Name of the Centre : Homi Bhabha Centre For Science Education
Name Of The Exam : INCho Indian National Chemistry Olympiad – 2018
Name Of The Subject : Chemistry
Document type : Sample Questions/Past Papers
Year : 2018
Website : http://olympiads.hbcse.tifr.res.in/how-to-prepare/past-papers/
HBCSE INCho Sample Question Paper
Question Paper of Indian National Chemistry Olympiad – 2018 Sample Questions and model Solutions is now available in the official website of Homi Bhabha Centre For Science Education.
Related : Homi Bhabha Centre For Science Education INBO Questions Paper 2018 : www.pdfquestion.in/29882.html
Instructions for students
** Write your Name and Roll No. at the top of the first pages of all problems.
** This examination paper consists of 30 pages of problems including answer boxes.
** Kindly check that the booklet has all the pages. If not, report to the invigilator immediately.
** Adequate space has been provided in the answersheet for you to write/calculate your answers. In case you need extra space to write, you may request for additional blank sheets from the invigilator. Remember to write your roll number on the extra sheets and get them attached to your answersheet.
** Use only a pen to write the answers in the answer boxes. Answers written in pencil will be penalized.
** All answers must be written in the appropriate boxes. Anything written elsewhere will not be considered for assessment.
** You must show the main steps in the calculations.
** For objective type question, mark X in the correct box. Some of the objective questions may have more than one correct answer.
** A copy of the Periodic Table of the Elements is provided at the back of this page.
** Do not leave the examination room until you are directed to do so.
Download Question Paper :
INCho QP : https://www.pdfquestion.in/uploads/INChO2018.pdf
Solutions : https://www.pdfquestion.in/uploads/INChO2018Sol.pdf
Model Questions
Problem 1 20 Marks :
Lead Acid Batteries :
Part A: Electrochemical processes in a lead acid cell
Conventional lead acid batteries used in cars and invertors are the most common rechargeable batteries in the market. A rechargeable lead acid cell consists of a positive electrode of Pb(s)/PbO2(s), and a negative electrode of Pb(s), both immersed in aqueous H2SO4 electrolyte.
Standard electrode potentials for some half-cell reactions at 298 K are given below.
1.1 Write the half-cell reactions that take place at cathode and at anode in above cell during discharge at 298 K. Write the overall discharge reaction and calculate the E° of this cell.
1.2 Calculate the standard enthalpy change and standard Gibbs energy change for the overall discharge reaction of the cell in 1.1.
In a galvanic cell, the chemical energy released during the discharge reaction drives the electrical work. The Gibbs energy change of the discharge reaction represents the maximum electrical work wmax(el) that can be extracted from the cell during discharge.
wmax(el) = ??G°rxn
Maximum work is obtained when the cell is operated reversibly, i.e., at a very low current.
Assume that the cell is designed to exchange heat with the surrounding to prevent overheating or overcooling.
1.3 For reversible operation of the cell at 298 K and all species in the cell in their standard states,
a) determine the amount of heat absorbed by the cell from the surrounding per mole of reaction as per the cell equation in 1.1.
b) what fraction of wmax(el) is obtained from the heat exchanged reversibly with the surroundings?
The concentration c of a species in a solution is related to its activity a as a = ? × c, where ? is the activity coefficient. For highly concentrated solutions, the reaction quotient Q must be expressed in terms of activities of the species (i.e., ? ? 1).
In commercial lead-acid batteries, highly concentrated H2SO4 (aq.) is used. Conventionally, the liquid and solid components in a chemical reaction are assumed to be in their ‘standard states’ with activity as 1. However, due to the high concentration of sulfuric acid in the electrolyte, the activity of water also cannot be taken as 1.
1.4 For the discharge reaction of cell in 1.1, write the expression for the cell EMF in terms of activities of the species involved in the reaction. Consider H2SO4 (aq.) to be completely dissociated and assume that only solids are in their standard states
The lead-acid cell is considered to be discharged when the current that can be drawn from it decreases below a certain value. Discharge is also accompanied by a decrease in H2SO4 concentration.
Activities of water and products of activities of relevant sulphuric acid species at 298 K in a lead acid cell are listed below at two concentrations of sulphuric acid.
1.5 Assuming the discharged and charged states correspond to 1.00 molal and 5.00 molal H2SO4, respectively, find the drop in the cell EMF values when it is discharged at 298 K.
Following are certain facts about the cell in Fig. 1.
(i) PbSO4(s) predominantly deposits on the electrodes and not in other parts of the cell.
(ii) PbSO4(s) deposits on both the electrodes.
(iii) Resistivity of PbSO4(s) deposits (3 × 109 O cm) is much higher than that of PbO2(s) (2 × 10?4 O cm).
(iv) Resistivity of PbO2(s) (2 × 10?4 O cm) is close to that of Pb(s) (0.2 × 10?4 O cm).
1.6 Which of the above facts makes the cell rechargeable? (Write Statement number(s) i – iv, as applicable)
Electrolysis of water may become a competing process during charging of the cell in Fig. 1. The relevant half-cell reactions are given below.
1.7 For the operation of the cell in Fig.1 at 298 K, indicate whether the given statement is correct or incorrect. (Mark X in the appropriate box)
a. During charging of this cell, positive terminal of the DC power source is connected to the electrode that gets reduced, and negative terminal to the electrode that gets oxidised.
b. The EMF of the cell depends upon the amounts of the Pb species in the cell.
c. After a long-time of the lead-acid cell overcharging, the evolution of oxygen on one terminal and that of hydrogen on the other terminal may result in the formation of an explosive mixture of gases.
d. Upon completion of the charging process, oxygen starts to form on the negative electrodes.
e. The total energy stored in the cell is limited by the moles of Pb(s) in the negative electrode or the moles of Pb4+ in the positive electrode, whichever amount is less.
