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Physical Chemistry XII · CBSE 2025–26

5-Mark Concepts
Physical Chemistry

Solutions · Electrochemistry · Chemical Kinetics — priority guide compiled from PYQs and top question banks.

3
Chapters
23
Total marks
6
Key 5-mark concepts
3
Always-asked topics

Overview

🎯 These topics appear in EVERY or NEARLY EVERY paper — prepare these first:

  1. Electrochemistry — EMF & Nernst Equation — cell reactions, Nernst equation derivation, standard electrode potential  (9 marks chapter — highest weightage in physical)
  2. Chemical Kinetics — Rate Law & Arrhenius — order from data, rate constant units, activation energy graph  (asked almost every year)
  3. Solutions — Colligative Properties & van't Hoff Factor — osmotic pressure, elevation of boiling point, depression of freezing point  (7 marks, numericals guaranteed)
9
Electrochemistry marks
7
Solutions marks
7
Chemical Kinetics marks
💧 Solutions
7 marks
  • Raoult's Law — ideal vs non-ideal solutions, positive/negative deviations
    5-mark: azeotrope explanation + deviation graph
  • Colligative properties — ΔTb, ΔTf, osmotic pressure derivations
    van't Hoff factor for electrolytes is very common
  • Numerical: molar mass from osmometry / cryoscopy data
    Always combined: formula → calculate → explain
⚡ Electrochemistry
9 marks
  • Galvanic cell — electrode reactions, cell notation, EMF calculation
    5-mark: Draw cell → write reactions → calculate E°cell
  • Nernst equation — derivation and numerical application
    Concentration effect on EMF — very frequent
  • Kohlrausch's Law — molar conductivity at infinite dilution
    Calculate Λ°m for weak electrolyte using strong electrolyte data
  • Faraday's laws — electrolysis calculations
    Mass deposited / volume of gas liberated numericals
⏱ Chemical Kinetics
7 marks
  • Rate law — determination of order from concentration-time data
    Table of [A] vs rate → find order → find k
  • Arrhenius equation — activation energy from k vs T data
    log k vs 1/T graph, Ea calculation numerical
  • First-order reaction — half-life, integrated rate law
    t½ = 0.693/k; calculate time for given fraction to react

🔥 Priority List — Most Repeated 5-Mark Topics

#TopicChapterStarsTip
1 Nernst Equation + EMF Calculation Electrochemistry ★★★★★ Write cell notation, electrode reactions, apply Nernst, calculate E_cell
2 Rate Law — Order Determination from Data Kinetics ★★★★★ Table method: double [A], check rate change → order; then find k with units
3 Colligative Properties — van't Hoff Factor Solutions ★★★★☆ π = iCRT or ΔTb = i·Kb·m; calculate i from degree of dissociation α
4 Arrhenius Equation — Activation Energy Kinetics ★★★★☆ log(k₂/k₁) = Ea/2.303R × (1/T₁ – 1/T₂); draw log k vs 1/T graph
5 Kohlrausch's Law — Molar Conductivity Electrochemistry ★★★★☆ Λ°m(CH₃COOH) = Λ°m(HCl) + Λ°m(CH₃COONa) – Λ°m(NaCl)
6 Raoult's Law — Deviations & Azeotropes Solutions ★★★☆☆ Positive deviation: A-B < A-A/B-B → higher VP; Negative: stronger A-B interaction

Concept Patterns — Fixed Templates

Electrochemistry — Nernst Equation
5 marks Most repeated in Chapter 2
Fixed Pattern
① Write the cell: Zn | Zn²⁺ || Cu²⁺ | Cu
② Write anode and cathode half-reactions
③ E°cell = E°cathode – E°anode
④ Apply Nernst: E = E° – (0.0591/n) × log Q
⑤ Calculate E_cell or concentration
Common Variants
→ "Find E_cell when [Zn²⁺] = 0.001M and [Cu²⁺] = 0.1M"
→ "What happens to EMF as cell discharges?"
→ "Calculate ΔG° from E°cell: ΔG° = –nFE°"
Chemical Kinetics — Rate Constant from Data
5 marks Table-based numericals
Fixed Pattern
① Write rate law: r = k[A]^x[B]^y
② Pick 2 rows: vary [A] keeping [B] constant → find x
③ Pick 2 rows: vary [B] keeping [A] constant → find y
④ Overall order = x + y; units of k
⑤ Substitute one row → solve for k
Common Variants
→ "Show that the reaction is first order in A"
→ "Find the rate when [A] is doubled"
→ "Half-life of first order: t½ = 0.693/k"
Solutions — Colligative Properties Numerical
5 marks Formula + van't Hoff factor
Fixed Pattern
① Identify colligative property: ΔTb, ΔTf, π, or ΔP
② For electrolyte: apply van't Hoff: i = 1 + (n–1)α
③ Substitute into formula: ΔTf = i·Kf·m
④ Solve for unknown (Kf, m, or molar mass)
⑤ Compare with ideal value to explain deviation
Common Variants
→ 0.1M KCl in water: i ≈ 2; ΔTf = 2 × 1.86 × 0.1 = 0.372K
→ Molar mass from osmotic pressure: M = ρRT/π
→ "Why does dissolution of NaCl lower freezing point more than glucose?"

