India 2019 — INChO2019-Question.pdf

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Problema 1

Question No 1 2 3 4 5 Total Marks 23 17 19 20 24 103 Marks Obtained

Signature of Examiner Structures of common -amino acids Indian National Chemistry Olympiad 2019 Roll No ©HBCSE, February 2, 2019

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Problem 1

23 marks Common and Uncommon Amino Acids Proteins are the most diverse class of biologically important compounds, derived from structurally varied $\alpha -amino$ acids. However, $\alpha -amino$ acids are not just found as the building blocks of proteins, but serve a variety of other functions too. This capacity of amino acids is derived due to their unique structure. Here, we will first look at the common properties of amino acids and then two special amino acids. Part I $\alpha -Amino$ acids have at least two pKa values, one of the –COOH group and the other of the conjugate acid of the amino group. The two pKa values of the amino acid proline are pKa1 = 1.95 and pKa2 = 10.64. 1.1 Write the predominant equilibria in aqueous solutions of proline at pH = 1.95 and 10.64.

$\alpha -Amino$ acids are usually characterized by their isoelectric pH (written as pI), which is the pH at which the degree of protonation at one functional group in the amino acid equals the degree of deprotonation at another functional group in the amino acid. At this pH, the amino acid shows minimum/zero electrical conductivity in aqueous solution and predominantly exists in electricallyneutral zwitter ionic form.

1.2 Based on the above definition of pI, derive a relation between pI, pKa1, pKa2 for proline. Indian National Chemistry Olympiad 2019 Roll No ©HBCSE, February 2, 2019

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1.3. Consider the following statements for the amino acid and mark X in the correct box/es.

a. The pKa of –COOH group in an $\alpha -amino$ acid is lower than the pKa of acetic acid.

True

False b. The pKa of –NH3

• group in an $\alpha -amino$ acid is lower than the pKa of –NH3
• group derived from a primary amine.

True False

1.4. When the side chain of the $\alpha -amino$ acid bears an additional acidic or basic functional group, they are classified as acidic or basic amino acids, respectively. The amino acid arginine has three pKa’s in the pH range 0 – 14. a. For arginine, the most likely pKa values are (Mark X in the correct box) i. 1.2, 2.0, 9.0

ii. 3.1, 5.0, 12.3 iii. 3.0, 6.7,12.1

iv. 2.2, 9.0, 12.5

b. In the following structure of arginine, circle the groups involved in proton exchange and assign appropriate pKa values based on your answer in 1.4a. Indian National Chemistry Olympiad 2019 Roll No ©HBCSE, February 2, 2019

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Zwitter Ion Zwitter ion + H+

c. Write the structure for the zwitter ionic form of arginine amino acid and also for the species that is formed by the addition of one H+ to this zwitter ion.

d. Calculate the pI value of arginine. Show the steps involved in calculations.

Chromatography is a technique used to separate species in a mobile phase (liquid or gaseous) on the basis of their distribution between stationery and mobile phases. The ion exchange chromatographic technique exploits the interaction between charged entities in the mobile phase and the charged groups present in the solid stationary phase—usually an ion exchange resin. The following figure indicates one type of ion exchange resin (X) that contains SO3 Na+ groups as shown. The counter ion Na+ can be exchanged with the positive ions present in the mobile phase and hence, the resin is called cation-exchange resin. Indian National Chemistry Olympiad 2019 Roll No ©HBCSE, February 2, 2019

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1.5. An equimolar mixture of tryptophan (A, pI = 5.88), histidine (B, pI = 7.6) and aspartic acid (C, pI = 2.98) was made in a buffer solution of pH = 6.0. This mixture was poured into column of X. The order in which these amino acids will come out of the column is (Mark X in the correct box) i) A, B, C

ii) C, A, B

iii) B, C, A iv) A, C, B

v) C, B, A

vi) B, A, C

Part II Keratin (hair protein) contains unusually large number of cysteine units. These units from different part of the protein chains on mild oxidation form disulphide linkages which are responsible for the 3-D structure of hair.

