India 2009 — INCHO-2009-paper.pdf

Fonte: gare di altri paesi/India/chimica/INCHO-2009-paper.pdf · Apri PDF · apri PDF p.3 Cluster: Termodinamica Soluzioni (stessa cartella): · · · · · · · · · · · · · ·

Problema 1

Problem 1

10 Marks Hydrogen atom

The electronic ground state of a hydrogen atom contains one electron in the first orbit. If sufficient energy is provided, this electron can be promoted to higher energy levels. The electronic energy of a hydrogen-like species (any atom/ions with nuclear charge Z and one electron) can be given as

1.1 The energy in Joule of an electron in the second orbit of H atom is

(0.5 mark) 1.2 The energy required to promote the ground state electron of H atom to the first excited state is

(1 mark) When an electron returns from a higher energy level to a lower energy level, energy is given out in the form of UV/Visible radiation. 1.3 Calculate the wavelength of light (nm) for the electronic transition of H atom from the first excited state to ground state.

(1 mark) Name of Student

Roll no. number quantum principal n , constant Rydberg R where n Z R n E H 2 2 H

= $-$

Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

4 constant is k where, Z 2 n k r =

In the model of hydrogen-like atom put forward by Niels Bohr (1913), the electron

orbits around the central nucleus. The Bohr radius of nth orbit of a hydrogen-like

species is given by

1.4 What is the principal quantum number, $n^{\prime }$ of the orbit of Be${}^{3+}$ that has the same Bohr radius as that of ground state hydrogen atom?

(1 mark) 1.5 The ratio of energy of an electron in the ground state Be${}^{3+}$ ion to that of ground state

H atom is

(A) 16

(B) 4

(C) 1

(D) 8

(1 mark)

The kinetic and potential energies of an electron in the H atom are given as

1.6 Calculate the following:

a) the kinetic energy (in eV) of an electron in the ground state of hydrogen atom

b) the potential energy (in eV) of an electron in the ground state of hydrogen

atom

(2.5 marks) r 2 e 0 $4\pi$ 1 P.E. and 2r 2 e 0 $4\pi$ 1 K.E. $ϵ$ $ϵ$ $-$

= $n^{\prime }$ = Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

5

1.7 A gaseous excited hydrogen-like species with nuclear charge Z can emit radiations of

six different photon energies.

a) The principal quantum number of the excited state is

(A) 6

(B) 5

(C) 4

(D) 3

(0.5 mark)

b) It was observed that when this excited species emits photons of energy = 2.64

eV when it comes to next lower energy state. Calculate the nuclear charge of

the species.

(1 mark)

The least energy required to remove an electron from a species is known as the

ionization energy (I.E.) of the species. The experimental I.E. of He atom is

24.58 eV. 1.8 Total energy required to remove two electrons from He is

(1.5 marks) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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N.. C H3 H C2H5 i) N H H5C2 C3H7 CH3 ii) + N.. H iii) N C2H5 H3C N iv)

Topic: Modern-Quantum Physics Metodi: Bohr Model & Quantization, Photon Energy Relation, de Broglie Relation Competenze: Mathematical Modeling, Physical Reasoning Fonte: Testo (PDF) — p.3

Problema 2

Problem 2

19 Marks Nitrogen containing compounds

Several naturally occurring important compounds such as alkaloids, amino acids,

proteins and peptides contain nitrogen. Nitrogen heterocycles in the form of

pyrimidine and purine bases, are essential features of nucleic acids, which are

responsible for storage of genetic information in an organism and for transmitting the

same to its progeny. 2.1 Which of the following amine/s is/are resolvable?

(1 mark) 2.2 The most acidic species amongst the following is

(1 mark)

Name of Student

Roll no.

N H H + i) N H H + ii) N H + iii) N N(CH3)2 H + iv) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

7 N NH2 Br2 , CH3COOH, $20{}^{\circ }\text{C}$ O H2 N CH3 CHO B BuLi ii) C i)

B

C N (C2H5)3N and

CH3C O Cl CH2N2 +

2.3 Draw the structures of the isolable products of the following reactions.

a)

b)

(2 marks) 2.4 Identify B and C in the following reaction sequence.

(2 marks) 2.5 Which of the following statements is true with respect to the following pair of

compounds?

D

E i) D is more basic than E ii) E is more basic than D iii) D and E are of comparable basicity

(1 mark) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

8 CH3I Ag2O (i) (ii) ,

H N CH3 F CH3CH2NH2 CH3I CH3CH2N(CH3)3I +

Ag2O CH2 = CH2 N(CH3)3 3 +

G CH3I Ag2O (i) (ii) ,

Alkaloids are naturally occurring biologically active nitrogeneous compounds,

isolated from plants. Structure determination of alkaloids involves a few general

steps, some of which are mentioned below.

