12th Standard Chemistry: Previous Year Mixed Test

Explanation. Sodium is a highly electropositive metal that cannot be reduced by carbon, so it is extracted by electrolysis of fused NaCl (Down's process). Iron and lead are obtained by carbon reduction and silver by cyanide leaching.

Explanation. Down group 13 the inert pair effect increases, so the ns² electrons are less readily used in bonding. Hence the stability of the +1 state increases Al < Ga < In < Tl.

Explanation. Partial hydrolysis gives XeOF₄ and XeO₂F₂, but complete hydrolysis gives xenon trioxide: XeF₆ + 3H₂O → XeO₃ + 6HF.

Explanation. Co³⁺ and Fe²⁺ are both 3d⁶ with 4 unpaired electrons, silver is the best conductor, and the 15% rule (Hume-Rothery) applies to substitutional alloys, so (a)-(c) are correct. A metal with a large negative E° loses electrons easily and is a powerful reducing agent, not an oxidising agent, so (d) is incorrect.

Explanation. Pt(II) complexes are square planar. A complex of type [Mabcd] has three geometrical isomers, depending on which ligand is trans to a chosen ligand (e.g. Py trans to NH₃, Br or Cl).

Explanation. In Frenkel defect a cation only moves from its lattice site to an interstitial site; no ion leaves the crystal, so the density does not change. Both the assertion and the reason are false (loss of cation-anion pairs is Schottky defect).

Explanation. Number of half-lives n = 560/140 = 4. Amount left = 1 g × (1/2)⁴ = 1/16 g.

Explanation. Sodium formate is a salt of a weak acid and a strong base; its hydrolysis gives a basic solution with pH = 7 + ½pKa + ½log C.

Explanation. Λm = κ × 1000 / C = (5.76 × 10⁻³ S cm⁻¹ × 1000 cm³ dm⁻³) / 0.5 mol dm⁻³ = 11.52 S cm² mol⁻¹.

Explanation. By the Hardy–Schulze rule, only an ion with charge opposite to that of the colloidal particles causes coagulation (sign), and its coagulating power increases with the magnitude of its charge.

Explanation. Lucas reagent replaces the –OH group by –Cl without changing the carbon skeleton: CH₃CH(OH)CH₃ + HCl → CH₃CHClCH₃ (2-chloropropane) + H₂O.

Explanation. Reductive ozonolysis of ethene gives two molecules of formaldehyde (X). Formaldehyde condenses with ammonia: 6HCHO + 4NH₃ → (CH₂)₆N₄ (hexamethylene tetramine, urotropine) + 6H₂O.

Explanation. Sn/HCl reduces nitrobenzene to aniline (A); diazotisation at 273 K gives benzene diazonium chloride (B); warming with water at 283 K hydrolyses it to phenol (C), C₆H₅OH.

Explanation. α- and β-D-glucose differ only in the configuration at C-1, the anomeric (hemiacetal) carbon of the cyclic form, so they are anomers.

Explanation. Antagonists bind to the receptor site and block its natural function, whereas agonists mimic the natural messenger and activate the receptor.

Explanation. 2H₂O₂ → 2H₂O + O₂. 48 g O₂ = 48/32 = 1.5 mol per minute. Two moles of water form for every mole of O₂, so the rate of formation of water = 2 × 1.5 = 3.0 mol min⁻¹.

Explanation. K₂Cr₂O₇ + 7H₂SO₄ + 6KI → 4K₂SO₄ + Cr₂(SO₄)₃ + 3I₂ + 7H₂O. One Cr₂O₇²⁻ gains 6 electrons, which oxidises 6 I⁻ to 3 I₂, so 3 moles of I₂ are liberated.

Explanation. Non-stick coating is Teflon (PTFE), made by polymerising tetrafluoroethylene, CF₂=CF₂. The paper prints "1,1,2,2-tetrafluoroethane" (as in the textbook); the intended monomer is tetrafluoroethene, so option (c) is the answer.

Explanation. Secondary amines react with nitrous acid to form N-nitrosoamines, which are yellow oily liquids. N-methylaniline is a secondary amine: C₆H₅NHCH₃ + HNO₂ → C₆H₅N(CH₃)–N=O + H₂O.

