India 2016 — INChO-2016.pdf

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Problema 1

Question No 1 2 3 4 5 Total Marks 20 24 17 17 24 102 Marks Obtained

Signature of Examiner Indian National Chemistry Olympiad Theory 2016 HBCSE, 30th January 2016

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Fundamental Constants

Avogadro number

NA = 6.022 x 1023 mol–1 Electronic charge

e = 1.602 x 10–19 C Molar gas constant

R = 8.314 J K–1mol–1

= 8.314 K Pa.dm3 K–1mol–1

= 0.082 L.atm K–1mol–1 1 atomic mass unit (1u)

= 931.5 MeV/C2 1 eV

= 1.602 x 10–19 J 1 cm–1

= 11.9 x 10–3 kJ mol–1 Rydberg constant

RH = 2.179 x 10–18 J Mass of electron

me = 9.109 x 10–31 kg Plancks constant

h = 6.625 x 10–34 Js Speed of light

c = 2.998 x 108 ms 1 Acceleration due to gravity g = 9.8 ms 2 Density of mercury

= 13.6 x 103 kg m 3 Faraday constant

F = 96485 C mol–1 Temperature

0 C = 273.15 K Indian National Chemistry Olympiad 2016

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6

Problem 1

20 Marks Sulphate dynamics in Wastewaters Wastewaters are one of the highly complex chemical systems in our environment. Understanding these complex chemical reactions is important for safe management of wastewater systems. Here we look at some of the chemical dynamics of sulphate ions in wastewaters. Under scarcity of stronger oxidizers such as O2 and NO3 , SO4 2 ions can act as oxidizing agents for organic matter in wastewater aiding its decomposition. Sulphate reduction is often facilitated by sulphate-reducing bacteria (SRB), which are usually present in sewage and thrive in an anaerobic (low oxygen) environment. Sulphate reduction can eventually lead to production of toxic H2S gas. The overall redox reaction can be represented by the equation: SO4 2 (aq) + 9H3O+(aq) + 8e HS (aq) + 13H2O $E^{\circ }$ = 0.22 V ----(1) HS thus produced gets converted into H2S. In sewage, organic matter is present predominantly as suspended solid particles, which can be represented as {CH2O}. This matter can act as food for SRB and as a reducing agent for sulphate. The chemical transformation in the process can be represented as SO4 2 (aq) + 2{CH2O}(s) + H3O+(aq) HS (aq) + 2CO2(aq) + 3H2O(l) $E^{\circ }$ = 0.26 V ----(2) For all parts in this problem, take temperature of the system to be 298 K. 1.1 Determine the change in standard Gibbs free energy, $G^{\circ }$, for reaction (2) in kJ.

(1 mark) 1.2 Write an expression for equilibrium constant, K, for reaction (2) in terms of molar

concentrations of the relevant chemical species.

(1 mark) 1.3 Calculate the value of K for reaction (2).

(1 mark) Name of Student

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1.4 A sewage entering a septic tank has [SO4 2 ] = 0.5 mM and pH = 7.0. Assuming (2) to be the

predominant redox reaction in the sewage, determine the concentration of dissolved H2S in

it when the system reaches equilibrium. (For H2S, pKa1 = 7.0 and pKa2 = 11.96).

Assume that (a) H2S and CO2 gases produced remain in the solution phase. (b) the carbonic acid produced here doesn’t undergo dissociation. (c) the sewage acts as buffer.

(2 marks)

Under aerobic conditions, CH4 is also often produced in the decomposition of sewage in

addition to H2S and CO2. 1.5 Following graphs show temperature dependence of solubilities of the three gases in

water, which includes the gas in dissolved form as well as its reaction products with water.

i) Label the boxes in each graph with appropriate gas.