Curling or straightening of hair involves rearranging these –S-S- linkages of keratin. Thus, the amino acids with mercapto (thiol) groups can act as reducing agents. 1.6. a. The simplest $\beta -mercapto-\alpha -amino$ acid reduces Cu(II) ions to Cu(I). Write a balanced chemical equation for this reaction.

Compound P, a $\beta -mercapto\alpha -amino$ acid with one chiral centre, is a biochemical breakdown product of the antibiotic penicillin. Excess of P when reacted with copper(II) chloride at its pI (6.2) gives a yellow precipitate of compound Q. Elemental analysis showed that Q contains 26.14% C, 5.26% H and 27.66% Cu. The compound Q (molar mass = 229.5 g $mol-1$) also has one water molecule of hydration. b. Deduce the molecular formula of Q showing the appropriate steps.

2R-SH R-S-S-R + 2H+ + 2e (O) Indian National Chemistry Olympiad 2019 Roll No ©HBCSE, February 2, 2019

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c. Draw the structure of P.

Wilson’s disease is a genetic disease of copper assimilation in humans. This condition is effectively treated using agents such as P that can chelate copper. Lewis bases like thioethers and thioelates make stable complexes with Cu(I) with 2 to 4 coordination at the metal center, in preference to Cu(II) ions. However, Cu(II) ions gives 4 to 6 co-ordinations with Lewis bases like amino, amido and carbonyl oxygen. d. Write the balanced chemical equation(s) for the reaction of P (excess) with copper(II) chloride at its pI = 6.2 leading to the formation of Q.

Part III: Thyroxine Thyroid hormones are derivatives of the amino acid tyrosine having iodine bound covalently to aromatic carbon atoms. There are two thyroid hormones: the principal one is thyroxine. Thyroxine’s chemical structure was deciphered in 1927 and was found to be optically active, but its stereochemical configuration was difficult to decipher. In 1934, researchers of Pathological Chemistry in a medical college in London were able to get the configuration of Thyroxine by the following method based on L-tyrosine. 1.7.i) The researchers used L-tyrosine to prepare Z via the following route. Complete the following sequence of reactions by drawing structures D – F with correct stereochemistry. Indian National Chemistry Olympiad 2019 Roll No ©HBCSE, February 2, 2019

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1. Dilute acid / heat Step (II) L-tyrosine K2CO3/ 2-butanone
2. FeSO4 / Ba(OH)2
3. C6H5COCl (1 equivalent)
4. SOCl2 Step (III)
5. C2H5OH Step (I)

ii) The reagent mixture FeSO4/ Ba(OH)2 in Step III is acting as

a) Reducing agent b) Precipitating agent

c) Complexing agent Indian National Chemistry Olympiad 2019 Roll No ©HBCSE, February 2, 2019

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H2 / Pd

1. NaNO2 / HCl, 0 - $5^{\circ }C$ CaCO3
2. 5 N HCl Step (IV) Step (V) iii) The Compound F was further treated as follows.

An aqueous solution of thyroxine (C15H11O4NI4) on treatment with Pd/CaCO3 in an atmosphere of H2 also gave the same compound Z on workup (the reaction mixture on acidification and evaporation). This Z had the same specific rotation as when obtained from L-tyrosine. iv) Draw any one possible structure of thyroxine consistent with the above information, with correct stereochemistry.

**During the exam, the following additional was communicated to the students. Q 1.7(iii) Draw the structures of compounds G and Z with stereochemistry. Indian National Chemistry Olympiad 2019

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Topic: Conservation of Energy, Thermodynamics, Electrostatics Metodi: Energy Conservation Method, Conservation Laws, First Law of Thermodynamics, Thermodynamic Cycle Analysis Competenze: Physical Reasoning, Mathematical Modeling, Diagrammatic Reasoning Fonte: Testo (PDF) — p.1

Problema 2

Problem 2

17 marks Boron Compounds through the Ages

Boron has been widely found in nature in form of Borax, known as “Buraq” in Arabic, suhaga in Hindi, and tankan in Sanskrit. In 1807, Sir Davy isolated the element boron from boric acid. However for another century, synthetic chemists found no interest in boron. Around 1910 in Germany, Alfred Stock prepared a series of compounds called boranes, which had high reactivities with many substances. Around World War II, chemists found boranes could be used for chemical storage of hydrogen for weather balloons and military needs. A popular compound for this purpose was NaBH4 which reacted with water to give hydrogen. 2.1 The gas obtained by reacting 100 g of NaBH4 with water at 25oC was used to fill a balloon. Write the balanced equation for the reaction of NaBH4 with water. Assuming the pressure inside the balloon to be approximately equal to outside pressure, estimate the volume of the balloon after the reaction is over. Show the main steps in calculations used to arrive at the answer.

