NEB Class 12 · Past paper
The complete NEB Class 12 2072 exam paper for Chemistry, all 33 questions with solved model answers.
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What are the features of tetrahedral hybridization? Write an example of it.
Features of tetrahedral (sp3) hybridization: one s and three p orbitals mix to give four equivalent sp3 hybrid orbitals directed to the four corners of a regular tetrahedron; the bond angle is 109 degrees 28 minutes (about 109.5 degrees); each orbital has 25% s and 75% p character.
Example: methane, $\ce{CH4}$, where carbon is sp3 hybridised and forms four $\ce{C-H}$ bonds at 109.5 degrees.
Which one has higher concentration and why? a) 80 gm/litre NaOH solution and 3 M NaOH solution. b) 5.3 gm/litre Na2CO3 and N/10 Na2CO3 solution.
Convert to the same units. a) $80$ g/L $\ce{NaOH}$ (M = 40): molarity $$ \begin{aligned} &= \dfrac{80}{40} \ &= 2 \end{aligned} $$ M. Compared with 3 M, the 3 M $\ce{NaOH}$ is more concentrated. b) $5.3$ g/L $\ce{Na2CO3}$ (M = 106): mol...
Define the Lewis concept of a base and point out its limitation.
Lewis base: a species that can donate a lone pair of electrons to form a coordinate bond, e.g. $\ce{NH3}$, $\ce{H2O}$, $\ce{OH-}$. Limitation: the Lewis concept is too general; it labels all electron-pair donors as bases even when they s...
Why does AgNO3 solution become bluish when a copper rod is dipped in it? (The standard reduction potentials of Cu and Ag are +0.3 V and +0.8 V respectively)
Copper has a lower (less positive) reduction potential (+0.34 V) than silver (+0.80 V), so copper is the stronger reducing agent and displaces silver from the solution: $$\ce{Cu(s) + 2AgNO3(aq) - Cu(NO3)2(aq) + 2Ag(s)}$$ The $\ce{Cu^2+}$...
What is meant by a state function? Give its example.
State function: a thermodynamic property whose value depends only on the present state of the system (its temperature, pressure, volume, composition) and not on the path taken to reach that state.
Examples: internal energy ($U$), enthalpy ($H$), entropy ($S$), Gibbs free energy ($G$), pressure, volume and temperature. (Heat and work are not state functions.)
Calculate dS and dG for the conversion of ice into water when they are in equilibrium at 0 degrees C (dH = 4 kJ/mol).
At the melting point (0 degrees C = 273 K) ice and water are in equilibrium, so the process is reversible and $$\Delta G = 0$$ At equilibrium $$ \begin{aligned} \Delta G &= \Delta H - T\Delta S \ &= 0 \end{aligned} $$ , hence $$ \begin{...
Define the terms i) activated complex ii) rate of reaction.
i) Activated complex: the unstable, high-energy intermediate arrangement of atoms formed at the top of the energy barrier (transition state) when reactant molecules collide effectively; it breaks down to give products. ii) Rate of reacti...
State Huckel's rule for aromaticity.
Huckel's rule: a planar, cyclic, fully conjugated ring system is aromatic if it contains $(4n+2)$ delocalised pi electrons, where $n = 0, 1, 2, 3, \ldots$ For example benzene has 6 pi electrons ($4\times1+2=6$, with $n=1$) and is aromati...
Give the major products in the following equations: i) CH3-CHBr-CH3 with Na/ether ii) CH3-CH2-Br with LiAlH4.
i) Wurtz reaction (2-bromopropane with sodium in dry ether) couples two units to give 2,3-dimethylbutane: $$\ce{2CH3-CHBr-CH3 + 2Na -[dry ether] (CH3)2CH-CH(CH3)2 + 2NaBr}$$ ii) $\ce{LiAlH4}$ reduces bromoethane to ethane (replaces
Write down the structural formula and IUPAC name of tert. butyl alcohol.
Tert-butyl alcohol is $\ce{(CH3)3C-OH}$ (a central carbon bearing three methyl groups and one $\ce{-OH}$). IUPAC name: 2-methylpropan-2-ol. It is a tertiary alcohol (the $\ce{-OH}$ carbon is joined to three other carbons).
