NEB Class 11 · Past paper
The complete NEB Class 11 2071 exam paper for Chemistry, all 33 questions with solved model answers.
Tap a question to open its answer.
12 g of carbon react with 4 g of hydrogen to produce 16 g of methane. Which chemical law do these data illustrate? State the law.
The data illustrate the law of conservation of mass (a special case of the law of definite proportions). $$ \begin{aligned} \ce{C + 2H2 - CH4} \ \text{mass of reactants} &= 12+4 \ &= 16\ \text{g} \ &= \text{mass of methane produced} ...
How many gold atoms are present in 0.6 g of 18-carat gold? 24-carat gold is taken as 100% pure. (atomic mass of gold = 197)
18-carat gold is $\dfrac{18}{24}$ pure, so $$ \begin{aligned} \text{mass of pure gold} &= 0.6\times\dfrac{18}{24} \ &= 0.45\ \text{g} \ n{\ce{Au}} &= \dfrac{0.45}{197} \ &= 2.284\times10^{-3}\ \text{mol} \ \text{atoms} &= 2.284\times...
Distinguish between crystalline and amorphous solids, giving an example of each.
Crystalline solids: particles are arranged in a regular, repeating three-dimensional pattern (long-range order); they have sharp melting points, definite geometry and are anisotropic. Example: $\ce{NaCl}$, diamond. Amorphous solids: part...
The boiling points of liquids P and Q are 70 degree C and 90 degree C respectively. Which of the two has the higher vapour pressure at 70 degree C? Give reason.
Liquid P (b.p. $70,^\circ\text{C}$) has the higher vapour pressure at $70,^\circ\text{C}$.
A liquid boils when its vapour pressure equals the external pressure. The liquid with the lower boiling point (P) is the more volatile, meaning its molecules escape more easily and it has a higher vapour pressure at any given temperature than Q, whose stronger intermolecular forces give it a higher boiling point and lower vapour pressure.
Write the electronic configuration for the atom having atomic number 19. What are the values of n and l of its outermost electron?
$Z=19$ is potassium: $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 4s^1}$. The outermost electron is $4s^1$: - Principal quantum number $n=4$ - Azimuthal quantum number $l=0$ (an $s$ orbital).
Which principle goes against the concept of Bohr's fixed orbits? State the principle.
Heisenberg's uncertainty principle goes against Bohr's idea of fixed, well-defined circular orbits. Statement: it is impossible to determine simultaneously and exactly both the position and the momentum (velocity) of a small moving parti...
What is a thermonuclear reaction? Write an example of it.
Thermonuclear reaction: a nuclear fusion reaction that takes place only at extremely high temperatures (millions of degrees), where light nuclei combine to form a heavier nucleus with release of enormous energy.
Example (the reaction that powers the sun and the hydrogen bomb): $$\ce{^2_1H + ^3_1H -> ^4_2He + ^1_0n} + \text{energy}$$
What is meant by intramolecular hydrogen bonding? Give an example of it.
Intramolecular hydrogen bonding: a hydrogen bond formed within the same molecule, between a hydrogen atom and a highly electronegative atom present in that molecule (as opposed to between two different molecules).
Example: ortho-nitrophenol, where the -OH hydrogen forms a hydrogen bond with the oxygen of the neighbouring $\ce{-NO2}$ group (forming a chelate ring). This is why o-nitrophenol is more volatile and less water-soluble than p-nitrophenol.
Write down the Lewis structure of the compound formed by two elements A and B whose outermost electrons are represented by 3s2 and 3s2 3p5 respectively.
Element A ($\ce{...3s^2}$) has 2 valence electrons; counting shells it is magnesium (Mg). Element B ($\ce{...3s^2 3p^5}$) has 7 valence electrons; it is chlorine (Cl). Mg loses 2 electrons and each Cl gains one, forming the ionic compoun...
Which one, N or O, has more electron affinity and why?
Oxygen has the higher (more negative) electron affinity than nitrogen. Nitrogen has a stable, exactly half-filled $2p^3$ configuration; adding an extra electron disturbs this stable arrangement and is resisted, so its electron affinity i...