0.0E+00 5.0E-06 1.0E-05 1.5E-05 2.0E-05 2.5E-05 3.0E-05 3.5E-05 X (Mole Fraction) Temperature (K) 3.5 $\times$ 10-5 3.0 $\times$ 10-5 2.5 $\times$ 10-5 2.0 $\times$ 10-5 1.5 $\times$ 10-5 1.0 $\times$ 10-5 5.0 $\times$ 10-6 0 0.0E+00 5.0E-04 1.0E-03 1.5E-03 2.0E-03 2.5E-03 X (Mole Fraction) Temperature (K) 0.0E+00 2.0E-04 4.0E-04 6.0E-04 8.0E-04 1.0E-03 X (Mole Fraction) Temperature (K) 1.0 $\times$ 10-3 8.0 $\times$ 10-4 6.0 $\times$ 10-4 4.0 $\times$ 10-4 2.0 $\times$ 10-4 0 2.5 $\times$ 10-3 2.0 $\times$ 10-3 1.5 $\times$ 10-3 1.0 $\times$ 10-3 0.5 $\times$ 10-4 0 Indian National Chemistry Olympiad 2016

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ii) Which of the three gases will escape from sewage the fastest?

(2 marks) 1.6 Septic tanks have a certain amount of air above the water level. In one such tank, the

average composition of the air was found to be N2 = 86%, O2 = 9%, H2O = 1.5%, CO2 =

1.5%, H2S = 1% and CH4 = 1% (where the percentages are by mass). Calculate the average

density of this air in kg m 3, assuming its pressure to be 1 atm and all gases to be ideal.

(2 marks) 1.7 Unless there is turbulence or mixing, gases entering a chamber do not immediately

diffuse uniformly in the chamber space. Initially they tend to accumulate in different

horizontal zones according to their relative density compared to the air present in the

chamber. Septic tanks are designed to have slow flow rate of sewage, therefore, do not have

any significant turbulence in air inside. In the tank described in 1.6, indicate the zones in the

air space (A, B, or C as shown below), in which each of the three gases (CH4, H2S, CO2)

escaping from the sewage would accumulate initially.

CH4

CO2

H2S

(1.5 marks) 1.8 Water vapour in this air condenses on the tank walls. H2S in the air then slowly dissolves in

these droplets. Since oxygen is available in immediate vicinity of the droplets (because of

high surface area to volume ratio as compared to sewage below), sulphur oxidizing bacteria

(SOB) start accumulating in these droplets. In this new aqueous system, SOB oxidize

sulphide species and produce sulphuric acid. The cement part in concrete walls can be

represented as (CaO)3(Al2O3), which is attacked by sulphuric acid. Write the balanced

reaction(s) responsible for sulphuric acid production in the water droplets and its possible

reaction/s with the cement in concrete. Sewage Air A B C Indian National Chemistry Olympiad 2016

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(1.5 marks) Wastewater from tanneries contains high amounts of SO4 2 and Cr(VI) species. Since Cr(VI)

species are highly toxic, such wastewater is often treated with reducing agents such as

sucrose to reduce Cr(VI) to Cr(III) before discharging. However, on exposure to air, Cr(III)

species may get oxidized back to Cr2O7 2 .

1.9 A tannery wastewater containing 50 mg L 1 Cr(III) and 100 mg L 1 SO4 2 having a pH = 7.0

is discharged in an open drain exposed to atmosphere. If atmospheric air contains

21.9% oxygen by mass, rest being predominantly nitrogen, calculate the ratio of Cr2O7 2

to Cr(III) after it has attained equilibrium with the atmospheric air.

O2 (g) + 4H3O+ (aq.) + 4e– 6 H2O

$E^{\circ }$ = 1.23 V

(3)

Cr2O7 2 (aq) + 14H3O+ (aq) + 6e– 2Cr3+(aq) + 21H2O $E^{\circ }$ = 1.36 V ---- (4)

(4 marks) Indian National Chemistry Olympiad 2016

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As wastewater flow in open drains, its volume slowly decreases due to evaporation of water.

In the open drain described in 1.9, the water volume reduced by 30% on travelling 1 km. 1.10 Determine the equilibrium concentrations of SO4 2 , Cr(III) and Cr2O7 2 in the wastewater after

this 1 km travel.

