NEB Class 12 · Past paper
The official NEB Class 12 model questions for Chemistry, all 76 questions with solved model answers.
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Why is no external indicator needed in a potassium permanganate titration, and what kind of titration is this called?
Potassium permanganate acts as its own indicator, so the titration is called a self indicator titration. The permanganate ion is intensely purple, while the manganese(II) ion it is reduced to in acidic medium is almost colourless in dilu...
Why is dilute sulphuric acid used to acidify the mixture, and why not hydrochloric acid or nitric acid?
An acidic medium is essential because permanganate needs eight hydrogen ions per ion reduced to go to manganese(II); in neutral or alkaline medium it would stop at manganese dioxide instead and the titre would be wrong. Sulphuric acid su...
Why must the oxalic acid solution be warmed before titration, while the Mohr's salt titration is carried out cold?
The reaction between permanganate and oxalate is slow at room temperature, so the purple colour would linger and the end point could not be judged; warming to about 60 to 70 degrees Celsius makes it fast enough to titrate. The reaction i...
Why can a solution of potassium permanganate not be prepared as a primary standard by direct weighing?
A primary standard must be obtainable pure, must be stable, and must not change composition on storage, and potassium permanganate fails on every count. The solid nearly always carries a little manganese dioxide as an impurity, so its tr...
What are concordant readings, and why do you average only these?
Concordant readings are two, and preferably three, consecutive titre values that agree with one another within 0.1 mL, which is the smallest division the burette can be read to. They show that the technique has settled down and that the same end point is being reached each time. The first titration is nearly always rough, because the analyst does not yet know roughly where the end point lies and overshoots it, so its value is recorded in the table for honesty but excluded from the mean. Averaging a rough reading in with good ones would drag the mean away from the true titre and carry that error straight into the calculated strength. Readings that refuse to become concordant point to a leaking stopcock, an air bubble, or a poorly judged colour.
Why is Mohr's salt preferred to ordinary ferrous sulphate, and why is its solution prepared in dilute sulphuric acid?
Mohr's salt, ferrous ammonium sulphate hexahydrate, is a double salt and is far more resistant to oxidation by air than ferrous sulphate, whose crystals become coated with a brown ferric layer on the shelf. It can therefore be weighed ou...
Why is sodium metal used in Lassaigne's test, and why must the fusion extract be alkaline?
In an organic compound nitrogen, sulphur and halogen are joined by covalent bonds and so give none of the ordinary ionic tests. Sodium is a powerful reducing agent and, on fusion, breaks those covalent bonds and converts the elements int...
Why is the sodium fusion extract boiled with dilute nitric acid before silver nitrate is added in the test for halogens?
If nitrogen or sulphur is present, the extract also contains cyanide and sulphide ion, and both of these give precipitates with silver nitrate: silver cyanide is white and silver sulphide is black. A white precipitate from cyanide would be read as chloride, so the result would be wrong. Acidifying with dilute nitric acid converts them to hydrogen cyanide and hydrogen sulphide, and boiling drives both gases out of the solution. Only the halide ion, which is not volatilised, remains. Silver nitrate added to the cooled solution then gives a precipitate that can be attributed to halogen alone: white and readily soluble in ammonium hydroxide for chloride, pale yellow and sparingly soluble for bromide, and yellow and insoluble for iodide.
A compound containing both nitrogen and sulphur gives a blood red colour rather than Prussian blue in Lassaigne's test. Why?
When both elements are present in the same molecule, the sodium fusion does not give separate sodium cyanide and sodium sulphide. Instead the two combine and the extract contains sodium thiocyanate. Thiocyanate ion does not build up the ferric ferrocyanide lattice that gives the Prussian blue colour; it forms ferric thiocyanate with ferric ion, which is blood red. So a blood red colouration on adding ferric chloride to the acidified extract is itself the evidence that nitrogen and sulphur are both present. If a large excess of sodium is used in the fusion, the thiocyanate is decomposed to cyanide and sulphide, and then the ordinary Prussian blue and nitroprusside tests are obtained separately instead.
How would you distinguish an aldehyde from a ketone at the bench?
Both give the same first result, an orange to red precipitate with 2,4-dinitrophenylhydrazine, which establishes only that a carbonyl group is present. They are separated by the fact that an aldehyde is easily oxidised and a ketone is no...
Why must Tollens' reagent be prepared fresh, and what precaution applies to it?
Tollens' reagent is ammoniacal silver nitrate, made by adding ammonium hydroxide to silver nitrate solution until the brown precipitate of silver oxide just redissolves as the diamminesilver(I) complex. On standing it decomposes, so an o...
In the analysis of an inorganic salt, why must the group reagents be added in a fixed order?
Each group reagent is chosen to precipitate its own group under the conditions of that group, but it will also precipitate any earlier group that is still in solution. Hydrogen sulphide in ammoniacal medium, the fourth group reagent, wou...
Write an example of a molecule having trigonal pyramidal geometry. What is the mode of hybridisation on central atom of the molecule?
Example: ammonia, $\ce{NH3}$. In $\ce{NH3}$ the central nitrogen atom is sp3 hybridised (four sp3 hybrid orbitals). Three orbitals form $\ce{N-H}$ sigma bonds and one holds a lone pair. The lone pair pushes the bonds together, so the sha...
What is titration error? How is it minimized?
Titration error: the small difference between the observed end point (colour change of the indicator) and the true equivalence point of the reaction. It makes the measured volume slightly more or less than the exact volume needed. Minimi...
Calculate the pH of 1x10^-8 M HCl.