Calculate the oxidation number of the underlined atom in the following compounds: a) N in NH4NO3 b) Cr in Cr2O7^2-
a) $\ce{NH4NO3}$: It contains two nitrogens in different environments. In $\ce{NH4+}$: $$ \begin{aligned} x+4(+1) &= +1 \ &\Rightarrow x = -3 \end{aligned} $$ In $\ce{NO3-}$: $$ \begin{aligned} x+3(-2) &= -1 \ &\Rightarrow x = +5 \end{...
Define equilibrium constant (Kc). What would be the expression for Kc for the reaction C(s) + CO2(g) <=> 2CO(g)?
Equilibrium constant $K_c$: at a given temperature it is the ratio of the product of the equilibrium molar concentrations of the products to that of the reactants, each raised to the power of its stoichiometric coefficient.
For
$$ \ce{C(s) + CO2(g) <=> 2CO(g)} $$
, the pure solid carbon is not included: $$K_c=\dfrac{[\ce{CO}]^2}{[\ce{CO2}]}$$
Why is water an excellent solvent for polar substances?
Water is a highly polar molecule with a large dipole moment and a high dielectric constant. When an ionic or polar solute is added, water molecules orient their $\delta+$ (H) and $\delta-$ (O) ends towards the oppositely charged ions and...
Give reason: a) CO2 is an acidic oxide b) Al2O3 is an amphoteric oxide.
(a) $\ce{CO2}$ is acidic because it is a non-metal oxide. It dissolves in water to give an acid, $$\ce{CO2 + H2O - H2CO3}$$ and it reacts with a base to give a salt and water, $$\ce{CO2 + 2NaOH - Na2CO3 + H2O}$$ (b) $\ce{Al2O3}$ is ampho...
Name any two gaseous compounds each which are responsible for a) photochemical smog b) greenhouse effect.
a) Photochemical smog: nitrogen dioxide ($\ce{NO2}$) and unburnt hydrocarbons (with ozone $\ce{O3}$ and PAN formed by sunlight). b) Greenhouse effect: carbon dioxide ($\ce{CO2}$) and methane ($\ce{CH4}$) (also $\ce{N2O}$ and CFCs).
Give a balanced chemical equation for the preparation of a) phosphine b) carbon monoxide.
a) Phosphine (warming white phosphorus with concentrated NaOH): $$\ce{P4 + 3NaOH + 3H2O ->[\Delta] PH3(g) + 3NaH2PO2}$$
b) Carbon monoxide (dehydration of formic acid by conc. $\ce{H2SO4}$): $$\ce{HCOOH ->[conc.\ H2SO4] CO(g) + H2O}$$
What is meant by hydrometallurgy? Give an example of it.
Hydrometallurgy: the extraction of a metal by first dissolving its ore in a suitable aqueous reagent to form a soluble complex/salt, then recovering the metal from the solution (usually by displacement with a more reactive metal or by electrolysis).
Example: silver is extracted by leaching its ore with sodium cyanide and then displacing it with zinc: $$ \begin{aligned} \ce{Ag2S + 4NaCN -> 2Na[Ag(CN)2] + Na2S} \ \ce{2Na[Ag(CN)2] + Zn -> Na2[Zn(CN)4] + 2Ag} \end{aligned} $$
Define the terms a) dead burnt plaster b) available chlorine.
a) Dead burnt plaster: when gypsum ($\ce{CaSO4.2H2O}$) is heated above about $200,^\circ\text{C}$, it loses all its water to give anhydrous calcium sulphate, $\ce{CaSO4}$. This form no longer sets with water (it will not rehydrate), and is called dead burnt plaster.
b) Available chlorine: the amount of chlorine (as a percentage by mass) that is liberated from a sample of bleaching powder on treatment with dilute acid. It is a measure of the bleaching power of the sample (good bleaching powder has about 35-38% available chlorine).
Give the IUPAC name of the following compounds: a) HC(triple bond)C-C(CH3)2-CH3 b) HOOC-CH(CH3)-CH2-NH2
a) $\ce{HCC-C(CH3)2-CH3}$: a but-1-yne chain with two methyl groups on C-3, 3,3-dimethylbut-1-yne. b) $\ce{HOOC-CH(CH3)-CH2-NH2}$: the -COOH carbon is C-1; C-2 bears a methyl group and C-3 bears an amino group, giving 3-amino-2-methylpro...
