NEB Class 12 · Past paper
The complete NEB Class 12 2070 exam paper for Chemistry, all 33 questions with solved model answers.
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Why do NH3 and BF3 have dissimilar geometries?
Both have a central atom bonded to three atoms, but the electron arrangement differs. In $\ce{NH3}$ nitrogen is sp3 hybridised with three bond pairs and one lone pair; the lone pair repels the bonds, giving a trigonal pyramidal shape (ab...
What volume of water should be added to 500 ml of 2N (f=0.98) Na2CO3 solution to make it exactly N/10?
Actual normality of the stock $$ \begin{aligned} &= 2\times 0.98 \ &= 1.96\ \text{N} \end{aligned} $$ Equivalents present $$ \begin{aligned} &= 1.96 \times 0.500 \ &= 0.98 \end{aligned} $$ eq (unchanged on dilution). Final volume for
Define Lewis acid and Lewis base giving one example of each.
Lewis acid: an electron-pair acceptor, e.g. $\ce{BF3}$, $\ce{AlCl3}$, $\ce{H+}$.
Lewis base: an electron-pair donor, e.g. $\ce{NH3}$, $\ce{H2O}$, $\ce{OH-}$.
They combine by forming a coordinate bond:
$$ \ce{BF3 + NH3 -> F3B<-NH3} $$
.
How many coulombs are required to deposit 81 gm of aluminium when the electrode reaction is Al^3+ + 3e- -> Al?
Moles of aluminium $$ \begin{aligned} &= \dfrac{81}{27} \ &= 3\ \text{mol} \end{aligned} $$ Each $\ce{Al^3+}$ needs 3 electrons, so electrons required $$ \begin{aligned} &= 3\times 3 \ &= 9\ \text{mol} \ Q &= 9 \times 96500 \ &= 8685...
State the first law of thermodynamics.
First law of thermodynamics (law of conservation of energy): energy can neither be created nor destroyed, only converted from one form to another. The total energy of an isolated system remains constant.
Mathematically, for a system absorbing heat $q$ and doing work $w$: $$\Delta U = q + w$$ where $\Delta U$ is the change in internal energy.
How would you predict the spontaneity of a reaction using the relation T.dS(total) = -dG(system)?
The relation is $\Delta G{system} = -T,\Delta S{total}$ (at constant T and P). Since $T$ is always positive, the sign of $\Delta G{system}$ is opposite to that of $\Delta S{total}$. - If $\Delta S{total} 0$, then $\Delta G{sys} < 0$: th...
Draw an energy profile diagram for catalyzed and uncatalyzed reactions.
The catalyst provides an alternative route with a lower activation energy ($E_a' < E_a$), so more molecules can cross the barrier and the reaction goes faster. The catalyst does not change $\Delta H$ (the energy of reactants and products is unchanged).
What happens when i) Benzene is heated with acetic anhydride in presence of anhydrous AlCl3 ii) Sodium benzoate is heated with soda-lime?
(i) Friedel-Crafts acylation: benzene gives acetophenone. $$\ce{C6H6 + (CH3CO)2O -[anhyd. AlCl3] C6H5COCH3 + CH3COOH}$$ (ii) Decarboxylation with soda-lime: sodium benzoate loses $\ce{CO2}$ (as $\ce{Na2CO3}$) to give benzene. $$\ce{C6H5C...
Convert 1-bromopropane to 2-bromopropane.
Eliminate then add HBr the other way (Markovnikov). Dehydrohalogenate with alcoholic $\ce{KOH}$ to give propene, then add $\ce{HBr}$ (Markovnikov addition puts Br on the middle carbon): $$ \begin{aligned} \ce{CH3CH2CH2Br -[alc. KOH] CH3-...
What is the laboratory test of ethanol?
Iodoform test: warm ethanol with iodine and sodium hydroxide (or sodium carbonate); a yellow precipitate of iodoform ($\ce{CHI3}$) with a characteristic smell confirms ethanol. $$\ce{C2H5OH + 4I2 + 6NaOH -> CHI3(s) + HCOONa + 5NaI + 5H2O}$$
(Also, ethanol warmed with ethanoic acid and conc. $\ce{H2SO4}$ gives the fruity smell of ethyl ethanoate, the ester test.)
Prepare CH3-O-CH2CH3 (methoxyethane) and C6H5-OCH3 (methoxybenzene) by Williamson's ether synthesis.
