NEB Class 11 · Past paper
The complete NEB Class 11 2070 exam paper for Chemistry, all 33 questions with solved model answers.
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Which of the following gases has the greater number of hydrogen atoms? 9 g of CH4 or 10 g of NH3.
Compare the moles of H atoms in each. $\ce{CH4}$: $$ \begin{aligned} n &= \dfrac{9}{16} \ &= 0.5625\ \text{mol} \ \text{H atoms} &= 0.5625\times4 \ &= 2.25\ \text{mol} \end{aligned} $$ $\ce{NH3}$: $$ \begin{aligned} n &= \dfrac{10}{17...
What mass of H2 gas will react with 22.4 litres of O2 at STP to produce 36 g of water?
$$\ce{2H2(g) + O2(g) - 2H2O(l)}$$ $22.4\ \text{L}$ of $\ce{O2}$ at STP $=1\ \text{mol O2}$. From the equation, $1$ mol $\ce{O2}$ reacts with $2$ mol $\ce{H2}$. $$ \begin{aligned} \text{mass of }\ce{H2} &= 2\times2 \ &= \mathbf{4\ g} \en...
Write any two physical properties of a liquid caused by surface tension.
Two properties arising from surface tension: 1. Spherical shape of liquid drops: surface tension minimises the surface area, and a sphere has the least surface area for a given volume, so free drops become spherical. 2. Capillary rise (c...
Distinguish between isotropic and anisotropic substances with one example of each.
Isotropic substances: those whose physical properties (refractive index, conductivity, etc.) are the same in all directions. Amorphous solids and liquids are isotropic. Example: glass. Anisotropic substances: those whose physical propert...
An atom 'A' has atomic number (Z = 29). Calculate the total number of s-electrons in A+.
$\ce{A}$ ($Z=29$) is copper: $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1}$. $\ce{A+}$ is formed by losing the $4s^1$ electron: $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10}}$ (28 electrons). $$ \begin{aligned} s\text{-electrons} &= \underbrace{2}...
State Pauli's exclusion principle and give an example.
Pauli's exclusion principle: no two electrons in the same atom can have all four quantum numbers ($n, l, m, s$) identical. Consequently an orbital can hold at most two electrons and these must have opposite spins. Example: in the $1s$ or...
What is meant by a nuclear reaction? Give an example. (OR: Write a covalent compound formed by nitrogen and oxygen and its Lewis structure.)
Nuclear reaction: a reaction that involves a change in the composition of the nucleus of an atom (its protons/neutrons), producing a different nucleus and releasing large energy, unlike ordinary chemical reactions which involve only elec...
Define polar covalent bond and give an example of it.
Polar covalent bond: a covalent bond formed between two atoms of different electronegativity, so the shared electron pair is drawn more towards the more electronegative atom. This unequal sharing gives the bond partial charges ($\delta+$ and $\delta-$) and a dipole.
Example: the $\ce{H-Cl}$ bond in hydrogen chloride, where Cl is $\delta-$ and H is $\delta+$.
How does hydrogen bonding affect the physical properties of a compound?
Hydrogen bonding raises the melting and boiling points of a compound far above those expected from its molar mass, because extra energy is needed to break the intermolecular H-bonds (e.g. $\ce{H2O}$ boils at $100,^\circ\text{C}$, much h...
On what basis does Mendeleev's periodic law differ from the modern periodic law?
Mendeleev's periodic law: the physical and chemical properties of elements are a periodic function of their atomic masses. Modern periodic law: the properties of elements are a periodic function of their atomic numbers (nuclear charge). ...
Find the oxidation number of nitrogen in a) Ammonia b) Ammonium sulphate.
a) Ammonia, $\ce{NH3}$: $$ \begin{aligned} x + 3(+1) &= 0 \ &\Rightarrow x = -3 \end{aligned} $$ N $=-3$. b) Ammonium sulphate, $\ce{(NH4)2SO4}$: The nitrogen is in the $\ce{NH4+}$ ion: $$ \begin{aligned} x + 4(+1) &= +1 \ &\Rightarrow...
What is meant by equilibrium constant (Kc)? Write a chemical equilibrium in which Kc becomes equal to Kp.
