Download 50 chemistry questions for class xii

50 Questions Chemistry class-XII 2015-2016
Q1
(i) Why is Frenkel defect not found in pure alkali metal halides.
(ii) What happens to the structure of CsCl when it is heated to about 760 k?
(iii) Fe3O4 is ferrimagnetic at room temperature and becomes paramagnetic at 850 K.Why?
(iv) When atoms are placed at the corners of all 12 edges, how many atoms are present per unit cell?
Ans→Because ions can not get into interstitial sites due to their larger size.(b) CsCl structure transforms
into NaCl structure.(c) due to randomization of spins at high temp.(d) at corners 8 x 1/8 = 1 atom
Q2 Explain intrinsic and extrinsic semiconductors with suitable examples.
Ans→ Extrinsic contains impurities(B doped with Si) & intrinsic no impurities ( Si and Ge)
Q3 A metal crystallises into two cubic phases FCC & BCC whose edge lengths are 3.5 & 3.0 A 0
respectively. Calculate the ratio of densities of FCC & BCC.
Ans→ d(fcc) = 4 x M / NA x (3.5)3, d(bcc) = 2 x M / NA(3)3
d(fcc) / d(bcc) = 4 x 33 / 2 x (3.5)3 = 1.26 ans
Q4 (a)Why is elevation in boiling point of water different in the following cases;
(i) 0.1 M NaCl solution (ii) 0.1 M sugar solution
(b)Which is more concentrated 1 M or 1 m solution & why?
(c)Two liquids A & B boil at 1450C 1900C respectively. Which of them will have higher V.P at 800C?
Ans→(a) 0.1M NaCl due to more number of ions (b) 1Molar because it contains solvent & solute
both(c) liq A
Q5 State Henry,S law for solubility of a gas in liquid.Explain the significance of Henry,S constant(KH).
At the same temperature, H2 is more soluble than He in water. Which of them will have higher value of
KH& Why?
Ans→ statement, Solubility α 1 / KH, H2 has lower value of KH
Q6 Molar mass of CH3CH2CHClCOOH = 122.5,
moles of CH3CH2CHClCOOH= 10/122.5 = 0.0816, m = 0.0816/ 0.250 kg = 0.3264
α = √ Ka/C = √ 1.4 x 10—3/ 0.3264 = 0.065,
i—1
i—1
i—1
α = --------, 0.065 = -------- = -------, i = 1.065
m—1
2—1
1
0
∆Tf = i x Kf x m = 1.065 x 1.86 x 0.3264 = 0.65 (ANS)
Q7 Which will have greater molar conductivity?
3
(i) Solution containing 1 mol of KCl in 200 ml or 1 mol of KCl in 500 ml.
(ii) Solution containing 1 mol of LiCl in 500 ml or 1 mol of KCl in 500 ml.
(b) Why is it not possible to determine Λ∞m for weak electrolytes by extrapolation?
Ans→(a) (i) 1 mol of KCl in 500 ml . (ii) 1 mol of KCl in 500 ml (b) graph does not intersect the conc
axis.
Q8 (a) Can a solution of 1 M CuSO4 be stored in a Ni vessel? E0(Cu+2/Cu) = 0.34 V, E0 (Ni+2/Ni) = -0.25V.
(b) Suggest a way to determine the Λ∞m value of water.
(c) Which reaction is feasible (i) I2 + 2e-→ 2I-, E0 = 0.54 V. (ii) Br2 + 2e- → 2Br-, E0 = 1.08 V.
Ans→ (a) No. because Cu reduces due to its high reduction potential.
(b) Λ∞m(H2O) → Λ∞m(HCl) + Λ∞m(NaOH) -Λ∞m(NaCl)
(c) reaction (ii) is feasible because E0 of Br2 is more than I2.
Q9 A solution of Ni(NO3)2 is electrolyzed between Pt electrodes using a current of 5 A for 20 minutes.
What mass of Ni is deposited at the cathode?
Ans→ Q = I x t = 5 x 20 x 60 = 6000 coulomb.
Ni+2 + 2e- = Ni ( atomic mass of Ni = 58.7)
2 x 96500 coulomb candeposite = 58.7gm Ni
6000 coulomb candeposite = 58.7 x 6000/2 x 96500 = 1.28gm(Ans)
Q10 The reaction A + B → C has zero order. What is the rate equation?
