Class 11 Chemistry — Complete Notes and Solved PYQs

Chapter 1: Some Basic Concepts of Chemistry

Mole Concept

Mole: 1 mole = 6.022 × 10²³ particles (Avogadro's Number)

Molar Mass: Mass of 1 mole of substance in grams (same number as atomic/molecular mass in amu)

Important Formulas:

Number of moles (n) = Given mass / Molar mass
                    = Given particles / Avogadro number
                    = Volume at STP / 22.4 L (for gases)

Molarity (M) = moles of solute / volume of solution (L)
Molality (m) = moles of solute / mass of solvent (kg)
Mole fraction = moles of component / total moles

Empirical vs Molecular Formula:

• Empirical: Simplest whole number ratio (CH₂O)
• Molecular: Actual formula (C₆H₁₂O₆) — Molecular = n × Empirical

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Chapter 2: Structure of Atom

Atomic Models:

| Model | Proposed by | Key Feature | Failure | |---|---|---|---| | Plum Pudding | Thomson (1904) | Electrons embedded in positive sphere | Geiger-Marsden experiment | | Nuclear | Rutherford (1911) | Dense +ve nucleus, electrons orbit | Electron should spiral into nucleus | | Bohr's Model | Bohr (1913) | Electrons in fixed orbits | Multi-electron atoms, Zeeman effect | | Quantum/Wave | Schrödinger | Electron probability clouds | — |

Bohr's Atomic Model (Hydrogen):

• Electrons orbit in fixed energy levels (n=1,2,3...)
• Energy of nth orbit: Eₙ = -13.6/n² eV
• Radius: rₙ = 0.529 × n² Å (Bohr radius)
• Electron jumps between levels → photon emitted/absorbed

Quantum Numbers (4 quantum numbers):

| Symbol | Name | Values | Describes | |---|---|---|---| | n | Principal | 1,2,3... | Shell/energy level | | l | Azimuthal | 0 to n-1 | Subshell (s,p,d,f) | | mₗ | Magnetic | -l to +l | Orbital orientation | | ms | Spin | +1/2 or -1/2 | Electron spin |

Orbital shapes: s=sphere, p=dumbbell, d=double dumbbell or cloverleaf

Electronic Configuration Rules:

• Aufbau Principle: Fill lowest energy orbitals first (1s<2s<2p<3s...)
• Pauli Exclusion: No two electrons have same four quantum numbers
• Hund's Rule: Maximize unpaired electrons in degenerate orbitals

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Chapter 3: Classification of Elements (Periodic Table)

Periodic Law: Properties of elements are periodic function of atomic number.

Periods: 7 horizontal rows. Period number = number of shells. Groups: 18 vertical columns. Group determines valence electrons.

Periodic Trends:

| Property | Across Period (→) | Down Group (↓) | |---|---|---| | Atomic Radius | Decreases | Increases | | Ionic Radius | Varies (cation < atom < anion) | Increases | | Ionization Energy | Increases | Decreases | | Electron Affinity | Increases (exceptions: noble gases, N) | Decreases | | Electronegativity | Increases (F is highest = 4.0) | Decreases | | Metallic Character | Decreases | Increases |

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Chapter 4: Chemical Bonding

Types of Bonds

Ionic Bond: Transfer of electrons between metal and non-metal.

• Example: NaCl (Na gives e⁻ to Cl)
• Properties: high mp/bp, conducts electricity when dissolved/melted, brittle

Covalent Bond: Sharing of electrons between non-metals.

• Single bond (1 pair), Double bond (2 pairs), Triple bond (3 pairs)
• Example: H₂, O₂, N₂, CH₄, CO₂

Coordinate (Dative) Bond: Both electrons provided by one atom.