(1.5 marks)

After this 1 km stretch, the wastewater in 1.10 enters a closed sewer containing sewage,

where oxygen is not available. Here, SRB would reduce sulphate to sulphide which can

reduce Cr2O7 2 back to Cr(III). 1.11 Write balanced chemical equations for the reactions and their standard electrochemical

potentials which would determine the concentrations of Cr(III) and Cr2O7 2 in this sewer,

assuming they are solely driven by sulphate-sulphide inter-conversion.

(2.5 marks) Indian National Chemistry Olympiad 2016

HBCSE, 30th

Topic: Thermodynamics, Kinetic Theory Metodi: First Law of Thermodynamics, Ideal Gas Law, Physical Modeling Competenze: Mathematical Modeling, Physical Reasoning Fonte: Testo (PDF) — p.1

Problema 2

Problem 2

24 Marks Olefin Chemistry

Formation of carbon - carbon double bond (C=C) is important in organic synthesis as it provides a route for introduction of a wide range of functional groups in the resulting olefins. The geometric isomers of olefins are distinguished using stereodescriptors E / Z. If the two groups of higher priority, according to the Cahn – Ingold – Prelog rules, are on the opposite sides of the double bond, the configuration is E. The other configuration is Z. 2.1 Give the IUPAC name with stereochemical descriptor (E / Z) for the following compound.

(2 marks) One of the most widely used methods for the formation of C=C is $\beta$ - elimination reaction of the following type.

Where X= -Cl, -OCOR, -+NR3 etc.

The spatial arrangement for a bimolecular elimination (E2) of the type shown above requires a trans geometry of the $\beta$ - hydrogen and the leaving group (X). In most of the cases, the most substituted olefin is the favored product. However, in the case of ammonium and sulphonate salts, the least substituted olefin is formed as a major product. 2.2 Identify the major products A-D in each of the following reactions, indicating the correct stereochemistry wherever applicable.

Name of Student

Roll No Indian National Chemistry Olympiad 2016

HBCSE, 30th January 2016

9 H H Et

(2.5 marks) Pyrolytic elimination is another synthetic route to generate alkenes. This reaction proceeds in a concerted manner through a cyclic transition state. The commonly used substrates in pyrolysis include carboxylate esters, sulfoxides, xanthates, etc. Such reactions proceed in a syn fashion, the hydrogen and the leaving group depart from the same side. 2.3 Identify the products F-H in the following reactions indicating the correct stereochemistry wherever applicable. Indian National Chemistry Olympiad 2016

HBCSE, 30th January 2016

10 iii) MeI iii) H O+

(3 marks) Fragmentation reactions similar to $\beta$ - elimination are useful for the preparation of olefins. One such reaction is shown below.

2.4 Identify the products I-K with proper stereochemistry, in the following reaction sequence which leads to an intermediate useful in the synthesis of the juvenile hormone, which regulates insect physiology. Indian National Chemistry Olympiad 2016

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(3.5 marks) One of the routes for the synthesis of E - olefins is the Julia olefination, where the starting compound is a sulfone having a good leaving group at the $\beta$ - position.

2.5 Sphingolipids are naturally occurring compounds which are believed to protect the cell surface from harmful environmental factors. Identify the products L-N in the following synthesis which yields the precursor of sphingosins, that forms the backbone of sphingolipids.

(2.5 marks) Diboranes add readily to alkenes to give monoalkyl, dialkyl or trialkyl organoborane compounds. Organoboranes can be converted to C-O, C-N and C-C bonds via initial nucleophilic attack followed by rearrangement as illustrated below. Indian National Chemistry Olympiad 2016

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(symmetric bicyclic) X XH R BOH

2.6 Complete the following reactions by identifying the compounds O-Q.

(3 marks) 2.7 Give the products R-T in the following reactions with correct stereochemistry wherever applicable.