Synthesis method for NaBH4 was developed by H. C. Brown in USA not to make boranes but to refine uranium. His Ph.D. supervisor Schlesinger had found that uranium forms a volatile complex [U(BH4)4] which could be used to separate isotopes of uranium. 2.2 Borohydride anion acts as a ligand in complexes where the metal ion coordinates to the B-H bond and not to any atomic center. Neutron diffraction studies of [U(BH4)4] in gas phase show the coordination number of U(IV) to be greater than 8, whereas in solid phase, it exists as a polymeric structure with U(IV) center having coordination number 14. Draw the most stable structures of [U(BH4)4] in gaseous and solid phase. You may use dot-wedge notations to represent bonds wherever needed. (Note: steric effects may play a major role here in determining the structures) Indian National Chemistry Olympiad 2019

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Gas phase

Solid phase

Due to developments of other better methods, [U(BH4)4] method was eventually not used for uranium refining, but it led to method developments for large scale synthesis of NaBH4, which became an important reagent in organic synthesis. In 1979, H. C. Brown received the Nobel prize in chemistry for the same. Another property of boron attracted the attention of nuclear scientists. One of its isotopes, 10B has a very high affinity to absorb neutrons. Therefore, boron rods could be used as moderators in nuclear reactors to control the fission reaction.

2.3 The average atomic weight of boron is 10.81 amu and consists of only two isotopes: 10B and 11B. The masses of these isotopes are 10.0129 amu and 11.0093 amu, respectively. What is the percentage of 10B atoms in natural Boron samples? Show the steps involved in calculations.

Solutions of boron compounds can also be used to control the number of neutrons (referred as Chemical shims) in nuclear reactors or in case of a nuclear accident.

2.4 Among saturated aqueous solutions of boric acid (H3BO3) and of borax (Na2B4O7.10H2O), which will be more effective as chemical shims? Show the calculations needed to arrive at the answer. The solubilities of Boric acid and Borax in water are 52 g L 1 and 58 g L 1, respectively. 10 B + 1 n [11B] 4 He + 7 Li + 2.79 MeV Indian National Chemistry Olympiad 2019

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P

Q

R

The neutron absorption property of 10B is also being used for cancer treatment (in Boron Neutron Capture Therapy, BNCT). If one can place a boron-containing molecule inside a cancer cell, then a neutron irradiation will react with boron and the emitted alpha particle can kill selectively the cancer cell, while sparing the normal cells lacking boron. One way of preparing 10B enriched molecules for BNCT applications is given below:

2.4 Identify P, Q & R.

Theoretical explainations of bonding in boranes had always been a challenge. The crystalline form of elemental boron consists of B12 icosahedra units, each of which in turn is bonded to neighboring icosahedra via boron-boron bonds. A typical structure of icosahedron is given here:

2.5 The maximum number of bonds for a boron atom in this crystalline form of boron is

More surprise came when scientists succeeded in preparing similar cage compounds with some of the B atoms replaced with C atoms having the same number of bonds. One such compound is C2- carborane, whose method of synthesis is indicated below. Indian National Chemistry Olympiad 2019

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S

T X = F/Cl/Br a) b) Acetonitrile

For BNCT, the tumour cells can be preferentially targeted by attaching a boron-containing molecule to the hormones, as some tumours depend strongly on hormones for growth. In one of the first studies, estrone was used for the purpose, as shown below.

