PSEB 11TH CHEMISTRY MARCH 2025 REAL PAPER FULLY SOLVED PDF-
PSEB 11TH CHEMISTRY MARCH 2025 REAL PAPER FULLY SOLVED PDF-
Here are the solutions to the 11th Chemistry Final Exam March 2025:
ONE MARK QUESTIONS
Q.1. MCQ
(i) Mass of 2.6 gram molecule of SO₂
Answer: (a) 166.4 g
Explanation:
- Molar mass of SO₂ = 32 (S) + 2 × 16 (O) = 64 g/mol
- Mass of 2.6 gram molecules = 2.6 × 64 = 166.4 g
(ii) The moles of sodium chloride in 250 cm³ of 0.50 M NaCl are
Answer: (c) 0.125 mol
Explanation:
- Moles = Molarity × Volume (in liters)
- Moles = 0.50 × 0.250 = 0.125 mol
(iii) Neutrons are present in all atoms except
Answer: (c) H
Explanation:
- Hydrogen (H) has no neutrons in its most common isotope (¹H).
(iv) Which of the following orbitals does not make sense?
Answer: (c) 2d
Explanation:
- The 2d orbital does not exist because the d-subshell starts from the 3rd energy level (n=3).
(v) Isotopes are atoms showing same
Answer: (d) atomic number
Explanation:
- Isotopes have the same atomic number (number of protons) but different mass numbers (due to different numbers of neutrons).
(vi) Maximum number of bonds between two atoms of a covalent bond can be
Answer: (c) 3
Explanation:
- The maximum number of bonds between two atoms is 3 (triple bond), as seen in molecules like N₂.
(vii) Which of the following bonds will be non-polar?
Answer: (c) F-F
Explanation:
- F-F bond is non-polar because both atoms have the same electronegativity.
(viii) A thermodynamic state function is a quantity
Answer: (b) whose value is independent of path
Explanation:
- A state function depends only on the initial and final states, not on the path taken.
(ix) The heat of solution depends upon
Answer: (d) All of these
Explanation:
- The heat of solution depends on the nature of solute, solvent, and concentration.
(x) According to Law of Mass Action, the rate of reaction is directly proportional to
Answer: (d) Molar concentration of reactants
Explanation:
- The rate of reaction is directly proportional to the product of the molar concentrations of the reactants.
TRUE/FALSE
(xi) Phosphorous (Z=15) has three unpaired electrons.
Answer: True
Explanation:
- Phosphorus has an electron configuration of [Ne] 3s² 3p³, with three unpaired electrons in the 3p orbital.
(xii) Diamond is more stable than graphite at room temperature and pressure.
Answer: False
Explanation:
- Graphite is more stable than diamond at room temperature and pressure.
(xiii) A catalyst increases the speed of equilibrium reaction and hence the amount of products formed increases.
Answer: False
Explanation:
- A catalyst speeds up both forward and reverse reactions equally, so it does not change the amount of products formed at equilibrium.
(xiv) Oxidation number of an element can be zero but valency is never zero.
Answer: True
Explanation:
- The oxidation number of an element in its elemental form is zero, but valency is never zero.
(xv) Hexane has four structural isomers.
Answer: False
Explanation:
- Hexane has five structural isomers.
COMPREHENSION
(xvi) Who observed the X-rays’ characteristics?
Answer: (a) Henry Moseley
Explanation:
- Henry Moseley observed the characteristic X-ray spectra of elements.
(xvii) The physical and chemical properties of an element are the periodic function of its
Answer: (c) Number of electrons
Explanation:
- The properties of elements are periodic functions of their atomic numbers, which determine the number of electrons.
(xviii) What does (Z) represent in this graph?
Answer: (b) Atomic number
Explanation:
- Z represents the atomic number in the graph.
(xix) Name the ore of uranium.
Answer: (c) Pitch blende
Explanation:
- Pitch blende is an ore of uranium.
(xx) What is the fundamental property of an element?
Answer: (b) Atomic number
Explanation:
- The atomic number is the fundamental property of an element.
TWO MARKS QUESTIONS
Q.2. Classify following substances as elements, compounds and mixtures: Water, tea, silver, steel, carbon dioxide and platinum.
Answer:
- Elements: Silver (Ag), Platinum (Pt)
- Compounds: Water (H₂O), Carbon dioxide (CO₂)
- Mixtures: Tea, Steel
Q.3. Out of sigma and pi bonds, which one is stronger bond and why?
Answer:
- Sigma bond (σ) is stronger than a pi bond (π) because sigma bonds are formed by head-on overlapping of orbitals, which results in a stronger bond. Pi bonds are formed by side-on overlapping, which is weaker.