Identify the major products A and B of the following: CH3-CH2-CH2-Br --(alc. KOH, heat)--> A --(HBr)--> B, and write their IUPAC names.
Alcoholic KOH first strips a molecule of HBr from the propyl bromide (dehydrohalogenation), leaving a double bond. $$\ce{CH3-CH2-CH2Br -[alc.\ KOH][\Delta] CH3-CH=CH2 + HBr}$$ So the product A is propene (prop-1-ene). Adding HBr across t...
How would you detect the presence of sulphur in an organic compound?
Sulphur is detected by Lassaigne's (sodium fusion) test. The compound is fused with sodium, converting sulphur to sodium sulphide: $$\ce{2Na + S - Na2S}$$ The aqueous extract is then tested: - Add sodium nitroprusside: a violet (purple) ...
Define gasoline additive and give an example of a gasoline additive.
Gasoline (petrol) additive: a substance added in small amount to petrol to improve its quality, chiefly to raise the octane number and reduce engine knocking, or to improve combustion and storage.
Example: tetraethyl lead, $\ce{Pb(C2H5)4}$ (an anti-knocking agent); modern additives include MTBE and ethanol.
Write the essential postulates of Bohr's model of an atom. How does this model correct the defect of Rutherford's atomic model?
Postulates of Bohr's model: 1. Electrons revolve around the nucleus only in certain fixed circular orbits (stationary states) of definite energy, without radiating energy. 2. Only those orbits are allowed in which the angular momentum is...
8 g of pure calcium is treated with 50 g of pure HCl to give CaCl2 and H2. a) Which is the limiting reactant? b) Calculate the mass of unreacted reactant left over. c) What mass of CaCl2 will be formed? d) How many moles of water will be produced if the whole H2 formed reacts with O2?
$$ \begin{aligned} \ce{Ca + 2HCl - CaCl2 + H2(g)} \ n{\ce{Ca}} &= \dfrac{8}{40} \ &= 0.20\ \text{mol} \ n{\ce{HCl}} &= \dfrac{50}{36.5} \ &= 1.37\ \text{mol} \end{aligned} $$ a) $0.20$ mol Ca needs only $0.40$ mol HCl, but $1.37$ mol...
Distinguish between oxidant and reductant. Balance the following equation by the oxidation number method: KMnO4 + HCl -> KCl + MnCl2 + H2O + Cl2
Oxidant (oxidising agent): a species that gains electrons (is itself reduced) and oxidises others. Reductant (reducing agent): a species that loses electrons (is itself oxidised) and reduces others. Balancing: Mn goes $+7\to+2$ (gains
Give a suitable reaction for the preparation of each of a) ethane b) ethyne. How does ethyne react with i) water ii) cuprous chloride? How would you convert ethyne into ethane?
a) Ethane (hydrogenation of ethene, or Wurtz): $$\ce{C2H4 + H2 -[Ni][\Delta] C2H6}$$ b) Ethyne (action of water on calcium carbide): $$\ce{CaC2 + 2H2O - Ca(OH)2 + C2H2(g)}$$ i) Ethyne + water (dilute $\ce{H2SO4}$, $\ce{HgSO4}$) gives ace...
Give a balanced chemical equation for the preparation of ammonia from nitrogen and hydrogen. What are the proper physical conditions for a better yield of ammonia? Write the action of ammonia on a) mercurous nitrate paper b) CuSO4 solution c) CuO.
Haber's synthesis: $$\ce{N2(g) + 3H2(g) <=[Fe] 2NH3(g)}\quad \Delta H=-ve$$ Conditions for better yield (Le-Chatelier): high pressure (~$200\ \text{atm}$, since gas moles fall $4\to2$), moderately low temperature (~$450,^\circ\text{C}$ ...
Write the chemistry of plaster of paris.
Plaster of paris is calcium sulphate hemihydrate, $\ce{CaSO4.\tfrac12 H2O}$ (i.e. $\ce{(CaSO4)2.H2O}$). Preparation: heating gypsum to about $120\text{-}130,^\circ\text{C}$ drives off part of the water: $$\ce{2(CaSO4.2H2O) -[120,^\circ...
Explain how bromine is manufactured from carnallite. Why are halides found in sea water?