At this very low concentration the $\ce{H+}$ from water cannot be ignored. Solution: let total $[\ce{H+}] = x$. Charge/mass balance with $Kw=10^{-14}$ gives: $$ \begin{aligned} x &= [\ce{H+}]{acid} + [\ce{OH-}] \ &= 10^{-8} + \frac{10^{...
What products would you expect at cathode and anode when aqueous NaCl is electrolysed using platinum electrodes?
In aqueous $\ce{NaCl}$ the ions present are $\ce{Na+, Cl-, H+, OH-}$. Cathode (reduction): $\ce{H+}$ (from water) is discharged in preference to $\ce{Na+}$, so hydrogen gas is evolved. $$\ce{2H2O(l) + 2e- - H2(g) + 2OH-(aq)}$$ Anode (oxi...
Define spontaneous process and give one example of it.
Spontaneous process: a physical or chemical change that takes place on its own under a given set of conditions without any continuous outside help. It proceeds in a definite direction accompanied by a decrease in Gibbs free energy ($\Delta G < 0$).
Example: flow of heat from a hot body to a cold body, or the rusting of iron: $$\ce{4Fe(s) + 3O2(g) -> 2Fe2O3(s)}$$
Calculate the enthalpy of formation in the following reactions: i) 2H2(g) + O2(g) -> 2H2O(l), H = -136 kcal ii) H2(g) + I2(g) -> 2HI(g), H = 24.8 kcal.
Enthalpy of formation is per one mole of the compound formed. (i) $$ \ce{2H2(g) + O2(g) - 2H2O(l)} $$ , $\Delta H = -136$ kcal for 2 mol water. $$ \begin{aligned} \Delta Hf(\ce{H2O}) &= \frac{-136}{2} \ &= -68\ \text{kcal/mol} \end{alig...
75% of a first order reaction is completed in 30 minutes. What time will it take to complete 50% of the reaction?
For a first order reaction the time is measured in half-lives, independent of concentration. 75% complete means one quarter is left, i.e. two half-lives: $$ \begin{aligned} 1 - 0.75 &= 0.25 \ &= \left(\tfrac{1}{2}\right)^2 \ &\Rightarr...
How would you convert sodium benzoate into acetophenone?
Heat sodium benzoate with soda-lime to get benzene, then carry out Friedel-Crafts acylation. $$ \begin{aligned} \ce{C6H5COONa + NaOH -[CaO][\Delta] C6H6 + Na2CO3} \ \ce{C6H6 + CH3COCl -[anhyd. AlCl3] C6H5COCH3 + HCl} \end{aligned} $$ Th...
What happens when i) Benzene diazonium chloride is treated with CuCl2 in the presence of HCl ii) Chlorobenzene is heated with chloral in acidic medium?
(i) With cuprous chloride/HCl (Sandmeyer / Gattermann type) the diazonium group is replaced by chlorine, giving chlorobenzene: $$\ce{C6H5N2Cl -[CuCl/HCl] C6H5Cl + N2(g)}$$ (ii) Chlorobenzene condenses with chloral (trichloroacetaldehyde)...
Propan-1-ol has higher boiling point than propan-2-ol though they have same molecular mass. Give reason.
Both are alcohols and form intermolecular hydrogen bonds, but propan-1-ol (a primary alcohol, $\ce{CH3-CH2-CH2-OH}$) has its $\ce{-OH}$ on a terminal carbon, so it is less crowded and its molecules pack and hydrogen-bond more effectively...
Convert ethoxyethane to methoxyethane.
Cleave the ether with HI to get ethanol and iodoethane, then use Williamson synthesis. $$\ce{C2H5-O-C2H5 + HI - C2H5OH + C2H5I}$$ Prepare sodium methoxide and react it with iodoethane: $$\ce{CH3ONa + C2H5I - CH3-O-C2H5 + NaI}$$ The produ...
Identify the major product X and mention its one important use (Methanal treated with NH3 gives X).
Methanal (formaldehyde) reacts with ammonia to give hexamethylenetetramine (urotropine), X = $\ce{(CH2)6N4}$: $$\ce{6HCHO + 4NH3 - (CH2)6N4 + 6H2O}$$ Use: it is used as a urinary antiseptic (urotropine) and as a hardener (with the explos...
Write an example of each of the following reactions: i) Carboxylation ii) Esterification.
(i) Carboxylation introduces a $\ce{-COOH}$ group. A common example is a Grignard reagent reacting with carbon dioxide, followed by acidic work-up. The Grignard reagent first adds to $\ce{CO2}$: $$\ce{CH3MgBr + CO2 - CH3COOMgBr}$$ Acidif...
Give resonance structure of nitrobenzene to show that nitro group is a meta-directing group.
The $\ce{-NO2}$ group is electron withdrawing. Its resonance places a positive charge on the ortho and para positions of the ring, so the electrophile avoids these and attacks the meta position (which keeps its electron density). In the ...
Give a suitable reaction for the preparation of methanamine from ethanamide. What happens when methanamine reacts with nitrous acid at low temperature?
Preparation (Hofmann bromamide degradation): ethanamide loses one carbon and gives methanamine. $$\ce{CH3CONH2 + Br2 + 4NaOH - CH3NH2 + Na2CO3 + 2NaBr + 2H2O}$$ With nitrous acid ($\ce{HNO2}$, i.e. $\ce{NaNO2/HCl}$) a primary aliphatic a...
Distinguish between dipeptide and polypeptide.
Dipeptide: formed by the union of two amino acid molecules through a single peptide ($\ce{-CO-NH-}$) bond, e.g. glycylalanine. It has one peptide linkage. Polypeptide: formed by the union of many (a large number of) amino acid molecules ...