Write the IUPAC name of CH3-O-CH(CH3)2 and use Williamson's synthesis method for its preparation.
$\ce{CH3-O-CH(CH3)2}$ is isopropyl methyl ether. IUPAC name: 2-methoxypropane. Williamson synthesis (sodium alkoxide + alkyl halide; choose the combination that avoids elimination, i.e. methyl halide with the secondary alkoxide): $$\ce{(...
How does benzaldehyde react with i) Conc. NaOH ii) Acetic anhydride?
i) Conc. NaOH (Cannizzaro reaction): benzaldehyde has no alpha-hydrogen, so it disproportionates into benzyl alcohol and sodium benzoate: $$\ce{2C6H5CHO + NaOH - C6H5CH2OH + C6H5COONa}$$ ii) Acetic anhydride (Perkin condensation, with so...
Predict the major products of the following reactions: i) (CH3CO)2O with LiAlH4 ii) (CH3COO)2Ca on heating.
i) $\ce{LiAlH4}$ reduces acetic anhydride to ethanol (the acyl groups are reduced to primary alcohol): $$\ce{(CH3CO)2O -[LiAlH4] 2CH3CH2OH}$$ ii) Calcium acetate on dry distillation (heating) gives propanone (acetone): $$\ce{(CH3COO)2Ca ...
Convert nitrobenzene into i) p-aminophenol ii) hydrazobenzene.
i) p-aminophenol (electrolytic reduction of nitrobenzene in strongly acidic medium; phenylhydroxylamine rearranges): $$ \begin{aligned} \ce{C6H5NO2 + 4[H] - C6H5NHOH} \ \ce{C6H5NHOH -[H+] HO-C6H4-NH2} \end{aligned} $$ (the product is p-...
How does aniline react with i) aqueous bromine ii) NaNO2 + HCl at low temperature?
i) Aqueous bromine: aniline gives a white precipitate of 2,4,6-tribromoaniline (the $\ce{-NH2}$ group is strongly activating and o/p directing): $$\ce{C6H5NH2 + 3Br2 - C6H2Br3NH2(s) + 3HBr}$$ ii) $\ce{NaNO2 + HCl}$ at 0 to 5 degrees (dia...
Define the terms i) zwitter ions ii) denaturation of protein.
i) Zwitter ion: a dipolar ion of an amino acid that carries both a positive ($\ce{-NH3+}$) and a negative ($\ce{-COO-}$) charge on the same molecule while remaining overall neutral, e.g. $\ce{^{+}H3N-CH2-COO^{-}}$ for glycine.
ii) Denaturation of protein: the loss of the natural (secondary and tertiary) structure of a protein, without breaking the peptide bonds, on heating or on treatment with acids, alkalis or heavy metals; the protein loses its biological activity (e.g. boiling of egg white).
What is saponification? Give an example of it.
Saponification: the alkaline hydrolysis of an ester (especially a fat or oil) with $\ce{NaOH}$/$\ce{KOH}$ to give an alcohol (glycerol) and the sodium/potassium salt of the fatty acid (soap).
Example: $$\ce{CH3COOC2H5 + NaOH -> CH3COONa + C2H5OH}$$ (For a fat, glyceryl tristearate gives glycerol and sodium stearate, i.e. soap.)
Write down the molecular formula of the monomers of i) Bakelite ii) Nylon-66.
i) Bakelite monomers: phenol $\ce{C6H5OH}$ and formaldehyde (methanal) $\ce{HCHO}$. ii) Nylon-66 monomers: hexamethylenediamine $\ce{H2N-(CH2)6-NH2}$ and adipic acid $\ce{HOOC-(CH2)4-COOH}$.
Write down the structural formula of each of the following: i) azo-dye ii) analgesic drug iii) pesticide iv) nitrogen fertilizer.
i) Azo-dye: aminoazobenzene / p-hydroxyazobenzene, $\ce{C6H5-N=N-C6H4OH}$ (contains the $\ce{-N=N-}$ azo link). ii) Analgesic drug: aspirin, $\ce{C6H4(OCOCH3)COOH}$ (2-acetoxybenzoic acid). iii) Pesticide: DDT, $\ce{(ClC6H4)2CH-CCl3}$. i...