Manufacture from carnallite: carnallite ($\ce{KCl.MgCl2.6H2O}$) is worked for potassium salts, leaving a bromide-rich bittern. The bittern is acidified and warmed, and chlorine gas is passed through it. Chlorine (more reactive) displaces bromine, which is swept out by air/steam and condensed: $$\ce{MgBr2 + Cl2 -> MgCl2 + Br2(g)}$$
Why halides occur in sea water: the halogens are very reactive, so they are never found free in nature; they occur only as their halide salts ($\ce{NaCl}$, $\ce{MgCl2}$, $\ce{KBr}$, etc.). These salts are highly soluble in water, so over geological time rivers have leached them from rocks and carried them to the sea, where they have accumulated - making sea water rich in chlorides, bromides and iodides.
Draw a self-explanatory sketch for the manufacture of sulphuric acid by the contact process and explain the principle and procedure involved. Why is it called the contact process? How does conc. H2SO4 react with i) sugar ii) copper? Write down the test reaction of the sulphate ion in aqueous solution.
Contact process (principle and procedure): 1. Burn sulphur (or roast pyrites) to $\ce{SO2}$: $$ \ce{S + O2 - SO2} $$ . 2. Purify and dry the gas, then oxidise it over $\ce{V2O5}$: $$\ce{2SO2(g) + O2(g) <=[V2O5][450,^\circ C] 2SO3(g)}\qu...
Explain the chemistry of the Castner-Kellner process for the manufacture of caustic soda. Write down the action of caustic soda with i) Zn ii) sulphur iii) ammonium chloride. How is caustic soda converted into washing soda?
Castner-Kellner process: concentrated brine is electrolysed in a rocking cell using a mercury cathode and graphite anodes. $$ \begin{aligned} \text{Anode: }\ce{2Cl- - Cl2(g) + 2e-} \ \text{Cathode: }\ce{Na+ + e- - Na}\ (\text{forms sodi...
What are the postulates of the kinetic molecular theory of gas? Distinguish between an ideal gas and a real gas. An evacuated glass vessel weighs 50 g when empty, 148 g when filled with a liquid of density 0.98 g/cc, and 50.5 g when filled with an ideal gas at 760 mmHg at 27 degree C. Calculate the volume of the ideal gas at STP.
Postulates of kinetic molecular theory:
Ideal vs real gas: an ideal gas obeys $PV=nRT$ at all temperatures and pressures, has point-mass molecules and no intermolecular forces. A real gas has finite molecular volume and intermolecular attractions, so it obeys the gas laws only at low pressure and high temperature and deviates otherwise (described by van der Waals' equation).
Numerical:
$$ \begin{aligned} \text{volume of vessel} &= \text{volume of liquid} \ &= \dfrac{148-50}{0.98} \ &= \dfrac{98}{0.98} \ &= 100\ \text{cc} \ &= 0.1\ \text{L} \ \text{mass of gas} &= 50.5-50 \ &= 0.5\ \text{g} \end{aligned} $$
at $T=300\ \text{K}$, $P=1\ \text{atm}$.
$$ \begin{aligned} \text{volume at STP} &= V\times\dfrac{273}{300} \ &= 0.1\times\dfrac{273}{300} \ &= \mathbf{0.091\ L\ (91\ mL)} \end{aligned} $$
Write short notes on any two: a) Classification of hydrocarbons b) Advantages of the modern periodic table c) Relation between Kp and Kc d) Lassaigne's test for nitrogen.
a) Classification of hydrocarbons: hydrocarbons are broadly aliphatic (open chain) and cyclic. Aliphatic are saturated (alkanes, $\ce{C_nH_{2n+2}}$) or unsaturated (alkenes $\ce{C_nH_{2n}}$, alkynes $\ce{C_nH_{2n-2}}$). Cyclic are alicyclic (e.g. cyclohexane) and aromatic (benzene and its derivatives, containing the benzene ring).
b) Advantages of the modern periodic table: based on atomic number (a fundamental property), it removes Mendeleev's anomalous pairs; isotopes get one place; it clearly separates metals, non-metals and metalloids and arranges elements into s, p, d, f blocks reflecting electronic configuration; periodicity of properties is well explained.