Mention one important biological function of each of the following: i) DNA ii) RNA iii) phospholipids iv) carbohydrate.
i) DNA: stores and transmits the hereditary (genetic) information and controls protein synthesis. ii) RNA: carries the genetic message from DNA and takes part in the actual synthesis of proteins (translation). iii) Phospholipids: main st...
What is meant by co-polymerisation? Write an example of such a polymer.
Co-polymerisation: a polymerisation in which two (or more) different monomers join together to form a single polymer chain called a copolymer. The product contains repeating units of both monomers. Example: Buna-S (SBR) rubber, made by c...
Write a structural formula and one major use of antipyretics.
Antipyretics are drugs that lower body temperature (reduce fever). A common example is paracetamol (acetaminophen): Structure: $\ce{HO-C6H4-NH-CO-CH3}$ (p-hydroxyacetanilide, an $\ce{-OH}$ and an $\ce{-NHCOCH3}$ group para on a benzene r...
Write down the molecular formula and one use of each of the following: i) Mohr's salt ii) Green vitriol.
i) Mohr's salt (ferrous ammonium sulphate): $\ce{FeSO4.(NH4)2SO4.6H2O}$. Use: it is a stable primary standard for $\ce{Fe^2+}$ in redox titrations (e.g. against $\ce{KMnO4}$). ii) Green vitriol (ferrous sulphate heptahydrate):
What happens when ZnO is i) dissolved in excess of caustic alkali ii) heated with cobalt nitrate?
(i) Being amphoteric, $\ce{ZnO}$ dissolves in excess caustic alkali to form soluble sodium zincate: $$\ce{ZnO + 2NaOH - Na2ZnO2 + H2O}$$ (ii) On heating $\ce{ZnO}$ moistened with cobalt nitrate solution, a green mass called Rinmann's gre...
What is meant by frosting of silver?
Frosting of silver is the process of giving silver articles a dull, white, matt (frosted) surface finish instead of a bright polish. The article is dipped in dilute nitric acid or treated so that a fine film of finely divided silver is d...
Define molality of a solution. Calculate the molality of one litre of 93% H2SO4 solution (weight by volume). The density of the solution is 1.84 g/mL.
Molality (m): the number of moles of solute dissolved per kilogram of solvent. $$m = \frac{\text{moles of solute}}{\text{mass of solvent (kg)}}$$ Calculation (basis: 1 litre = 1000 mL of solution): - 93% w/v means 930 g $\ce{H2SO4}$ in 1...
What is meant by i) ionic product of water ii) solubility product constant (Ksp)? The solubility product of BaSO4 is 1x10^-10. Will a precipitate form if equal volumes of 2x10^-3 M BaCl2 and 2x10^-4 M Na2SO4 solutions are mixed?
(i) Ionic product of water ($Kw$): the product of the molar concentrations of $\ce{H+}$ and $\ce{OH-}$ ions in water, $Kw=[\ce{H+}][\ce{OH-}] = 10^{-14}$ at 25 degrees. (ii) Solubility product ($K{sp}$): for a sparingly soluble salt, the...
Define the terms i) standard electrode potential ii) electrochemical series. The cost of electricity required to deposit 1 g of Mg is Rs. 6. How much would it cost to deposit 10 g of Al? (At. wt. of Al = 27)
(i) Standard electrode potential: the potential of an electrode measured against the standard hydrogen electrode when the ion concentration is 1 M, gas pressure 1 atm and temperature 298 K.
(ii) Electrochemical series: the arrangement of electrodes (elements) in order of their increasing standard reduction potentials. It helps predict the strength of oxidising/reducing agents and the feasibility of redox reactions.
Calculation: charge (and hence cost) needed is proportional to the number of gram-equivalents deposited.
$$ \begin{aligned} \ce{Mg} &= 24/2 \ &= 12 \end{aligned} $$
; equivalents in 1 g $= 1/12$.
$$ \begin{aligned} \ce{Al} &= 27/3 \ &= 9 \end{aligned} $$
; equivalents in 10 g $= 10/9$.
$$ \begin{aligned} \text{Cost} &= 6 \times \frac{10/9}{1/12} \ &= 6 \times \frac{10}{9}\times 12 \ &= 6 \times 13.33 \ &= \textbf{Rs. 80} \end{aligned} $$
Write down the chemistry of calomel.
Calomel is mercurous chloride, $\ce{Hg2Cl2}$, a white insoluble powder.
Preparation: by subliming a mixture of mercuric chloride and mercury, or by precipitation: $$ \begin{aligned} \ce{HgCl2 + Hg -> Hg2Cl2} \ \ce{Hg2(NO3)2 + 2NaCl -> Hg2Cl2(s) + 2NaNO3} \end{aligned} $$
Action of ammonia (test): calomel turns black because it disproportionates into finely divided mercury (black) and a white mercuric amido compound: $$\ce{Hg2Cl2 + 2NH3 -> Hg(NH2)Cl + Hg + NH4Cl}$$ The black colour (free $\ce{Hg}$) confirms a mercurous salt.
Uses: in the calomel electrode (a reference electrode) and, formerly, in medicine as a purgative.
Write an example of each of the following: i) DNP test ii) Rosenmund's reduction iii) Aldol condensation iv) Tollen's test v) Cannizzaro's reaction.
i) DNP (2,4-DNP) test (for a carbonyl group), gives an orange-yellow precipitate: $$\ce{CH3CHO + H2N-NH-C6H3(NO2)2 - CH3CH=N-NH-C6H3(NO2)2 + H2O}$$ ii) Rosenmund's reduction (acid chloride to aldehyde): $$\ce{CH3COCl + H2 -[Pd/BaSO4] CH3...