Explanation. Boric acid is a weak monobasic Lewis acid. It does not donate its own proton; the electron-deficient boron accepts OH⁻ from water: B(OH)₃ + H₂O → [B(OH)₄]⁻ + H⁺.

Explanation. Particles moving to the cathode are positively charged, so the anions cause coagulation. By the Hardy–Schulze rule, higher anion charge means greater coagulating power: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻, i.e. (iii) > (ii) > (i) > (iv).

Explanation. The F–F bond is weaker than the Cl–Cl bond (about 158 vs 242 kJ mol⁻¹), so the assertion is false. The reason is also false: the small fluorine atom has greater repulsion between its lone pairs, which is why its bond is weak.

Explanation. In fluorite (CaF₂), Ca²⁺ ions form an fcc lattice and F⁻ ions occupy all tetrahedral voids. Each Ca²⁺ is surrounded by 8 F⁻ and each F⁻ by 4 Ca²⁺, giving 8 and 4.

Explanation. The sequence of α-amino acids is the primary structure. The secondary structure is the regular, hydrogen-bonded folding (fixed configuration) of the polypeptide backbone; the α-helix is only one example of it (β-sheet is another), so (b) is the complete answer.

Explanation. Ionisation of water is endothermic, so Kw increases as temperature rises. Only option (c), 2.71 × 10⁻¹⁴, is larger than 1.00 × 10⁻¹⁴.

Explanation. Br⁻ outside the bracket makes the complex ion +1. NO₂⁻ is −1, H₂O and NH₃ are neutral: x + (−1) = +1, so x = +2.

Explanation. 1 F = 96500 C is the charge of 1 mole (6.022 × 10²³) of electrons. 9650 C = 0.1 F, so number of electrons = 0.1 × 6.022 × 10²³ = 6.022 × 10²².

Explanation. The –NO₂ group at the para position stabilises the phenoxide ion by both −I and −R effects, so 4-nitrophenol is the most acidic. In 2-nitrophenol intramolecular hydrogen bonding makes the proton harder to lose, 3-nitrophenol has only the −I effect, and Cl is a much weaker electron-withdrawing group.

Explanation. CH₃Br + KCN → CH₃CN (A); acid hydrolysis gives CH₃COOH (B); PCl₅ converts the –COOH group to –COCl, giving CH₃COCl, acetyl chloride (C).

Explanation. Leaching gives the soluble complex Na[Ag(CN)₂]. Zinc, being more electropositive, displaces silver: 2Na[Ag(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Ag.

Explanation. Monoclinic sulphur crystallises with unequal axes a ≠ b ≠ c and angles α = γ = 90°, β ≠ 90°, which are exactly the parameters of the monoclinic system. So both statements are true and the reason explains why it is classed as monoclinic.

Explanation. The complex ion is anionic, so the metal name ends in -ate (aluminate). Three oxalate ligands give 'trioxalato', and x + 3(−2) = −3 gives Al in the +3 state: potassium trioxalato aluminate (III).

Explanation. Down group 17 the H–X bond becomes longer and weaker, so HI releases H⁺ most easily. HI is the strongest acid among the hydrogen halides.

Explanation. Fullerene, graphite and graphene have sp² hybridised carbon atoms. Dry ice is solid CO₂, where carbon (O=C=O) is sp hybridised.

Explanation. RNA contains the purines adenine and guanine and the pyrimidines cytosine and uracil. Thymine occurs in DNA, not RNA.

Explanation. Aspirin is obtained by acetylating the –OH group of salicylic acid with acetic anhydride, so it is acetyl salicylic acid (2-acetoxybenzoic acid).

Explanation. In acid medium \(MnO_4^-\) is reduced to \(Mn^{2+}\) and oxalic acid is oxidised: \(5C_2O_4^{2-} + 2MnO_4^- + 16H^+ \rightarrow 2Mn^{2+} + 10CO_2 + 8H_2O\). The product is carbon dioxide.