What happens when corrosive sublimate is i) treated with excess KI solution? ii) heated with excess SnCl2 solution?
Corrosive sublimate is mercuric chloride, $\ce{HgCl2}$. (i) With excess KI: the scarlet $\ce{HgI2}$ first formed dissolves in excess $\ce{KI}$ to give potassium tetraiodomercurate(II): $$ \begin{aligned} \ce{HgCl2 + 2KI - HgI2(s) + 2KCl}...
Why does silver nitrate produce a permanent black stain on the skin? Write an important use of silver nitrate.
The proteins and organic matter of the skin reduce $\ce{AgNO3}$ to finely divided metallic silver, which is black and cannot be washed off, so the stain is permanent. $$\ce{2AgNO3 ->[reduction by skin] 2Ag(black) + ...}$$
Use: silver nitrate is used as lunar caustic (in medicine to cauterise wounds) and as a laboratory reagent to test for halide ions.
Write the chemical reactions involved in the zone of reduction of the blast furnace during the extraction of iron.
In the zone of reduction (middle of the furnace, about 400 to 900 degrees) carbon monoxide reduces the iron oxides to iron in stages: $$ \begin{aligned} \ce{3Fe2O3 + CO - 2Fe3O4 + CO2} \ \ce{Fe3O4 + CO - 3FeO + CO2} \ \ce{FeO + CO - Fe...
Define the terms i) Electrochemical equivalent ii) Standard electrode potential. How many coulombs are required to produce i) 80 gm of aluminium from molten Al2O3 ii) 24 gm of magnesium from MgCl2?
i) Electrochemical equivalent (z): the mass of a substance deposited or liberated at an electrode by the passage of one coulomb of electricity.
ii) Standard electrode potential: the potential of an electrode measured against the standard hydrogen electrode when the ion concentration is 1 M, at 1 atm and 298 K.
Charge required:
(i) 80 g Al ($\ce{Al^3+ + 3e- -> Al}$): moles
$$ \begin{aligned} &= 80/27 \ &= 2.963 \end{aligned} $$
; electrons
$$ \begin{aligned} &= 2.963\times3 \ &= 8.889\ \text{mol} \ Q &= 8.889\times96500 \ &= 8.58\times10^{5}\ \text{C} \end{aligned} $$
(ii) 24 g Mg ($\ce{Mg^2+ + 2e- -> Mg}$): moles
$$ \begin{aligned} &= 24/24 \ &= 1 \end{aligned} $$
; electrons $=2$ mol.
$$ \begin{aligned} Q &= 2\times96500 \ &= 1.93\times10^{5}\ \text{C} \end{aligned} $$
Define heat of formation. The heats of combustion of methane, carbon and hydrogen are -210, -94 and -68 kcal respectively. Calculate the heat of formation of methane.
Heat of formation: the enthalpy change when one mole of a compound is formed from its elements in their standard states.
For
$$ \ce{C(s) + 2H2(g) -> CH4(g)} $$
, by Hess's law:
$$ \begin{aligned} \Delta H_f(\ce{CH4}) &= \Delta H_c(\ce{C}) + 2,\Delta H_c(\ce{H2}) - \Delta H_c(\ce{CH4}) \ &= (-94) + 2(-68) - (-210) \ &= -94 - 136 + 210 \ &= -20\ \text{kcal/mol} \end{aligned} $$
The heat of formation of methane is $-20$ kcal/mol (exothermic).
What is meant by normality factor? How many ml of conc. HNO3 of specific gravity 1.41 containing 69% by mass are required to prepare 500 ml of 0.5N HNO3?
Normality factor (f): the ratio of the actual normality of a solution to its intended (labelled) normality; it corrects for the difference between the calculated and true strength.
Calculation: equivalents of $\ce{HNO3}$ needed
$$ \begin{aligned} &= 0.5\times0.500 \ &= 0.25 \end{aligned} $$
eq. Since $\ce{HNO3}$ is monobasic, eq. wt. $=63$:
$$ \begin{aligned} \text{mass of HNO3 needed} &= 0.25\times63 \ &= 15.75\ \text{g} \end{aligned} $$
Mass of pure $\ce{HNO3}$ per mL of conc. acid
$$ \begin{aligned} &= 1.41\times0.69 \ &= 0.973\ \text{g/mL} \ V &= \frac{15.75}{0.973} \ &= 16.2\ \text{mL} \end{aligned} $$
About 16.2 mL of the concentrated acid is required (diluted to 500 mL).