Write down the laboratory method of preparation of trichloromethane from ethanol. What product would you obtain when trichloromethane is treated with acetone?
Preparation of trichloromethane (chloroform) from ethanol uses the haloform reaction with bleaching powder ($\ce{Ca(OCl)Cl}$), which supplies both $\ce{Cl2}$ and $\ce{Ca(OH)2}$. Ethanol is first oxidised to ethanal: $$\ce{CH3CH2OH + Cl2 ...
Write the structural formula with IUPAC name of each of the secondary and tertiary alcohol for C4H10O. How would you prepare these alcohols from CH3MgBr?
Secondary alcohol: butan-2-ol, $\ce{CH3-CH2-CH(OH)-CH3}$. Tertiary alcohol: 2-methylpropan-2-ol, $\ce{(CH3)3C-OH}$. Preparation using $\ce{CH3MgBr}$ (Grignard reagent). For butan-2-ol, add the Grignard reagent to an aldehyde (propanal) t...
Describe the laboratory method of preparation of pure and dry nitrobenzene. The compound obtained by heating phenol with zinc dust is treated with alc. KCN; what major product would you obtain? Identify the compounds in the given reaction sequence.
Laboratory preparation of nitrobenzene: benzene is nitrated with a nitrating mixture of concentrated $\ce{HNO3}$ and concentrated $\ce{H2SO4}$ at 55 to 60 degrees (kept below 60 degrees to avoid m-dinitrobenzene). $$\ce{C6H6 + HNO3 -[con...
i) How are primary, secondary and tertiary amines separated from their mixture by Hoffmann's method? ii) An aliphatic haloalkane A gives compound B when heated with alc. NaOH. B reacts with HBr to give major product C. On heating C with sodium in dry ether it yields 2,3-dimethylbutane. What product is obtained when C is subjected to ozonolysis? Identify A, B, C.
(i) Hoffmann's method (separation of amines): the mixture is treated with diethyl oxalate. The primary amine forms a solid oxamide (dialkyl oxamide), the secondary amine forms a liquid oxamic ester, and the tertiary amine does not react ...
Define the terms i) half-life period ii) rate law iii) instantaneous rate iv) zero-order reaction. How do the surface area of reactant and a catalyst affect the rate? For 2A + B2 -> 2AB the data are: (0.50,0.50,1.6x10^-4), (0.50,1.00,3.2x10^-4), (1.00,1.00,3.2x10^-4) for [A],[B2],rate. Find the overall order and rate constant.
Definitions.
i) Half-life period: the time in which the concentration of a reactant falls to half its initial value.
ii) Rate law: the experimentally determined equation expressing the rate as a function of the molar concentrations of the reactants, e.g. rate $=k[A]^x[B]^y$.
iii) Instantaneous rate: the rate of reaction at a particular instant, given by the slope $-\dfrac{d[R]}{dt}$ at that moment.
iv) Zero-order reaction: one whose rate is independent of the concentration of the reactant (rate $=k$).
Effect of surface area / catalyst: increasing the surface area of a solid reactant (finely divided) exposes more particles, so the rate increases. A catalyst provides an alternative path of lower activation energy, so more molecules cross the barrier and the rate increases (it is not consumed).
Order from data:
Overall order
$$ \begin{aligned} &= 0 + 1 \ &= \textbf{1} \end{aligned} $$
. Rate law: rate $=k[B_2]$.
$$ \begin{aligned} k &= \frac{\text{rate}}{[B_2]} \ &= \frac{1.6\times10^{-4}}{0.50} \ &= 3.2\times10^{-4}\ \text{s}^{-1} \end{aligned} $$
Write short notes on any two: i) Extraction of blister copper from copper pyrites ii) Manufacture of steel by open hearth process iii) Gibbs free energy change and prediction of spontaneity iv) Laboratory preparation of anhydrous methanoic acid.
i) Blister copper from copper pyrites ($\ce{CuFeS2}$): the concentrated ore is roasted, then smelted with silica to form copper matte ($\ce{Cu2S + FeS}$); slag ($\ce{FeSiO3}$) is removed. In the Bessemer converter, air is blown through t...
Define neutralization reaction with suitable examples.
Neutralization is the reaction between an acid and a base to give a salt and water only. The $\ce{H+}$ of the acid combines with the $\ce{OH-}$ of the base to form water. $$ \begin{aligned} \ce{HCl + NaOH - NaCl + H2O} \ \ce{H2SO4 + 2KO...
Differentiate between crystalline and amorphous solid.
Crystalline solid: particles are arranged in a regular, repeating three-dimensional pattern (long-range order); has a sharp, definite melting point; is anisotropic; e.g. $\ce{NaCl}$, diamond. Amorphous solid: particles are arranged irreg...
State Graham's law of diffusion.
Graham's law of diffusion: at constant temperature and pressure, the rate of diffusion (or effusion) of a gas is inversely proportional to the square root of its density (or molar mass). $$ \begin{aligned} r \propto \frac{1}{\sqrt{d}} \q...
What is absolute zero? Why is it not attainable?
Absolute zero is the lowest possible temperature, $0\ \text{K} = -273.15\ \text{degrees C}$, at which the volume (and pressure) of an ideal gas would theoretically become zero and molecular motion would cease. Not attainable because ever...
What observations led Rutherford to the following conclusions? i) The atomic centre is positively charged. ii) Most of the space inside the atom is hollow.