Williamson synthesis: sodium alkoxide/phenoxide + alkyl halide. Methoxyethane: $$\ce{CH3ONa + C2H5I - CH3-O-C2H5 + NaI}$$ Methoxybenzene (anisole): use sodium phenoxide (phenol cannot be used as the aryl halide, so the aryl part supplies...
How does methanal react with i) NH3 ii) Conc. NaOH?
(i) With ammonia: methanal gives hexamethylenetetramine (urotropine): $$\ce{6HCHO + 4NH3 - (CH2)6N4 + 6H2O}$$ (ii) With conc. NaOH (Cannizzaro reaction): methanal has no alpha-hydrogen, so it undergoes self oxidation-reduction to methano...
Write the functional isomer of ethanoic acid and its IUPAC name.
Ethanoic acid is $\ce{CH3COOH}$ (molecular formula $\ce{C2H4O2}$). A functional isomer (same formula, different functional group) is the ester methyl methanoate, $\ce{HCOOCH3}$. IUPAC name: methyl methanoate (an ester, whereas ethanoic a...
Why is the -NO2 group meta-directing towards electrophilic aromatic substitution?
The nitro group is strongly electron withdrawing (by resonance and induction). Its resonance pulls electron density out of the ring and places a partial positive charge on the ortho and para carbons, deactivating them most. The meta posi...
How are sulphanilic acid and acetanilide prepared from aniline?
Acetanilide (acetylation of aniline with acetic anhydride): $$\ce{C6H5NH2 + (CH3CO)2O - C6H5NHCOCH3 + CH3COOH}$$ Sulphanilic acid (heating aniline with conc. $\ce{H2SO4}$; the anilinium hydrogen sulphate rearranges on heating to give p-a...
Distinguish between sugars and non-sugars.
Sugars: low-molecular-mass carbohydrates that are sweet, crystalline and soluble in water, e.g. glucose, fructose, sucrose. Many are reducing (they reduce Tollen's/Fehling's reagent). Non-sugars: high-molecular-mass carbohydrates (polysa...
What is a peptide linkage? Give an example of a dipeptide.
Peptide linkage: the amide bond ($\ce{-CO-NH-}$) formed when the $\ce{-COOH}$ group of one amino acid condenses with the $\ce{-NH2}$ group of another, with loss of water. Example of a dipeptide: glycylglycine, formed from two glycine mol...
What are chemical fertilizers? Give two examples.
Chemical (artificial) fertilizers are manufactured substances added to the soil to supply plant nutrients such as nitrogen, phosphorus and potassium, to increase crop yield. Examples: urea $\ce{CO(NH2)2}$ (nitrogenous) and superphosphate...
Write a method of preparation of each of the following polymers: i) Bakelite ii) PVC.
(i) Bakelite (condensation polymer of phenol and formaldehyde): $$\ce{C6H5OH + HCHO -[H+/OH-] HO-C6H4-CH2OH}$$ On further heating this loses water and cross-links to the network resin bakelite. (ii) PVC (addition polymer of vinyl chlorid...
What is Nessler's reagent? How is it prepared?
Nessler's reagent is an alkaline solution of potassium tetraiodomercurate(II), $\ce{K2[HgI4]}$ in $\ce{KOH}$. It is used to detect ammonia/ammonium ion (gives a brown precipitate).
Preparation: add potassium iodide to mercuric chloride until the red $\ce{HgI2}$ first formed just redissolves in excess $\ce{KI}$, then make alkaline with $\ce{KOH}$: $$ \begin{aligned} \ce{HgCl2 + 2KI -> HgI2(s) + 2KCl} \ \ce{HgI2 + 2KI -> K2[HgI4]} \end{aligned} $$
What happens when i) Zinc white is heated with cobalt nitrate ii) Metallic zinc is dissolved in hot conc. NaOH?
(i) Zinc white ($\ce{ZnO}$) moistened with cobalt nitrate and heated gives a green mass, Rinmann's green (cobalt zincate), a test for zinc: $$\ce{ZnO + Co(NO3)2 -[\Delta] CoZnO2 + 2NO2(g) + 1/2 O2(g)}$$ (ii) Zinc dissolves in hot concent...
Why is the open-hearth process more advantageous than the Bessemer process for the manufacture of steel?
The open-hearth process is preferred because: - The composition of the steel can be checked and adjusted during the slow process, so steel of accurate, uniform composition is obtained (the Bessemer process is too fast to control). - Scra...
How is ethoxyethane prepared in the laboratory in a pure and dry state?