2.6 Draw the structures of S and T.

2.7 Boron compounds are not only used for therapeutic but also for diagnostic applications. A class of pyrrole-containing boron compounds are used as dyes to stain and image various (transparent) organelles inside the living cells, which are ordinarily difficult to see. The B N bonds in these compounds are very stable even under biochemical condition, and hence the compounds do not interfere with cellular processes. Two representative examples of such dyes are given below:

Acetonitrile C2-carborane Indian National Chemistry Olympiad 2019

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i. What is the oxidation state of boron in a? ii. What is the geometry around boron in a? iii. Out of all three halides (X = F/Cl/Br) in the class of compounds a, which is the most stable species in aqueous conditions? iv. For a given X (among the stable compounds in aqueous solutions), which dye among a and b will preferentially stain fat tissues? Indian National Chemistry Olympiad 2019

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Reduced Species

Resonance Structures

B A Hennosides

Topic: Nuclear & Particle Physics Metodi: Ideal Gas Law, Physical Modeling Competenze: Mathematical Modeling, Physical Reasoning Fonte: Testo (PDF) — p.10

Problema 3

Problem 3

19 Marks Chemistry behind Henna – Lawsone

Mehendi or henna is known to us as colourant for hands, hair, and fabrics. The orange colour observed from henna is due to a compound called lawsone.

Part I: Properties of Lawsone Henna leaves, however, do not contain lawsone, but have a class of compounds called hennosides. When the plant cells are ruptured by crushing the leaves, hennosides undergo enzymatic cleavage to generate one equivalent of glucose (C6H12O6) and a triol A which on air oxidation produces lawsone. Compound A also gives a positive test with 2, 4-Dinitrophenylhydrazine. 3.1 Draw structures for the possible hennosides and of the triol A.

3.2 Which of the following common substances can promote the hydrolysis of hennoside? (Mark X in the correct box) a) Lemon juice

b) Glucose syrup

c) NaHCO3

3.3 Lawsone undergoes tautomerisation in water. Draw the structure of its tautomer B.

Lawsone is yellow in colour at pH below 3.6, above which it undergoes de-protonation in solutions and becomes orange in colour. In strong solutions of alkalies, lawsone undergoes reduction forming a species that is highly stable due to resonance. 3.4 Draw the species obtained by 1 e reduction of deprotonated lawsone. Also draw most stable resonance structures of this reduced species.

Lawsone Indian National Chemistry Olympiad 2019

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Juglone (5-hydroxy derivative of 1,4-naphthalene dione) is a compound structurally similar to lawsone, and occurs in root, leaves, and bark of walnut. Juglone and lawsone, both have intramolecular hydrogen bonding. 3.5 Draw the structures of lawsone and juglone with intramolecular hydrogen bonding.

3.6 Planar molecules would be present in (Mark X in the correct box/es) Lawsone

Juglone

3.7 Out of lawsone and juglone, the compound that is more acidic is (Mark X in the correct box/es) Lawsone

Juglone

Part II: Lawsone in Polymers Lawsone-based polymers can be used for dyeing of wool and other textiles. A monomer (C) for these polymers was prepared by the following method: Lawsone (0.4 g) was dissolved in 10 mL water-free tetrahydrofuran (THF) under argon gas at room temperature. After 10 min, 0.12 mL (density = 1.12 g cm 3) 2-propenoyl chloride was added to this solution, followed by the addition of 0.254 g triethylamine (Mol. Mass = 101.2 g $mol-1$). A deep red color appeared immediately. The reaction was allowed to undergo completion for 4 h. The solvent was then removed by evaporation. Next, 30 mL of toluene was added to the mixture, and this solution was shaken with dilute sulfuric acid, and then the aqueous layer was removed. After removing toluene by evaporation, a dark red product (C) was obtained with a yield of 95%. 3.8 Based on the above procedure, give the balanced chemical equation for the reaction leading to formation of C. Also write the reaction conditions (in box above the arrow) necessary for the formation of C.

3.9 In the above synthesis procedure, triethyl amine acts as a (Mark X in the correct box) a) Base

b) Catalyst

c) Nucleophile

d) Acid Indian National Chemistry Olympiad 2019

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C Toluene, AIBN (radical initiator) Ar, 80oC, 72 h 3.10 In the above synthesis procedure, which of the reagents is the limiting reagent? Show all the steps in the calculations that help in indicating the limiting reagent.

3.11 Give the reaction involving dilute H2SO4 taking place when it is shaken with the reaction mixture. Specify the phase in which this reaction occurs.

