Comprehensive CBSE Class 12 Chemistry: 100 Essential 5-Mark Study Notes

PHYSICAL CHEMISTRY

• Buffer Solutions and the Henderson–Hasselbalch Equation

  • A buffer solution is a system that resists drastic changes in $pH$ when small amounts of an acid or a base are added. It typically consists of a weak acid and its conjugate base (acidic buffer) or a weak base and its conjugate acid (basic buffer).
  • The Henderson–Hasselbalch equation for an acidic buffer is derived from the acid dissociation constant $K_a$: $K_a = \frac{[H^+][A^-]}{[HA]}$. Taking basic logarithms and rearranging: $pH = pK_a + \log\left(\frac{[\text{Salt}]}{[\text{Acid}]}\right)$.
  • For basic buffers: $pOH = pK_b + \log\left(\frac{[\text{Salt}]}{[\text{Base}]}\right)$, where $pH = 14.0 - pOH$.

• Raoult’s Law and Ideal Solutions

  • Raoult’s Law states that for a solution of volatile liquids, the partial vapour pressure of each component ($P_i$) in the solution is directly proportional to its mole fraction ($\chi_i$): $P_i = P_i^0 \chi_i$.
  • An Ideal Solution obeys Raoult’s law over the entire range of concentration. Characteristics include $\Delta H_{mix} = 0$ and $\Delta V_{mix} = 0$. Interaction forces between $A-B$ are similar to $A-A$ and $B-B$.

• Colligative Properties: Vapour Pressure and Boiling Point

  • Relative Lowering of Vapour Pressure: $\frac{P_A^0 - P_s}{P_A^0} = i \chi_{solute}$. It is the ratio of the lowering of vapour pressure to the vapour pressure of the pure solvent.
  • Elevation of Boiling Point ($\Delta T_b$): Boiling point increases when a non-volatile solute is added because vapour pressure decreases. Equation: $\Delta T_b = i K_b m$, where $K_b$ is the molal elevation constant (ebullioscopic constant) and $m$ is molality.

• Osmotic Pressure (\Pi)

  • Osmotic pressure is the excess pressure that must be applied to a solution to prevent the flow of solvent into it through a semi-permeable membrane.
  • The expression derived from the Van't Hoff equation is $\Pi = iCRT$, where $C$ is molarity ($mol\,dm^{-3}$), $R$ is the gas constant ($0.0821\,L\,atm\,K^{-1}\,mol^{-1}$), and $T$ is temperature in Kelvin.

• Molarity vs. Molality

  • Molarity (M): Number of moles of solute per $dm^3$ of solution. It is temperature-dependent because volume changes with temperature.
  • Molality (m): Number of moles of solute per kilogram of solvent ($mol\,kg^{-1}$). It is independent of temperature as mass does not change with varying temperatures.

• Nernst Equation

  • The Nernst equation relates the reduction potential of an electrochemical cell to the standard electrode potential, temperature, and activities of the chemical species: $E_{cell} = E^\circ_{cell} - \frac{RT}{nF} \ln Q$.
  • At $298\,K$, this simplifies to: $E_{cell} = E^\circ_{cell} - \frac{0.0591}{n} \log Q$.
  • Applications: Determination of unknown electrode potentials, $\text{pH}$, and equilibrium constants ($K_c$).

• Lead–Acid Battery

  • Anode: Spongy lead ($Pb$); Cathode: Grid of lead packed with lead dioxide ($PbO_2$); Electrolyte: $38\%$ solution of sulphuric acid ($H_2SO_4$).
  • Discharge reactions:
    • Anode: $Pb_{(s)} + SO_{4\,(aq)}^{2-} \rightarrow PbSO_{4\,(s)} + 2e^-$
    • Cathode: $PbO_{2\,(s)} + SO_{4\,(aq)}^{2-} + 4H^+_{(aq)} + 2e^- \rightarrow PbSO_{4\,(s)} + 2H_2O_{(l)}$

• Conductivity and Dilution

  • Conductivity ($\kappa$) decreases with dilution because the number of ions per unit volume decreases.
  • Molar conductivity ($\Lambda_m$) increases with dilution. For strong electrolytes, it increases linearly (Debye-Hückel-Onsager equation); for weak electrolytes, it increases steeply near infinite dilution as degree of dissociation ($\alpha$) increases.

• Fuel Cells

  • Fuel cells are galvanic cells that convert the chemical energy of a fuel (like $H_2$, $CH_4$, $CH_3OH$) directly into electrical energy.
  • The $H_2-O_2$ fuel cell uses porous carbon electrodes with catalysts. Reactions produce water, making it eco-friendly. Unlike conventional cells, fuel cells do not go dead as long as fuel is supplied.

