# Chemistry: Solved Previous Year’s Question Paper for NCERT/CBSE  Class 12th Chemistry Paper Code (HSE2KROXI 9318—A)

## Chemistry: Solved Previous Year’s Question Paper for NCERT/CBSE  Class 12th Chemistry Paper Code (HSE2KROXI 9318—A)

Question: What is van’t hoff factor? How does Vant Hoff factor help in determining the degree of dissociation and association? (5 Marks)

Answer: The Van’t Hoff factor is defined as the relationship between the actual concentration of particles and the calculated concentration of a substance. The van’t Hoff factor is expressed as the ratio of the actual number of ions in a solution to the number of ions that would be present if the solute had not dissociated.

The Vant Hoff factor helps in determining the degree of dissociation and association by providing an equation that can be used to calculate the extent to which a substance has dissociated or been associated. By taking into account the number of particles that are produced when a molecule dissociates and the concentration of the solution, the Vant Hoff factor can be used to calculate the degree of dissociation. This can then be used to predict the degree of ionization of the substance and the extent to which it will interact with other ions in a solution. Similarly, the Vant Hoff factor can also be used to calculate the degree of association, which measures the extent to which molecules have bonded together.

Question: State and explain Roults law. Why does a solution of ethanol and cyclohexane show positive deviation from ideal behaviour (5 Marks)

Answer: Raoult’s Law states that the partial vapor pressure of a component of an ideal mixture of liquids is equal to the mole fraction of the component multiplied by the vapor pressure of the pure component.

Mathematically, Raoult’s law equation is written as;

P solution = Χ solvent P0 solvent

Where,

P solution = vapour pressure of the solution
Χ solvent  = mole fraction of the solvent
P0 solvent = vapour pressure of the pure solvent

This law is used to calculate the vapour pressure of a liquid mixture at a given temperature. It is based on the assumption that the vapour pressure of each component is the same as that of the pure component and is independent of the others.

The positive deviation from ideal behaviour in a solution of ethanol and cyclohexane is due to the presence of hydrogen bonding between the ethanol molecules. Hydrogen bonds are much stronger than other intermolecular forces, and this causes the molecules to be attracted to each other more strongly than in a solution with only non-polar molecules. This causes the solution to have a higher boiling point and a lower vapour pressure than expected from the ideal behaviour.

Question: How is chlorine prepared from  HCL? What happens when chlorine is treated with

i.  Sulphur 2. Slaked lime 3. Hot alkalies 4. Turpentile oil (5 Marks)

Answer: Chlorine can be prepared from HCl (hydrochloric acid) by electrolysis. In this process, HCl is placed in a cell and subjected to an electric current. This causes the HCl to break down into hydrogen gas and chlorine gas. The chlorine gas can then be collected and used as desired.

1. When chlorine is treated with sulphur, it forms a yellow, poisonous gas called chlorine sulphide.

2. When chlorine is treated with slaked lime, it forms calcium hypochlorite.

3. When chlorine is treated with hot alkalies, it forms hydrochloric acid and sodium chloride.

4. When chlorine is treated with turpentine oil, it forms a yellow-green gas called phosgene.

Question: Name the elements of group 16. Why are they called chalogens? Explain why oxygen exhibits an oxidation state of -2  while other members of the family show oxidation of +2, +4, and +6. (5 Marks)

Answer: The elements of group 16 are oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). They are called chalogens because they all form at least one common compound with chlorine (Cl) that has the same oxidation number for each element.

The oxidation state of oxygen is -2 because oxygen has an electronegativity of 3.5 and an atomic number of 8. Oxygen has 6 valence electrons and it prefers to have 8 electrons in its outer shell. This means that oxygen has to share two electrons with other atoms to achieve a stable octet. Since it’s sharing two electrons, it has an oxidation state of -2. Other members of the family typically show oxidation states of +2, +4, and +6 because they have different electronegativity values and different atomic numbers.

Question: Name the elements of the 3d series. Give their electronic configuration. Explain why transitional elements form variable oxidation states. (5 Marks)

Answer: The elements of the 3d series are Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), and Zinc (Zn).

Transitional elements form variable oxidation states due to their electron configurations. These elements have partially filled d orbitals, which allow them to easily gain and lose electrons. This ability to gain and lose electrons gives them the flexibility to form different oxidation states, which is why these elements are known for their variable oxidation states.

Question: How is k2cr2o7 prepared from chromite ore? What happens when k2cr2o7 reacts with (5 Marks)

i. Stanous chloride

ii. Sodium sulphide

iii. A chloride and conc. H2so4

Answer: Chromite ore can be used to prepare potassium dichromate (K2Cr2O7) by first converting it into ferrous chromate (FeCrO4). This can be done by roasting the ore in a furnace and then adding concentrated sulfuric acid to convert it into ferrous sulfate (FeSO4). The ferrous sulfate can then be heated with potassium hydroxide (KOH) to form potassium dichromate.

i. When K2Cr2O7 reacts with stannous chloride, it forms a black precipitate of CrO2Cl2.

ii. When K2Cr2O7 reacts with sodium sulphide, it forms a black precipitate of CrS.

iii. When K2Cr2O7 reacts with chloride and conc. H2SO4 forms a yellow precipitate of CrCl3.