Preparation (from ethanol using excess conc. $\ce{H2SO4}$ at 140 degrees, continuous etherification): $$ \begin{aligned} \ce{C2H5OH + H2SO4 - C2H5HSO4 + H2O} \ \ce{C2H5HSO4 + C2H5OH -[413 K] C2H5-O-C2H5 + H2SO4} \end{aligned} $$ Ethanol...
Write any three methods of preparation of iodoethane. What happens when iodoethane is heated with i) Sodium in presence of dry ether ii) alc. NaOH?
Three preparations of iodoethane: 1. From ethanol with red P and iodine: $$ \ce{3C2H5OH + PI3 - 3C2H5I + H3PO3} $$ 2. From ethene by adding $\ce{HI}$: $$ \ce{CH2=CH2 + HI - C2H5I} $$ 3. By halide exchange (Finkelstein): $$ \ce{C2H5Cl + N...
What happens when nitrobenzene is reduced in acidic, neutral, alkaline and electrolytic conditions?
Nitrobenzene gives different products depending on the medium: - Acidic ($\ce{Sn/HCl}$): complete reduction to aniline. $$\ce{C6H5NO2 + 6[H] - C6H5NH2 + 2H2O}$$ - Neutral ($\ce{Zn/NH4Cl}$): stops at phenylhydroxylamine. $$\ce{C6H5NO2 + 4...
Define normality. 0.8 gm of a divalent metal was dissolved in 100 ml of 1.28N HCl and the solution was diluted to 200 ml. Then 50 ml of the solution required 54.6 ml of 0.22N NaOH for neutralization. Find the atomic weight of the metal.
Normality: the number of gram-equivalents of solute dissolved per litre of solution.
Calculation:
Total $\ce{HCl}$ taken
$$ \begin{aligned} &= 1.28 \times 0.100 \ &= 0.128 \end{aligned} $$
eq (in 200 mL after dilution).
The 50 mL portion is one quarter of 200 mL, and its unreacted $\ce{HCl}$ is neutralized by NaOH:
$$ \begin{aligned} \text{NaOH} &= 0.22 \times \frac{54.6}{1000} \ &= 0.012012\ \text{eq} \ &= \text{excess HCl in 50 mL} \end{aligned} $$
Excess $\ce{HCl}$ in whole 200 mL
$$ \begin{aligned} &= 0.012012 \times 4 \ &= 0.048048\ \text{eq} \end{aligned} $$
$\ce{HCl}$ that reacted with the metal
$$ \begin{aligned} &= 0.128 - 0.048 \ &= 0.0800 \end{aligned} $$
eq $=$ equivalents of metal.
$$ \begin{aligned} \text{Equivalent weight} &= \frac{0.8}{0.0800} \ &= 10 \ &\Rightarrow \text{Atomic weight} = 10 \times 2 \ &= \textbf{20} \end{aligned} $$
What is meant by degree of ionization? 0.41 g of NaOH is placed in 100 ml of 0.1N H2SO4. Find the pH of the resulting solution.
Degree of ionization: the fraction of the total number of molecules of an electrolyte that actually dissociate into ions in solution ($\alpha = $ ionized fraction). Calculation: Moles of $$ \begin{aligned} \ce{NaOH} &= \dfrac{0.41}{40} ...
Represent graphically the variation of equivalent conductivity of a strong electrolyte and a weak electrolyte with concentration. Why do their equivalent conductivities vary differently with dilution?
Reason:
Write the preparations, properties and uses of blue vitriol. OR How is mercury extracted from its ore and refined?
Blue vitriol is copper sulphate pentahydrate, $\ce{CuSO4.5H2O}$. Preparation: dissolve copper oxide or copper carbonate (or scrap copper with hot dilute $\ce{H2SO4}$ and air) in dilute sulphuric acid, then crystallise: $$\ce{CuO + H2SO4 ...
a) How is nitrobenzene prepared in the laboratory in pure and dry state? b) What are amines? How would you separate 1 deg, 2 deg and 3 deg amines from their mixture by Hofmann's method?
a) Nitrobenzene: benzene is warmed with a nitrating mixture of conc. $\ce{HNO3}$ and conc. $\ce{H2SO4}$ at 55 to 60 degrees: $$\ce{C6H6 + HNO3 -[conc. H2SO4] C6H5NO2 + H2O}$$ The product is washed with water and dilute $\ce{NaOH}$, dried...