How would you obtain blister copper from copper pyrites?
Copper pyrites ($\ce{CuFeS2}$) is treated as follows: 1. Concentration by froth flotation. 2. Roasting in air removes some sulphur and moisture and oxidises part of the iron: $$\ce{2CuFeS2 + O2 - Cu2S + 2FeS + SO2(g)}$$ 3. Smelting with ...
Give the chemical reaction for the preparation of trichloromethane from ethanal. What happens when it is heated with silver powder? Identify the product A and write its IUPAC name.
Trichloromethane (chloroform) from ethanal: chlorination followed by hydrolysis (haloform reaction): $$ \begin{aligned} \ce{CH3CHO + 3Cl2 -> CCl3CHO + 3HCl} \ \ce{2CCl3CHO + Ca(OH)2 -> 2CHCl3 + (HCOO)2Ca} \end{aligned} $$
Chloroform heated with silver powder gives ethyne (acetylene), product A: $$\ce{2CHCl3 + 6Ag ->[\Delta] C2H2 + 6AgCl}$$
A is ethyne (acetylene); IUPAC name: ethyne.
Suggest any three suitable chemical reactions for the preparation of ethanoic acid. How is ethanoic acid converted into methanoic acid?
Three preparations of ethanoic acid. Oxidation of ethanol: $$\ce{CH3CH2OH -[K2Cr2O7/H2SO4] CH3COOH}$$ Oxidation of ethanal: $$\ce{CH3CHO + [O] - CH3COOH}$$ Hydrolysis of methyl cyanide: $$\ce{CH3CN + 2H2O -[H+] CH3COOH + NH3}$$ Ethanoic ...
An aliphatic compound A reacts with SOCl2 to give B. B on reduction with H2 in presence of Pd/BaSO4 gives C. When HCN is added to C, it produces D. On hydrolysis of D in acidic medium, E is formed. Compound C gives the iodoform test and produces a silver mirror with Tollen's reagent. Identify A, B, C, D, E and the reactions involved.
C gives the iodoform test and Tollen's test, so C is ethanal (acetaldehyde), $\ce{CH3CHO}$ (a methyl aldehyde). Working back and forward: - C is made from B by Rosenmund reduction ($\ce{H2}$, Pd/BaSO4), so B = ethanoyl chloride,
What is meant by i) common ion effect ii) solubility product constant (Ksp)? Explain the common ion effect and the solubility product principle in qualitative salt analysis. What will be the resulting pH of a solution prepared by mixing 200 ml of aqueous HCl (pH = 2) with 300 ml of aqueous NaOH (pH = 12)?
i) Common ion effect: the suppression of the ionization of a weak electrolyte when a strong electrolyte having a common ion is added (e.g. adding $\ce{NH4Cl}$ to $\ce{NH4OH}$ lowers $[\ce{OH-}]$).
ii) Solubility product ($K_{sp}$): for a sparingly soluble salt, the product of the molar concentrations of its ions in a saturated solution, each raised to its stoichiometric power.
In qualitative analysis: the common ion effect and $K_{sp}$ control group precipitation. For group II, $\ce{H2S}$ is passed in the presence of dilute $\ce{HCl}$; the common $\ce{H+}$ suppresses ionization of $\ce{H2S}$, keeping $[\ce{S^2-}]$ low so that only the very insoluble sulphides (small $K_{sp}$) precipitate. For group III, $\ce{NH4Cl}$ (common $\ce{NH4+}$) lowers $[\ce{OH-}]$ from $\ce{NH4OH}$ so only hydroxides with the smallest $K_{sp}$ (Fe, Al, Cr) precipitate.