Equilibrium constant $K_c$: for a reversible reaction at equilibrium, $K_c$ is the ratio of the product of the molar concentrations of the products to that of the reactants, each raised to its stoichiometric coefficient. For
$$ \ce{aA + bB <=> cC + dD} $$
, $K_c = \dfrac{[\ce{C}]^c[\ce{D}]^d}{[\ce{A}]^a[\ce{B}]^b}$.
$K_p = K_c(RT)^{\Delta n}$, so $K_p = K_c$ when $\Delta n = 0$ (equal gaseous moles on both sides). Example: $$\ce{H2(g) + I2(g) <=> 2HI(g)}\quad (\Delta n = 0)$$
Classify the following oxides: N2O5, NO, BaO2, Fe2O3.
How is oxygen converted into ozone? Mention one major use of ozone.
Conversion: passing a silent electric discharge through cold, dry oxygen in an ozoniser converts part of it to ozone: $$\ce{3O2 -[silent\ electric\ discharge] 2O3}$$ Use: ozone is used to purify (sterilise) drinking water and air; it is ...
Write the molecular formula of each of the following: a) Hypophosphorous acid b) Hypo c) Borax d) Silica gel.
How is acid rain formed?
Acid rain forms when the oxides of sulphur and nitrogen released by burning fossil fuels dissolve in atmospheric moisture. The $\ce{SO2}$ is first oxidised to $\ce{SO3}$, $$\ce{2SO2 + O2 - 2SO3}$$ which then dissolves in rain water to gi...
Write a chemical reaction involved in a) carbon reduction process b) alumino-thermite process.
a) Carbon reduction (smelting; the metal oxide is reduced by coke): $$\ce{ZnO + C -[\Delta] Zn + CO(g)}$$ b) Alumino-thermite (Goldschmidt) process (a less easily reduced oxide is reduced by aluminium, highly exothermic): $$\ce{Fe2O3 + 2...
Starting from sodium, how would you prepare Na2CO3.10H2O?
$$ \begin{aligned} \ce{2Na + 2H2O - 2NaOH + H2(g)} \ \ce{2NaOH + CO2 - Na2CO3 + H2O} \end{aligned} $$ On crystallisation the carbonate takes up water of crystallisation: $$\ce{Na2CO3 + 10H2O - Na2CO3.10H2O}$$ (washing soda).
Write down the IUPAC name of the following compounds: a) Cl3C-CHO b) OHC-CH=CH-CHO
a) $\ce{Cl3C-CHO}$ (chloral): 2,2,2-trichloroethanal. b) $\ce{OHC-CH=CH-CHO}$: a four-carbon chain with a $\ce{-CHO}$ at each end and a double bond between C-2 and C-3, but-2-enedial.
A haloalkane 'X' undergoes Wurtz reaction to give 2,3-dimethylbutane as the major product. Identify 'X' and write the chemical reaction.
2,3-dimethylbutane is a symmetrical alkane $\ce{(CH3)2CH-CH(CH3)2}$; joining two identical $\ce{(CH3)2CH-}$ units means X is 2-chloropropane (isopropyl chloride), $\ce{(CH3)2CHCl}$. $$\ce{2(CH3)2CHCl + 2Na -[dry\ ether] (CH3)2CH-CH(CH3)2...
Explain why a) sodium extract is alkaline in nature b) organic compounds are combustible.
a) In Lassaigne's test the organic compound is fused with excess sodium metal. Any unused sodium reacts with water during extraction to give sodium hydroxide, $$ \ce{2Na + 2H2O - 2NaOH + H2} $$ , and the sodium salts formed (NaCN,
What is the function of tetraethyl lead (TEL) in gasoline?
Tetraethyl lead, $\ce{Pb(C2H5)4}$, is added to petrol as an anti-knocking agent. It suppresses the premature, uncontrolled explosion (knocking) of the fuel-air mixture in the engine cylinder, raising the octane number and giving smooth, efficient combustion. (Because its lead exhaust is toxic, it is now largely replaced by lead-free additives.)