Explanation. Aniline undergoes N-acetylation: \(C_6H_5NH_2 + (CH_3CO)_2O \rightarrow C_6H_5NHCOCH_3 + CH_3COOH\). The product is acetanilide.

Explanation. NaCl is a salt of a strong acid (HCl) and a strong base (NaOH), so neither ion reacts with water. The other salts contain a weak acid anion or a weak base cation and are hydrolysed.

Explanation. Primary cells cannot be recharged: the Leclanche cell and the mercury cell. The Ni-Cd cell and the lead storage battery are rechargeable secondary cells.

Explanation. The nucleophile \(CN^-\) attacks the electrophilic carbonyl carbon of acetone and the product adds \(H^+\) on oxygen, giving acetone cyanohydrin. This is nucleophilic addition.

Explanation. The unit of rate constant is \((mol\ L^{-1})^{1-n}\ s^{-1}\). A unit of \(s^{-1}\) means 1 − n = 0, so n = 1: first order.

Explanation. In electrolytic refining the impure metal is made the anode and a thin strip of pure metal the cathode. Copper dissolves from the impure anode and deposits as pure copper on the cathode.

Explanation. In the Haber process, \(Al_2O_3\) acts as a promoter and increases the activity of the iron catalyst. \(H_2S\) and \(As_2O_3\) are catalyst poisons.

Explanation. Pyridinium chlorochromate (PCC) is a mild oxidising agent that converts a primary alcohol only to the aldehyde. Acidified dichromate or KMnO₄ oxidise it further to the carboxylic acid.

Explanation. Milk of Magnesia is a suspension of \(Mg(OH)_2\), a weak base that neutralises excess HCl in the stomach, so it is an antacid.

Explanation. A catalyst provides an alternative path with lower activation energy; state functions such as entropy, internal energy and enthalpy of the reaction are unchanged.

Explanation. In the Hall-Heroult process, alumina dissolved in molten cryolite is electrolysed to obtain aluminium.

Explanation. 1.73 BM = \(\sqrt{n(n+2)}\) with n = 1. \(Cu^{2+}\) is \(3d^9\) with one unpaired electron; \(Co^{2+}\) (\(d^7\), high spin) has 3, while \(Ti^{4+}\) (\(d^0\)) and square planar \([Ni(CN)_4]^{2-}\) have none.

Explanation. Butter is a gel (liquid dispersed in solid). Whipped cream is a foam, mist is an aerosol of liquid in gas, and smoke is an aerosol of solid in gas.

Explanation. HCl is a strong acid that supplies \(H^+\) ions, the ion common to the equilibrium, so it suppresses the dissociation of acetic acid.

Explanation. Each carbon in \(C_{60}\) is bonded to three other carbons in five- and six-membered rings, so it is \(sp^2\) hybridised.

Explanation. Like the lanthanoids, the actinoids most commonly show the +3 oxidation state, though they also show other states.

Explanation. DIBAL-H selectively reduces the nitrile group to an imine, which hydrolyses to the aldehyde, leaving the C=C double bond untouched. Both statements are true and the reason explains the assertion.

Explanation. Radon is radioactive and is used as a source of gamma rays (for example in radiotherapy).

Explanation. The standard electrode potential of the SHE is taken as 0 V by convention; other electrode potentials are measured against it.

Explanation. When NaCl is heated in sodium vapour, anion vacancies are occupied by electrons (F-centres), giving a metal excess defect. AgBr shows Frenkel and Schottky defects and FeO shows metal deficiency.

Explanation. DNA contains adenine, guanine, cytosine and thymine; uracil occurs only in RNA.

Explanation. Ethane-1,2-diol (ethylene glycol) lowers the freezing point of water and is used as anti-freeze in automobile radiators.

Explanation. An aromatic aldehyde condenses with a primary amine with loss of water to form an imine \(ArCH=NR\), called a Schiff's base (e.g. benzaldehyde + aniline gives benzylidene aniline).

Explanation. A group-15 element has 5 valence electrons (\(ns^2np^3\)); in the 3rd period n = 3, so the configuration is \(1s^2 2s^2 2p^6 3s^2 3p^3\) (phosphorus).

