List of top Chemistry Questions asked in CBSE Class XI

What do you understand by isoelectronic species? Name a species that will be isoelectronic with each of the following atoms or ions. 

1. F^- 
2. Ar 
3. Mg²⁺ 
4. Rb⁺

Does the number of moles of reaction products increase, decrease or remain same when each of the following equilibria is subjected to a decrease in pressure by increasing the volume?

1. \(PCl_5 (g) ⇋ PCl_3 (g) + Cl_2 (g)\)
2. \(CaO (s) + CO_2 (g) ⇋ CaCO_3 (s)\)
3. \(3Fe (s) + 4H_2O (g) ⇋ Fe_3O_4 (s) + 4H_2 (g)\)

Describe the change in hybridisation (if any) of the Al atom in the following reaction.
\(AlCl_3+Cl^- \rightarrow AlCl_4^-\)

The first ionization constant of H₂S is 9.1 × 10^–8. Calculate the concentration of HS^– ion in its 0.1M solution. How will this concentration be affected if the solution is 0.1M in HCl also? If the second dissociation constant of H₂S is 1.2 × 10^–13, calculate the concentration of S₂^– under both conditions.

At 700 K, equilibrium constant for the reaction:
\(H_2 (g) + I_2 (g) ⇋ 2HI (g)\)
is 54.8. If 0.5 mol L^–1 of HI(g) is present at equilibrium at 700 K, what are the concentration of H₂(g) and I₂(g) assuming that we initially started with HI(g) and allowed it to reach equilibrium at 700 K?

Using s, p, d notations, describe the orbital with the following quantum numbers.

1. n=1, l=0
2. n = 3, l=1
3. n = 4, l =2
4. n=4, l=3

The ejection of the photoelectron from the silver metal in the photoelectric effect experiment can be stopped by applying the voltage of 0.35 V when the radiation 256.7 nm is used. Calculate the work function for silver metal.

Convert the following into basic units:

1. 28.7 pm
2. 15.15 pm
3. 25365 mg

Give a brief description of the principles of the following techniques taking an example in each case.(a) Crystallisation (b) Distillation (c) Chromatography.

Consider the following species: N^3- , O^2-, F^-, Na⁺, Mg²⁺ and Al³⁺ 

1. What is common in them? 
2. Arrange them in the order of increasing ionic radii.

The first ionization enthalpy values (in kJmol^-1 ) of group 13 elements are :

B	Al	Ga	In	Tl
801	577	579	558	589

How would you explain this deviation from the general trend?

The enthalpy of combustion of methane, graphite and dihydrogen at 298 K are, -890.3 kJ mol^-1, -393.5 kJ mol^-1, and -285.8 kJ mol^-1 respectively. Enthalpy of formation of CH₄(g) will be

Determine the empirical formula of an oxide of iron, which has 69.9% iron and 30.1% dioxygen by mass.

Why does the following reaction occur?
\(XeO_6^{4-}(aq) + 2F^ - (aq) + 6H^ + (aq) \rightarrow XeO_3(g) + F_2(g) + 3H_2O(l)\) What conclusion about the compound \(Na_4XeO_6\)(of which \(XeO_6^{4-}\) is a part) can be drawn from the reaction.

What is the maximum number of emission lines when the excited electron of a H atom in n = 6 drops to the ground state?

The ionization constant of acetic acid is 1.74 × 10^–5. Calculate the degree of dissociation of acetic acid in its 0.05 M solution. Calculate the concentration of acetate ion in the solution and its pH.

Which of the following represents the correct IUPAC name for the compounds concerned? 
(a) 2,2 Dimethylpentane or 2-Dimethylpentane 
(b) 2,4,7-Trimethyloctane or 2,5,7-Trimethyloctane 
(c) 2-Chloro-4 methylpentane or 4-Chloro-2-methylpentane 
(d) But-3-yn-1-ol or But-4-ol-1-yne.

A 25 watt bulb emits monochromatic yellow light of wavelength of 0.57µm. Calculate the rate of emission of quanta per second.

Calcium carbonate reacts with aqueous \(HCl\) to give \(CaCl_2 \) and \(CO_2\) according to the reaction, \(CaCO_3 (s) + 2 HCl (aq) → CaCl_2 (aq) + CO_2(g) + H_2O(l)\). What mass of \(CaCO_3\) is required to react completely with \(25 \ mL\) of \(0.75 \ M\) \(HCl\)?

Draw formulas for the first five members of each homologous series beginning with the following compounds.\((a) H–COOH (b) CH_3COCH_3 (c) H–CH=CH_2\)

A sample of pure PCl₅ was introduced into an evacuated vessel at 473 K. After equilibrium was attained, concentration of PCl₅ was found to be \(0.5×10^{–1}\ mol L^{–1}\). If value of K_c is \(8.3×10^{–3}\), what are the concentrations of PCl₃ and Cl₂ at equilibrium?
\(PCl_5 (g) ⇋ PCl_3 (g) + Cl_2(g)\)

Nitric oxide reacts with Br₂ and gives nitrosyl bromide as per reaction given below:
\(2NO (g) + Br_2 (g) ⇋ 2NOBr (g)\)
When 0.087 mol of NO and 0.0437 mol of Br₂ are mixed in a closed container at constant temperature, 0.0518 mol of NOBr is obtained at equilibrium. Calculate equilibrium amount of NO and Br₂ .

For an isolated system, ∆U=0, what will be ∆S ?

Classify the following reactions in one of the reaction type studied in this unit
a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{HS}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}+\mathrm{Br}^{-}\)
b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}+\mathrm{HCl} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{ClC}-\mathrm{CH}_{3}\)
c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{HO}^{-} \rightarrow \mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{Br}^{-}\)
d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH}+\mathrm{HBr} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CBrCH}_{2} \mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O}\)

Chlorine is prepared in the laboratory by treating manganese dioxide \((MnO_2)\) with aqueous hydrochloric acid according to the reaction
\(4 HCl (aq) + MnO_2(s) → 2H_2O (l) + MnCl_2(aq) + Cl_2 (g)\).
How many grams of HCl react with 5.0 g of manganese dioxide?