For the reaction Ca(OH)2 + 2NH4Cl(aq) -> CaCl2(aq) + 2NH3 + 2H2O(l), 7 g of pure Ca(OH)2 and 7 g of pure NH4Cl are mixed. a) Find the limiting reactant. b) Calculate the mole of unreacted reactant left over. c) How many grams of CaCl2 are formed? d) What volume of NH3 gas is produced at 27 degree C and 1.5 atmospheric pressure?
$$ \begin{aligned} \ce{Ca(OH)2 + 2NH4Cl - CaCl2 + 2NH3 + 2H2O} \ n{\ce{Ca(OH)2}} &= \dfrac{7}{74} \ &= 0.0946\ \text{mol} \ n{\ce{NH4Cl}} &= \dfrac{7}{53.5} \ &= 0.1308\ \text{mol} \end{aligned} $$ a) $0.0946$ mol $\ce{Ca(OH)2}$ need...
What is meant by oxidation number? Balance the following equation by oxidation number or ion-electron method indicating the oxidant and reductant: NaOH + S -> Na2S + Na2S2O3 + H2O
Oxidation number: the charge an atom would carry if all its bonding electrons were assigned to the more electronegative atom; it may be positive, negative or zero.
Here sulphur ($0$) disproportionates: part is reduced to $\ce{Na2S}$ (S $=-2$) and part is oxidised to $\ce{Na2S2O3}$ (S $=+2$). So sulphur is both the oxidant and the reductant.
$$ \ce{S^0 + 2e- -> S^{2-}} $$
(gain $2e^-$).
$$ \ce{2S^0 -> S2^{2+}\ (in\ S2O3^{2-}) + 4e-} $$
... balancing electrons gives: $$\ce{6NaOH + 4S -> 2Na2S + Na2S2O3 + 3H2O}$$ Check: Na $6=6$; S $4=2+2$; O $6=3+3$; H $6=6$. Balanced.
What are the conclusions made by Rutherford from his alpha-ray scattering experiment about the structure of atom? Mention its limitation.
Conclusions:
Limitation: it could not explain the stability of the atom (a revolving electron should radiate energy and collapse into the nucleus) nor the discrete line spectra of elements.
Define homologous series and write its important characteristics. What is the IUPAC name of the first member of the ketone series?
Homologous series: a series of organic compounds having the same functional group and the same general formula, in which successive members differ by a $\ce{-CH2-}$ group ($14\ \text{u}$).
Characteristics:
First ketone: the smallest ketone has three carbons, $\ce{CH3-CO-CH3}$, IUPAC name propanone (propan-2-one).
Write a balanced chemical reaction for the preparation of ammonia by Haber's process. How does ammonia react with a) Sodium b) Chlorine c) CuSO4 solution?
Haber's process ($200\ \text{atm}$, ~$450,^\circ\text{C}$, finely divided Fe catalyst with Mo promoter): $$\ce{N2(g) + 3H2(g) <=[Fe][450,^\circ C] 2NH3(g)}\quad \Delta H = -ve$$ a) With sodium (hot), sodamide forms: $$\ce{2Na + 2NH3 - ...
How is bromine manufactured from carnallite? Write any one use of bromine.
Manufacture: the bittern (mother liquor left after crystallising potassium salts from carnallite $\ce{KCl.MgCl2.6H2O}$) is rich in magnesium bromide. It is acidified and warmed, and chlorine gas is passed through it, which displaces brom...
Give the chemistry of quick-lime. What happens when water is added to quick-lime?
Quicklime is calcium oxide, $\ce{CaO}$, made by heating limestone: $$\ce{CaCO3 -[\Delta] CaO + CO2(g)}$$ It is a white basic oxide; with acids it gives salts, e.g. $$ \ce{CaO + 2HCl - CaCl2 + H2O} $$ , and with $\ce{SiO2}$ it forms slag ...
Sketch a well-labelled diagram for the manufacture of sodium hydroxide and explain the principle involved in the process. How does it react with a) CO2 b) Zn? Write any two uses of sodium hydroxide.
Manufacture (Castner-Kellner process): brine ($\ce{NaCl}$ solution) is electrolysed in a cell with a flowing mercury cathode and graphite anodes. $$ \begin{aligned} \text{Anode: }\ce{2Cl- - Cl2(g) + 2e-} \ \text{Cathode: }\ce{Na+ + e- -...
Starting from iron pyrites, how would you obtain dilute sulphuric acid by the contact process? Explain with a labelled diagram. Give the action of conc. H2SO4 on a) sucrose b) HI. What is the test of the sulphate ion?