Q.4. The equilibrium constant for a reaction is 10. What will be the value of ΔG⁰? R = 8.314 J K⁻¹ mol⁻¹, T = 300 K
Answer:
- ΔG⁰ = -RT ln K
- ΔG⁰ = -8.314 × 300 × ln(10)
- ΔG⁰ = -8.314 × 300 × 2.303
- ΔG⁰ = -5748.1 J/mol or -5.748 kJ/mol
OR
Enthalpy and entropy changes of a reaction are 40.63 kJ mol⁻¹ and 108.8 J K⁻¹ mol⁻¹ respectively. Predict the feasibility of the reaction at 270°C.
Answer:
- ΔG = ΔH – TΔS
- T = 270°C = 543 K
- ΔG = 40.63 × 10³ – 543 × 108.8
- ΔG = 40630 – 59064.4
- ΔG = -18434.4 J/mol
- Since ΔG is negative, the reaction is feasible.
Q.5. State and explain first law of thermodynamics. Give its mathematical form.
Answer:
- First Law of Thermodynamics: Energy cannot be created or destroyed, only transferred or converted from one form to another.
- Mathematical Form: ΔU = q – W
- ΔU = Change in internal energy
- q = Heat added to the system
- W = Work done by the system
Q.6. What is Kc for the following equilibrium when the equilibrium concentration of each substance is [SO₂] = 0.60 M, [O₂] = 0.82 M, and [SO₃] = 1.90 M?
2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)
Answer:
- Kc = [SO₃]² / ([SO₂]² [O₂])
- Kc = (1.90)² / ((0.60)² × 0.82)
- Kc = 3.61 / (0.36 × 0.82)
- Kc = 3.61 / 0.2952
- Kc ≈ 12.23
OR
Calculate the pH of a sample of soft drink whose hydronium ion concentration is 3.8 × 10⁻³ M.
Answer:
- pH = -log[H₃O⁺]
- pH = -log(3.8 × 10⁻³)
- pH ≈ 2.42
Q.7. (a) State law of mass action.
Answer:
- The rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants, each raised to the power of their stoichiometric coefficients.
(b) Define equilibrium constant.
Answer:
- The equilibrium constant (Kc) is the ratio of the product of the concentrations of the products to the product of the concentrations of the reactants, each raised to the power of their stoichiometric coefficients, at equilibrium.
Q.8. The compound AgF₂ is unstable compound. However, if formed, the compound acts as a very strong oxidizing agent, why?
Answer:
- AgF₂ is unstable because silver typically forms +1 oxidation state, but in AgF₂, it is in +2 oxidation state, which is highly unstable. However, if formed, it acts as a strong oxidizing agent because it can easily gain electrons to return to the more stable +1 oxidation state.
Q.9. Arrange the following metals in the order in which they displace each other from the solution of their salts: Al, Cu, Fe, Mg, Zn
Answer:
- Mg > Al > Zn > Fe > Cu
- This is based on the reactivity series, where more reactive metals can displace less reactive metals from their compounds.
OR
Define terms: Oxidation, reduction, oxidizing agent, and reducing agent according to electronic concept.
Answer:
- Oxidation: Loss of electrons.
- Reduction: Gain of electrons.
- Oxidizing agent: A substance that accepts electrons (gets reduced).
- Reducing agent: A substance that donates electrons (gets oxidized).
Q.10. What are electrophiles and nucleophiles? Explain with examples.
Answer:
- Electrophiles: Electron-deficient species that accept electrons. Example: H⁺, BF₃.
- Nucleophiles: Electron-rich species that donate electrons. Example: OH⁻, NH₃.
OR
How will you differentiate between oxidation number and valency?
Answer:
- Oxidation Number: The charge an atom would have if electrons were transferred completely. It can be positive, negative, or zero.
- Valency: The combining capacity of an atom, determined by the number of electrons it can lose, gain, or share. It is always a whole number.
Q.11. What effect does branching of an alkane chain has on its boiling point?
Answer:
- Branching decreases the boiling point of alkanes because it reduces the surface area and weakens the van der Waals forces between molecules.
Q.12. State the following laws of chemical combination and give one example in each case:
(a) Law of constant composition
Answer:
- A pure compound always contains the same elements in the same proportion by mass.
- Example: Water (H₂O) always has a 1:8 ratio of hydrogen to oxygen by mass.
(b) Law of multiple proportions
Answer:
- When two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers.
- Example: CO and CO₂ (carbon combines with oxygen in a 1:1 and 1:2 ratio).
Q.13. Write the favourable factors for the formation of ionic bond.
Answer:
- Low ionization energy of the metal.
- High electron affinity of the non-metal.
- Large electronegativity difference between the two atoms.
- High lattice energy of the resulting ionic compound.
Q.14. Define Hess’s law of constant heat summation.
Answer:
- Hess’s law states that the total enthalpy change for a reaction is the same, regardless of the number of steps in which the reaction is carried out.
Q.15. Name two tests to test the presence of double bond in a compound.