c) Relation between $K_p$ and $K_c$: $K_p = K_c(RT)^{\Delta n}$, $\Delta n=$ (gaseous product moles $-$ gaseous reactant moles). If $\Delta n=0$, $K_p=K_c$; for
$$ \ce{N2 + 3H2 <=> 2NH3} $$
, $\Delta n=-2$, so $K_p=K_c(RT)^{-2}$.
d) Lassaigne's test for nitrogen: the compound is fused with sodium, converting N to $\ce{NaCN}$. The extract is boiled with $\ce{FeSO4}$ and acidified with $\ce{HCl}$; a Prussian-blue colour confirms nitrogen:
$$ \ce{6NaCN + FeSO4 -> Na4[Fe(CN)6] + Na2SO4} $$
;
$$ \ce{3Na4[Fe(CN)6] + 4FeCl3 -> Fe4[Fe(CN)6]3 + 12NaCl} $$
.
18-carat gold is pure, so $$ \begin{aligned} \text{mass of pure gold} &= 0.6\times\dfrac{18}{24} \ &= 0.45\ \text{g} \ n{\ce{Au}} &= \dfrac{0.45}{197} \ &= 2.284\times10^{-3}\ \text{mol} \ \text{atoms} &= 2.284\times...
Crystalline solids: particles are arranged in a regular, repeating three-dimensional pattern (long-range order); they have sharp melting points, definite geometry and are anisotropic. Example: , diamond. Amorphous solids: part...
Liquid P (b.p. ) has the higher vapour pressure at .
A liquid boils when its vapour pressure equals the external pressure. The liquid with the lower boiling point (P) is the more volatile, meaning its molecules escape more easily and it has a higher vapour pressure at any given temperature than Q, whose stronger intermolecular forces give it a higher boiling point and lower vapour pressure.
is potassium: . The outermost electron is : - Principal quantum number - Azimuthal quantum number (an orbital).
Thermonuclear reaction: a nuclear fusion reaction that takes place only at extremely high temperatures (millions of degrees), where light nuclei combine to form a heavier nucleus with release of enormous energy.
Example (the reaction that powers the sun and the hydrogen bomb):
Intramolecular hydrogen bonding: a hydrogen bond formed within the same molecule, between a hydrogen atom and a highly electronegative atom present in that molecule (as opposed to between two different molecules).
Example: ortho-nitrophenol, where the -OH hydrogen forms a hydrogen bond with the oxygen of the neighbouring group (forming a chelate ring). This is why o-nitrophenol is more volatile and less water-soluble than p-nitrophenol.
Element A () has 2 valence electrons; counting shells it is magnesium (Mg). Element B () has 7 valence electrons; it is chlorine (Cl). Mg loses 2 electrons and each Cl gains one, forming the ionic compoun...
Oxygen has the higher (more negative) electron affinity than nitrogen. Nitrogen has a stable, exactly half-filled configuration; adding an extra electron disturbs this stable arrangement and is resisted, so its electron affinity i...
a) : It contains two nitrogens in different environments. In : In : $$ \begin{aligned} x+3(-2) &= -1 \ &\Rightarrow x = +5 \end{...
Equilibrium constant : at a given temperature it is the ratio of the product of the equilibrium molar concentrations of the products to that of the reactants, each raised to the power of its stoichiometric coefficient.
For
, the pure solid carbon is not included:
Water is a highly polar molecule with a large dipole moment and a high dielectric constant. When an ionic or polar solute is added, water molecules orient their (H) and (O) ends towards the oppositely charged ions and...
(a) is acidic because it is a non-metal oxide. It dissolves in water to give an acid, and it reacts with a base to give a salt and water, (b) is ampho...
a) Photochemical smog: nitrogen dioxide () and unburnt hydrocarbons (with ozone and PAN formed by sunlight). b) Greenhouse effect: carbon dioxide () and methane () (also and CFCs).
a) Phosphine (warming white phosphorus with concentrated NaOH):
b) Carbon monoxide (dehydration of formic acid by conc. ):
Hydrometallurgy: the extraction of a metal by first dissolving its ore in a suitable aqueous reagent to form a soluble complex/salt, then recovering the metal from the solution (usually by displacement with a more reactive metal or by electrolysis).