Ans→ rate = k(A)0(B)0
Q11 Define activation energy of a reaction.
Ans→ Extra amount of energy that is supplied from outside so that colliding particles must produce
Effective Collisions.
Q12 Give an example of a pseudo first order reaction.
Ans→CH3COOC2H5 + H2O → CH3COOH + C2H5OH
Q13 How does the value of rate constant vary with reactant concentration ?
Ans→α 1/ Cn-1
Q14 Identify the order of reaction.
Ans→Zero order
Q15 Define the following terms: (i) Zeta potential (ii) Kraft temperature (iii) Emulsion
Ans→(i) potential difference between the fixed layer and the diffused layer of opposite charges is called
the electrokinetic potential or zeta potential.
(ii)The formation of micelles takes place only above a particular temperature called Kraft temperature
(Tk) and above a particular concentration called critical micelle concentration (CMC).
(iii) These are liquid-liquid colloidal systems, i.e., the dispersion of finely divided droplets in another
liquid. If a mixture of two immiscible or partially miscible liquids is shaken, a coarse dispersion of one
liquid in the other is obtained which is called emulsion.
Q16 What is Hardy Schulze rule? What modification do you suggest in this law?
Ans→ The power of coagulation is directly proportional to the fourth power of active ion. Law must be
modified as; When equimolar proportions of two oppositely charged sols are mixed, they mutually
neutralize their charge and both get coagulated
Q17 Discuss the effect of temperature and pressure on adsorption of gases by solids
Ans→Book
Q18 Explain what is observed when?
(i) An electrolyte is added to ferric hydroxide sol.
(ii) An emulsion is subjected to centrifugation.
(iii) Direct current is passed through a colloidal solution.
Ans→(a) Coagulation (b) de-emulsification (c) electrophoresis.
Q19 What is pyrometallurgy?
Ans→ The process of extraction of metal by heating the metal oxide with a suitable reducing agent is
called thermal reduction or pyrometallurgy
Q20 What is the role of cryolite in electrometallurgy of Aluminium?
Ans→ To reduce the melting point of alumina.
Q21 Out of C and CO, Which is better reducing agent at 673K&Why?
Ans→ When C reacts with O2 two reactions are possible; C + O2 → CO2 (1) 2C+O2→2CO (2).
It is clear from Ellingham diagram that at 673K, ∆G0 for the oxidation of CO is more –ive than the
reaction 1 & 2. Therefore CO is better reducing agent.
Q22 Metals do not occur in nature as nitrates. Why?
Ans→ Nitrates of almost all metals are soluble in water.
Q23 How is nitric acid manufactured by Ostwald process? Give all chemical reactions involved in the
process.
Ans→Catalytic
oxidation
of
NH3
by
atmospheric
oxygen
Q24
Q25
Q26
Q27
Q28
Q29
Q30
Q31
Q32
Nitric oxide thus formed combines with O2 to give NO2
NO2 so formed, dissolved in water to give Nitric acid
Give reason for the followings:
(a) Nitrogen does not form pentahalides.
(b) All five bonds in PCl5 are not equivalent.
(c) SF6can not be hydrolysed easily.
Ans→(A) Absence of vacant d orbital
(b) Axial bond pairs suffer more repulsion than equatorial bond pairs.
(c) Sulphur is sterically protected.
Arrange the following as indicated:
3
(a) HF ,HCl , HBr , HI ( decreasing order of acidic character)
(b) NH3, PH3, AsH3, SbH3( decreasing order of themal stability)
(c) H2O, H2S, H2Se, H2Te ( decreasing order of reducing property)
Ans→(a) HF <HCl<HBr< HI (b) (b) NH3> PH3> AsH3> SbH3(c) H2O< H2S< H2Se< H2Te
What is lanthaniodcontraction? What is its reason? What are its consequences?
Ans→ BooK
Account for the following;
(a) The enthalpies of atomisation of the transition metals are very high.
(b) Transition metals generally form coloured ions.
(c) Transition metals and many of their compounds show paramagnetic behaviour.
Ans→(a) due to unpaired electrons (b) due to d – d transition (c) due to unpaired electrons
Describe the preparation of K2Cr2O7 from chromite ore. What is the effect of increasing pH on a solution
of potassium dichromate?