(3 marks) Indian National Chemistry Olympiad 2016

HBCSE, 30th January 2016

13 equivalent While olefins are nucleophilic, presence of an electron withdrawing group attached to the olefinic carbon renders it electrophilic. Such olefins can undergo conjugate substitution reaction in the presence of good leaving groups at the $\beta$ - position. 2.8 Give the product of the following reaction.

(1 mark) 2.9 Complete the synthesis of the anti - ulcer drug Tagamet, by identifying U-W.

(3.5 marks) Indian National Chemistry Olympiad 2016

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Topic: Nuclear & Particle Physics, Astrophysics Metodi: Physical Modeling Competenze: Physical Reasoning Fonte: Testo (PDF) — p.8

Problema 3

Problem 3

17 marks

Chemical Kinetics and Nuclear Reactions Part A The reaction between BrO3 ̄ and Br ̄ in acidic solution results in the formation of Br2 and H2O and the rate law is given by,

r = d [ BrO3 ] = k [BrO3 ] [Br ] [H+] 2

dt

3.1 Write the stoichiometric equation corresponding to this reaction.

(1 mark)

3.2 Write the total order of the reaction.

(1 mark)

3.3 Simplify the rate law for the same reaction if [BrO3 ]0 = 2.5 $\times$ 10 3 M, [H+]0 = 1.0 M and

[Br ]0 = 0.28 M where the subscript zero refers to initial concentration.

(1 mark)

To study the kinetics of the above reaction, a student added phenol to the above reaction mixture, which undergoes the following reaction:

Br2 + C6H5OH BrC6H4OH + Br

To detect the completion of the above monobromination reaction, the student added a drop of dilute methyl orange solution to the reaction flask. Methyl orange gets bleached by Br2 and becomes colourless immediately even at very low Br2 concentrations.

Monobromination of phenol, (C6H5OH) is a much faster reaction than the bromate-bromide reaction. The student prepared three reaction flasks as given in the table below. The student measured the time, t, it takes for the solution at different concentrations of phenol to turn colourless.

Name of Student

Roll No Indian National Chemistry Olympiad 2016

HBCSE, 30th January 2016

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*BrO3 is added in the last just before starting the experimental run.

3.4 On doubling the concentration of C6H5OH, (in the above table) t increased by more than a factor of two because (put X against the correct option/s) i) the decolouration reaction of methyl orange slowed down at higher concentration of phenol ii) the bromination of phenol slowed down at higher concentration of phenol iii) at higher concentration of phenol, BrC6H4OH started converting to Br2C6H3OH iv) the rate of bromate-bromide reaction decreases with progress of reaction overtime. v) at higher concentration of phenol, the simplified rate law derived in 3.3 is no longer valid

(2 marks)

3.5 Calculate the rate constant, k, of bromate-bromide reaction using the data given in the above

table. Show all important steps in the calculations.

(3 marks)

Flask No. [C6H5OH] (M) [Br ] (M) H+ (M) BrO3

• (M) Time taken for decolourization, t (sec) 1 4 $\times$ 10 3 0.28 1.0 2.5 $\times$ 10 3 11 2 2 $\times$ 10 3 0.28 1.0 2.5 $\times$ 10 3 4 3 1 $\times$ 10 3 0.28 1.0 2.5 $\times$ 10 3 1.5 Indian National Chemistry Olympiad 2016

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16 3.6 Another way to do this experiment is to keep the phenol concentration fixed and vary

[BrO3 ]0. In such an experiment, (mark X against the correct option/s)

(i) t will increase as [BrO3 ]0 increases

(ii) k can be obtained from the initial slope of [BrO3 ] vs t plot fit to an exponential

curve

(iii) the experiment become less accurate as [BrO3 ]0/[C6H5OH]0 decreases

(iv) t will be independent of [Br ]0

(v) k can be obtained using the slope of 1/t vs [BrO3 ] plot fit to a line

(2 marks) Part B 3.7 Radioactive 238U92 emits alpha and beta particles and forms stable 206Pb82.

i) Write the balanced overall nuclear reaction.