a) Explain the Cannizzaro reaction and Perkin condensation. What happens when propanone is treated with 2,4-dinitrophenylhydrazine? b) In the sequence A -(NH3)-> B -(Br2/aq.KOH)-> C -(HNO2)-> D -(NaOH/I2)-> E, compound E produces ethyne when heated with silver powder. Identify A, B, C, D, E.
a) Cannizzaro reaction: an aldehyde with no alpha-hydrogen, with conc. alkali, undergoes self oxidation-reduction: $$\ce{2HCHO + NaOH - CH3OH + HCOONa}$$ Perkin condensation: an aromatic aldehyde condenses with an acid anhydride in the p...
a) How is the free energy change of a reaction related to the enthalpy change and entropy change? Discuss the criteria of spontaneity and non-spontaneity on the basis of free energy change. b) For 2X + Y -> X2Y the data are: (0.10, 0.10, 1.3x10^-4), (0.10, 0.20, 2.6x10^-4), (0.20, 0.20, 1.04x10^-3) for [X], [Y], rate. i) Find the order with respect to X, Y and overall. ii) Find the rate constant with units. iii) Find the initial rate when [X] and [Y] are 1 M and 0.5 M.
a) The Gibbs free energy change is $$\Delta G = \Delta H - T\Delta S$$ - $\Delta G < 0$: reaction is spontaneous. - $\Delta G = 0$: system at equilibrium. - $\Delta G 0$: reaction is non-spontaneous. A negative $\Delta H$ (exothermic) an...
Write short notes on any two: a) Chemistry of white vitriol b) Rusting of iron c) Distinction between 1 deg, 2 deg and 3 deg alcohols by Victor Meyer's method d) Hess's law of constant heat summation.
b) Rusting of iron: this is an electrochemical corrosion that needs both water and oxygen. The iron acts as the anode, where it is oxidised:
$$\ce{Fe -> Fe^2+ + 2e-}$$
At the cathode, dissolved oxygen is reduced:
$$\ce{O2 + 2H2O + 4e- -> 4OH-}$$
The $\ce{Fe^2+}$ is then oxidised further to hydrated ferric oxide, which is rust:
$$ \begin{aligned} \ce{4Fe(OH)2 + O2 + 2H2O -> 4Fe(OH)3} \ \ce{2Fe(OH)3 -> Fe2O3.3H2O} \end{aligned} $$
Rust ($\ce{Fe2O3.xH2O}$) is a loose, brown, flaky solid. It is prevented by painting, greasing, galvanising or sacrificial protection.
d) Hess's law of constant heat summation: the total enthalpy change of a reaction is the same whether it takes place in one step or in several steps, provided the initial and final states are the same. It follows from the first law (enthalpy is a state function) and lets us add thermochemical equations to find unknown heats of reaction. For example, $\Delta H_{overall} = \Delta H_1 + \Delta H_2 + \Delta H_3$.
Both have a central atom bonded to three atoms, but the electron arrangement differs. In nitrogen is sp3 hybridised with three bond pairs and one lone pair; the lone pair repels the bonds, giving a trigonal pyramidal shape (ab...
Actual normality of the stock Equivalents present eq (unchanged on dilution). Final volume for
Lewis acid: an electron-pair acceptor, e.g. , , .
Lewis base: an electron-pair donor, e.g. , , .
They combine by forming a coordinate bond:
.
Moles of aluminium Each needs 3 electrons, so electrons required $$ \begin{aligned} &= 3\times 3 \ &= 9\ \text{mol} \ Q &= 9 \times 96500 \ &= 8685...
First law of thermodynamics (law of conservation of energy): energy can neither be created nor destroyed, only converted from one form to another. The total energy of an isolated system remains constant.
Mathematically, for a system absorbing heat and doing work : where is the change in internal energy.
The relation is (at constant T and P). Since is always positive, the sign of is opposite to that of . - If , then : th...
The catalyst provides an alternative route with a lower activation energy (), so more molecules can cross the barrier and the reaction goes faster. The catalyst does not change (the energy of reactants and products is unchanged).
(i) Friedel-Crafts acylation: benzene gives acetophenone. (ii) Decarboxylation with soda-lime: sodium benzoate loses (as ) to give benzene. $$\ce{C6H5C...
Eliminate then add HBr the other way (Markovnikov). Dehydrohalogenate with alcoholic to give propene, then add (Markovnikov addition puts Br on the middle carbon): $$ \begin{aligned} \ce{CH3CH2CH2Br -[alc. KOH] CH3-...
Iodoform test: warm ethanol with iodine and sodium hydroxide (or sodium carbonate); a yellow precipitate of iodoform () with a characteristic smell confirms ethanol.