Explanation. Bauxite, the chief ore of aluminium, is hydrated aluminium oxide, \(Al_2O_3\cdot nH_2O\).

Explanation. For a first order reaction, 75% completion takes two half-lives (100 → 50 → 25). So \(2t_{1/2} = 60\) min, giving \(t_{1/2} = 30\) min for 50% completion.

Explanation. Reduction in acidic medium (Sn/HCl) converts nitrobenzene to aniline. In neutral medium (\(Zn/NH_4Cl\)) it gives N-phenylhydroxylamine, and in alkaline medium bimolecular products such as azoxy, azo or hydrazo benzene are formed.

Explanation. Periodic acid cleaves the C–C bond of a 1,2-diol; ethylene glycol gives two molecules of methanal: \(HOCH_2CH_2OH + HIO_4 \rightarrow 2HCHO + HIO_3 + H_2O\).

Explanation. In \(K_4[Fe(CN)_6]\), Fe is +2; in \([Fe(CN)_3(NH_3)_3]\), x + 3(−1) + 0 = 0 gives +3; in \([Fe(CO)_5]\), CO is neutral so Fe is 0.

Explanation. \(HPO_4^{2-}\) can donate a proton (forming \(PO_4^{3-}\)) and accept a proton (forming \(H_2PO_4^-\)), so it is amphiprotic. HCl can only donate; \(Br^-\) and \(SO_4^{2-}\) can only accept.

Explanation. Borax is a salt of a strong base (NaOH) and a weak acid (boric acid); it hydrolyses in water to give NaOH, so the solution is basic: \(Na_2B_4O_7 + 7H_2O \rightarrow 2NaOH + 4H_3BO_3\).

Explanation. In the acid hydrolysis of sucrose, the catalyst (dil. HCl) and the reactants are all in the same (aqueous) phase. The other three use solid catalysts (Ni, Fe, \(V_2O_5\)/Pt) with gaseous or liquid reactants, so they are heterogeneous.

Explanation. The nucleophile \(CN^-\) attacks the electrophilic carbonyl carbon of acetone and HCN adds across the C=O bond, so it is a nucleophilic addition reaction.

Explanation. Lanthanoids commonly lose the two 6s electrons and one 5d (or 4f) electron, so +3 is their most common and stable oxidation state.

Explanation. One Faraday is the charge of one mole of electrons: \(F = N_A \times e = 6.022\times10^{23} \times 1.602\times10^{-19} \approx 96500\ C\ mol^{-1}\).

Explanation. Glycine (\(H_2N-CH_2-COOH\)) has two hydrogen atoms on its α-carbon, so it has no chiral carbon; the other amino acids listed are chiral.

Explanation. FeO is found as \(Fe_{0.95}O\): some \(Fe^{2+}\) ions are missing and electrical neutrality is kept by \(Fe^{3+}\) ions, which is a metal deficiency defect. Heated ZnO shows a metal excess defect instead.

Explanation. Fog consists of tiny water droplets (dispersed phase: liquid) spread in air (dispersion medium: gas), so it is a liquid-in-gas colloid (aerosol).

Explanation. Colloidal particles scatter light, so the path of the beam becomes visible. This scattering is the Tyndall effect.

Explanation. Impure Zr is converted to volatile \(ZrI_4\), which is then decomposed on a hot filament to give pure Zr. This iodide decomposition method is the Van Arkel process.

Explanation. Cream is an emulsion (liquid in liquid), butter is a gel (liquid in solid), whipped cream is a foam (gas in liquid) and ink is a sol (solid in liquid).

Explanation. DNA contains the pyrimidines cytosine and thymine; uracil occurs only in RNA, while adenine and guanine are purines. (The paper prints "Thiamine", a misprint for thymine.)

Explanation. pH = 0 means \([H_3O^+] = 10^0 = 1\) M, a very high hydrogen ion concentration, so the solution is strongly acidic.

Explanation. Phenol forms a violet coloured iron-phenoxide complex with neutral \(FeCl_3\); this is the test for phenols.