(a) Zeisel method: It is used to determine the presence and number of methoxyl

groups. Here an alkaloid is heated with HI at its boiling point ($126{}^{\circ }\text{C}$) when

methyl iodide is formed, which is absorbed in ethanolic AgNO3 and the AgI

formed is separated and weighed.

(b) Hoffmann exhaustive methylation method: It is used to determine the

skeleton of an alkaloid. It involves the exhaustive methylation of an amine

with CH3I followed by heating the resulting quaternary ammonium salt with

moist Ag2O when elimination takes place as represented below.

Note: During this reaction, the less stable alkene is formed.

2.6 Identify the products ‘G’and ‘H’ obtained in the Hoffmann exhaustive methylation

and degradation of N-methylpiperidine (F) in the following sequence. Note: H

absorbs above 200 nm in UV.

(2 marks)

Papaverine (C20H21NO4) is an optically inactive alkaloid. Goldschmidt and co-

workers established the structure of papaverine in 1888 and their work is a classic

example of the application of oxidative degradation to structure determination. Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

9 OH COOH Conc. H2SO4 CH3I/ NaOH Veratric acid i. ii. fuse with excessNaOH dil. acid, i. ii. dil. acid excess

2.7 Papaverine reacts with four equivalents of HI at $126{}^{\circ }\text{C}$ to give papaveroline as one of the products. The formula of papaveroline is i) C16H13NO4

ii) C16H9NO4

iii) C16H14NO4

iv) C16H15NO4

(1 mark)

Papaverine, when oxidized with hot conc. KMnO4 is decomposed into smaller

fragments viz; veratric acid, C9H10O4 (I), metahemipinic acid (J), pyridine

tricarboxylic acid (K) and 6,7-dimethoxyisoquinoline -1- carboxylic acid. 2.8 Verartic acid (I) is prepared from p-hydroxybenzoic acid by the following sequence.

Identify the missing intermediates.

(3 marks) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

10 N CH3 hot KMnO4 L

i) COOH COOH MeO MeO COOH COOH OMe MeO iii) iv) COOH OMe MeO COOH

K i) COOH MeO MeO COOH

Metahemipinic acid J, (C10H10O6) on heating with CaO, gives veratrole, which is

also formed by heating veratric acid (I) with CaO. J forms an anhydride on heating

and forms only one monoester. 2.9 Structure of metahemipinic acid is

(1 mark) 2.10 The pyridine tricarboxylic acid K is produced when lepidine L (4-methylquinoline)

is oxidized. Draw the structure of K.

(1 mark) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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CH2 N OMe OMe OMe MeO i) CH2 N MeO MeO OMe OMe ii) MeO N CH2 MeO OMe OMe iii) MeO N CH2 OMe OMe OMe iv)

2.11 Based on the above observations, the structure of papaverine is

(2 marks)

Oxidation of papaverine with hot dilute KMnO4 gives papaverinic acid (C16H13NO7)

which forms an oxime. 2.12 Structure of papaverinic acid is

(2 marks) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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i) CO2

ii) SiO2

Topic: Conservation of Energy, Thermodynamics, Geometric Optics Metodi: Physical Modeling Competenze: Diagrammatic Reasoning, Physical Reasoning Fonte: Testo (PDF) — p.6

Problema 3

Problem 3

17 marks

Chemistry of silicon

Silicon is the second most abundant element (~27.2 %) in the earth’s crust after

oxygen (45.5 %). Carbon, silicon, germanium, tin and lead constitute the group 14

of the periodic table. Chemistry of silicon is distinctly different from that of carbon.

For example, under standard conditions CO2 is a gas whereas SiO2 is a covalent solid. 3.1 Draw the structures of CO2 and SiO2.

(1.5 marks) 3.2. The reason for the distinct difference in the properties of CO2 and SiO2 is

a) carbon is more electronegative than O and in case of SiO2

oxygen is more electronegative than silicon

b) carbon has small size and forms a $\pi$ bond with good overlap whereas

silicon has larger size hence has a poor $\pi$ overlap

c) carbon has only ‘p’ orbitals and lacks ‘d’ orbitals whereas

silicon has ‘d’ orbitals

d) first ionization potential of carbon is higher than that of silicon

(1086 kJ ${\text{mol}}^{-1}$ for C and 786 kJ ${\text{mol}}^{-1}$ for Si)

(1 mark)

Name of Student

Roll no. Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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3.3 Glass, made of SiO2, Na2SiO3 and CaSiO3, is attacked by hydrofluoric acid with

formation of SiF6${}^{2-}$ anion. The analogous CF6${}^{2-}$ anion does not exist. The reason/s

is/are a) carbon is more electronegative than silicon b) silicon has larger atomic size than carbon c) silicon has 3d orbitals which form an sp3d2 hybrid orbitals d) carbon and fluorine have comparable atomic sizes

(2 marks) 3.4 Group 14 elements have tendency to catenate. The first three members C, Si and Ge

show significant catenation. Arrange these elements in decreasing order of their

catenation tendency.