Ans→4FeCr2O4 + 8Na2CO3(or NaOH) + 7O2 → 8Na2CrO4 + 2Fe2O3 +8CO2(or 8H2O)
2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O
Na2Cr2O7 + KCl
→ K2Cr2O7 + 2 NaCl
H
On increasing the P dichromate ion (orange) will change into chromate ion (yellow).
K2Cr2O7 + KOH → K2CrO4 + H2O
What is meant by chelate effect? Give an example.
Ans→ The complexes formed by chelating ligands like (en) and EDTA are more stable than formed by
monodentate ligands like H2O and NH3. This enhanced stability of complexes containing ligands is called
chelate effect. Example [PtCl2(en)]
Define the following terms; (a) Homoleptic complexes (b) Coordination entity (c) ligands
Ans→(a)The complex in which metal ion is bonded to only one kind of donor ligands. eg [Co(NH3)]+3
(b) The central metal ion together with ligands directly attached to it is termed as coordination sphere. It
is written in square bracket. example [Co(NH3)]+3
(a) Geometrical isomerism is not possible in tetrahedral complexes. Why?
(b) Give the evidence that [Co (NH3)5Cl]SO4 and [Co(NH3)5 SO4]Cl are ionisational isomers.
Ans→(a) All positions are cis,There is no trans position.
(b) They give different types of ions SO4-2 and Cl- ions respectively.
Write IUPAC names of
(a) CH3-NH-CH(CH3)2
(b) m-BrC6H4NH2 .
Ans→(a) N-methyl propan-2-amine (b) 3-bromo benzenamine
Q33 Explain Why (a)Alkyl halides, though polar,are immiscible with water.
(b) Grignard reagents should be prepared under anhydrous conditions
(c) o-nitrophenol is more steam volatile than p-nitrophenol.
(d) o-methoxyphenol is less acidic than o-nitrophenol.
Ans→(a) Alkyl halides are polar compounds and they do not form H-bonding with water.
(b) They react with water.
R-Mg-X + HOH → R-H + Mg(OH) X
(c) o-nitro phenol is steam volatile due to intramolecular H- bonding and intermolecular H- bonding
in p-nitro phenol.
(d) o-methoxy phenol is less acidic because of +I effect in it.
Q34 Write the equations for the following reactions.
(a) Riemer-Tiemann reaction
(b) Finkelstein reaction
Ans→Reimer-Tiemann reaction; Phenol + CHCl3+ NaOH + Heat → Salicylaldehyde.
Phenol + CCl4 + NaOH + Heat → Salicylic acid.
Finkelstein reaction ; CH3CH2Br +NaI → CH3CH2I + NaBr OR CH3CH2Cl +NaI → CH3CH2I + NaCl
Q35 Arrange the following in increasing order of basic strength .
(a) Pri, Sec , Ter amine. (gas phase) .
(b) CH 3 NH 2 , CH 3 NHCH 3 , (CH 3 ) 3 N (aqueous form)
Ans→(a) Pri< Sec < Ter amine. (b) Sec >Pri> Ter.
Q36 Account for the following
(a) There are two –NH2 groups in semicarbazides.However, only one is involved in the formation of
semicabazones.
(b) FCH2COOH is stronger than ClCH2COOH.
Ans→(a) methylamine gives carbylamine test. (b) benzylamine reacts with HNO2 to form C6H5CH2N2Cl
which is unstable and form C6H5CH2OH with water.
Q37 An organic compound A (C2H6O) reacts with sodium to form compound B with evolution of hydrogen
and gives a yellow compound C when treated with iodine &NaOH. On heating with conc H2SO4 at 413
K, it gives a compound D(C4H10O) which on treatment with HI at 373K gives E. D is also obtained when
B is heated with E.Identify the compounds and write the equations involved.
Ans→
Q38 Name the reagents used in the following reactions.
(a) Oxidation of a primary alcohol to aldehyde.
(b) Bromination of phenol to 2.4.6 tribromophenol.
(c) butan-2-one to butan-2-ol.
Ans→(a) Pyridinium Chloro Chromate PCC in CH2Cl2 (b) Br2/ H2O(c) H2/ Ni, LiAlH4, NaBH4
Q39 Account for the following (a) Ortho-nitrophenol is more acidic than ortho-methoxyphenol/.