(1 mark)

ii) An uranium ore sample is found to contain 1.19 g of 238U92 and 1.03 g of 206Pb82. (Assume that there was no lead initially in the ore). a) What mass of helium has escaped from the ore sample?

(1 mark)

b) How old is the ore? Half of 238U92 converts to 206Pb82 in 1.0 billion years.

(1 mark)

3.8 239U92 decays to 239Pu through the formation of an intermediate nucleus 239Np, the successive decay constants being k1 and k2. i) Write the rate equation for decay of both 239U and 239Np.

(1.5 marks) Indian National Chemistry Olympiad 2016

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The concentration of 239Np can be expressed by the following expression.

[239Np] = k1 [239U] (e – k 1 t – e k 2 t)

k2 – k1

ii) Simplify the expression using suitable approximation if the half lives for the decay of the two nuclei are 23.5 minutes and 2.35 days, respectively.

(1mark)

iii) Following graph shows the concentration profiles for U, Np and Pu as a function of time. Label each curve with the corresponding species.

(1mark)

iv) State whether the following statement is true or false.

Nuclear disintegration rate approximately doubles for every 10oC rise in temperature.

(Mark X the appropriate choice).

True

False

(0.5 mark)

time

Concentration Indian National Chemistry Olympiad 2016

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18 i) PhLi (excess) CH COOD/D + ii) H O/H + Photochemical

Topic: Nuclear & Particle Physics Metodi: Radioactive Decay Law, Approximation & Series Expansion, Differential Equations Competenze: Mathematical Modeling, Graph Linearization Fonte: Testo (PDF) — p.14

Problema 4

Problem 4

17 Marks Synthesis of Natural Products

Nonconjugated dienes are important intermediates in natural product synthesis. One of the simple ways to synthesize these compounds is outlined below. 4.1 Identify products A and B in the following synthesis of a 1, 4 diene.

(1.5 marks) Di-pi methane rearrangement is an important photochemical reaction in which nonconjugated dienes form an ene-substituted cyclopropane, as shown below.

Zizaene is a naturally occurring terpene found in vetiver oil and used for the synthesis of - vetivone that is used in perfumeries. It is prepared from compound C.

Name of Student

Roll No Indian National Chemistry Olympiad 2016

HBCSE, 30th January 2016

19 ii) H O Me S/Et O more stable isomer of D gives positive , - DNP test

4.2 Draw the structure of the precursor of C, if it was to be prepared by the di-pi methane

rearrangement.

(1 mark) Vinyl cyclopropane to cyclopentene rearrangement (shown below) is an important method for the synthesis of five membered carbocycles. This rearrangement can be performed under thermal, photochemical or catalytic activation. Under thermal and photochemical conditions, this rearrangement proceeds via formation of the most stable biradical.

The vinyl cyclopropane rearrangement is a key step in the synthesis of Zizaene. The synthesis of Zizaene is outlined below. 4.3 Draw the structures of D – F in the following reaction sequence.

(2.5 marks) Indian National Chemistry Olympiad 2016

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20 Intermediate G % C = . % H = . J K L M N m-CPBA Na/Hg Compound F on reaction with LiN(i-Pr)2 (LDA) (LDA is a bulky base) and PhS-SPh gives compound G, which on further oxidation using NaIO4 gives intermediate H and product I.

4.4 Identify G, H and I.

(2 marks) 4.5 Draw the structures of J and K in the following reaction. i) B H / THF J K i) PhSH ii) (CH OH) / H + ii) H O , NaOH iii) TsCl , pyridine

(3 marks)

Compound N is an intermediate for the synthesis of Zizaene. It is synthesized as shown

below.

i) LDA, THF ii) PhS-SPh 120 o C I G [ H ] NaIO 4 F Indian National Chemistry Olympiad 2016

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L M %C = . %H = . %O = . %C= . %H = . N Base Oxalic Acid CH I ( eq) Zizaene C H ONa BuLi Zizaene 4.6 Identify L, M and N.

