CBSE Class 12 Board Exam Chemistry Syllabus

What are lanthanoid contraction and one of its consequences?

Lanthanoid contraction is the steady decrease in atomic/ionic radii across the lanthanoid series due to poor shielding by 4f electrons. Consequence: Zr and Hf (and other 4d/5d pairs) have nearly identical sizes and similar properties.

Why do transition metals and their compounds often act as good catalysts?

Because they exhibit variable oxidation states (allowing easy electron transfer) and can form intermediate complexes / provide surfaces for adsorption.

Which transition metal has the highest melting point in the 3d series and why are these melting points high?

Chromium-region/manganese region aside, high melting points arise from strong metallic bonding involving unpaired d-electrons; in the 3d series the maximum is around the middle (V, Cr). (Cr/V show very high values.)

What is the general electronic configuration of d-block elements?

(n−1)d^(1–10) ns^(0–2).

Define ligand and denticity in coordination chemistry.

A ligand is an ion or molecule that donates a lone pair to a central metal atom/ion. Denticity is the number of donor (coordinating) atoms a single ligand uses to bind the metal (e.g., monodentate, bidentate).

Define coordination number of a central metal atom.

It is the number of ligand donor atoms directly bonded (sigma) to the central metal ion.

State Werner's concept of primary and secondary valencies.

Primary valency is the oxidation state (ionizable, satisfied by negative ions); secondary valency is the coordination number (non-ionizable, satisfied by ligands and fixed in space, giving geometry).

What is a chelate, and why are chelate complexes extra stable?

A chelate is a ring complex formed when a polydentate ligand binds the metal at two or more points. They are more stable than comparable monodentate complexes due to the chelate effect (favourable entropy).

According to Crystal Field Theory, how do d-orbitals split in an octahedral field?

They split into a lower-energy triply degenerate t₂g set (dxy, dyz, dzx) and a higher-energy doubly degenerate eg set (dz², dx²−y²), separated by Δo.

Differentiate strong-field and weak-field ligands with respect to spin.

Strong-field ligands (large Δo, e.g., CN⁻, CO) cause pairing → low-spin complexes. Weak-field ligands (small Δo, e.g., H₂O, F⁻) favour unpaired electrons → high-spin complexes.

Name the type of isomerism shown by [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br.

Ionisation isomerism (they give different ions in solution).

What is the IUPAC name of K₄[Fe(CN)₆]?

Potassium hexacyanidoferrate(II).

In qualitative inorganic analysis, which cations are precipitated in Group I and by what reagent?

Group I cations are Pb²⁺, Ag⁺, Hg₂²⁺ (Hg₂²⁺/Ag⁺/Pb²⁺), precipitated as chlorides by dilute HCl.

Which reagent precipitates Group II cations in inorganic salt analysis, and as what?

H₂S gas in the presence of dilute HCl precipitates Group II cations (e.g., Cu²⁺, Pb²⁺, As³⁺) as their sulphides.

What confirms the presence of ammonium ion (NH₄⁺) in qualitative analysis?

Heating the salt with NaOH releases ammonia gas, which has a pungent smell, turns moist red litmus blue, and gives white fumes with HCl rod.

What is the brown ring test used to detect, and what is the brown ring?

It detects the nitrate ion (NO₃⁻). A brown ring [Fe(H₂O)₅NO]²⁺ forms at the junction when conc. H₂SO₄ is added along the side of a tube containing the nitrate and FeSO₄.

What is the order of reactivity of haloalkanes in SN1 reactions, and why?

Tertiary > Secondary > Primary, because SN1 proceeds via a carbocation intermediate, and tertiary carbocations are the most stable.

Distinguish SN1 and SN2 mechanisms by their rate law and stereochemistry.

SN1: rate = k[substrate] (first order), proceeds via carbocation, gives racemization. SN2: rate = k[substrate][nucleophile] (second order), single-step backside attack, gives inversion of configuration (Walden inversion).

What is the Wurtz reaction?

Two molecules of an alkyl halide react with sodium metal in dry ether to form a higher symmetrical alkane: 2R–X + 2Na → R–R + 2NaX.

Why is chlorobenzene less reactive than chloroethane toward nucleophilic substitution?

Due to resonance, the C–Cl bond in chlorobenzene has partial double-bond character (shorter, stronger), and sp² carbon plus electron-rich ring repel nucleophiles, making substitution difficult.

What is the Finkelstein reaction?

Conversion of an alkyl chloride/bromide to an alkyl iodide by treatment with NaI in dry acetone: R–X + NaI → R–I + NaX (NaCl/NaBr precipitates).

How is the acidity of phenol explained compared to alcohols?

Phenol is more acidic because the phenoxide ion formed on losing H⁺ is resonance-stabilized (negative charge delocalized into the ring), whereas alkoxide ions are not.

What is the Williamson ether synthesis?

