydrogen atoms of aldehydes and ketones acidic in nature ?
Write the product(s) of the following :
(I) One mol of ethanal is treated with 1 mol of $\mathrm{CH_3OH}$ in the presence of dry HCl gas.
(II) Benzaldehyde is treated with conc. NaOH.
(III) Ethanoic acid is heated in the presence of $\mathrm{P_2O_5}$.
Write a simple chemical test to distinguish between Ethanal and Propanal.
Write the name of the reagent to transform Allyl alcohol to Propanal.
The half-life period of a radioactive element is $\mathrm{1.5\times10^{10}}$ years. Calculate the time in which the activity of the element is reduced to 75% of its original value. [Given : log 2 = 0·30, log 3 = 0·48, log 4 = 0·60]
Give reasons for the following :
(a) Haloalkanes react with KCN to form alkyl cyanides, while with AgCN they form isocyanides.
(b)
is more reactive towards $\mathrm{S_N2}$ displacement as compared to
.
(c) Grignard reagents should be prepared under anhydrous conditions.
An organic compound (A) with molecular formula $\mathrm{C_3H_5N}$ on reaction with $\mathrm{C_6H_5MgBr}$ followed by hydrolysis, gives a compound (B). Compound (B) forms an orange-red precipitate with 2,4-DNP reagent and does not give iodoform test. It neither reduces Tollens' or Fehling's reagent nor does it decolourise bromine water. On drastic oxidation with chromic acid it gives a carboxylic acid (C) having molecular formula $\mathrm{C_7H_6O_2}$. Identify the compounds (A), (B) and (C). Write the reactions of compound (A) with $\mathrm{C_6H_5MgBr}$ followed by hydrolysis to give compound (B).
Why are low spin tetrahedral complexes rarely formed ?
Name two coordination compounds which are important in biological systems.
What is meant by chelate effect ? Give an example.
How will you obtain the following from aniline ? Write the chemical equations involved :
(a) Sulphanilic acid
(b) 2,4,6-Tribromoaniline
(c) Acetanilide
Explain why, on addition of 1 mol of KCl to 1 litre of water, the boiling point of water increases, while the addition of 1 mol of methyl alcohol to 1 litre of water decreases the boiling point.
An antifreeze solution is prepared by dissolving 31 g of ethylene glycol (Molar mass = 62 g $\mathrm{mol^{-1}}$) in 600 g of water. Calculate the freezing point of the solution. ($\mathrm{K_f}$ for water = 1·86 K kg $\mathrm{mol^{-1}}$)