pH calculation:
$\ce{H+}$ from HCl: $[\ce{H+}]=10^{-2}$ M, moles
$$ \begin{aligned} &= 10^{-2}\times0.200 \ &= 2\times10^{-3} \end{aligned} $$
$\ce{OH-}$ from NaOH: $[\ce{OH-}]=10^{-2}$ M (pH 12 means pOH 2), moles
$$ \begin{aligned} &= 10^{-2}\times0.300 \ &= 3\times10^{-3} \end{aligned} $$
Excess
$$ \begin{aligned} \ce{OH-} &= 3\times10^{-3}-2\times10^{-3} \ &= 1\times10^{-3} \end{aligned} $$
mol in total 500 mL:
$$ \begin{aligned} [\ce{OH-}] &= \frac{10^{-3}}{0.5} \ &= 2\times10^{-3}\ \text{M},\quad pOH \ &= 2.70,\quad pH \ &= 14-2.70 \ &= 11.30 \end{aligned} $$
How is pure and dry aniline prepared in the laboratory? Identify A, B, C, D in the sequence: A -(Zn, heat)-> B -(CH3Cl/AlCl3)-> C -(oxidation)-> D. Compound D on reaction with zinc amalgam in acid gives toluene.
Preparation of aniline: nitrobenzene is reduced with tin and conc. HCl, and the aniline is liberated with $\ce{NaOH}$ and separated by steam distillation, then dried over $\ce{KOH}$: $$\ce{C6H5NO2 + 6[H] -[Sn/HCl] C6H5NH2 + 2H2O}$$ React...
Describe Victor Meyer's method to distinguish propan-2-ol and 2-methyl propan-2-ol. Why is phenol more acidic than alcohol? How would you convert ethanal into propanone and vice-versa?
Victor Meyer's method: convert the alcohol to the iodide (red P + $\ce{I2}$), then to the nitro compound with $\ce{AgNO2}$, and finally treat with nitrous acid ($\ce{HNO2}$) followed by $\ce{NaOH}$. Propan-2-ol (secondary) gives a nitrolic-type product that turns the solution blue with $\ce{NaOH}$, whereas 2-methylpropan-2-ol (tertiary) cannot form the required nitro compound (it has no alpha-H on the nitro carbon), so it gives no colour and stays colourless. The blue colour therefore identifies the secondary alcohol and no colour the tertiary alcohol.
Why phenol is more acidic than an alcohol: when phenol loses $\ce{H+}$ it gives a phenoxide ion whose negative charge is delocalised (resonance-stabilised) over the benzene ring, so the ion is stable and phenol ionises readily. An alkoxide from an alcohol has no such delocalisation and is further destabilised by the electron-releasing alkyl group, so alcohols are far weaker acids.
Ethanal to propanone. Add $\ce{CH3MgBr}$ to ethanal and hydrolyse to get propan-2-ol:
$$\ce{CH3CHO ->[CH3MgBr] CH3CH(OH)CH3}$$
then oxidise the secondary alcohol to the ketone:
$$\ce{CH3CH(OH)CH3 + [O] -> CH3COCH3}$$
Propanone to ethanal. Reduce propanone to propan-2-ol:
$$\ce{CH3COCH3 ->[H2/Ni] CH3CH(OH)CH3}$$
dehydrate it to propene:
$$\ce{CH3CH(OH)CH3 ->[-H2O] CH3CH=CH2}$$
and ozonolyse the propene to give ethanal (along with methanal):
$$\ce{CH3CH=CH2 ->[O3][Zn/H2O] CH3CHO + HCHO}$$
Write short notes on any two: a) Order and molecularity of reaction b) Rusting of iron c) Chemistry of zinc white d) Distinction of 1 deg, 2 deg and 3 deg alcohol by Victor-Meyer's method.
a) Order and molecularity:
c) Chemistry of zinc white: zinc white is zinc oxide, $\ce{ZnO}$, a white amphoteric powder that turns yellow on heating and white again on cooling. It can be prepared by burning zinc in air:
$$\ce{2Zn + O2 -> 2ZnO}$$
or by heating zinc carbonate:
$$\ce{ZnCO3 ->[\Delta] ZnO + CO2}$$
Being amphoteric, it dissolves in acids:
$$\ce{ZnO + 2HCl -> ZnCl2 + H2O}$$
and also in alkalis:
$$\ce{ZnO + 2NaOH -> Na2ZnO2 + H2O}$$
It is used as a white pigment (it does not blacken with $\ce{H2S}$) and in zinc ointment.