• Electrolysis

  • Molten NaCl: Cathode produces $Na$ metal; Anode produces $Cl_2$ gas.
  • Aqueous CuSO4 (Inert electrodes): Cathode produces $Cu_{(s)}$ ($Cu^{2+} + 2e^- \rightarrow Cu$); Anode produces $O_2$ gas ($2H_2O \rightarrow O_2 + 4H^+ + 4e^-$) because water is easier to oxidize than $SO_4^{2-}$.

• Chemical Kinetics: Rate Laws

  • First-Order Integrated Rate Law: $k = \frac{2.303}{t} \log\left(\frac{[A]_0}{[A]}\right)$.
  • Half-life ($t_{1/2}$): The time taken for concentration to reduce to half its initial value. For first-order: $t_{1/2} = \frac{0.693}{k}$.
  • Second-Order reaction: Rate law is $\frac{1}{[A]} - \frac{1}{[A]_0} = kt$; Half-life: $t_{1/2} = \frac{1}{k[A]_0}$.

• Arrhenius Equation

  • Expression: $k = A e^{-E_a/RT}$, where $A$ is the frequency factor and $E_a$ is activation energy.
  • Logarithmic form: $\log\left(\frac{k_2}{k_1}\right) = \frac{E_a}{2.303R} \left[ \frac{T_2 - T_1}{T_1 T_2} \right]$.

• Order vs. Molecularity

  • Order: Experimental quantity; can be fractional or zero; sum of powers of concentrations in the rate law.
  • Molecularity: Theoretical value; number of reacting species colliding simultaneously; always a whole number ($1$, $2$, or $3$); cannot be zero or fractional.

• Surface Chemistry: Adsorption

  • Physisorption: Weak Van der Waals forces, reversible, low enthalpy ($20-40\,kJ\,mol^{-1}$), multi-molecular layers.
  • Chemisorption: Strong chemical bonds, irreversible, high enthalpy ($80-240\,kJ\,mol^{-1}$), unimolecular layer.
  • Freundlich Adsorption Isotherm: $\frac{x}{m} = k P^{1/n}$, where $x$ is mass of gas adsorbed on mass $m$ of adsorbent.

• Colloids and Catalysis

  • Homogeneous Catalysis: Reactants and catalyst are in the same phase.
  • Heterogeneous Catalysis: Reactants and catalyst are in different phases (e.g., solid catalyst for gaseous reactants).
  • Colloids: Dispersed phase and dispersion medium. Properties include Tyndall effect (scattering of light) and Brownian movement (zig-zag motion of particles).

INORGANIC CHEMISTRY

• Metallurgy and Extraction

  • Blast Furnace (Iron): Reduction of haematite ($Fe_2O_3$). Coke acts as fuel and reducing agent. Limestone ($CaCO_3$) acts as flux to remove silica ($SiO_2$) as slag ($CaSiO_3$).
  • Mond Process: Refining Nickel. Nickel is heated with $CO$ to form volatile Nickel Tetracarbonyl: $Ni + 4CO \xrightarrow{330-350\,K} Ni(CO)_4$. Decomposition at $450-470\,K$ yields pure nickel.
  • Bayer’s Process: Digestion of Bauxite with hot concentrates of $NaOH$ to form Sodium Aluminate, leaving impurities behind. Alumina is then precipitated as $Al(OH)_3$.
  • Zone Refining: Based on the principle that impurities are more soluble in the melt than in the solid state of the metal. Used for semiconductors like $Si$ and $Ge$.

• p-Block Elements (Group 15-18)

  • Inert Pair Effect: Reluctance of s-electrons to participate in bonding due to poor shielding of d and f electrons. Heavier elements prefer lower oxidation states (e.g., $Pb^{2+}$ over $Pb^{4+}$).
  • Interhalogen Compounds: Compounds of type $XX'$ ($ClF$, $IBr$, $IF_5$). They are more reactive than parent halogens (except $F_2$) due to weaker $X-X'$ bonds.
  • Oxoacids of Sulphur: Includes $H_2SO_4$ (Sulphuric acid), $H_2SO_3$ (Sulphurous acid), and $H_2S_2O_8$ (Marshall's acid). Acidity increases with the oxidation state of Sulphur.

• d- and f-Block Elements

  • Lanthanide Contraction: Gradual decrease in atomic and ionic size with increasing atomic number due to poor shielding of $4f$ electrons. Consequences: similar radii for 4d and 5d series (e.g., $Zr$ and $Hf$), difficulty in separating lanthanides.
  • Transitions metals: Form coloured complexes due to $d-d$ transitions and exhibit paramagnetism due to unpaired electrons. Magnetic moment: $\mu = \sqrt{n(n+2)}\,BM$.