Contact process from iron pyrites ($\ce{FeS2}$): 1. Roast pyrites to get $\ce{SO2}$: $$ \ce{4FeS2 + 11O2 -[\Delta] 2Fe2O3 + 8SO2} $$ . 2. Purify and dry the $\ce{SO2}$, then catalytically oxidise it: $$\ce{2SO2(g) + O2(g) <=[V2O5][450,^...
Explain how Charle's law gave the concept of the absolute scale of temperature. Derive the relation PV = nRT. A hydrocarbon CxHy has a mass ratio of hydrogen to carbon of 1:10.5. One litre of the hydrocarbon at 127 degree C and 1 atm pressure weighs 2.8 g. Find the molecular formula of the hydrocarbon.
Absolute scale from Charles's law: at constant pressure the volume of a gas decreases by $\tfrac{1}{273}$ of its $0,^\circ\text{C}$ value for each degree of cooling. Extrapolating, volume would become zero at $-273,^\circ\text{C}$; sin...
Write short notes on any two: a) Laboratory preparation of ethane b) Isomerism in organic compounds (structural isomers only) c) Relation between Kp and Kc d) Disadvantages of Mendeleev's periodic table.
a) Laboratory preparation of ethane: by Wurtz reaction, methyl iodide (or bromide) is heated with sodium in dry ether: $$\ce{2CH3I + 2Na ->[dry\ ether] CH3-CH3 + 2NaI}$$ (Also by electrolysis of sodium acetate solution - Kolbe's method.)
b) Structural isomerism: compounds with the same molecular formula but different arrangements of atoms. Types include chain (n-butane vs isobutane), position (1-propanol vs 2-propanol), functional (ethanol vs dimethyl ether) and metamerism. E.g. $\ce{C2H6O}$ exists as ethanol $\ce{C2H5OH}$ and dimethyl ether $\ce{CH3OCH3}$.
c) Relation between $K_p$ and $K_c$: $K_p = K_c(RT)^{\Delta n}$, where $\Delta n =$ (gaseous product moles $-$ gaseous reactant moles). If $\Delta n = 0$ then $K_p = K_c$; for
$$ \ce{N2 + 3H2 <=> 2NH3} $$
, $\Delta n = -2$ so $K_p = K_c(RT)^{-2}$.
d) Disadvantages of Mendeleev's table: no fixed place for hydrogen; isotopes had no separate place; some heavier elements were placed before lighter ones (anomalous pairs like Ar-K, Co-Ni, Te-I); dissimilar elements were grouped together (subgroups); and the position of the lanthanides could not be explained.
Compare the moles of H atoms in each. : : $$ \begin{aligned} n &= \dfrac{10}{17...
of at STP . From the equation, mol reacts with mol . $$ \begin{aligned} \text{mass of }\ce{H2} &= 2\times2 \ &= \mathbf{4\ g} \en...
() is copper: . is formed by losing the electron: (28 electrons). $$ \begin{aligned} s\text{-electrons} &= \underbrace{2}...
Pauli's exclusion principle: no two electrons in the same atom can have all four quantum numbers () identical. Consequently an orbital can hold at most two electrons and these must have opposite spins. Example: in the or...
Polar covalent bond: a covalent bond formed between two atoms of different electronegativity, so the shared electron pair is drawn more towards the more electronegative atom. This unequal sharing gives the bond partial charges ( and ) and a dipole.
Example: the bond in hydrogen chloride, where Cl is and H is .
Hydrogen bonding raises the melting and boiling points of a compound far above those expected from its molar mass, because extra energy is needed to break the intermolecular H-bonds (e.g. boils at , much h...
a) Ammonia, : N . b) Ammonium sulphate, : The nitrogen is in the ion: $$ \begin{aligned} x + 4(+1) &= +1 \ &\Rightarrow...
Equilibrium constant : for a reversible reaction at equilibrium, is the ratio of the product of the molar concentrations of the products to that of the reactants, each raised to its stoichiometric coefficient. For
, .
, so when (equal gaseous moles on both sides). Example:
Conversion: passing a silent electric discharge through cold, dry oxygen in an ozoniser converts part of it to ozone: Use: ozone is used to purify (sterilise) drinking water and air; it is ...