(Also, ethanol warmed with ethanoic acid and conc. gives the fruity smell of ethyl ethanoate, the ester test.)
Williamson synthesis: sodium alkoxide/phenoxide + alkyl halide. Methoxyethane: Methoxybenzene (anisole): use sodium phenoxide (phenol cannot be used as the aryl halide, so the aryl part supplies...
(i) With ammonia: methanal gives hexamethylenetetramine (urotropine): (ii) With conc. NaOH (Cannizzaro reaction): methanal has no alpha-hydrogen, so it undergoes self oxidation-reduction to methano...
Ethanoic acid is (molecular formula ). A functional isomer (same formula, different functional group) is the ester methyl methanoate, . IUPAC name: methyl methanoate (an ester, whereas ethanoic a...
Acetanilide (acetylation of aniline with acetic anhydride): Sulphanilic acid (heating aniline with conc. ; the anilinium hydrogen sulphate rearranges on heating to give p-a...
Peptide linkage: the amide bond () formed when the group of one amino acid condenses with the group of another, with loss of water. Example of a dipeptide: glycylglycine, formed from two glycine mol...
Chemical (artificial) fertilizers are manufactured substances added to the soil to supply plant nutrients such as nitrogen, phosphorus and potassium, to increase crop yield. Examples: urea (nitrogenous) and superphosphate...
(i) Bakelite (condensation polymer of phenol and formaldehyde): On further heating this loses water and cross-links to the network resin bakelite. (ii) PVC (addition polymer of vinyl chlorid...
Nessler's reagent is an alkaline solution of potassium tetraiodomercurate(II), in . It is used to detect ammonia/ammonium ion (gives a brown precipitate).
Preparation: add potassium iodide to mercuric chloride until the red first formed just redissolves in excess , then make alkaline with :
(i) Zinc white () moistened with cobalt nitrate and heated gives a green mass, Rinmann's green (cobalt zincate), a test for zinc: (ii) Zinc dissolves in hot concent...
Preparation (from ethanol using excess conc. at 140 degrees, continuous etherification): Ethanol...
Three preparations of iodoethane: 1. From ethanol with red P and iodine: 2. From ethene by adding : 3. By halide exchange (Finkelstein): $$ \ce{C2H5Cl + N...
Nitrobenzene gives different products depending on the medium: - Acidic (): complete reduction to aniline. - Neutral (): stops at phenylhydroxylamine. $$\ce{C6H5NO2 + 4...
Normality: the number of gram-equivalents of solute dissolved per litre of solution.
Calculation:
Total taken
eq (in 200 mL after dilution).
The 50 mL portion is one quarter of 200 mL, and its unreacted is neutralized by NaOH:
Excess in whole 200 mL
that reacted with the metal
eq equivalents of metal.
Degree of ionization: the fraction of the total number of molecules of an electrolyte that actually dissociate into ions in solution ( ionized fraction). Calculation: Moles of $$ \begin{aligned} \ce{NaOH} &= \dfrac{0.41}{40} ...
Reason:
Blue vitriol is copper sulphate pentahydrate, . Preparation: dissolve copper oxide or copper carbonate (or scrap copper with hot dilute and air) in dilute sulphuric acid, then crystallise: $$\ce{CuO + H2SO4 ...
a) Nitrobenzene: benzene is warmed with a nitrating mixture of conc. and conc. at 55 to 60 degrees: The product is washed with water and dilute , dried...
a) Cannizzaro reaction: an aldehyde with no alpha-hydrogen, with conc. alkali, undergoes self oxidation-reduction: Perkin condensation: an aromatic aldehyde condenses with an acid anhydride in the p...
a) The Gibbs free energy change is - : reaction is spontaneous. - : system at equilibrium. - : reaction is non-spontaneous. A negative (exothermic) an...
b) Rusting of iron: this is an electrochemical corrosion that needs both water and oxygen. The iron acts as the anode, where it is oxidised:
At the cathode, dissolved oxygen is reduced:
The is then oxidised further to hydrated ferric oxide, which is rust:
Rust () is a loose, brown, flaky solid. It is prevented by painting, greasing, galvanising or sacrificial protection.
d) Hess's law of constant heat summation: the total enthalpy change of a reaction is the same whether it takes place in one step or in several steps, provided the initial and final states are the same. It follows from the first law (enthalpy is a state function) and lets us add thermochemical equations to find unknown heats of reaction. For example, .