Question: How is acetic acid Prepared from (5 Marks)

i. Primary Alcohol

ii. From nitriles

iii. By the carbonation of Grignard’s reagent.

Give the reaction of acetic acid with Pcl3 and Chlorine in presence of Phosphoric acid

Answer: i. Primary alcohol can be oxidized with a strong oxidizing agent such as potassium dichromate to form acetic acid.

ii. Nitriles can be hydrolyzed to form an amide, which can then be hydrolyzed further to form acetic acid.

iii. Grignard’s reagent can be reacted with carbon dioxide to form acetic acid.

Acetic acid with PCl3 and chlorine in presence of phosphoric acid will form a reaction which is known as Friedel-Crafts acylation reaction. In this reaction, acetic acid reacts with PCl3 and chlorine in presence of phosphoric acid as a catalyst to form an acyl chloride. This acyl chloride then reacts with an aromatic compound to form an aromatic ester.

The overall reaction equation is as follows:

Acetic acid + PCl3 + Cl2 + H3PO4 → Acyl Chloride + HCl

Question: Write short notes on (5 Marks)

i. Perkins reaction

ii. Canizzaro’s reaction

iii. Claisen Reaction

Ans: i. Perkins Reaction: It is an organic reaction in which an aldehyde or ketone is reduced to alcohol using sodium amalgam as a reductant in the presence of an acid catalyst. It was developed by George Washington Carver Perkins in 1887.

ii. Canizzaro’s Reaction: It is an organic reaction in which aldehydes and ketones are reduced to primary alcohols by an alkaline solution of formaldehyde and sodium hydroxide. It was discovered by Cannizzaro in 1853.

iii. Claisen Reaction: It is an organic reaction in which an allyl ester is reacted with a base to form an allyl alcohol. It was developed by Rainer Ludwig Claisen in 1887.

Question: Explain with Examples the dielectric properties of solids (3 Marks)

Answer: Dielectric properties of solids refer to their ability to store electrical energy in an electric field. These properties are determined by the material’s ability to allow electric charges to move through it, and they can be used to measure the electrical resistance of a material. The most common example of a dielectric material is a capacitor, which can store energy between two conductors.

Question: Explain the mechanism of corrosion (3 Marks)

Answer: Corrosion is a natural process in which metal is oxidized and converted into a more stable form. Corrosion occurs when an exposed metal surface interacts with oxygen, water, or other chemicals in the environment. This process is known as oxidation and causes the metal to break down into its chemical components. The most common type of corrosion is rust, which is a reddish-brown oxide that forms on iron and steel due to the reaction of iron and oxygen in the presence of water or moisture. Other metals such as aluminium, copper, and zinc can also corrode, but generally at a slower rate than iron and steel. Corrosion can be prevented by coating the metal surface with a protective coating such as paint, galvanizing, or anodizing.

Question: Define the rate of a reaction; Give its unit. what is the effect of a catalyst on reaction rate? (3 Marks)

Answer: The rate of a reaction is the speed at which a chemical reaction occurs.

It is typically measured in terms of the amount of product produced over a certain period, usually expressed as moles of product per second, or moles per second. The unit of reaction rate is moles per second (mol/s).

The effect of a catalyst on reaction rate is to increase the rate of the reaction by lowering the activation energy required for the reaction to take place. Catalysts can also increase reaction rates by increasing the concentration of reactants, increasing the surface area available for reactants to react, or decreasing the reaction time.

Question: Define the following terms with examples (3 Marks)

i. Co-ordination complex

ii. Bidentate ligand

Answer: i. A coordination complex is an ion or molecule that contains a central metal atom or ion bonded to surrounding molecules or ions, which are called ligands. An example of a coordination complex is [Co(NH3)6]3+, which is a cobalt coordination complex with six ammonia molecules bonded to the cobalt atom.

ii. A bidentate ligand is a molecular species that binds to a central metal atom or ion via two distinct binding sites. Examples of bidentate ligands include ethylenediamine (en), diethylenetriamine (dien) and cyclohexanediamine (cxd).

Question: Starting from Bromoethane how will you get (3 Marks)

(a) Ethyl nitrate

(b) ethanamine

(c) Ethane thiol

Answer: (a) Bromoethane → Ethanoic acid → Ethyl nitrate

(b) Bromoethane → Ethanol → Ethanamine

(c) Bromoethane → Ethane → Ethane thiol

Question: What are Vitamins? Name the sources and diseases caused by Vitamin B6 (3 Marks)

Answer: Vitamins are organic compounds that are essential in small amounts for the normal functioning of the body.

They can be obtained from a variety of food sources, including fruits, vegetables, dairy products, grains, and meats.

Sources of Vitamin B6 include fortified cereals, bananas, avocados, potatoes, nuts, and meats. Deficiency of Vitamin B6 can lead to anemia, depression, confusion, irritability, and skin lesions. Severe Vitamin B6 deficiency can cause nerve damage and an increased risk of heart disease.