Explanation. The chain has 1 + 4 + 1 = 6 carbon atoms with \(-NH_2\) groups on C-1 and C-6, so the name is hexane-1,6-diamine. Hexamethylene diamine is only the common name.

Explanation. 9650 C = 9650/96500 = 0.1 mol of electrons. Number of electrons = \(0.1 \times 6.022\times10^{23} = 6.022\times10^{22}\).

Explanation. In diamond each carbon is bonded tetrahedrally to four other carbons (\(sp^3\)). Fullerene, graphite and graphene contain \(sp^2\) carbons.

Explanation. Carbonylation of methanol with CO over a Rh/Ir complex catalyst (Monsanto process) gives acetic acid. The paper misprints the reactant as \(CH_3-CHO\); the intended reactant is methanol.

Explanation. In FeO some \(Fe^{2+}\) sites are vacant and the charge is balanced by \(Fe^{3+}\) ions, so the crystal has fewer metal ions (metal deficiency). ZnO shows metal excess.

Explanation. Hydration of acetylene with dil. \(H_2SO_4\)/\(HgSO_4\) gives acetaldehyde (X). Being an aldehyde with a \(CH_3CO-\) group it gives iodoform, Tollen's and Fehling tests; the Victor Meyer test is for alcohols, so X does not give it.

Explanation. Arrhenius equation: \(\log k = \log A - \frac{E_a}{2.303RT}\), so a plot of log k (or ln k) against 1/T is a straight line with negative slope. Only graph (b) shows this; (a) is a curve and (c) has a positive slope.

Explanation. Xenon forms fluorides and oxides because its outer electrons are loosely held. Neon is very small with a very high ionisation energy, so \(NeF_2\) is not formed.

Explanation. Six monodentate \(CN^-\) ligands are bonded to the central \(Fe^{2+}\) ion, so its coordination number is 6.

Explanation. Fluorine is the strongest oxidising agent; the borax bead test identifies coloured metal ions; aluminium is the most abundant metal in the earth's crust; sulphur (a chalcogen) occurs in volcanic ash.

Explanation. Wolframite is magnetic while tinstone (cassiterite) is non-magnetic, so they are separated by electromagnetic separation.

Explanation. Scandium (3d¹4s²) loses all three valence electrons to give the stable Sc³⁺ ion, so it shows only the +3 oxidation state.

Explanation. Therapeutic index is the ratio of the lethal dose to the therapeutic dose; it measures the medicinal value (safety) of a drug.

Explanation. HCOONa and KCN are salts of a weak acid and a strong base, so they hydrolyse to give basic solutions; anilinium chloride is a salt of a weak base and a strong acid, so its solution is acidic.

Explanation. A pairs with T and G pairs with C, so ATGCTTGA pairs with TACGAACT.

Explanation. Methyl groups are electron-releasing and increase the electron density on nitrogen, while –Br, –Cl and –NO₂ withdraw electrons; hence 2,4-dimethyl aniline is the most basic.

Explanation. Mn changes from +7 in \(MnO_4^-\) to +2, gaining 5 electrons per ion, so 5 F are required per mole.

Explanation. As an aromatic aldehyde without α-hydrogen, p-N,N-dimethylaminobenzaldehyde takes part in (crossed) benzoin condensation, and –CHO is indeed meta directing, but its directing influence in ring substitution does not explain benzoin condensation. The item is loosely framed (the electron-donating –NMe₂ group hinders self-condensation); (c) is the intended answer.

Explanation. Laptops and mobile phones use light, rechargeable lithium-ion batteries.

Explanation. Hyponitrous acid is HO–N=N–OH, i.e. \(H_2N_2O_2\); \(HNO_2\) is nitrous acid.

Explanation. The methoxide ion attacks the methyl halide in a single step from the back side, displacing the halide: an \(S_N2\) reaction.

Explanation. The packing efficiency of a bcc unit cell is 68%, so the vacant space is 100 − 68 = 32%.

Explanation. By definition, the time taken for the concentration of a reactant to fall to half its initial value is the half-life period.

Explanation. Sulphonation of phenol at low temperature (about 280 K) gives mainly o-phenol sulphonic acid; at 373 K the para isomer predominates.