(3.5 marks)

Ylide is a species with a positive charge on a hetero atom and a negative charge on the

adjacent carbon atom. Wittig reaction is a reaction which involves the phosphorous ylide.

4.7 Identify O and P. Draw the structure of Zizaene.

(3 marks)

4.8 How many stereocenters does Zizaene have?

(0.5 marks) Indian National Chemistry Olympiad 2016

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Metodi: Physical Modeling Competenze: Physical Reasoning Fonte: Testo (PDF) — p.18

Problema 5

Problem 5

24 Marks Nickel and its chemistry Commercially important nickel ores include its sulfide, which also contains iron (II) and copper (I) sulfides. The concentrated ore is smelted in presence of silica which helps in removal of iron as slag (Step I). The upper silvery layer of Cu2S and lower black layer of Ni2S3 remaining with the ore are then mechanically separated. The Ni2S3 is roasted in air to give a green solid (Step-II), which on subsequent treatment with water gas (mixture of CO and H2 gas) at 50 oC gives a volatile complex and other byproducts (Step-III). This complex on thermal decomposition in air at 230 oC gives pure solid Ni (Step-IV). 5.1 i) Write the balanced chemical equations for Steps I IV.

(2.5 marks) ii) Which of the following statements is / are true for Step III (Mark X in the correct box/es): a) Only H2 from water gas reacts with Ni ions and reduces it b) H2 from water gas reduces Ni ions and then CO complexes with

the reduced form of Ni c) Only CO from water gas reduces Ni ions d) Both CO and H2 from water gas act as reducing agents

(1mark)

Step I: Step II: Step III: Step IV:

Name of Student

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Pure nickel forms alloys with various metals such as As, Al, Cd, Cr, Zn etc. and some of these have

well defined crystal structure. The unit cell of NixAsy is shown in the following figure.

Diagram sourced from Structural Inorganic Chemistry, A F Wells, 5th Reprint edition, 2012, Oxford University Press, Oxford. 5.2 i) Using the above diagram identify the coordination number and geometries of Ni and As. (Mark X in the correct box)

(2 marks) ii) Find the values of x and y in NixAsy. x = y =

(0.5 mark)

Nickel forms various alloys such as stainless steel, nichrome and cupranickel which have

various applications in industries.

Stainless steel has the following approximate composition (w/w): 73% Fe, 18% Cr and 8% Ni. An industrial sample of stainless steel was analyzed for its nickel content using chemical methods. One of the standard quantitative methods for estimation of nickel utilizes two moles of Element Coordination No. Coordination geometry Ni 6 4 Octahedral Trigonal Prismatic Square planar Tetrahedral

As 6 4 Octahedral Trigonal Prismatic Square planar Tetrahedral

As Ni Indian National Chemistry Olympiad 2016

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chelating bidentate ligand, dimethylglyoxime [DMG, CH3C(NOH)C(NOH)CH3]. Nickel precipitates as a neutral complex (C8H14O4N4Ni), dark pink in color. 5.3 i) Draw the structure of the complex formed.

(1 mark) 1.4 g of stainless steel sample was treated with aqua-regia to dissolve all the contents of the sample. The solution was treated for removal of iron and chromium ions and was then diluted to 100 mL using distilled water. 10 mL of this diluted solution when treated with DMG gave 0.0532 g of the precipitate of the complex.

ii) Calculate the % of nickel in the sample taken for analysis. Show all important steps of calculations.

(2 marks)

The stability of nickel complexes can be understood from their formation constant, K. The formation constants of the following Ni complexes X and Y at 25 oC are

a) NiSO4.7H2O + excess NH3 $\to$ X

ln K = 8.61

Equation I

b) X + 2(H2NCH2CH2NH2) $\to$ Y

ln K = 9.67

Equation II 5.4 i) Draw the structure/s of all possible isomers of Y and encircle the optically active isomer/s if any. Indian National Chemistry Olympiad 2016

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(2 marks) ii) Calculate the entropy change during the formation of Y in Equation II at 25 oC, if the heat of

reaction for Equation II at constant pressure is –12.1 kJ mol 1.