One student tried this synthesis on a rainy day, and coincidently forgot to attach the Ar gas supply and left the reaction flask open. She could not observe the red product C in the flask. 3.12 Write the balanced equation for the side reaction due to which the product C was not formed.

The monomer C obtained can undergo polymerization in presence of radical initiators. 3.13 Give the structure of the product of the following polymerization reaction of C.

3.14 For each of the given statements below, mark X in the appropriate box.

True False (i) Lawsone and Juglone will have the same physical properties.

(ii) Lawsone can act as an acid-base indicator.

(iii) Lawsone-metal ion system can be used as an oxidizing agent. Indian National Chemistry Olympiad 2019

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Topic: Thermodynamics, Nuclear & Particle Physics Metodi: First Law of Thermodynamics, Thermodynamic Cycle Analysis, Ideal Gas Law, Radioactive Decay Law Competenze: Mathematical Modeling Fonte: Testo (PDF) — p.15

Problema 4

Problem 4

20 Marks Gold Refining

Even though gold is found in elemental form in nature, separating the element from other minerals found in its ores is a difficult process. Earlier, this separation was achieved by using liquid mercury or chlorine gas. Around 1887, leaching metallic gold in cyanide solution in air was found to be the most effective method which is still used in many parts of the world, including in India. Part I: Cyanidation Method This dissolution of gold in NaCN solution is an oxidation-reduction process resulting in a strong complex of cyanide ion with Au+ ion. This reaction is considered to proceed via two chemical pathways, wherein H2O2 is formed in one pathway and consumed in the second. 4.1 Write balanced chemical reactions for the two leaching pathways of Au in aqueous CN solution in presence of air leading to the formation of the gold complex.

Solubility of O2 in water at room temperature and 1 atm pressure is very low (8.2 mg $L-1$). Interestingly, this process is economical even with this concentration of oxygen. Therefore, cyanide concentrations used for effective leaching are also not very high. 4.2 Assuming that the leaching reactions proceed to completion faster than the diffusion of atmospheric O2 in water, what minimum concentration of NaCN (in g $L-1$) is desirable for maximum gold leaching at room temperature and 1 atm?

4.3 Write the most preferable geometry and spin magnetic moment ( spin only) value of the gold cyanide complex formed. Indian National Chemistry Olympiad 2019

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The reduction potential for Au+(aq) ion $(E^{\circ }Au$ + /Au= +1.69 V) is much higher than that of O2, indicating that gold is stable with respect to oxidation by oxygen at all pH values. Presence of strong complexing agents such as CN , which can stabilize gold in either +1 or +3 oxidation states, significantly shifts the reduction potential of gold. Cyanide ions affect the equilibria of the reaction Au+(aq) + $e-$ ⇌ Au(s) due to the following additional equilibrium: [Au CN $2]-(aq$) ⇌ Au+(aq) + $2CN-(aq$)

pK = 38.3 (at $25^{\circ }C$) 4.4 Find the $E^{\circ }$ value for the following half-cell reaction and write its Nernst equation. Au CN 2 $-$ aq + $e-$ ⇌ Au (s) + $2CN-(aq$)

Electrochemical potentials of several redox systems change with pH. This dependence is graphically represented in form of the EH (potential) vs pH diagrams. The reduction potential EH of a half-cell reaction relative to Standard Hydrogen Electrode is plotted on the y-axis with pH on the x-axis. Regions on different sides of plotted lines indicate domains of E and pH values at which specific species will be predominant (due to thermodynamic stability) under equilibrium conditions. These diagrams are based on the Nernst equation (based only on thermodynamics), but give no information about the reaction kinetics. The EH vs pH diagrams of (a) Au, (b) Ag, (c) Pd and (d) Pt in 0.02 M NaCN solution at $25^{\circ }C$ are shown below. The H2O stability limits are represented by the two dotted lines which correspond to the equilibria:

O2 (g) + 4H+ (aq) + 4e Ý 2H2O (aq)

2H+(aq) + 2e Ý H2 (g) Regions 1 and 2 represent stability domains for O2 and H2 (both at unit activity), respectively. Indian National Chemistry Olympiad 2019

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4.5 Based on the information given in above diagrams for Au, Ag, Pd, and Pt at pH = 7, (i) identify the metals which can be leached out using NaCN solution in presence of O2.