A different type of lead-based galvanic cell is given below. This cell has methane-sulphonic acid in which Pb2+ has high solubility. The electrolyte consists of Pb2+ ions dissolved in aqueous CH3SO3H in a cell having two inert electrodes. When external power supply is connected to this cell, Pb2+ ions are reduced at one electrode and oxidized at the other electrode, with the products depositing on the respective electrodes.
1.8 Write the half-cell reactions that take place at cathode and at anode in the cell in Fig. 2 during discharge at 298 K. Write the overall discharge reaction and calculate the E° of this cell.
1.9 For the cell in Fig. 2, indicate whether the given statements are true or false
(Mark X in the appropriate box). True False
a. Assuming that the volume of the electrolyte in this cell is kept constant, the pH of the electrolyte decreases during discharge of the cell.
b. The EMF of the cell increases with increasing Pb2+ ion concentration in the cell electrolyte.
c. The energy stored in the cell depends on the total amount of the Pb species in the cell.
Part B: Re-using spent batteries from cars and inverters
Recently a team of scientists showed that electrolyte for the lead acid cell in Figure 2 can be obtained by recycling the electrodes of a car battery. For this, the positive and negative plates of the spent battery (Fig.1) are thoroughly washed, separately dried and crushed into powders. Addition of excess aqueous methanesulfonic acid to these powders yields the electrolyte for the new lead acid cell (Fig.2).
1.10 Write balanced equations for the reactions involved in this conversion.
Some small-scale recyclers recover metallic Pb(s) from the electrodes of spent car batteries and throw away the PbSO4(s) slurry from the cell. It has been shown that a compound X useful for fabricating Pb-based thin film solar cells can be obtained from the slurry without high energy inputs. For this synthesis, the PbSO4(s) slurry is stirred overnight with a near-saturated solution of salt A. This yields a white precipitate B, which is collected by filtration.
1.11 Upon heating in a test tube, A decomposes into three colourless gases: an acidic gas, a basic gas and a neutral gas. Inserting a burning stick inside the test tube extinguished it, while a rod dipped in HCl brought close to the mouth of test tube generates white fumes. Identify A and hence B. Next, a weak monoprotic acid C is used to dissolve B. 150 mg sample of C is completely neutralised by 25 mL of 0.1 M NaOH solution.
1.12 Calculate the molar mass of C. Identify C.
1.13 When excess of a halide salt that is used for treatment of hyper-thyroidism is added to the solution above, the yellow salt X precipitates out. Identify X.
Problem 2 : 25 marks
When Rain meets the Soil
Part I: The fragrance of soil
When rain falls on soil after a dry period, a sweet earthy smell is produced known as the fragrance of first rain, which can be detected by camels even at a distance of 50 km. This scent has been captured in an increasingly famous scent “mitti ka itir” produced in Kannauj, India. This smell is due to the compounds geosmin (A) and methyl-isoborneol (B) produced by the actinobacteria present in soils. On combustion, 0.455 g of ‘A’ yields 1.318 g of CO2.
2.1 Calculate the percentage of carbon in ‘A’. Show the calculations involved.
‘A’ undergoes dehydration easily under acidic condition to form an odourless hydrocarbon Argosmin ‘C’. 0.455g of A reacts with sodium to liberate hydrogen gas which occupies 28.01 cm3 at STP.
2.2 Calculate the empirical formula of A. Show the calculations involved.
2.3 Draw the structures of the intermediate compounds and hence of E in the boxes provided.
2.4 Draw the structures of D and C.
2.5 A has three chiral centres. Draw all possible structural isomers of A consistent with the above information. (no stereo isomers are required)
2.6 Draw the structure of F and hence A. The compound „B? can be synthesized by the following route.
2.7 Draw structures for G and H in the boxes provided.
Part II: If the rain is acidic,…
As the water in clouds and rain drops equilibrates with atmosphere, it dissolves gases from the atmosphere. Usually, rain is slightly acidic because of dissolved CO2 leading to a pH sometime as low as 5.6. The acidity of rain water in industrial areas is higher with pH as low as 3.5 to even 2 in some regions.
Natural water bodies often have carbonates of Na+, K+, Ca2+, and Mg2+, dissolved from carbonate rocks, whose buffering action neutralizes acid in the rain. This buffering mechanism is important for sustaining aquatic life.
The carbonate buffer system is composed of various species and the equilibria between these species are governed by the following reactions.
2.8 Based on the graph, answer the following questions:
a. For what pH range(s), we cannot obtain buffers based on carbonate species?
b. What volume of 0.03 M H2SO4 would change the pH of 1 L 0.004 M Na2CO3 by 1 unit.
c. Buffer capacity of a Na2CO3 solution would
i) Increase with the concentration of Na2CO3(aq)
ii) Remain unaffected by the concentration of Na2CO3(aq)
iii) Decrease with the concentration of Na2CO3(aq)
Next, let us do some simple calculations to understand the buffering action of a CaCO3 solution. A saturated solution of CaCO3 (solubility = 1.1 × 10-4 M) was prepared in distilled water. Assume that the distilled water was free of any dissolved gases and any other cations.
2.9 Calculate the pH of the saturated CaCO3 solution. (Hint: amount of H2CO3 (aq) formed under these conditions would be negligible.)
2.10 Determine what volume of 0.03 M H2SO4 would be required to lower the pH of 300 mL of saturated CaCO3 solution by 1unit. Assume that the volume change of the CaCO3 solution due to acid addition is small.