Acid rain forms when the oxides of sulphur and nitrogen released by burning fossil fuels dissolve in atmospheric moisture. The is first oxidised to , which then dissolves in rain water to gi...
a) Carbon reduction (smelting; the metal oxide is reduced by coke): b) Alumino-thermite (Goldschmidt) process (a less easily reduced oxide is reduced by aluminium, highly exothermic): $$\ce{Fe2O3 + 2...
On crystallisation the carbonate takes up water of crystallisation: (washing soda).
a) (chloral): 2,2,2-trichloroethanal. b) : a four-carbon chain with a at each end and a double bond between C-2 and C-3, but-2-enedial.
2,3-dimethylbutane is a symmetrical alkane ; joining two identical units means X is 2-chloropropane (isopropyl chloride), . $$\ce{2(CH3)2CHCl + 2Na -[dry\ ether] (CH3)2CH-CH(CH3)2...
a) In Lassaigne's test the organic compound is fused with excess sodium metal. Any unused sodium reacts with water during extraction to give sodium hydroxide, , and the sodium salts formed (NaCN,
Tetraethyl lead, , is added to petrol as an anti-knocking agent. It suppresses the premature, uncontrolled explosion (knocking) of the fuel-air mixture in the engine cylinder, raising the octane number and giving smooth, efficient combustion. (Because its lead exhaust is toxic, it is now largely replaced by lead-free additives.)
a) mol need...
Oxidation number: the charge an atom would carry if all its bonding electrons were assigned to the more electronegative atom; it may be positive, negative or zero.
Here sulphur () disproportionates: part is reduced to (S ) and part is oxidised to (S ). So sulphur is both the oxidant and the reductant.
(gain ).
... balancing electrons gives: Check: Na ; S ; O ; H . Balanced.
Conclusions:
Limitation: it could not explain the stability of the atom (a revolving electron should radiate energy and collapse into the nucleus) nor the discrete line spectra of elements.
Homologous series: a series of organic compounds having the same functional group and the same general formula, in which successive members differ by a group ().
Characteristics:
First ketone: the smallest ketone has three carbons, , IUPAC name propanone (propan-2-one).
Haber's process (, ~, finely divided Fe catalyst with Mo promoter): a) With sodium (hot), sodamide forms: $$\ce{2Na + 2NH3 - ...
Manufacture: the bittern (mother liquor left after crystallising potassium salts from carnallite ) is rich in magnesium bromide. It is acidified and warmed, and chlorine gas is passed through it, which displaces brom...
Quicklime is calcium oxide, , made by heating limestone: It is a white basic oxide; with acids it gives salts, e.g. , and with it forms slag ...
Manufacture (Castner-Kellner process): brine ( solution) is electrolysed in a cell with a flowing mercury cathode and graphite anodes. $$ \begin{aligned} \text{Anode: }\ce{2Cl- - Cl2(g) + 2e-} \ \text{Cathode: }\ce{Na+ + e- -...
Contact process from iron pyrites (): 1. Roast pyrites to get : . 2. Purify and dry the , then catalytically oxidise it: $$\ce{2SO2(g) + O2(g) <=[V2O5][450,^...
Absolute scale from Charles's law: at constant pressure the volume of a gas decreases by of its value for each degree of cooling. Extrapolating, volume would become zero at ; sin...
a) Laboratory preparation of ethane: by Wurtz reaction, methyl iodide (or bromide) is heated with sodium in dry ether: (Also by electrolysis of sodium acetate solution - Kolbe's method.)
b) Structural isomerism: compounds with the same molecular formula but different arrangements of atoms. Types include chain (n-butane vs isobutane), position (1-propanol vs 2-propanol), functional (ethanol vs dimethyl ether) and metamerism. E.g. exists as ethanol and dimethyl ether .
c) Relation between and : , where (gaseous product moles gaseous reactant moles). If then ; for
, so .
d) Disadvantages of Mendeleev's table: no fixed place for hydrogen; isotopes had no separate place; some heavier elements were placed before lighter ones (anomalous pairs like Ar-K, Co-Ni, Te-I); dissimilar elements were grouped together (subgroups); and the position of the lanthanides could not be explained.