(2 marks)

Raney nickel is a versatile hydrogenation catalyst. It is prepared by leaching out Al selectively from Ni-Al alloy powder under controlled condition by treating with concentrated solution of caustic soda. The slurry of finely divided spongy nickel particles is thus obtained which is then washed with large quantities of water to make the particles free from caustic soda. 5.5 Write balanced chemical equation for leaching process during the preparation of catalyst.

(1 mark)

Green nickel oxide NiO can be doped with colorless lithium oxide, Li2O, to give a black solid LixNi1-xO which acts as a semiconductor.

5.6 Which of the following statement(s) is/are correct for the above observations? (Mark X in the correct box)

i) LixNi1-xO exhibits both cationic and anionic vacancies

ii) LixNi1-xO exhibits Schottky defect iii) Doping of NiO with lithium induces mixed valency of nickel iv) NiO becomes a p-type semiconductor

(1mark) Indian National Chemistry Olympiad 2016

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26 Case I

Case II

Case III

Mixed metal oxides of transition metals often form spinels having the general formula AIIBIII 2O4 where the oxide ions form a close packed cubic lattice with eight tetrahedral holes and four octahedral holes per AIIBIII 2O4 unit. In a normal spinel 1/8th of the tetrahedral holes are occupied by A (II) whereas B (III) resides in octahedral sites. In inverse spinel A (II) and one half of B (III) occupy the octahedral holes while the other half of B (III) occupy the tetrahedral holes. The structure becomes ‘normal’ (AIIBIII 2O4) or ‘inverse’ spinel (BIIIAIIBIIIO4) depending upon the octahedral site preference energy which is the difference of the crystal field stabilization energy (CFSE) of the same cation in the corresponding octahedral and tetrahedral field of the ligand.

Metal ion $\Delta 0$ (cm 1) $\text{Deltat}$ (cm 1)

Co3+

20,760 9,226

Fe3+

14,000 6,220

Ni2+

8,500 3,780

$(\Delta 0$ and $\text{Deltat}$ imply the octahedral and tetrahedral splitting parameters respectively for oxide ions which will provide a moderately weak field similar to that of water) 5.7 On the basis of the above data, calculate the CFSE for

Case I: i) Co(III) in octahedral ii) Co(III) in tetrahedral sites Case II: i) Fe (III) in octahedral ii) Fe(III) in tetrahedral sites Case III : i) Ni(II) in octahedral ii) Ni(II) in tetrahedral sites

(Calculations involving pairing energies can be neglected for this question and only CFSE values can be calculated)

(2.5 marks) Indian National Chemistry Olympiad 2016

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iii) Calculate the octahedral site preference energy from 5.7 for all the above three cases.

(1 mark)

iv) Based on the above octahedral site preference energy calculations, classify the oxides as

normal and inverse spinels. (Mark X in the correct box)

Compound Normal Inverse NiFe2O4

NiCo2O4

(1 mark) 5.8 One of the widely used batteries of nickel is Ni-Cd battery. This battery uses high purity NiOOH (nickel oxyhydroxide) as anode. The typical synthesis of NiOOH is as follows. In the first step, concentrated NaOH (30% w/v) is reacted with gaseous Cl2 for sufficient time to generate hypochlorite. i) Write balanced chemical equation for this reaction.

(0.5 mark)

The resultant reaction mixture is then treated with excess of solid NaOH with vigorous stirring which leads to precipitation of NaCl. The precipitate is filtered and the sufficient amount of Ni(OH)2 is added to the filtrate with constant stirring. Black colored NiOOH

Topic: Thermodynamics Metodi: First Law of Thermodynamics, Physical Modeling Competenze: Mathematical Modeling, Diagrammatic Reasoning Fonte: Testo (PDF) — p.22