Question: Name the Monomers of nylon-6. How nylon-6 is prepared (3 Marks)

Answer: The monomers of nylon-6 are caprolactam.

Nylon-6 is prepared by a process known as polycondensation. This process involves the reaction of a diamine and a dibasic acid. The diamine and dibasic acid are reacted in a condensation reaction, which produces water and a linear polymer chain. The linear polymer chain is then further polymerized to form the nylon-6 polymer.

Question: What is the Nerst equation? What is the Significance of each term in the equation? (2 Marks)

Answer: The Nernst equation is a thermodynamic equation that relates the potential of a redox reaction (the electromotive force, abbreviated as E) to the activities of the species involved in the reaction.

It is given by the equation: E = E° – (RT/nF) * ln(Q) where E° is the standard potential, R is the universal gas constant, T is the temperature, n is the number of electrons exchanged in the reaction, F is the Faraday constant, and Q is the reaction quotient.

The term E° represents the standard potential of the reaction, which is the potential of the reaction when all reactants and products are at unit activity. The term (RT/nF) is a constant, which is the same for all redox reactions. The term ln(Q) represents the reaction quotient, which is a measure of the ratio of the concentrations of the reactants to the concentrations of the products in the reaction. The reaction quotient is also influenced by the temperature, so the Nernst equation must be solved for a given temperature to obtain the standard potential.

Question: How is activated energy affected by the rise in temperature? (2 Marks)

Answer: As temperature rises, the number of activated energy increases. This is because the increased temperature gives molecules more energy, which allows them to move faster and react more quickly. As a result, more energy is needed to activate a reaction and complete the transition from the reactants to the products.

Question: Define the following terms (any two) (2 Marks)

(i) Polling (ii) Liquation (iii) Leaching

Answer: (i) Polling: Polling is a process used to separate different components in a material by evaporating them. This is usually done in a vacuum with the material placed in a chamber and the vacuum pressure adjusted accordingly.

(ii) Liquation: Liquation is a process where a solid is heated to a temperature where it melts and then cooled to form a liquid. This process is used to separate different components of a material based on their melting points.

(iii) Leaching: Leaching is a process that uses a solvent to extract components from a material. The solvent is usually a liquid, such as water or acid, and it is used to dissolve the components from the material so that they can be separated and isolated.

Question: How is Xenon Hexa-fluoride prepared? Give its reaction with H₂O. (2 Marks)

Answer: Xenon Hexa-fluoride (XeF6) is prepared by passing a gaseous mixture of fluorine and xenon over heated magnesium fluoride (MgF2) at 500°C:

Xe + 3F2 → XeF6

Xenon Hexa-fluoride (XeF6) reacts with water (H2O) in a very exothermic reaction to form hydrofluoric acid (HF) and xenon tetrafluoride (XeF4):

XeF6 + 6H2O → 4HF + XeF4

Question: Define Tranquillzer? Give the structure of any Tranquillzer. (2 Marks)

Answer: Tranquillizers are drugs used to reduce anxiety, stress and tension. They work by affecting the brain chemicals that are responsible for these emotions.

Commonly prescribed tranquillizers include benzodiazepines such as Valium, Xanax, Klonopin, and Ativan.

Structure of Valium (diazepam):

Valium is a benzodiazepine with the molecular structure of 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one. The molecular formula is C16H13ClN2O and the molecular weight is 284.75 g/mol.

Question: Starting from phenol how phenolphthalein is obtained? (2 Marks)

Answer: Phenolphthalein is obtained from phenol by a process known as esterification. This involves reacting phenol with an acid such as phosphoric acid, sulfuric acid, or hydrochloric acid, to form the ester phenolphthalein. The esterification reaction requires heat, and the resulting product is then distilled to isolate phenolphthalein.

Question: Give Hinsberg’s test by which three main types of amines can be identified. (2 Marks)

Answer: Hinsberg’s test is an important tool for identifying three main types of amines: primary, secondary, and tertiary amines. Primary amines react with chloroform (CHCl3) to form a white precipitate, whereas secondary and tertiary amines react with chloroform to form an orange-red solution. The reaction involves the formation of an iminium ion with the amine, which can then be identified by a test such as silver nitrate.

(Objective Type Questions)

(a) Fill in the

(i) In h.c.p the co-ordination number is ————–

(ii) Addition of HCN to a carbonyl compound is an example of ——— Answer: Nucleophilic addition reaction.

(b) Tick mark the true/false statement given below

(i) In nucleic acid the sequence is base—phosphate—-sugar (true/false)

(ii) the poisonous gas which can be prepared from chlorine gas is mustard gas (true/false)

(c) Give a one-word substitution to the following

(i) Name the reagent which is used during leaching to dissolve the ore particles in Bauxite ore

(ii) Name the sweetener which is 2000 times sweeter than sucrose

(d) (i) Write the structure of D.D.T

(ii) write the structure of N-Ethyl-N-Methyl ethanamine

(i) Milk can be preserved by adding a few drops of:

(a) Formic acid solution

(b) Formaldehyde solution

(c) Acetic acid solution

(d) Acetaldehyde solution