Class 12 Chemistry Important Questions

Very Short Answer Questions (1 Mark)

Ques. Define Molar Conductivity.

Ans. It is defined as the conductivity of an electrolytic solution divided by its molar concentration. It measures the total conducting power of all the ions produced by dissolving one mole of electrolyte in a solution.

Ques. What type of semiconductor is formed when silicon is doped with arsenic? (Delhi 2010)

Ans. n-type semiconductor

Ques. Some liquids form azeotropes when mixed. What do you mean by azeotropes? (Delhi 2014)

Ans. Azeotrope may be defined as a liquid mixture with a definite composition that boils like a pure liquid without changing its composition.

Ques. What are the reagents used in the following reaction?

CH₃COOH →CH₃COCl

Ans. Carboxylic acids are converted into acid chloride by reacting them with phosphorus pentachloride. So reagent used is PCl₅

Ques. Write the formula of Sodium dicyanidoaurate(I) as per IUPAC norms.

Ans. Na[Au(CN)₂ ]

Ques. What is the name of the monomer of Teflon?

Ans. Tetrafluoroethylene(CF₂=CF₂)

Ques. Write the formula of Tetraaminechloridonitrito-N-platinum(IV) sulfate as per IUPAC norms.

Ans. [Pt(NH₃)₄Cl(NO₂)]SO₄

Ques. What is the name of the monomers of Buna-N?

Ans. Buna-N has copolymers as Acrylonitrile (CH₂=CH-CN) and 1,3-butadiene(CH₂=CH-CH=CH₂)

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Short Answer Questions (2 Marks)

Ques. Define Lanthanoid Contraction. What are its two features?

Ans. Lanthanoid contraction refers to the decrease in size of lanthanoid ions when their atomic number goes on increasing.

Features:

1. Similarity of second and third transition groups is found there.
2. Ionization potential of the group doesn't show a regular trend.

Ques. How are the below-mentioned colloidal solutions prepared? 

1. Sulfur in water

Ans. Sulfur sol is made by oxidizing H₂S with SO₂ as shown below

SO₂ + H₂S→ 2S + H₂O

2. Gold in water

Ans. Gold sol is made by reducing AuCl₃ with formaldehyde as shown in the reaction below

2AuCl₃ + 3 HCHO + 3H₂O → 2Au(sol) + 3HCOOH + 6HCL

3. Explain briefly Gabriel's Phthalimide reaction

Ans. Gabriel's Phthalimide reaction is the reaction of phthalimide with ethanolic KOH, which results in potassium salt of phthalimide. Then it is heated with an alkyl halide to get N-alkyl phthalimide. After that its reaction with hydrazine(Alkyline hydrolysis) gives a primary amine. Here secondary and tertiary amines are not formed as alkylation is avoided in excess. Also, the aromatic primary amines are not formed because aryl halides don't perform nucleophilic substitution with anion formed by phthalimide.

Ques. Explain the Coupling Reaction briefly.

Ans. The coupling reaction is the name given to a reaction that occurs when diazonium salt reacts with phenols and aromatic amines which results in forming azo compounds having a general formula Ar-N=N-Ar. A coupling reaction with phenol occurs in a mild alkaline medium.

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Long Answer Questions (3 Marks)

Ques. a) What type of isomerism is shown by complex [Co(NH₃)₅(SCN)]²⁺ ?

Ans. Linkage Isomerism is shown by the above-mentioned complex

b) Why is [NiCl₄]^2- paramagnetic while Ni(CN₄]^2- is diamagnetic?(Atomic Number of Ni=28)

Ans. The reason lies in the type of ligands. Cl⁻ is a weak ligand so is unable to pair up paired 3d electrons. Hence [NiCl₄]^2- is paramagnetic.

CN⁻ is a strong field ligand and can hence pair unpaired 3d electrons. Because of unpaired electrons, Ni(CN₄]^2- is diamagnetic.

Ques. Explain briefly the principle of refining metals by the following processes

1. Zone Refining.

Ans. The principle for refining through zone refining is that impurities are more soluble in metal melt than in its solid state.

2. Vapour Phase Refining

Ans. The principle behind vapor phase refining is that crude metal is separated from impurities by first converting it into a volatile compound at a low temperature by mixing with a reagent and then decomposing the volatile compound formed above to get pure metal at a high temperature.

Ques. An element has a bcc structure having a cell edge length of 250 pm. Suppose its density is 8.0 g cm^-3, find the molar mass and radius of the atom of an element.

Ans. Here we are given

a = 250 pm = 250*10^-10 cm, d = 8g cm^-3, z = 2 (for bcc), M = ?

