Chemistry: Solved Previous Year’s Question Paper for NCERT/CBSE Class12th

Chemistry: Solved Previous Year’s Question Paper for NCERT/CBSE Class12th

Short Answer Type Questions (Each carry 3 Marks)

Question: Discuss Amorphous solids

Answer: Amorphous solids are materials that lack long-range order and lack a crystalline structure. Examples of amorphous solids include glasses, plastic, and certain polymers. These materials are generally non-crystalline, and they do not have a repeating structure like crystal lattices do. They are typically soft and ductile and have a disordered arrangement of atoms. Typically, amorphous solids are formed when a material is cooled quickly, which prevents the formation of a crystalline structure. Amorphous solids can also be formed through chemical reactions, such as the formation of glass and polymers. Amorphous solids are typically harder and less brittle than crystalline solids, and they often have a higher melting point. In addition, they are often non-porous and have greater chemical stability.

Question: Explain Activation energy

Answer: Activation energy is the minimum energy required for a chemical reaction to occur. It is the energy barrier that must be overcome to start a chemical reaction. Activation energy is also known as the energy of activation or threshold energy. It is the energy that must be supplied to reactants for them to form products. Activation energy can be provided by heat, light, or catalysts. It is an important concept in the understanding of chemical kinetics and reaction rates.

Question: Explain Tyndal effect

Answer: The Tyndall effect is a phenomenon in which light is scattered by particles in a colloid or a very fine suspension. When a beam of light passes through a colloid, it is scattered in all directions by the suspended particles, resulting in the beam becoming visible through its scattering of light. The amount of scattering depends on the frequency of the light and the size of the particles. The Tyndall effect is named after the 19th-century British physicist John Tyndall, who first discovered and studied it.

Question: Give the electronic configuration of group 16 elements.

Answer: The group 16 elements of the periodic table are oxygen, sulfur, selenium, tellurium, and polonium.

The electronic configuration of each element is as follows:

Oxygen: [He] 2s2 2p4

Sulfur: [Ne] 3s2 3p4

Selenium: [Ar] 3d10 4s2 4p4

 Tellurium: [Kr] 4d10 5s2 5p4

Polonium: [Xe] 4f14 5d10 6s2 6p4

Question: Define ligand and its types

Answer: A ligand is a molecule or an ion that binds to a central atom in a coordination complex. Ligands form coordinate covalent bonds to the metal atom or ion, and the resulting complex is called a coordination complex.

There are two types of ligands: monodentate and polydentate. Monodentate ligands are ligands that bind to only one atom, while polydentate ligands can bind to two or more atoms.

Question: Discuss dehydration of Ethyl alcohol

Answer: Dehydration of ethyl alcohol is a common laboratory procedure used to form an alkene product. The dehydration of ethyl alcohol is usually conducted by heating the alcohol in the presence of an acidic catalyst, such as sulfuric acid, phosphoric acid, or hydrochloric acid. The acid catalyst increases the rate of dehydration by protonating the alcohol’s hydroxyl group, making it more susceptible to elimination. The dehydration reaction involves the elimination of water from the alcohol, forming an alkene product. The dehydration of ethyl alcohol is an exothermic reaction, meaning it releases energy as heat.

Question: Give the preparation of Potassium dichromate from Chromite ore.

Answer: 1. Chromite ore is heated with a concentrated solution of sodium hydroxide at a temperature of 500-600°C.

2. The resulting sodium chromate is then treated with potassium chloride to form potassium dichromate.

3. The reaction is as follows: 2Na2CrO4 + 2KCl → K2Cr2O7 + 2NaCl 4. The resulting potassium dichromate is then filtered and dried.

Question: Discuss three chemical properties of Glucose

Answer: 1. Glucose is a reducing sugar, meaning that it can donate electrons during a redox reaction. This can be observed in Benedict’s test, where glucose will produce a brick-red colour when heated with Benedict’s reagent.

2. Glucose is a monosaccharide, meaning it is composed of single molecules, and it can react with other molecules to form disaccharides and polysaccharides, such as maltose and starch.

3. Glucose is highly soluble in water, which is important for its role in metabolic processes in living organisms.

Very Short Answer Type Questions (Each carry 2 Marks)

Question:  Define the Rate of reaction

Answer: The rate of reaction is the measure of how quickly reactants are converted into products in a chemical reaction. It is usually expressed as the concentration of a product that is formed in a given amount of time.

Question: Explain the Froth Floatation process

Answer: Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic ones. This is used in mineral processing, paper recycling and waste-water treatment industries. Historically this was first used in the mining industry, where it was one of the great enabling technologies of the 20th century.

Question: Why do d-block elements act as catalysts?

Answer: D-block elements act as catalysts because of their ability to form strong bonds with other elements, allowing them to speed up reactions by binding to reactants and forming intermediates. Additionally, the d-block elements have multiple oxidation states which allow them to bridge between different reactants and products, thus facilitating the reaction.

Question: Give two substitution reactions of Haloarenes.

Answer: 1. Friedel-Crafts Alkylation: This reaction involves the addition of an alkyl halide to an aromatic ring. For example, bromobenzene can be alkylated with chloroethane to form ethylbenzene.

2. Friedel-Crafts Acylation: This reaction involves the addition of an acid chloride to an aromatic ring. For example, bromobenzene can be acylated with benzoyl chloride to form benzophenone.

Question: Give two methods of preparation of Alkyl halides

Answer: 1. Alkyl halides can be prepared by the reaction of an alkyl halide with an alkyl halide. This reaction is known as a halogen exchange reaction.

2. Alkyl halides can also be prepared by the reaction of alcohols with hydrogen halides. This reaction is known as the halogenation of alcohol.

Question: Explain the basic structure of amines

Answer: Amines are organic compounds that contain nitrogen atoms bonded to hydrogen atoms or organic groups. The general form of an amine is R-NH2, where R is an organic group.

 Question: Give two uses of Nylon 6,6.

Answer: 1. Nylon 6,6 is used in the manufacturing of carpets, fabrics, and textiles due to its superior strength and durability.

2. Nylon 6,6 is also used in the automotive and industrial sectors as an engineering plastic. It is used in the production of gears, bearings, and other machine components due to its high wear and heat resistance.

Question: What are Analgesics? Give an example

Answer: Analgesics are medications that are used to relieve pain. An example of an analgesic is acetaminophen (Tylenol).

Objective Type Questions (Each carries 1 Marks)

(i) In f.c.c total number of atoms per unit cell is ———–

Answer: 8

(ii) The functional group in Carboxylic acids is ————

Answer: Carboxyl (COOH).

(iii) Oxide Ores are concentrated by Gravity process  (True/False)

Answer: True

(iv) Alcohols have high B.P. due to hydrogen bonding  (True/False)

Answer: True

(v) Amines are acidic in nature (True/False)

Answer: False

(vi) Give the functional group in Isocyanides

Answer: Isocyanides contain the functional group -N=C=O.

(vii) Define Carbohydrates

Answer: Carbohydrates are molecules composed of carbon, hydrogen, and oxygen atoms. They are an important energy source and are found in a wide variety of foods, such as grains, fruits, vegetables, and dairy products. They are commonly referred to as sugars, starches, and fibres.

(viii) Synthetic rubber is

(a) Polythene (b) Isoprene (c) Neoprene (d) PVC

Answer: C. Neoprene

(ix) Synthetic detergents can be used in

(a) Hard water (b) Soft water (c) Hard and soft water (D) None of the above

Answer: C. Hard and soft water

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