• Coordination Compounds

  • Werner’s Theory: Distinguishes between primary valency (ionizable/oxidation state) and secondary valency (non-ionizable/coordination number).
  • Isomerism: Includes structural (linkage, ionization, solvate) and stereoisomerism (geometrical and optical).
  • Bonding in $[Fe(CN)_6]^{4-}$: Hexacyanoferrate(II). $Fe^{2+}$ undergoes $d^2sp^3$ hybridization (inner orbital, octahedral, diamagnetic) due to strong-field $CN^-$ ligands.
  • Chelate Effect: Enhanced stability of a complex containing chelating ligands (ligands that bind via multiple atoms to form a ring) compared to similar complexes with monodentate ligands.

ORGANIC CHEMISTRY

• Haloalkanes and Haloarenes

  • $S_N1$ Mechanism: Two steps, involves carbocation intermediate, reactivity order: $3^\circ > 2^\circ > 1^\circ$, results in racemization.
  • $S_N2$ Mechanism: One step, involves pentacoordinate transition state, reactivity order: $1^\circ > 2^\circ > 3^\circ$, results in Walden inversion.
  • Environmental Impact: Chlorofluorocarbons (CFCs) cause ozone depletion; Polyhalogen compounds like $DDT$ and $\text{carbon tetrachloride}$ are toxic and persistent in the environment.

• Alcohols, Phenols, and Ethers

  • Dehydration Mechanism: Protonation of alcohol $\rightarrow$ Carbocation formation (slowest step) $\rightarrow$ Elimination of proton to form alkene.
  • Acidity of Phenols: Phenols are more acidic than alcohols because the phenoxide ion is stabilized by resonance, spreading the negative charge over the benzene ring.
  • Ethers: Prepared by Williamson Synthesis ($R-X + R'-ONa \rightarrow R-O-R' + NaX$). Reaction with $HI$: primary alkyl groups form alkyl iodide via $S_N2$, tertiary groups form iodide via $S_N1$.

• Aldehydes, Ketones, and Carboxylic Acids

  • Nucleophilic Addition: Aldehydes are more reactive than ketones due to less steric hindrance and electronic (inductive) effects.
  • HVZ Reaction: Hell–Volhard–Zelinsky reaction involves treating carboxylic acids with $X_2$ and red phosphorus to substitute the alpha-hydrogen with a halogen.
  • Oxidation: Aldehydes reduce Tollen’s reagent (silver mirror test) and Fehling’s solution; ketones do not.

• Amines and Biomolecules

  • Basicity of Amines: In the gas phase: $3^\circ > 2^\circ > 1^\circ > NH_3$. In aqueous phase: Order changes due to hydration and steric effects (e.g., for methyl amines: $2^\circ > 1^\circ > 3^\circ$).
  • Gabriel Phthalimide Synthesis: Synthesis of pure primary aliphatic amines. Aromatic amines cannot be made this way because aryl halides do not undergo nucleophilic substitution with phthalimide easily.
  • Carbohydrates: Classified as monosaccharides (glucose), disaccharides (sucrose), and polysaccharides (starch). Reducing sugars contain free aldehyde or ketone groups.
  • Biomolecules: DNA (double helix, deoxyribose, ATGC) vs RNA (single strand, ribose, AUGC). Proteins take shapes like alpha-helix or beta-pleated sheets.

BOARD-IMPORTANT / VALUE-BASED

• Green Chemistry and Environment

  • Green Chemistry: Design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances.
  • Reverse Osmosis: Application of pressure greater than osmotic pressure to push solvent from solution to pure solvent side. Used extensively in water desalination.
  • Polymers: Addition (polyethene) vs Condensation (nylon-6,6). Biodegradable polymers like PHBV and Nylon-2-nylon-6 reduce the ecological footprint of plastics.

• Chemistry in Daily Life

  • Medicines: Antiseptics (applied to living tissue, e.g., Dettol/0.2% phenol) vs Disinfectants (applied to inanimate objects, e.g., 1% phenol).
  • Detergents: Anionic, cationic, and non-ionic. They can work in hard water, unlike soaps which form scum (calcium/magnesium salts).
  • Corrosion: Electrochemical phenomenon. Prevention via galvanization (zinc coating), painting, or using sacrificial anodes.
  • Hardness of Water: Temporary (bicarbonates) removed by boiling; Permanent (chlorides/sulphates) removed by Calgon’s method or ion-exchange resins.

• Industrial and Biological Roles

  • Catalysts: Speed up Industrial reactions (e.g., Iron in Haber’s process for $NH_3$) by lowering the activation energy barrier.
  • Blood Buffer: The mixture of carbonic acid ($H_2CO_3$) and bicarbonate ion ($HCO_3^-$) maintains blood $pH$ at approximately $7.4$.
  • Coordination Compounds: Chlorophyll (Mg complex for photosynthesis), Haemoglobin (Fe complex for oxygen transport), Cisplatin (Pt complex for cancer treatment).
  • Retrosynthesis: A logical approach to organic synthesis where the target molecule is broken down into simpler precursor structures (synthons).