Ques. For the reaction:2NH₃(g) N₂(g) +3H₂(g) , Rate=k

1. What is the order and molecularity of the reaction?

Ans. It is a zero-order reaction and its molecularity is two

2. What is the unit of k?

Ans. molL^-1t^-1

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Very Long Answer Questions (5 Marks)

Ques. How do you convert the following:

1. Phenol to toluene

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1. Formaldehyde to Ethanol

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Ques. A first-order reaction takes only 20 min for 25% decomposition. After 75% of the reaction is completed, what will be the time taken?

Ans. 

Ques. What will be the emf of the following cell at 25 ⁰C Ag(s)|Ag⁺(10⁻³-3 M)|| Cu²⁺(10⁻¹M)|Cu(s)? Given E⁰cell=+0.46V

and log 10ⁿ = n

Ans. Cathode: Cu²⁺ + 2e⁻ → Cu

Anode: 2Ag → Ag⁺ + 2e⁻

Cell Reaction: Cu²⁺ +2Ag → Cu + 2Ag⁺

E⁰ = + 0.46 V

Ques. Answer the following.

1. What do you mean by fuel cells? Explain briefly the electrode reactions involved in H₂-O₂ fuel cell working.

Ans. Fuel cells are defined as cells that convert fuel’s chemical energy into electrical energy.

The electrode reactions involved in this are as

Anode:

[H₂ + 2OH⁻ → 2H₂O + 2e⁻] *2

Cathode:

O₂ + H₂O +4e⁻ → 4OH⁻ 

Net Reaction:

2H₂ + O₂ → 2H₂O

2. Represent a Galvanic cell in which the following reaction occurs.

Zn + Cu²⁺ → Zn²⁺ + Cu

Ans. Galvanic cells with the above reaction can be represented as

MnO₄⁻ (Aq) + Br⁻ (aq) MnO₂(s) +BrO₃⁻ (aq) 

Ques. Define the following terms

1. Mole Fraction
2. Isotonic Solution
3. Vant Hoff factor
4. Ideal Solution
5. Molarity (Delhi 2012)

Ans. The following terms can be defined as – 

1. Mole Fraction: Is is defined as the number of moles of one component divided by the number of moles in the mixture.
2. Isotonic Solution: Isotonic solutions are those solutions: If two solutions have the same osmotic pressure at the given temperature
3. Vant Hoff factor: It can be expressed as

\(i = \frac{normal molar mass}{abnormal molar mass}\)

500. Ideal solution: Ideal solution is that solution which obeys Roult’s law under all conditions.

5. Molarity: Molarity is defined as the number of moles of solute dissolved in a one-liter solution. It depends on temperature and can be expressed as 

\(M = \frac{\omega \times 1000}{mol.mass \times V}\)

Ques. Define Roult’s Law. Apply Roult’s law to show how it is formulated for solutions of nonvolatile solutes. (Comptt.Delhi 2019) 

Ans. Roult’s law states that “for a solution of volatile liquids the partial pressure of each component in the solution is directly proportional to its mole fraction”.

Therefore for component 1:p₁=p₁⁰ X₁

[ Where p₁⁰ is the vapor pressure of pure component 1]

for component 1: p₂=p₂⁰ X₂

According to dalton’s law of partial pressures

P(total) = P₁+P₂⇒P(T)=p₁⁰ X₁ + p₂⁰ X₂

⇒P(T) = p₁⁰(1-X₂) + p₂⁰ X₂

⇒P(T) = p₁⁰+ (p₂⁰-p₁⁰) X₂

Ques. What is Kohlrausch’s Law of independent migration of ions? What are its applications?

Ans. Kohlrausch’s law states that at infinite dilution, each ion migrates independently and makes its contribution to the total molar conductivity of an electrolyte irrespective of the nature of the other associated ion.

Thus Λ₀=λ⁰₊ + λ⁰₋

Where

Λ₀= Molar conductivity of electrolyte at an initial concentration

λ⁰₊ = Molar conductivity of cation at an initial concentration

λ⁰_- = Molar conductivity of anion at an initial concentration

Example:

Λ₀(HCl) = λ⁰₊(H⁺) + λ⁰₋(Cl⁻)

Where 

Λ₀(HCl) =Molar conductivity of HCl at an initial concentration

λ⁰₊(H⁺) =Molar conductivity of protons at an initial concentration

λ⁰₋(Cl⁻) =Molar conductivity of chloride ions at an initial concentration

Application:

The molar conductivity of electrolytes at zero concentration is calculated using this law. The molar conductivity of acetic acid can be calculated using the molar conductivity of HCl, sodium acetate, and sodium chloride as shown in the following expression

Λ₀(CH₃COOH)=Λ₀(HCl) +Λ₀(CH₃COONa) - Λ₀(NaCll)

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