Strategy — Physical Chemistry

⚡ Electrochemistry

  • Always write formula first, then substitute
  • Nernst: always note n (electrons transferred)
  • Kohlrausch: write the addition formula before calculating
  • ΔG° = –nFE° and K = 10^(nE°/0.0591) — know both
  • Faraday: Q = It, moles of e⁻ = Q/96500

⏱ Chemical Kinetics

  • Kinetics data table: eliminate variables methodically
  • Arrhenius: log k vs 1/T — slope = –Ea/2.303R
  • Units of k: mol^(1-n) L^(n-1) s⁻¹ where n = order
  • First-order half-life is independent of concentration
  • Draw log k vs 1/T graph for graph-based questions

💧 Solutions

  • van't Hoff factor i: know i for MgCl₂ (i=3), NaCl (i=2)
  • Positive deviation → minimum boiling azeotrope
  • Negative deviation → maximum boiling azeotrope
  • Osmotic pressure: π = CRT (C in mol/L)
  • Always identify unit of Kf/Kb (K kg mol⁻¹)

Model Answer Cards — 5-Mark Q&A

Electrochemistry
Derive the Nernst equation for a cell and calculate EMF
How it's asked
  1. "Derive Nernst equation. Calculate E_cell for Zn–Cu cell when [Zn²⁺] = 0.001M, [Cu²⁺] = 0.1M. E°cell = 1.10V" [5]
  2. "State Nernst equation. What is the effect of concentration on EMF?" [3+2]
  3. "Calculate ΔG° and equilibrium constant K for a cell with E°cell = 1.10V (n=2)" [5]
Show Model Answer 5 marks
1 mark
Cell Notation & Reactions:
Zn | Zn²⁺(c₁) || Cu²⁺(c₂) | Cu
Anode: Zn → Zn²⁺ + 2e⁻
Cathode: Cu²⁺ + 2e⁻ → Cu
2 marks
Nernst Equation:
E_cell = E°_cell – (RT/nF) × ln Q
At 298 K: E = E° – (0.0591/n) × log([Zn²⁺]/[Cu²⁺])
Derived from: ΔG = ΔG° + RT ln Q, and ΔG = –nFE
2 marks
Calculation:
E = 1.10 – (0.0591/2) × log(0.001/0.1)
E = 1.10 – (0.02955) × log(0.01)
E = 1.10 – (0.02955) × (–2)
E_cell = 1.10 + 0.059 = 1.159 V
Mark split: Cell (1) + Nernst derivation (2) + Calculation (2)Total: 5/5
Electrochemistry
State Kohlrausch's Law and calculate Λ°m for acetic acid
How it's asked
  1. "State Kohlrausch's law. Given Λ°m(HCl)=426, Λ°m(NaCl)=126, Λ°m(CH₃COONa)=91 S cm² mol⁻¹, find Λ°m(CH₃COOH)" [5]
  2. "How is molar conductivity of a weak electrolyte determined?" [5]
Show Model Answer 5 marks
1 mark
Kohlrausch's Law: At infinite dilution, molar conductivity of an electrolyte equals the sum of individual ionic conductivities of its cation and anion.
Λ°m = ν₊λ°₊ + ν₋λ°₋
2 marks
Why needed for weak electrolytes: Weak electrolytes (CH₃COOH) don't fully dissociate even at low concentrations, so the graph of Λm vs √c cannot be extrapolated to zero concentration. Λ°m must be calculated indirectly.
2 marks
Calculation:
Λ°m(CH₃COOH) = Λ°m(CH₃COONa) + Λ°m(HCl) – Λ°m(NaCl)
= 91 + 426 – 126
= 391 S cm² mol⁻¹
Law (1) + Explanation (2) + Calculation (2)Total: 5/5
Kinetics
Determine order of reaction from experimental data table
How it's asked
  1. "From the following data, determine order with respect to A and B, overall order, and rate constant k" [5]
  2. "Show that the reaction is first order in each reactant. Find k and its units." [5]
Show Model Answer 5 marks
1 mark
Rate Law: r = k[A]^x[B]^y. To find x: keep [B] constant, vary [A], observe rate change.
2 marks
Finding orders (example data):