(ii) arrange the leachable metals identified in (i) in descending order of ease of leaching using the above cyanidation process at the given pH.

4.6 The most suitable pH for Au extraction using NaCN solution in presence of O2 is

Many gold ores also contain copper minerals. Thus, during cyanidation, some copper minerals readily dissolve forming several copper(I)-cyanide complexes. These can reduce the process efficiency (and economics) due to high cyanide consumption, leading to reduced gold leach rates and poor gold recovery. Indian National Chemistry Olympiad 2019

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The pH dependency of the speciation of copper (I) cyanide complexes is shown in the following plot, where ai is the ratio between concentrations of a cyanide complex and of total dissolved copper.

4.7 Write the formulae of copper(I) cyano complexes most likely to be extracted along with Au during the cyanidation process at the most suitable pH condition (identified in 4.6 above).

To separate the dissolved gold complexes from dissolved copper complexes, activated carbon is added to the leached solution. The Au(I) cyanide complex ions preferentially move into the pores of activated carbon compared to the Cu(I) cyanide complex. 4.8 Mark the correct reason/s for the above observation from the following options (Mark X in the correct box/es) (a) Lower hydration of Au(I) cyanide complex anion.

(b) Lower hydration of Cu(I) cyanide complex anion. (c) Higher charge density of Au(I) cyanide complex anion. (d) Higher charge density of Cu(I) cyanide complex anion. The gold complexes are then removed from activated carbon by elution with a NaOH/NaCN solution, from which gold is recovered by electrowinning. Part II: Thiourea Method The use of thiourea, NH2CSNH2, as an extracting agent for precious metals like gold has shown promise as it is less toxic compared to the reagents used in the cyanidation process. 4.9 Write the balanced chemical equation for reaction of Au with thiourea in presence of ferric ion (which is used as an oxidant) leading to the formation of the gold complex.

… dicyano Copper(I) complex —— tricyano Copper(I) complex tetracyano Copper(I) complex Indian National Chemistry Olympiad 2019

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One of the factors that prevents the economic viability of thiourea use in gold leaching is the irreversible loss of thiourea by oxidation. 4.10 Write the structure of the product formed by oxidation of thiourea in presence of ferric ions in the following (balanced) reaction. 2CS(NH2)2 + 2Fe3+

• 2H+ + 2Fe2+

When thiourea is separately mixed with Fe3+ solution, the oxidation reaction was found to be secondorder reaction with a rate constant of 1.55 $\times$ 10 6 m3mol 1s 1 at 303 K. The activation energy for the process was found to be 79 kJ mol 1. For the leaching reaction of Au by thiourea and Fe3+, activation energy is 8.5 kJ mol 1. 4.11 Determine the ratios of the rate constants at temperature 303 K to 295 K for the leaching and the thiourea oxidation reactions. Based on the ratios, deduce whether Au recovery per kg of thiourea will be more at 295 K or at 303 K under identical conditions? Show steps to arrive at the answers.

The Au-thiourea complex is separated from the leaching solution using ion-exchange resins. Ion exchange resins are polymeric compounds having side groups, with which some of the ions in the solution can bind or can exchange for an ion associated with the resin. 4.12 Of the following, the ion-exchange resin(s) that will lead to adsorption of Au-thiourea complex are (Mark X in the correct box/es) Indian National Chemistry Olympiad 2019

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The Au-thiourea complex is extracted from the ion exchange resin into a solution and then pure gold is obtained by electrowinning of this solution. In this process, current is passed through an inert anode, the metal is deposited onto the cathode and thiourea remains in the electrolyte. The amount of thiourea is determined by titrating with KIO3 solution in the presence of starch indicator. 4.13 Thiourea is oxidized by potassium iodate in acid solution and forms [CS(NH2)2]2 2+. Write down the balanced equation for the associated reaction.