From the alpha-particle scattering experiment (thin gold foil): i) Positively charged centre (nucleus): a very few alpha particles were deflected through large angles and a very small number bounced straight back. Only a dense, positivel...
Calculate the current required in amperes to deposit 10 gram of Ag in 2 hours (at. wt. of Ag = 108).
Silver is univalent, so its equivalent weight $=108$. Charge required (Faraday's first law): $$ \begin{aligned} Q &= \frac{\text{mass}}{\text{eq. wt.}}\times 96500 \ &= \frac{10}{108}\times 96500 \ &= 8935\ \text{C} \end{aligned} $$ Ti...
Differentiate between the modern periodic table and Mendeleev's periodic table.
Mendeleev's table: elements arranged in increasing order of atomic mass; periodicity based on atomic mass; had anomalous pairs (e.g. Ar before K) and no fixed place for isotopes; only about 63 elements known. Modern (long-form) table: el...
Define Lewis acid and Lewis base giving one example of each.
Lewis acid: a species that can accept a lone pair of electrons (electron-pair acceptor), e.g. $\ce{BF3}$, $\ce{AlCl3}$, $\ce{H+}$.
Lewis base: a species that can donate a lone pair of electrons (electron-pair donor), e.g. $\ce{NH3}$, $\ce{H2O}$, $\ce{OH-}$.
Example of their reaction:
$$ \ce{BF3 + NH3 -> F3B<-NH3} $$
(a coordinate bond forms).
Find the oxidation number of i) C in C2H2O4 ii) S in Na2S2O7.
(i) C in $\ce{C2H2O4}$ (oxalic acid). Let C = $x$; H = $+1$, O = $-2$: $$ \begin{aligned} 2x + 2(+1) + 4(-2) &= 0 \ &\Rightarrow 2x - 6 = 0 \ &\Rightarrow x = +3 \end{aligned} $$ Oxidation number of C $= +3$. (ii) S in $\ce{Na2S2O7}$ (...
What is the importance of calculating the normality factor of solutions during titration?
The normality factor (f) tells how much the actual strength of a prepared solution differs from its intended (labelled) normality, because it is hard to weigh out an exact amount, especially of a secondary standard. Its importance: the t...
What happens when conc. H2SO4 is reacted with sugar?
Concentrated sulphuric acid is a powerful dehydrating agent. It removes the elements of water from sugar (sucrose, $\ce{C12H22O11}$), leaving a black spongy mass of carbon (charring): $$\ce{C12H22O11 -[conc. H2SO4] 12C + 11H2O}$$ Heat is...
How do you perform the test of HNO3 and HCl in the laboratory?
Test for $\ce{HCl}$ (chloride): add silver nitrate solution; a white precipitate of $\ce{AgCl}$ forms that is soluble in ammonia. $$\ce{HCl + AgNO3 - AgCl(s) + HNO3}$$ Test for $\ce{HNO3}$ (nitrate, brown-ring test): add freshly prepared...
'Every ore is a mineral but every mineral is not an ore.' Comment on the statement.
A mineral is any naturally occurring substance in which a metal is found (combined or free). An ore is a mineral from which the metal can be extracted conveniently and profitably. So every ore is a mineral (it contains the metal), but a ...
Write the molecular formula of each of the following: i) blue vitriol ii) green vitriol iii) white vitriol iv) washing soda.
i) Blue vitriol (copper sulphate pentahydrate): $\ce{CuSO4.5H2O}$ ii) Green vitriol (ferrous sulphate heptahydrate): $\ce{FeSO4.7H2O}$ iii) White vitriol (zinc sulphate heptahydrate): $\ce{ZnSO4.7H2O}$ iv) Washing soda (hydrated sodium c...
Give a functional isomer of C3H8O and write its IUPAC name.
$\ce{C3H8O}$ can be an alcohol or an ether (functional isomers, same formula but different functional groups). - Alcohol: propan-1-ol, $\ce{CH3-CH2-CH2-OH}$. - Ether (functional isomer): methoxyethane, $\ce{CH3-O-CH2-CH3}$, IUPAC name me...
Write down the IUPAC name of the following: i) (CH3)2C=C(CH3)2 ii) HOOC-CH=CH-COOH.
i) $\ce{(CH3)2C=C(CH3)2}$ has a four-carbon double-bonded chain with methyl groups on C-2 and C-3: 2,3-dimethylbut-2-ene. ii) $\ce{HOOC-CH=CH-COOH}$ is a four-carbon chain with a $\ce{-COOH}$ at each end and a double bond in the middle: ...
Write an example of each of the following reactions: i) coupling reaction ii) Friedel-Crafts reaction.
i) Coupling reaction (a diazonium salt couples with an aromatic phenol/amine to give an azo dye): $$\ce{C6H5N2Cl + C6H5OH - C6H5-N=N-C6H4OH + HCl}$$ (p-hydroxyazobenzene, an orange dye). ii) Friedel-Crafts reaction (alkylation/acylation ...
Write a reaction to show the iodoform test.
Ethanol (or any $\ce{CH3CH(OH)-}$ / $\ce{CH3CO-}$ compound) gives a yellow precipitate of iodoform with $\ce{I2}$ and $\ce{NaOH}$: $$\ce{C2H5OH + 4I2 + 6NaOH - CHI3(s) + HCOONa + 5NaI + 5H2O}$$ The yellow precipitate of $\ce{CHI3}$ (iodo...
List any two examples of phosphatic fertilizer.