(b) Alcohols are weaker acid than water.
(c) Alcohols are more soluble in water than hydrocarbons
Ans→(a) Due to +I effect of OCH3 group, H+can not be released easily from ortho-methoxyphenol.
(b) Due to +I effect of alkyl group in alcohol, H+can not be released easily from ortho-methoxyphenol.
(c) alcohols forms H-bonding with water.
Q40 Give equations for the following reactions.
(a) Dilute HNO3 with phenol.
(b) Bromine with CS2 with phenol.
(c) 1-phenylethanol from a suitable alkene.
Ans→(a) O-nitrophenol and P-nitrophenol (b) O-bromophenol and P-bromophenol
(c)
Q41 What are reducing and non-reducing sugars? Give one example of each.
Ans→ Sugars which can reduce the Fehling,S solution or Tollen,S reagent are called reducing sugars.
Examples- all monosachharides, ribose,mannose,andgalactose.
Sugars which can not reduce the Fehling,S solution or Tollen,S reagent are called non reducing
sugars.
Examples-maltose,lactose and sucrose.
Q42 What happens when D-glucose is treated with the following reagents?
(a) HI
(b) Bromine water
(c) conc HNO3
Ans→(a) glucose + HI→ n-hexane (b) glucose + Br2(aq)→ gluconic acid[ COOH—(CH2OH)4—
CH2OH].
(c) glucose + Conc HNO3 → glucaric or sachharic acid [COOH—(CH2OH)4—COOH].
Q43 What are essential and non-essential amino acids? Give one example of each type.
Ans→ Essential-Which can not be synthesized in our body.Valine and Leucine
Non-essential- Which can be synthesized in our body.Glycine and Alanine.
Q44 Name the polymer used for making nonstick utencils.
Name→(I) Teflon, Melamine
Q45 Give one point of difference between thermosets &thermoplasts.
Ans→BooK
Q46
--- (--CH2-CH---) n--│
C6H5
Is the above polymer homopolymer or copolymer? Is it an addition or
condensation?
Ans→ Homo & addition.
Q47 What is the difference between polyacrylate& polyester?
Ans→ polyacrylate—homo,addition,polymerization across C=C bond.
Polyester—copolymer,condensation,polymerization through ester linkage.
Q48 Why do drugs designed to cure some ailment in one organ in the body not affect the other parts? Explain
with an example.
Ans→ An important point about the receptors is that they are not disturbed evenly around the body.
For example- heart has more β adrenergic receptors than α adrenergic receptors .This means
that a drug designed to interact with β adrenergic receptors will act on heart rather than a tissue
which are rich in α adrenergic receptors .
Q49 Define the following terms with one example(Any two)
(a) Antipyretics (b) Antioxidants (c) Antifertility drugs
Ans→ Antipyretics—The chemical substances used to bring down the body temperature are called
antipyretics.Paracetamol.Phenacetin, Analgin.
Antioxidants--The substance which when added to the fat and fat containing foods prevents their
oxidation and thus prolongs their life.Withoutthem,fatty foods such as potato chips can not be
stored for long time. Examples—Butylatedhydroxyanisol(BHA), Butylatedhydroxy toluene
(BHT), Propyl gallate(PG),Tertiary butyl hydroquinone(TBHQ),
Antifertility drugs—The chemicals which are used to control pregnancy are called antifertility
drugs.They are mostly hormonal contraceptives and are available for females only.They are
gonadal steroids(Oestrgen and Progesteron).
Q50 Account for the followings:
(a) The use of aspartame is limited to cold foods and drinks.
(b) Synthetic detergents are called soapless soaps.
(c) Medicines should not be taken without consulting doctors.
Ans→(a) It decomposes on heating. It nearly 100 times as sweet as cane sugar.Chemically it is methyl
ester of dipeptide of Aspartic acid and phenylalanine.
(b) Synthetic detergents are sodium compounds of benzene sulphonic acids whereas soaps are
sodium compounds of higher fatty acids.Soaps are based on oils. Synthetic detergents are based on
hydrocarbons.
(c) Medicines should not be taken without consulting doctors because their wrong choice and over
dose can cause havoc and may even prove to be fatal.