Chemistry Question Paper

SECTION – A

I. Answer ALL the questions. (10 × 2 = 20 M)

1. Graphite is a good conductor. Explain.

Answer: Graphite is a good conductor of electricity because it has a unique structure where each carbon atom is bonded to three other carbon atoms, forming hexagonal sheets. These sheets are arranged in layers with weak van der Waals forces between them, allowing the layers to slide over each other. Each carbon atom in the layer has one free electron, which can move freely along the layer, facilitating the conduction of electricity.

2. The empirical formula of a compound is CH₂O. Its molecular weight is 90. Calculate the molecular formula of the compound.

Answer:
The empirical formula mass of CH₂O = 12 + 2(1) + 16 = 30 g/mol.
Given molecular mass = 90 g/mol.
The ratio of the molecular mass to the empirical formula mass = 90 / 30 = 3.
Therefore, the molecular formula is (CH₂O)₃, which is C₃H₆O₃.

3. Give the hybridization of carbon in:
• Diamond: The carbon atoms in diamond are sp³ hybridized, forming a tetrahedral structure with four sigma bonds to other carbon atoms.
• Graphite: The carbon atoms in graphite are sp² hybridized, forming a planar structure with one free electron in each carbon atom’s p orbital.
• Fullerene: The carbon atoms in fullerene (C₆₀) are sp² hybridized, forming a spherical structure with alternating single and double bonds.
4. Write the structure of:
• Trichloroethanoic acid: The structure is CHCl₂COOH, where a carboxyl group (–COOH) is attached to a two-carbon chain, and two chlorine atoms are attached to the first carbon.
• p-nitrobenzaldehyde: The structure is C₆H₄(NO₂)CHO, where a nitro group (–NO₂) is attached to the para position of a benzene ring, and an aldehyde group (–CHO) is attached to the ring.
5. What is PAN? What effect is caused by it?

Answer: PAN (Peroxyacetyl Nitrate) is a chemical compound that is formed from the reaction of acetic acid and nitrogen oxides in the atmosphere. PAN is a significant air pollutant and is a component of photochemical smog. It causes respiratory problems and irritation of the eyes.

6. Define the terms COD and BOD.

Answer:
COD (Chemical Oxygen Demand): It is the amount of oxygen required to oxidize organic matter in water by chemical reagents.
BOD (Biochemical Oxygen Demand): It is the amount of oxygen consumed by microorganisms to decompose the organic matter in water over a specific period, usually 5 days at 20°C.

7. What is Boltzmann’s constant? Give its value.

Answer: Boltzmann’s constant (k) is a fundamental physical constant relating the average kinetic energy of particles in a gas to the temperature of the gas. Its value is 1.38 × 10⁻²³ J/K.

8. What happens when magnesium metal is burnt in air?

Answer: When magnesium metal is burnt in air, it reacts with oxygen to form magnesium oxide (MgO). The reaction is highly exothermic and produces a bright white flame.
2 Mg + O₂ → 2 MgO

9. Write the biological importance of Na⁺ and K⁺ ions.

Answer: Sodium (Na⁺) and potassium (K⁺) ions are essential for maintaining the electrochemical gradient across cell membranes, which is crucial for processes like nerve transmission, muscle contraction, and maintaining cellular fluid balance. The sodium-potassium pump helps maintain this gradient by actively transporting Na⁺ out of the cell and K⁺ into the cell.

10. Calculate the pH of 0.05M Ba(OH)₂.

Answer: Ba(OH)₂ is a strong base and dissociates completely in water:
Ba(OH)₂ → Ba²⁺ + 2 OH⁻
The concentration of OH⁻ ions is 0.10 M (since 0.05 M Ba(OH)₂ dissociates into 2 OH⁻ ions per formula unit).
pOH: -log[OH⁻] = -log(0.10) = 1
pH: 14 – pOH = 14 – 1 = 13

SECTION – B

II. Answer any SIX of the following questions. (6 × 4 = 24 M)

1. Write the postulates of the Kinetic Molecular Theory of gases.

Answer:
The postulates are:
1. Gases consist of a large number of molecules in constant random motion.
2. The volume of the individual gas molecules is negligible compared to the total volume of the gas.
3. Gas molecules move in straight lines and only change direction when they collide with each other or the walls of the container.
4. Collisions between gas molecules are perfectly elastic (no energy is lost).
5. The average kinetic energy of gas molecules is directly proportional to the temperature in Kelvin.