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Topic: Conservation of Energy, Thermodynamics, Magnetism Metodi: Energy Conservation Method, Conservation Laws, First Law of Thermodynamics, Thermodynamic Cycle Analysis Competenze: Mathematical Modeling, Physical Reasoning, Experimental Data Analysis Fonte: Testo (PDF) — p.18

Problema 5

Problem 5

24 marks Phosphate and Struvite Part I: Struvite from Phosphate Phosphorus is an essential element in living organisms, critical for bones and biomolecules like glyophosphates, DNA, RNA, ATP, etc. For plants also, phosphorus fertilizers are considered important (among NPK fertilizers) – the most common being the orthophosphate salts. On the other hand, high concentrations of phosphates in rivers and lakes is a serious environmental concern as these cause toxic algal blooms, ‘dead zones’ and health hazards. Important sources of phosphorus in water bodies include sewage discharges and run-offs from agricultural fields. Since phosphorus is mostly present in wastewaters as phosphates, one of the parameters defining the performance of a sewage treatment plant is the phosphate reduction in the wastewater during the treatment. Concentration of phosphorus (as phosphates) in a solution can be estimated by adding MgSO4 and NH3. Mass of the resulting precipitate MgNH4PO4.6H2O i.e. magnesium ammonium phosphate hexahydrate (MAP, also known as struvite) indicates the concentration of phosphorus in the sample. 5.1 If 100 mL of a standard phosphate salt solution yields 10.496 mg of MAP precipitate after addition of excess of MgSO4 and NH3, calculate the phosphorus content (in mg L‒1) of the sample.

5.2 Phosphorus content in a wastewater sample was determined by the MAP method as well as a standard spectroscopic method. The estimated phosphorus content by the MAP method had an error of 2%. The error in a measurement is defined as

Error = |Measured value – Actual Value| / Actual Value Chemical analysis of the precipitate showed that the molar ratio of Mg2+:NH4 +:PO4 3 deviated from 1:1:1. Further the precipitate also had some MgKPO4.6H2O (MKP), due to high K+ concentration in the wastewater which could explain the 2% error. What is the molar ratio of MAP:MKP in the precipitate? Indian National Chemistry Olympiad 2019

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23 5.3 On heating, a pure sample of MAP undergoes change in mass due to gradual loss of volatile components. The mass percentage of solid remaining at each temperature is given as follows:

The figures in the brackets indicate the approximate % mass loss at 205 K and 315 K, respectively. The solid at 205 K is a single compound A, whereas at 315 K is a mixture of A and another compound B in approximately equal mass ratio. Identify A and B, and their molar masses. Show steps needed to arrive at answer.

Part II: Precipitation Conditions for Struvite Sewage often contains NH4

• and Mg2+ ions along with phosphates in appreciable concentrations. Thus, struvite precipitation is common in sewage, causing blockage in pipes. Precipitation of struvite from a solution, however, highly depends on the pH, as several ionic equilibria affect its dissolution equilibrium. To understand the precipitation conditions for struvite, we shall first try to understand some of the equilibria involved. Consider an element that can exist in form of multiple species $X1,\dots$, Xn in a system. The relative percentage of the specie Xn is given as

For example, phosphate can exist as PO4 3‒, HPO4 2‒, H2PO4 ‒, and H3PO4 in solutions depending on the pH.

100 X X X % n n n

n (51.08%) (52.72%) Indian National Chemistry Olympiad 2019

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24 5.4 Given that H3PO4 has pKa1 = 2.30, pKa2 = 7.20, and pKa3 = 12.32, estimate the relative ratios [PO4 3‒] : [H2PO4 ‒] at pH = 7, and [PO4 3‒] : [HPO4 2‒] at pH = 11.

Another relevant pH-dependent process is the equilibrium between ammonium ions and ammonia

NH4 +(aq) ⇌ NH3(aq) + H+(aq) pKa = 9.24 5 .5 Assuming no ammonia escapes from the solution at any time, plot a graph of % NH4

• as a function of pH, by determining the required coordinates corresponding to the following three points a) pH = pKa (point Z1) b) pH = 11.0 (point Z2) c) pH where %NH4 +~ 99% (point Z3) Show calculations for the coordinates of the three points. Also, label the three points on the curve. Indian National Chemistry Olympiad 2019

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5.6 Depending on the pH of the solution, Mg2+ ions may hydrolyze to water soluble MgOH+ species or precipitate as Mg(OH)2.