An ether is prepared by the SN2 reaction of a sodium alkoxide with a primary alkyl halide: R–O⁻Na⁺ + R'–X → R–O–R' + NaX.

What is Kolbe's reaction (Kolbe's carboxylation)?

Sodium phenoxide reacts with CO₂ under pressure followed by acidification to give salicylic acid (2-hydroxybenzoic acid).

Name the reaction: phenol + CHCl₃ + NaOH → salicylaldehyde.

Reimer–Tiemann reaction.

How are primary, secondary, and tertiary alcohols distinguished by the Lucas test?

With Lucas reagent (conc. HCl + ZnCl₂): tertiary alcohols give immediate turbidity, secondary give turbidity in ~5 minutes, and primary give no turbidity at room temperature.

What is the Cannizzaro reaction?

Aldehydes lacking an α-hydrogen, when treated with concentrated alkali, undergo disproportionation: one molecule is oxidized to a carboxylate and another reduced to an alcohol.

State the test that distinguishes aldehydes from ketones using Tollens' reagent.

Aldehydes reduce Tollens' reagent (ammoniacal AgNO₃) to give a silver mirror; ketones do not react. This is the silver-mirror test.

What is the aldol condensation reaction?

Aldehydes/ketones with α-hydrogen, in dilute base, combine to form a β-hydroxy aldehyde/ketone (aldol), which on heating loses water to give an α,β-unsaturated carbonyl compound.

Why are carboxylic acids more acidic than phenols?

Because the carboxylate ion is stabilized by resonance over two equivalent oxygen atoms (greater delocalization), making the carboxylic acid release H⁺ more readily than phenol.

What is the HVZ (Hell–Volhard–Zelinsky) reaction?

Carboxylic acids having an α-hydrogen react with Cl₂/Br₂ in the presence of red phosphorus to give α-halocarboxylic acids.

How are amines classified as primary, secondary, and tertiary?

By the number of hydrogen atoms of ammonia replaced by alkyl/aryl groups: primary (RNH₂, one), secondary (R₂NH, two), tertiary (R₃N, three).

Why are aliphatic amines more basic than ammonia, while aromatic amines (aniline) are less basic?

Alkyl groups are electron-donating, increasing electron density on N (more basic). In aniline, the lone pair on N is delocalized into the ring, reducing availability for protonation (less basic).

What is the carbylamine reaction and what does it test for?

Primary amines heated with chloroform and alcoholic KOH give foul-smelling isocyanides (carbylamines). It is a test for primary amines (secondary and tertiary do not respond).

What is Hofmann's bromamide degradation reaction?

A primary amide treated with Br₂ and aqueous/alcoholic NaOH (KOH) gives a primary amine with one fewer carbon atom: R–CONH₂ → R–NH₂.

What happens when aniline reacts with NaNO₂ and HCl at 0–5 °C?

It forms benzenediazonium chloride (diazotisation): C₆H₅NH₂ → C₆H₅N₂⁺Cl⁻.

Classify carbohydrates into the three main groups with one example each.

Monosaccharides (e.g., glucose), oligosaccharides such as disaccharides (e.g., sucrose), and polysaccharides (e.g., starch).

What is the difference between a reducing and a non-reducing sugar? Give an example of each.

Reducing sugars have a free aldehyde/ketone group and reduce Tollens'/Fehling's reagent (e.g., glucose, maltose). Non-reducing sugars do not (e.g., sucrose).

What are essential and non-essential amino acids?

Essential amino acids cannot be synthesized by the body and must be obtained from diet; non-essential amino acids can be synthesized by the body.

What is the difference between DNA and RNA in terms of sugar and bases?

DNA contains deoxyribose sugar and bases A, T, G, C; RNA contains ribose sugar and bases A, U (uracil instead of thymine), G, C.

Define the primary and secondary structure of proteins.

Primary structure is the specific sequence of amino acids linked by peptide bonds. Secondary structure is the local folding (α-helix or β-pleated sheet) stabilized by hydrogen bonds.

What is denaturation of a protein?

The loss of the native secondary and tertiary structure (by heat, acid, etc.) while the primary structure remains intact, causing loss of biological activity (e.g., coagulation of egg white).

In qualitative organic analysis, how is nitrogen detected (Lassaigne's test)?

In Lassaigne's extract, sodium fusion converts N to NaCN; adding FeSO₄ then Fe³⁺ and acid gives a Prussian blue colour, confirming nitrogen.

How are sulphur and the unsaturation (C=C) of an organic compound detected in functional-group tests?

Sulphur: Lassaigne's extract gives violet colour with sodium nitroprusside (or black PbS with lead acetate). Unsaturation: decolourization of bromine water or pink KMnO₄ (Baeyer's reagent).

What is the iodoform test and which groups give a positive result?

Treatment with I₂/NaOH gives a yellow precipitate of iodoform (CHI₃). Positive for compounds containing CH₃CO– (methyl ketones) or CH₃CH(OH)– groups, and ethanol/acetaldehyde.