Solutions and Electrochemistry (NEET Chemistry)
Why This Chapter Matters
Solutions and Electrochemistry carry 6-10 marks in NEET Chemistry. Colligative properties, Raoult's law, Faraday's laws, and cell EMF calculations are all regularly tested.
Core Concepts
1. Types of Solutions and Concentration Terms
Concentration expressions:
Molarity (M) = moles of solute / litres of solution (changes with temperature)
Molality (m) = moles of solute / kg of solvent (temperature-independent — preferred for colligative properties)
Mole fraction (χ) = moles of component / total moles
Normality (N) = equivalents of solute / litre of solution
2. Raoult's Law and Vapour Pressure
Raoult's Law (ideal solutions):
p_A = χ_A × p°_A (partial pressure = mole fraction × pure vapour pressure)
For binary solution: p_total = χ_A p°_A + χ_B p°_B
Relative lowering of vapour pressure:
(p° - p)/p° = χ_solute = n_solute/(n_solute + n_solvent)
For dilute solutions: ≈ n_solute/n_solvent = W/M × M_s/W_s
Non-ideal solutions:
Positive deviation (A-B < A-A, B-B): p > Raoult's prediction. Example: alcohol-water.
Negative deviation (A-B > A-A, B-B): p < Raoult's prediction. Example: chloroform-acetone.
Azeotrope: mixture boiling at constant composition (cannot separate by distillation).
3. Colligative Properties
Depend only on number of solute particles, NOT on their nature.
Lowering of vapour pressure: Δp = χ_B × p°_A
Elevation of boiling point: ΔTb = Kb × m | Kb = molal boiling point elevation constant (ebullioscopic constant)
Depression of freezing point: ΔTf = Kf × m | Kf = molal freezing point depression constant (cryoscopic constant)
Osmotic pressure (π): π = MRT = nRT/V (van't Hoff equation)
van't Hoff factor (i):
i = observed colligative property / expected colligative property
For non-electrolytes: i = 1
For electrolytes: i > 1 (dissociation increases particles)
For association: i < 1
ΔTb = i × Kb × m | ΔTf = i × Kf × m | π = iMRT
4. Electrochemistry
Electrochemical cells:
Galvanic/Voltaic cell: chemical → electrical energy (spontaneous)
Electrolytic cell: electrical → chemical energy (non-spontaneous, needs external power)
Standard cell EMF:
E°cell = E°cathode - E°anode (reduction potentials)
E°cell > 0 → spontaneous reaction
Nernst Equation:
E_cell = E°cell - (RT/nF)lnQ
At 25°C: E_cell = E°cell - (0.0591/n)logQ
Relationship: ΔG = -nFE | ΔG° = -nFE° = -RT lnK
E°cell > 0 → ΔG° < 0 → K > 1 (spontaneous)
Faraday's Laws of Electrolysis:
1st law: Mass deposited ∝ Quantity of charge (Q = It)
2nd law: Mass deposited ∝ Equivalent weight
W = (M/nF) × Q = (E/F) × Q
where E = equivalent weight = M/n (n = valence factor)
1 Faraday = 96500 C = charge of 1 mole of electrons
Conductance:
Specific conductance (κ): conductance per unit length per unit area
Molar conductance (Λm): κ × 1000/M (units: S·cm²·mol⁻¹)
Λm increases with dilution for both strong and weak electrolytes.
Λm at infinite dilution (Λ°m):
Strong electrolytes: extrapolate graph
Weak electrolytes: use Kohlrausch's law: Λ°m = Σλ°(ions)
PYQs (NEET)
NEET 2023: 0.1 M glucose solution has osmotic pressure at 300K? (R = 0.083 L·bar·K⁻¹·mol⁻¹)
π = MRT = 0.1 × 0.083 × 300 = 2.49 bar
NEET 2022: How many seconds to deposit 1.08 g of silver (M=108) from AgNO₃ solution using 2A current?
Q = (M/nF) × mass → mass = Q × M/nF = I×t×M/nF
t = mass × nF/(M × I) = 1.08 × 1 × 96500 / (108 × 2) = 483 seconds
NEET 2021: For a reaction, E°cell = 0.59 V at 25°C with n=2. What is ΔG°?
ΔG° = -nFE° = -2 × 96500 × 0.59 = -113870 J = -113.9 kJ