Exp 1: [A]=0.1, [B]=0.1, r=2×10⁻⁴
Exp 2: [A]=0.2, [B]=0.1, r=4×10⁻⁴ → rate doubles when [A] doubles → x=1
Exp 3: [A]=0.1, [B]=0.2, r=4×10⁻⁴ → rate doubles when [B] doubles → y=1
Overall order = 1+1 = 2
2 marks
Rate constant k:
k = r / ([A][B]) = 2×10⁻⁴ / (0.1 × 0.1)
k = 0.02 L mol⁻¹ s⁻¹
Units for 2nd order: mol⁻¹ L s⁻¹
Rate law setup (1) + Finding orders (2) + k calculation + units (2)Total: 5/5
Kinetics
Arrhenius equation — calculate activation energy
How it's asked
  1. "Rate constant doubles from 300K to 310K. Calculate activation energy." [5]
  2. "Plot log k vs 1/T. Explain how activation energy is obtained from the graph." [5]
Show Model Answer 5 marks
1 mark
Arrhenius Equation:
k = A × e^(–Ea/RT)
A = pre-exponential (frequency) factor; Ea = activation energy
2 marks
Two-temperature form:
log(k₂/k₁) = (Ea / 2.303R) × (1/T₁ – 1/T₂)
Given: k₂/k₁ = 2, T₁=300K, T₂=310K
log 2 = (Ea / 2.303×8.314) × (1/300 – 1/310)
2 marks
Calculation:
0.3010 = (Ea / 19.14) × (310–300)/(300×310)
0.3010 = (Ea / 19.14) × 1.075×10⁻⁴
Ea = 0.3010 × 19.14 / 1.075×10⁻⁴ ≈ 53.6 kJ mol⁻¹
Equation (1) + Two-temp formula (2) + Calculation (2)Total: 5/5
Solutions
Calculate depression of freezing point and van't Hoff factor for an electrolyte
How it's asked
  1. "0.5g of KCl dissolved in 100g water. ΔTf = 0.240°C. Find van't Hoff factor and degree of dissociation. Kf = 1.86K kg mol⁻¹" [5]
  2. "Calculate osmotic pressure of 0.2M glucose at 300K. R = 0.083 L bar mol⁻¹ K⁻¹" [3+2]
Show Model Answer 5 marks
1 mark
Formula & Given:
ΔTf = i × Kf × m
m = (0.5/74.5) / 0.1 kg = 0.0672 mol kg⁻¹
ΔTf(observed) = 0.240°C
2 marks
Find i:
i = ΔTf / (Kf × m) = 0.240 / (1.86 × 0.0672)
i = 0.240 / 0.1250 = 1.92
ΔTf(normal) = 1.86 × 0.0672 = 0.125°C
2 marks
Degree of dissociation:
KCl → K⁺ + Cl⁻ (n=2 ions)
i = 1 + (n–1)α → 1.92 = 1 + (2–1)α
α = 0.92 (92% dissociation)
Formula + molality (1) + Find i (2) + Degree of dissociation (2)Total: 5/5
Solutions
State Raoult's Law. Explain positive and negative deviations with examples.
How it's asked
  1. "What are positive and negative deviations from Raoult's Law? Give one example of each. Draw vapour pressure vs composition graph." [5]
  2. "Explain the formation of maximum boiling and minimum boiling azeotropes." [5]
Show Model Answer 5 marks
1 mark
Raoult's Law: Vapour pressure of a volatile component is proportional to its mole fraction in the solution.
pA = xA × p°A (ideal solution — A–B interactions = A–A = B–B)
2 marks
Positive Deviation: A–B interactions < A–A and B–B → escaping tendency increases → observed VP > Raoult's value
ΔHmix > 0 (endothermic), ΔVmix > 0
Example: Ethanol + cyclohexane, acetone + CS₂
→ Minimum boiling azeotrope (ethanol 95% + water 5%, bp 78.2°C)
2 marks
Negative Deviation: A–B interactions > A–A and B–B → escaping tendency decreases → observed VP < Raoult's value
ΔHmix < 0 (exothermic), ΔVmix < 0
Example: CHCl₃ + acetone (H-bonding between them), HNO₃ + water
→ Maximum boiling azeotrope (HNO₃ 68% + water, bp 393.5K)
Raoult's law (1) + Positive deviation + azeotrope (2) + Negative deviation + azeotrope (2)Total: 5/5