Mg(OH)2 (s) ⇌ Mg2+(aq) + 2OH–(aq) Ksp = 5.61 $\times$ 10 12 Mg2+(aq) + OH–(aq) ⇌ MgOH+(aq)

K1 = 3.98 $\times$ 102 Consider a solution of 0.002 M MgCl2 prepared in a buffer of pH = 11. Determine %Mg2+ (relative to all soluble Mg species) in this solution. Show the steps involved.

Precipitation of struvite at any given pH will take place only if the ionic product (I.P., product of concentrations of the three ions) exceeds its solubility product (Ksp of struvite = 5.0 $\times 10-12$). I. P. = [Mg2+][NH4 +][PO4 3–] 5.7 Suppose a sewage water sample contains 2 mM total dissolved Mg2+, 30 mM total dissolved NH3 and 3 mM total soluble phosphate at pH = 7. (i) Determine the I. P. values at pH = 7 and pH = 11. Assume no other ionic equibria exists involving the three ions, other than those mentioned in this problem. Assume the temperature to be same for all calculations.

0 10 20 30 40 50 60 70 80 90 100 6 7 8 9 10 11 12 13 % Ammonium Ion pH Indian National Chemistry Olympiad 2019

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26

(ii) In the graph below, plot log (I.P.) values for pH = 7 and pH = 11 based on your calculations in 5.7 (i). Two points corresponding to pH = 8.5 and 9.5 are given here. Plot the dependence of log (I.P.) vs pH based on the four points and thus, deduce the pH range in which struvite will precipitate under equilibrium conditions. Assume the temperature to be same for all calculations.

pH Range: -14 -13 -12 -11 -10 -9 -8 -7 6 7 8 9 10 11 12 log I.P. pH Indian National Chemistry Olympiad 2019

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27 5.8 The dissolution of struvite has a peculiarity that it is endothermic at lower temperature and exothermic above a certain temperature. Which of the following graph/s qualitatively indicates this property of struvite (mark X in the correct box/es)

Controlled precipitation of struvite in sewage treatment plants is being used in some countries to recover valuable phosphorus for use as a slow-release fertilizer. In this approach, the sewage water (usually at pH ~ 7.0) is retained in a reactor and subject to conditions which promote struvite precipitation. The struvite precipitation in reaction reduces the phosphorus content in the effluent as well as the precipitation and clogging in pipes. 5.9 Given below are four interventions possible in the sewage tank. State the effect of each of these on the struvite precipitation (Write the statement number i-iv, as applicable for each option) (i) no effect

(ii) increase in precipitation

(iii) decrease in precipitation

(iv) need more information. Condition

Effect a) Addition of mineral acid

b) Addition of water c) Passing carbon dioxide d) Partial Removal of struvite precipitate

Solubility d) pKsp pKsp a) b) c)

T

1/T Ksp

T

1/T 1 H hydrogen 1.008 [1.0078, 1.0082] 1 18 3 Li lithium 6.94 [6.938, 6.997] 4 Be beryllium 9.0122 11 Na sodium 22.990 12 Mg magnesium 24.305 [24.304, 24.307] 19 K potassium 39.098 20 Ca calcium 40.078(4) 37 Rb rubidium 85.468 38 Sr strontium 87.62 38 Sr strontium 87.62 55 Cs caesium 132.91 55 Cs caesium 132.91 56 Ba barium 137.33 87 Fr francium 88 Ra radium 5 B boron 10.81 [10.806, 10.821] 13 Al aluminium 26.982 31 Ga gallium 69.723 49 In indium 114.82 81 Tl thallium 204.38 [204.38, 204.39] 6 C carbon 12.011 [12.009, 12.012] 14 Si silicon 28.085 [28.084, 28.086] 32 Ge germanium 72.630(8) 50 Sn tin 118.71 82 Pb lead 207.2 7 N nitrogen

Topic: Thermodynamics Metodi: Ideal Gas Law, Physical Modeling, Approximation & Series Expansion Competenze: Mathematical Modeling, Graph Linearization, Physical Reasoning Fonte: Testo (PDF) — p.24