Example: silver is extracted by leaching its ore with sodium cyanide and then displacing it with zinc:
a) Dead burnt plaster: when gypsum () is heated above about , it loses all its water to give anhydrous calcium sulphate, . This form no longer sets with water (it will not rehydrate), and is called dead burnt plaster.
b) Available chlorine: the amount of chlorine (as a percentage by mass) that is liberated from a sample of bleaching powder on treatment with dilute acid. It is a measure of the bleaching power of the sample (good bleaching powder has about 35-38% available chlorine).
a) : a but-1-yne chain with two methyl groups on C-3, 3,3-dimethylbut-1-yne. b) : the -COOH carbon is C-1; C-2 bears a methyl group and C-3 bears an amino group, giving 3-amino-2-methylpro...
Alcoholic KOH first strips a molecule of HBr from the propyl bromide (dehydrohalogenation), leaving a double bond. So the product A is propene (prop-1-ene). Adding HBr across t...
Sulphur is detected by Lassaigne's (sodium fusion) test. The compound is fused with sodium, converting sulphur to sodium sulphide: The aqueous extract is then tested: - Add sodium nitroprusside: a violet (purple) ...
Gasoline (petrol) additive: a substance added in small amount to petrol to improve its quality, chiefly to raise the octane number and reduce engine knocking, or to improve combustion and storage.
Example: tetraethyl lead, (an anti-knocking agent); modern additives include MTBE and ethanol.
a) mol Ca needs only mol HCl, but mol...
Oxidant (oxidising agent): a species that gains electrons (is itself reduced) and oxidises others. Reductant (reducing agent): a species that loses electrons (is itself oxidised) and reduces others. Balancing: Mn goes (gains
a) Ethane (hydrogenation of ethene, or Wurtz): b) Ethyne (action of water on calcium carbide): i) Ethyne + water (dilute , ) gives ace...
Haber's synthesis: Conditions for better yield (Le-Chatelier): high pressure (~, since gas moles fall ), moderately low temperature (~ ...
Plaster of paris is calcium sulphate hemihydrate, (i.e. ). Preparation: heating gypsum to about drives off part of the water: $$\ce{2(CaSO4.2H2O) -[120,^\circ...
Manufacture from carnallite: carnallite () is worked for potassium salts, leaving a bromide-rich bittern. The bittern is acidified and warmed, and chlorine gas is passed through it. Chlorine (more reactive) displaces bromine, which is swept out by air/steam and condensed:
Why halides occur in sea water: the halogens are very reactive, so they are never found free in nature; they occur only as their halide salts (, , , etc.). These salts are highly soluble in water, so over geological time rivers have leached them from rocks and carried them to the sea, where they have accumulated - making sea water rich in chlorides, bromides and iodides.
Contact process (principle and procedure): 1. Burn sulphur (or roast pyrites) to : . 2. Purify and dry the gas, then oxidise it over : $$\ce{2SO2(g) + O2(g) <=[V2O5][450,^\circ C] 2SO3(g)}\qu...
Postulates of kinetic molecular theory:
Ideal vs real gas: an ideal gas obeys at all temperatures and pressures, has point-mass molecules and no intermolecular forces. A real gas has finite molecular volume and intermolecular attractions, so it obeys the gas laws only at low pressure and high temperature and deviates otherwise (described by van der Waals' equation).
Numerical:
at , .
a) Classification of hydrocarbons: hydrocarbons are broadly aliphatic (open chain) and cyclic. Aliphatic are saturated (alkanes, ) or unsaturated (alkenes , alkynes ). Cyclic are alicyclic (e.g. cyclohexane) and aromatic (benzene and its derivatives, containing the benzene ring).
b) Advantages of the modern periodic table: based on atomic number (a fundamental property), it removes Mendeleev's anomalous pairs; isotopes get one place; it clearly separates metals, non-metals and metalloids and arranges elements into s, p, d, f blocks reflecting electronic configuration; periodicity of properties is well explained.
c) Relation between and : , (gaseous product moles gaseous reactant moles). If , ; for
, , so .
d) Lassaigne's test for nitrogen: the compound is fused with sodium, converting N to . The extract is boiled with and acidified with ; a Prussian-blue colour confirms nitrogen:
;
.