Features of tetrahedral (sp3) hybridization: one s and three p orbitals mix to give four equivalent sp3 hybrid orbitals directed to the four corners of a regular tetrahedron; the bond angle is 109 degrees 28 minutes (about 109.5 degrees); each orbital has 25% s and 75% p character.
Example: methane, , where carbon is sp3 hybridised and forms four bonds at 109.5 degrees.
Convert to the same units. a) g/L (M = 40): molarity M. Compared with 3 M, the 3 M is more concentrated. b) g/L (M = 106): mol...
Lewis base: a species that can donate a lone pair of electrons to form a coordinate bond, e.g. , , . Limitation: the Lewis concept is too general; it labels all electron-pair donors as bases even when they s...
Copper has a lower (less positive) reduction potential (+0.34 V) than silver (+0.80 V), so copper is the stronger reducing agent and displaces silver from the solution: The ...
State function: a thermodynamic property whose value depends only on the present state of the system (its temperature, pressure, volume, composition) and not on the path taken to reach that state.
Examples: internal energy (), enthalpy (), entropy (), Gibbs free energy (), pressure, volume and temperature. (Heat and work are not state functions.)
At the melting point (0 degrees C = 273 K) ice and water are in equilibrium, so the process is reversible and At equilibrium , hence $$ \begin{...
Huckel's rule: a planar, cyclic, fully conjugated ring system is aromatic if it contains delocalised pi electrons, where For example benzene has 6 pi electrons (, with ) and is aromati...
i) Wurtz reaction (2-bromopropane with sodium in dry ether) couples two units to give 2,3-dimethylbutane: ii) reduces bromoethane to ethane (replaces
Tert-butyl alcohol is (a central carbon bearing three methyl groups and one ). IUPAC name: 2-methylpropan-2-ol. It is a tertiary alcohol (the carbon is joined to three other carbons).
is isopropyl methyl ether. IUPAC name: 2-methoxypropane. Williamson synthesis (sodium alkoxide + alkyl halide; choose the combination that avoids elimination, i.e. methyl halide with the secondary alkoxide): $$\ce{(...
i) Conc. NaOH (Cannizzaro reaction): benzaldehyde has no alpha-hydrogen, so it disproportionates into benzyl alcohol and sodium benzoate: ii) Acetic anhydride (Perkin condensation, with so...
i) reduces acetic anhydride to ethanol (the acyl groups are reduced to primary alcohol): ii) Calcium acetate on dry distillation (heating) gives propanone (acetone): $$\ce{(CH3COO)2Ca ...
i) p-aminophenol (electrolytic reduction of nitrobenzene in strongly acidic medium; phenylhydroxylamine rearranges): (the product is p-...
i) Aqueous bromine: aniline gives a white precipitate of 2,4,6-tribromoaniline (the group is strongly activating and o/p directing): ii) at 0 to 5 degrees (dia...
i) Zwitter ion: a dipolar ion of an amino acid that carries both a positive () and a negative () charge on the same molecule while remaining overall neutral, e.g. for glycine.
ii) Denaturation of protein: the loss of the natural (secondary and tertiary) structure of a protein, without breaking the peptide bonds, on heating or on treatment with acids, alkalis or heavy metals; the protein loses its biological activity (e.g. boiling of egg white).
Saponification: the alkaline hydrolysis of an ester (especially a fat or oil) with / to give an alcohol (glycerol) and the sodium/potassium salt of the fatty acid (soap).
Example: (For a fat, glyceryl tristearate gives glycerol and sodium stearate, i.e. soap.)
i) Bakelite monomers: phenol and formaldehyde (methanal) . ii) Nylon-66 monomers: hexamethylenediamine and adipic acid .
i) Azo-dye: aminoazobenzene / p-hydroxyazobenzene, (contains the azo link). ii) Analgesic drug: aspirin, (2-acetoxybenzoic acid). iii) Pesticide: DDT, . i...
Corrosive sublimate is mercuric chloride, . (i) With excess KI: the scarlet first formed dissolves in excess to give potassium tetraiodomercurate(II): $$ \begin{aligned} \ce{HgCl2 + 2KI - HgI2(s) + 2KCl}...
The proteins and organic matter of the skin reduce to finely divided metallic silver, which is black and cannot be washed off, so the stain is permanent.