Phosphatic fertilizers supply phosphorus (as phosphate) to plants. Two examples: 1. Superphosphate of lime (calcium dihydrogen phosphate with calcium sulphate), $\ce{Ca(H2PO4)2 + CaSO4}$. 2. Triple superphosphate, $\ce{Ca(H2PO4)2}$. (Bon...
Draw the structure of DDT and BHC.
DDT (p,p'-dichlorodiphenyltrichloroethane): two p-chlorophenyl groups attached to a $\ce{CH}$ that also carries a $\ce{CCl3}$ group, written condensed as $\ce{(ClC6H4)2CH-CCl3}$. BHC (benzene hexachloride, also called gammexane/lindane):...
State Boyle's and Charles' law. Prove that PV = nRT.
Boyle's law: at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure, $V \propto \dfrac{1}{P}$. Charles' law: at constant pressure, the volume of a fixed mass of gas is directly proportional t...
How does Bohr's theory explain the origin of the hydrogen spectrum? Name the different spectral lines with a labelled diagram.
Origin: in Bohr's model the electron revolves only in fixed circular orbits (energy levels). When energy is supplied the electron jumps to a higher orbit (excited state); being unstable it falls back to a lower orbit and emits the energy...
State the modern periodic law. What are the advantages of the modern periodic table over Mendeleev's periodic table?
Modern periodic law: the physical and chemical properties of the elements are a periodic function of their atomic numbers. Advantages over Mendeleev's table: 1. Arrangement by atomic number removes the mass anomalies (e.g. Ar/K, Co/Ni, T...
What is a standard solution? What volume of deci-normal HCl is required to neutralize 25 ml of NaOH containing 8 g NaOH in one litre of solution?
Standard solution: a solution whose concentration (normality/molarity) is exactly and accurately known.
Calculation: normality of the $\ce{NaOH}$ solution (equivalent weight of $\ce{NaOH}=40$):
$$ \begin{aligned} N_{NaOH} &= \frac{8}{40\times 1} \ &= 0.2\ \text{N} \end{aligned} $$
Deci-normal $\ce{HCl}$ is $0.1$ N. Using $N_1V_1 = N_2V_2$ for neutralization:
$$ \begin{aligned} 0.1 \times V_{HCl} &= 0.2 \times 25 ; \ &\Rightarrow ; V_{HCl} = \frac{0.2\times 25}{0.1} \ &= 50\ \text{mL} \end{aligned} $$
Write the chemistry of white vitriol.
White vitriol is zinc sulphate heptahydrate, $\ce{ZnSO4.7H2O}$, a white crystalline solid. Preparation: by dissolving zinc, zinc oxide or zinc carbonate in dilute sulphuric acid, then crystallising: $$ \begin{aligned} \ce{Zn + H2SO4 - Zn...
What is fermentation? How do you prepare ethyl alcohol from molasses?
Fermentation: the slow decomposition of a complex organic substance (usually a carbohydrate) into simpler ones by the action of enzymes produced by microorganisms (yeast).
Ethyl alcohol from molasses: molasses is diluted and the enzymes in yeast act stepwise. $$\ce{C12H22O11 + H2O ->[invertase] C6H12O6 + C6H12O6}$$ (glucose + fructose) $$\ce{C6H12O6 ->[zymase] 2C2H5OH + 2CO2(g)}$$ The fermented liquid (wash) contains about 15% alcohol, which is concentrated by fractional distillation to give rectified spirit (about 95% ethanol).
Give the advantages and disadvantages of using chemical fertilizers.
Advantages: - Supply plant nutrients (N, P, K) in a concentrated, quick-acting form, raising crop yield. - Composition is known and can be matched to soil needs. - Easy to store, transport and apply. Disadvantages: - Overuse makes the so...
Starting from iron pyrites, how would you obtain dilute sulphuric acid by the contact process? Explain with a labelled diagram. Give the action of conc. H2SO4 with HI. What is the test of sulphate ion?
Contact process (from iron pyrites $\ce{FeS2}$): 1. Roasting the pyrites to get sulphur dioxide: $$\ce{4FeS2 + 11O2 - 2Fe2O3 + 8SO2(g)}$$ 2. Catalytic oxidation of $\ce{SO2}$ to $\ce{SO3}$ over vanadium pentoxide at about 450 degrees: $$...
What are the postulates of the kinetic molecular theory of gases? Distinguish between an ideal and a real gas. A given mass of gas occupies 100 ml at 20 degrees C and 650 mm pressure. Find its volume at STP.
Postulates of kinetic molecular theory:
Ideal vs real gas: an ideal gas obeys $PV=nRT$ at all temperatures and pressures and has zero molecular volume and no intermolecular forces; a real gas has finite molecular volume and intermolecular attractions, so it obeys the gas laws only at high temperature and low pressure and deviates otherwise.
Calculation (combined gas law), $P_1=650$ mm, $V_1=100$ mL, $T_1=293$ K; STP: $P_2=760$ mm, $T_2=273$ K:
$$ \begin{aligned} V_2 &= V_1\times\frac{P_1}{P_2}\times\frac{T_2}{T_1} \ &= 100\times\frac{650}{760}\times\frac{273}{293} \ &= 79.7\ \text{mL} \end{aligned} $$
Explain how aniline is prepared in the laboratory. Write the action of aniline with: i) chloroform (CHCl3) ii) carbonyl chloride (COCl2) iii) benzene diazonium chloride iv) aqueous Br2.
Laboratory preparation of aniline: nitrobenzene is reduced with tin and concentrated hydrochloric acid; the aniline formed as its salt is liberated with $\ce{NaOH}$ and steam distilled. $$\ce{C6H5NO2 + 6[H] -[Sn/HCl] C6H5NH2 + 2H2O}$$ Re...