Answer:
- Bromine water test: The compound decolorizes bromine water.
- Baeyer’s test: The compound reacts with alkaline KMnO₄, turning it from purple to brown.
SECTION – C
Q.16. Which one of the following will have the largest number of atoms?
(i) 1 g Au (s) (ii) 1 g Na (s) (iii) 1 g Li (s)
Answer:
- 1 g Li (s) will have the largest number of atoms because lithium has the smallest molar mass, so 1 g of Li contains more atoms than 1 g of Na or Au.
OR
The molecular mass of an organic compound is 78 and its percentage composition is 92.4% C and 7.6% H. Determine the molecular formula of the compound.
Answer:
- Empirical Formula:
- C: 92.4 / 12 = 7.7
- H: 7.6 / 1 = 7.6
- Ratio: C:H = 7.7:7.6 ≈ 1:1
- Empirical Formula: CH
- Molecular Formula:
- Empirical mass = 12 + 1 = 13
- n = 78 / 13 = 6
- Molecular Formula: C₆H₆
Q.17. List some postulates of Bohr’s model of an atom.
Answer:
- Electrons revolve around the nucleus in fixed orbits called energy levels.
- Electrons do not radiate energy while in stationary orbits.
- Energy is emitted or absorbed when electrons jump from one orbit to another.
- The angular momentum of electrons is quantized.
Q.18. Discuss the following terms and account for the variation in groups and periods of the periodic table:
(i) Electron gain enthalpy
Answer:
- Electron gain enthalpy is the energy change when an electron is added to a neutral atom.
- It generally increases across a period and decreases down a group.
(ii) Ionization enthalpy
Answer:
- Ionization enthalpy is the energy required to remove an electron from a neutral atom.
- It generally increases across a period and decreases down a group.
Q.19. Explain four types of organic reactions with examples.
Answer:
- Substitution Reaction:
- Example: CH₄ + Cl₂ → CH₃Cl + HCl
- Addition Reaction:
- Example: CH₂=CH₂ + H₂ → CH₃-CH₃
- Elimination Reaction:
- Example: CH₃-CH₂Br → CH₂=CH₂ + HBr
- Rearrangement Reaction:
- Example: CH₃-CH₂-CH₂⁺ → CH₃-CH⁺-CH₃
OR
Suggest and explain a suitable technique of separating naphthalene from kerosene present in a mixture.
Answer:
- Sublimation: Naphthalene sublimes on heating, while kerosene does not. The mixture can be heated, and naphthalene will sublime and can be collected on a cold surface.
SECTION – D
Q.20. Write short notes on:
(i) Friedel-Crafts reaction
Answer:
- It is a reaction where an alkyl or acyl group is introduced into an aromatic ring using an alkyl or acyl halide in the presence of a Lewis acid catalyst (e.g., AlCl₃).
(ii) Markovnikoff’s and anti-Markovnikoff’s rule
Answer:
- Markovnikoff’s Rule: In the addition of HX to an alkene, the hydrogen attaches to the carbon with more hydrogen atoms.
- Anti-Markovnikoff’s Rule: In the presence of peroxides, the hydrogen attaches to the carbon with fewer hydrogen atoms.
(iii) Wurtz reaction
Answer:
- It is a reaction where two alkyl halides react with sodium in dry ether to form a higher alkane.
- Example: 2CH₃Br + 2Na → CH₃-CH₃ + 2NaBr
OR
What are alkenes? Why do these show geometrical isomerism? Explain the mechanism of electrophilic addition reactions to alkenes.
Answer:
- Alkenes: Hydrocarbons with at least one carbon-carbon double bond.
- Geometrical Isomerism: Due to restricted rotation around the double bond, alkenes can have cis and trans forms.
- Electrophilic Addition Mechanism:
- Step 1: The double bond attacks an electrophile (e.g., H⁺), forming a carbocation.
- Step 2: A nucleophile (e.g., Br⁻) attacks the carbocation, forming the final product.
Q.21. Explain the difference between sigma bond and pi bond.
Answer:
- Sigma Bond (σ): Formed by head-on overlapping of orbitals. It is stronger and allows free rotation.
- Pi Bond (π): Formed by side-on overlapping of orbitals. It is weaker and restricts rotation.
OR
(a) Explain with the help of suitable example, polar covalent bond.
Answer:
- A polar covalent bond is formed when two atoms with different electronegativities share electrons unequally.
- Example: In HCl, chlorine is more electronegative than hydrogen, so the bond is polar.
(b) Write the resonance structures of SO₂, NO₂, and NO₃⁻
Answer:
- SO₂:
- O=S=O ↔ O⁻-S⁺=O
- NO₂:
- O=N=O ↔ O⁻-N⁺=O
- NO₃⁻:
- O=N(-O⁻)-O ↔ O⁻-N(+O)-O
This concludes the solutions to the 11th Chemistry Final Exam March 2025.