2. Balance the following reaction in acidic medium by the ion-electron method.

Answer: Please provide the specific reaction to be balanced.

3. a) Write the relation between Kp and Kc.

Answer:
The relationship between the equilibrium constants Kp (for partial pressures) and Kc (for concentrations) is given by:
Kp = Kc (RT)Δn
where R is the gas constant, T is the temperature in Kelvin, and Δn is the change in the number of moles of gas (moles of products – moles of reactants).

4. b) What is a conjugate acid-base pair? Illustrate with examples.

Answer:
A conjugate acid-base pair consists of two species that differ by one proton (H⁺). For example, in the acid-base reaction:
HCl ↔ H⁺ + Cl⁻
HCl is the acid and Cl⁻ is its conjugate base.

5. State and explain Hess’s Law of constant heat summation. Give an example.

Answer:
Hess’s Law states that the total heat change in a chemical reaction is the sum of the heat changes in the steps into which the reaction can be divided. Example: The formation of CO₂ from C can occur via two steps, and the total enthalpy change is the sum of the enthalpy changes of the individual steps.

6. Explain the structure of diborane.

Answer:
Diborane (B₂H₆) has a unique structure with two boron atoms, each bonded to three hydrogen atoms. Two of the hydrogen atoms bridge the two boron atoms, creating a 3-center 2-electron bond. The molecule has an unusual electron-deficient structure.

7. Write two oxidizing and two reducing properties of H₂O₂.

Answer:
Oxidizing properties:
1. H₂O₂ can oxidize iodide ions to iodine (I⁻ → I₂).
2. H₂O₂ can oxidize Fe²⁺ to Fe³⁺.
Reducing properties:
1. H₂O₂ can reduce potassium permanganate (MnO₄⁻ → Mn²⁺).
2. H₂O₂ can reduce chlorine (Cl₂ → Cl⁻).

8. Explain the following reactions:
• Wurtz reaction: A reaction where two alkyl halides react with sodium to form a higher alkane.
  2 R-X + 2 Na → R-R + 2 NaX
• Ozonolysis: A reaction where ozone reacts with an alkene to form ozonides, which are further cleaved to produce aldehydes or ketones.
  R-CH=CH₂ + O₃ → R-CHO + O=CH₂

SECTION – C

III. Answer any TWO of the following questions. (2 × 8 = 16 M)

1. How are the quantum numbers n, l, ml, and ms arrived and explain the significance of quantum numbers?

Answer:
Quantum numbers describe the properties of electrons in atoms. The four quantum numbers are:
1. **n (Principal Quantum Number)**: Defines the energy level of an electron.
2. **l (Azimuthal Quantum Number)**: Defines the shape of the orbital.
3. **ml (Magnetic Quantum Number)**: Defines the orientation of the orbital.
4. **ms (Spin Quantum Number)**: Describes the direction of the electron’s spin.

2. Define IP1, IP2. Why is IP2 > IP1 for a given atom? Discuss the factors that affect IP of an element.

Answer:
**IP1**: First Ionization Energy is the energy required to remove the first electron.
**IP2**: Second Ionization Energy is the energy required to remove the second electron.
IP₂ > IP₁ because after the first electron is removed, the remaining electrons are more tightly bound to the nucleus due to increased effective nuclear charge.
Factors affecting Ionization Energy: atomic size, nuclear charge, electron shielding, and electron configuration.

3. What do you understand by hybridization? Explain different types of hybridization involving s and p orbitals.

Answer:
Hybridization is the mixing of atomic orbitals to form new hybrid orbitals that can explain the bonding in molecules.
Types of hybridization:
1. **sp**: Linear geometry, e.g., BeCl₂.
2. **sp²**: Trigonal planar geometry, e.g., BF₃.
3. **sp³**: Tetrahedral geometry, e.g., CH₄.
4. **sp³d**: Trigonal bipyramidal geometry, e.g., PCl₅.
5. **sp³d²**: Octahedral geometry, e.g., SF₆.