Write short notes on any two: a) Homologous series b) Law of mass action c) Faraday's second law of electrolysis d) Laboratory preparation of chloroform.
b) Law of mass action: at constant temperature, the rate of a chemical reaction is directly proportional to the product of the molar concentrations (active masses) of the reactants, each raised to the power of its coefficient. For $$ \ce...
Example: ammonia, . In the central nitrogen atom is sp3 hybridised (four sp3 hybrid orbitals). Three orbitals form sigma bonds and one holds a lone pair. The lone pair pushes the bonds together, so the sha...
At this very low concentration the from water cannot be ignored. Solution: let total . Charge/mass balance with gives: $$ \begin{aligned} x &= [\ce{H+}]{acid} + [\ce{OH-}] \ &= 10^{-8} + \frac{10^{...
In aqueous the ions present are . Cathode (reduction): (from water) is discharged in preference to , so hydrogen gas is evolved. Anode (oxi...
Spontaneous process: a physical or chemical change that takes place on its own under a given set of conditions without any continuous outside help. It proceeds in a definite direction accompanied by a decrease in Gibbs free energy ().
Example: flow of heat from a hot body to a cold body, or the rusting of iron:
Enthalpy of formation is per one mole of the compound formed. (i) , kcal for 2 mol water. $$ \begin{aligned} \Delta Hf(\ce{H2O}) &= \frac{-136}{2} \ &= -68\ \text{kcal/mol} \end{alig...
Heat sodium benzoate with soda-lime to get benzene, then carry out Friedel-Crafts acylation. Th...
(i) With cuprous chloride/HCl (Sandmeyer / Gattermann type) the diazonium group is replaced by chlorine, giving chlorobenzene: (ii) Chlorobenzene condenses with chloral (trichloroacetaldehyde)...
Both are alcohols and form intermolecular hydrogen bonds, but propan-1-ol (a primary alcohol, ) has its on a terminal carbon, so it is less crowded and its molecules pack and hydrogen-bond more effectively...
Cleave the ether with HI to get ethanol and iodoethane, then use Williamson synthesis. Prepare sodium methoxide and react it with iodoethane: The produ...
Methanal (formaldehyde) reacts with ammonia to give hexamethylenetetramine (urotropine), X = : Use: it is used as a urinary antiseptic (urotropine) and as a hardener (with the explos...
(i) Carboxylation introduces a group. A common example is a Grignard reagent reacting with carbon dioxide, followed by acidic work-up. The Grignard reagent first adds to : Acidif...
The group is electron withdrawing. Its resonance places a positive charge on the ortho and para positions of the ring, so the electrophile avoids these and attacks the meta position (which keeps its electron density). In the ...
Preparation (Hofmann bromamide degradation): ethanamide loses one carbon and gives methanamine. With nitrous acid (, i.e. ) a primary aliphatic a...
Dipeptide: formed by the union of two amino acid molecules through a single peptide () bond, e.g. glycylalanine. It has one peptide linkage. Polypeptide: formed by the union of many (a large number of) amino acid molecules ...
Antipyretics are drugs that lower body temperature (reduce fever). A common example is paracetamol (acetaminophen): Structure: (p-hydroxyacetanilide, an and an group para on a benzene r...
i) Mohr's salt (ferrous ammonium sulphate): . Use: it is a stable primary standard for in redox titrations (e.g. against ). ii) Green vitriol (ferrous sulphate heptahydrate):
(i) Being amphoteric, dissolves in excess caustic alkali to form soluble sodium zincate: (ii) On heating moistened with cobalt nitrate solution, a green mass called Rinmann's gre...
Molality (m): the number of moles of solute dissolved per kilogram of solvent. Calculation (basis: 1 litre = 1000 mL of solution): - 93% w/v means 930 g in 1...
(i) Ionic product of water (): the product of the molar concentrations of and ions in water, at 25 degrees. (ii) Solubility product (): for a sparingly soluble salt, the...
(i) Standard electrode potential: the potential of an electrode measured against the standard hydrogen electrode when the ion concentration is 1 M, gas pressure 1 atm and temperature 298 K.
(ii) Electrochemical series: the arrangement of electrodes (elements) in order of their increasing standard reduction potentials. It helps predict the strength of oxidising/reducing agents and the feasibility of redox reactions.
Calculation: charge (and hence cost) needed is proportional to the number of gram-equivalents deposited.
; equivalents in 1 g .
; equivalents in 10 g .
Calomel is mercurous chloride, , a white insoluble powder.
Preparation: by subliming a mixture of mercuric chloride and mercury, or by precipitation:
Action of ammonia (test): calomel turns black because it disproportionates into finely divided mercury (black) and a white mercuric amido compound: The black colour (free ) confirms a mercurous salt.
Uses: in the calomel electrode (a reference electrode) and, formerly, in medicine as a purgative.
i) DNP (2,4-DNP) test (for a carbonyl group), gives an orange-yellow precipitate: ii) Rosenmund's reduction (acid chloride to aldehyde): $$\ce{CH3COCl + H2 -[Pd/BaSO4] CH3...
Preparation of trichloromethane (chloroform) from ethanol uses the haloform reaction with bleaching powder (), which supplies both and . Ethanol is first oxidised to ethanal: $$\ce{CH3CH2OH + Cl2 ...
Secondary alcohol: butan-2-ol, . Tertiary alcohol: 2-methylpropan-2-ol, . Preparation using (Grignard reagent). For butan-2-ol, add the Grignard reagent to an aldehyde (propanal) t...