• Example: NH₃→BF₃, H₃O⁺

VSEPR Theory

Electron pairs repel each other and arrange to minimize repulsion. Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair

| Electron Pairs | Lone Pairs | Shape | Example | |---|---|---|---| | 2 | 0 | Linear (180°) | BeCl₂, CO₂ | | 3 | 0 | Trigonal planar (120°) | BF₃, SO₃ | | 4 | 0 | Tetrahedral (109.5°) | CH₄, CCl₄ | | 4 | 1 | Trigonal pyramidal (107°) | NH₃ | | 4 | 2 | Bent/Angular (104.5°) | H₂O | | 5 | 0 | Trigonal bipyramidal | PCl₅ | | 6 | 0 | Octahedral (90°) | SF₆ |

Hybridization

| Hybridization | Shape | Bond Angle | Example | |---|---|---|---| | sp | Linear | 180° | BeCl₂, acetylene | | sp² | Trigonal planar | 120° | BF₃, ethylene | | sp³ | Tetrahedral | 109.5° | CH₄, water | | sp³d | Trigonal bipyramidal | 90°/120° | PCl₅ | | sp³d² | Octahedral | 90° | SF₆ |

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Chapter 6: Thermodynamics

System: The part being studied. Surroundings: Everything else. Universe = System + Surroundings

Thermodynamic Quantities:

• Internal Energy (U): Total energy of system
• Enthalpy (H): H = U + PV (heat at constant pressure)
• Entropy (S): Measure of disorder/randomness
• Gibbs Free Energy (G): G = H - TS (spontaneity predictor)

First Law of Thermodynamics: Energy can neither be created nor destroyed. ΔU = q + w (q = heat, w = work) For constant pressure: q = ΔH

Standard Enthalpy Changes:

• ΔH°f — enthalpy of formation
• ΔH°combustion — enthalpy of combustion (always negative)
• Hess's Law: ΔH is path-independent (state function)

Spontaneity — Gibbs Equation: ΔG = ΔH - TΔS

| ΔH | ΔS | ΔG | Spontaneous? | |---|---|---|---| | - | + | Always - | Always (at all T) | | + | - | Always + | Never | | - | - | Depends on T | At low T | | + | + | Depends on T | At high T |

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Chapter 7: Equilibrium

Chemical Equilibrium: Rate of forward reaction = rate of reverse reaction. Dynamic equilibrium: Both reactions occurring simultaneously at equal rates.

Equilibrium Constant Kc: For aA + bB ⇌ cC + dD: Kc = [C]^c [D]^d / [A]^a [B]^b

Relationship: Kp = Kc(RT)^Δn where Δn = (c+d)-(a+b)

Le Chatelier's Principle: When equilibrium is disturbed, it shifts to counteract the disturbance.

| Change | Effect | |---|---| | Increase concentration of reactant | Shifts forward → | | Decrease concentration of product | Shifts forward → | | Increase pressure | Shifts to fewer moles side | | Increase temperature | Shifts in endothermic direction |

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Solved PYQs

Q1 (2023): Calculate number of molecules in 4g of hydrogen gas. Molar mass H₂ = 2 g/mol Moles = 4/2 = 2 mol Molecules = 2 × 6.022×10²³ = 1.2044 × 10²⁴ molecules

Q2 (2022): Write electronic configuration of Fe (Z=26) and Fe²⁺. Fe: [Ar] 3d⁶ 4s² Fe²⁺: [Ar] 3d⁶ (loses 4s electrons first)

Q3 (2024): Predict shape of PCl₃ using VSEPR. P has 5 valence electrons. 3 used for bonding with Cl, 1 lone pair. Total electron pairs = 4 (tetrahedral arrangement) → Trigonal pyramidal shape (like NH₃), bond angle ~107°.

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Quick Revision Checklist

• Mole = 6.022×10²³ particles
• Molarity formula: M = n/V
• Bohr model: Eₙ = -13.6/n² eV
• 4 quantum numbers: n, l, mₗ, ms
• Aufbau, Pauli, Hund's rules
• Periodic trends: radius decreases → period, increases ↓ group
• VSEPR shapes: tetrahedral 109.5°, trigonal 120°, linear 180°
• Hybridization: sp=linear, sp²=planar, sp³=tetrahedral
• First law: ΔU = q + w
• Gibbs: ΔG = ΔH - TΔS (negative = spontaneous)
• Le Chatelier's principle applications