Use: silver nitrate is used as lunar caustic (in medicine to cauterise wounds) and as a laboratory reagent to test for halide ions.
i) Electrochemical equivalent (z): the mass of a substance deposited or liberated at an electrode by the passage of one coulomb of electricity.
ii) Standard electrode potential: the potential of an electrode measured against the standard hydrogen electrode when the ion concentration is 1 M, at 1 atm and 298 K.
Charge required:
(i) 80 g Al (): moles
; electrons
(ii) 24 g Mg (): moles
; electrons mol.
Heat of formation: the enthalpy change when one mole of a compound is formed from its elements in their standard states.
For
, by Hess's law:
The heat of formation of methane is kcal/mol (exothermic).
Normality factor (f): the ratio of the actual normality of a solution to its intended (labelled) normality; it corrects for the difference between the calculated and true strength.
Calculation: equivalents of needed
eq. Since is monobasic, eq. wt. :
Mass of pure per mL of conc. acid
About 16.2 mL of the concentrated acid is required (diluted to 500 mL).
Copper pyrites () is treated as follows: 1. Concentration by froth flotation. 2. Roasting in air removes some sulphur and moisture and oxidises part of the iron: 3. Smelting with ...
Trichloromethane (chloroform) from ethanal: chlorination followed by hydrolysis (haloform reaction):
Chloroform heated with silver powder gives ethyne (acetylene), product A:
A is ethyne (acetylene); IUPAC name: ethyne.
Three preparations of ethanoic acid. Oxidation of ethanol: Oxidation of ethanal: Hydrolysis of methyl cyanide: Ethanoic ...
C gives the iodoform test and Tollen's test, so C is ethanal (acetaldehyde), (a methyl aldehyde). Working back and forward: - C is made from B by Rosenmund reduction (, Pd/BaSO4), so B = ethanoyl chloride,
i) Common ion effect: the suppression of the ionization of a weak electrolyte when a strong electrolyte having a common ion is added (e.g. adding to lowers ).
ii) Solubility product (): for a sparingly soluble salt, the product of the molar concentrations of its ions in a saturated solution, each raised to its stoichiometric power.
In qualitative analysis: the common ion effect and control group precipitation. For group II, is passed in the presence of dilute ; the common suppresses ionization of , keeping low so that only the very insoluble sulphides (small ) precipitate. For group III, (common ) lowers from so only hydroxides with the smallest (Fe, Al, Cr) precipitate.
pH calculation:
from HCl: M, moles
from NaOH: M (pH 12 means pOH 2), moles
Excess
mol in total 500 mL:
Preparation of aniline: nitrobenzene is reduced with tin and conc. HCl, and the aniline is liberated with and separated by steam distillation, then dried over : React...
Victor Meyer's method: convert the alcohol to the iodide (red P + ), then to the nitro compound with , and finally treat with nitrous acid () followed by . Propan-2-ol (secondary) gives a nitrolic-type product that turns the solution blue with , whereas 2-methylpropan-2-ol (tertiary) cannot form the required nitro compound (it has no alpha-H on the nitro carbon), so it gives no colour and stays colourless. The blue colour therefore identifies the secondary alcohol and no colour the tertiary alcohol.
Why phenol is more acidic than an alcohol: when phenol loses it gives a phenoxide ion whose negative charge is delocalised (resonance-stabilised) over the benzene ring, so the ion is stable and phenol ionises readily. An alkoxide from an alcohol has no such delocalisation and is further destabilised by the electron-releasing alkyl group, so alcohols are far weaker acids.
Ethanal to propanone. Add to ethanal and hydrolyse to get propan-2-ol:
then oxidise the secondary alcohol to the ketone:
Propanone to ethanal. Reduce propanone to propan-2-ol:
dehydrate it to propene:
and ozonolyse the propene to give ethanal (along with methanal):
a) Order and molecularity:
c) Chemistry of zinc white: zinc white is zinc oxide, , a white amphoteric powder that turns yellow on heating and white again on cooling. It can be prepared by burning zinc in air:
or by heating zinc carbonate:
Being amphoteric, it dissolves in acids:
and also in alkalis:
It is used as a white pigment (it does not blacken with ) and in zinc ointment.