Laboratory preparation of nitrobenzene: benzene is nitrated with a nitrating mixture of concentrated and concentrated at 55 to 60 degrees (kept below 60 degrees to avoid m-dinitrobenzene). $$\ce{C6H6 + HNO3 -[con...
Definitions.
i) Half-life period: the time in which the concentration of a reactant falls to half its initial value.
ii) Rate law: the experimentally determined equation expressing the rate as a function of the molar concentrations of the reactants, e.g. rate .
iii) Instantaneous rate: the rate of reaction at a particular instant, given by the slope at that moment.
iv) Zero-order reaction: one whose rate is independent of the concentration of the reactant (rate ).
Effect of surface area / catalyst: increasing the surface area of a solid reactant (finely divided) exposes more particles, so the rate increases. A catalyst provides an alternative path of lower activation energy, so more molecules cross the barrier and the rate increases (it is not consumed).
Order from data:
Overall order
. Rate law: rate .
i) Blister copper from copper pyrites (): the concentrated ore is roasted, then smelted with silica to form copper matte (); slag () is removed. In the Bessemer converter, air is blown through t...
Neutralization is the reaction between an acid and a base to give a salt and water only. The of the acid combines with the of the base to form water. $$ \begin{aligned} \ce{HCl + NaOH - NaCl + H2O} \ \ce{H2SO4 + 2KO...
Crystalline solid: particles are arranged in a regular, repeating three-dimensional pattern (long-range order); has a sharp, definite melting point; is anisotropic; e.g. , diamond. Amorphous solid: particles are arranged irreg...
Absolute zero is the lowest possible temperature, , at which the volume (and pressure) of an ideal gas would theoretically become zero and molecular motion would cease. Not attainable because ever...
Silver is univalent, so its equivalent weight . Charge required (Faraday's first law): Ti...
Lewis acid: a species that can accept a lone pair of electrons (electron-pair acceptor), e.g. , , .
Lewis base: a species that can donate a lone pair of electrons (electron-pair donor), e.g. , , .
Example of their reaction:
(a coordinate bond forms).
(i) C in (oxalic acid). Let C = ; H = , O = : Oxidation number of C . (ii) S in (...
Concentrated sulphuric acid is a powerful dehydrating agent. It removes the elements of water from sugar (sucrose, ), leaving a black spongy mass of carbon (charring): Heat is...
Test for (chloride): add silver nitrate solution; a white precipitate of forms that is soluble in ammonia. Test for (nitrate, brown-ring test): add freshly prepared...
i) Blue vitriol (copper sulphate pentahydrate): ii) Green vitriol (ferrous sulphate heptahydrate): iii) White vitriol (zinc sulphate heptahydrate): iv) Washing soda (hydrated sodium c...
can be an alcohol or an ether (functional isomers, same formula but different functional groups). - Alcohol: propan-1-ol, . - Ether (functional isomer): methoxyethane, , IUPAC name me...
i) has a four-carbon double-bonded chain with methyl groups on C-2 and C-3: 2,3-dimethylbut-2-ene. ii) is a four-carbon chain with a at each end and a double bond in the middle: ...
i) Coupling reaction (a diazonium salt couples with an aromatic phenol/amine to give an azo dye): (p-hydroxyazobenzene, an orange dye). ii) Friedel-Crafts reaction (alkylation/acylation ...
Ethanol (or any / compound) gives a yellow precipitate of iodoform with and : The yellow precipitate of (iodo...
Phosphatic fertilizers supply phosphorus (as phosphate) to plants. Two examples: 1. Superphosphate of lime (calcium dihydrogen phosphate with calcium sulphate), . 2. Triple superphosphate, . (Bon...
DDT (p,p'-dichlorodiphenyltrichloroethane): two p-chlorophenyl groups attached to a that also carries a group, written condensed as . BHC (benzene hexachloride, also called gammexane/lindane):...
Boyle's law: at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure, . Charles' law: at constant pressure, the volume of a fixed mass of gas is directly proportional t...
Standard solution: a solution whose concentration (normality/molarity) is exactly and accurately known.
Calculation: normality of the solution (equivalent weight of ):
Deci-normal is N. Using for neutralization:
White vitriol is zinc sulphate heptahydrate, , a white crystalline solid. Preparation: by dissolving zinc, zinc oxide or zinc carbonate in dilute sulphuric acid, then crystallising: $$ \begin{aligned} \ce{Zn + H2SO4 - Zn...
Fermentation: the slow decomposition of a complex organic substance (usually a carbohydrate) into simpler ones by the action of enzymes produced by microorganisms (yeast).
Ethyl alcohol from molasses: molasses is diluted and the enzymes in yeast act stepwise. (glucose + fructose) The fermented liquid (wash) contains about 15% alcohol, which is concentrated by fractional distillation to give rectified spirit (about 95% ethanol).
Contact process (from iron pyrites ): 1. Roasting the pyrites to get sulphur dioxide: 2. Catalytic oxidation of to over vanadium pentoxide at about 450 degrees: $$...
Postulates of kinetic molecular theory:
Ideal vs real gas: an ideal gas obeys at all temperatures and pressures and has zero molecular volume and no intermolecular forces; a real gas has finite molecular volume and intermolecular attractions, so it obeys the gas laws only at high temperature and low pressure and deviates otherwise.
Calculation (combined gas law), mm, mL, K; STP: mm, K:
Laboratory preparation of aniline: nitrobenzene is reduced with tin and concentrated hydrochloric acid; the aniline formed as its salt is liberated with and steam distilled. Re...