(0.5 mark) 3.5 The order in 3.4 can be explained on the basis of

a) atomic size

b) electronegativity

c) bond strength

d) non-metallic character

(1 mark) 3.6 In a silicon manufacturing unit, silicon is obtained by heating 100 kg of pure sand

with 45 kg of high grade coke in an electric furnace. Write the balanced equation of

the reaction.

(1 mark) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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3.7 Silicon can be purified by converting it into volatile SiCl4 (b.p. 58 ${}^{\circ }C$) which is

purified by fractional distillation. SiCl4 can then be converted into Si using

molecular hydrogen. Write the balanced equations for the reactions involved.

(1 mark)

Silicon is widely used in semiconductor industry in which the purity required is of the

order of 1 part per billion. This can be achieved using “zone refining technique”. The

technique involves moving a heater coil across an impure Si rod. 3.8 In zone refining, silicon is purified as a) portions of Si rod are heated and cooled so that impurities are

differentially evaporated b) impurities are more soluble in liquid phase than in solid c) impurities are less soluble in liquid phase than in solid d) impurities are insoluble in molten Si and can be separated

(1 mark)

The reactivities of CCl4 and SiCl4 are different. For example SiCl4 can be easily hydrolysed and is prone to substitution reactions, whereas CCl4 is inert. 3.9 The observed difference is because a) carbon atom has smaller size hence substitution is not possible b) carbon is more electronegative than silicon c) silicon has low lying unoccupied orbitals d) C-Cl bond is stronger than Si-Cl bond

(1 mark) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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Silicon has high affinity for oxygen to form silicates having SiO4 units. Silicates can

have chain or cyclic structures. 3.10 Draw the structure of a cyclic silicate having structural formula of $[{\text{Si}}_6{\text{O}}_{18}{]}^{n-}$. Also

determine the value of n. [Hint: SiO4 can be shown as unit]

(4 marks) 3.11 Draw the structure of the anion present in pyroxene (MgCaSi2O6).

(1 mark)

Silicones are important synthetic polymers which find extensive applications due to their chemical inertness and water repelling nature. They are produced via the following reactions.

Si A C6H5Cl Cu + 2 370 K A O H2 HCl B Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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3.12 Identify A and the corresponding linear polymer B.

(2 marks) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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HO H3C H H H H3C A

Topic: Thermodynamics, Gravitation Metodi: Physical Modeling Competenze: Diagrammatic Reasoning, Physical Reasoning Fonte: Testo (PDF) — p.12

Problema 4

Problem 4

17 Marks Natural compounds and intermediates

Cholesterol (A) is the most abundant compound from steroids in animals. It is the

precursor of all other steroids. It is an important component of the cell membrane.

[Note: Thick solid wedge indicates a bond coming above the plane, while broken

wedge indicates a bond below the plane]

4.1 The total number of stereoisomers possible for A is

a) 32

b) 256

c) 128

d) 64

(0.5 mark) 4.2 Draw the structure of the ester of cholesterol with a fatty acid containing 16 carbon

atoms with a double bond at the C4 in E configuration.

(1 mark)

Name of Student

Roll no. Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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4.3 a) Complete the structure of the product, with stereochemistry, of the reaction of B2H6

with cholesterol in tetrahydrofuran, followed by H2O2 in an alkaline medium.

(1 mark)

b) The 3D perspective formula of the skeleton of the steroid obtained in a is given

below. Complete the structure of the product obtained in a, by placing the

substituents at the appropriate places with the correct stereochemistry.

(2 marks)

c) Complete the structures of the products (with stereochemistry) of the reaction of A

with

i) H3O+ (room temperature)

(0.5 mark) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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Intermediate D OH H B +

C

ii) peroxyacetic acid , CH3COOOH

(0.5 mark)

iii) Br2 / H2O

(1 mark) 4.4 Among the natural products, terpenes are interesting due to a variety of reactions

they undergo. Borneol (B) C10H18O, a terpene, on treatment with a BrØnsted acid

undergoes a reaction to form F, (C10H16). Identify the structures of $C\to F$.

(1 mark)

$-H2O$ Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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F

Intermediate E [Note: Conversion of $D\to E$ involves formation of a more stable species] CH3 O H5C2O O O O H3CO G H +

Intermediate D

(1 mark)

$-$ H+

(1 mark)

Diels-Alder reaction is an important reaction useful in the synthesis of cyclohexene

derivatives. It involves the reaction of an electron rich “diene” with an electron

deficient olefin referred to as “dienophile”.

Two different approaches have been designed to synthesize cortisone, a steroid.

These require intermediates G and H. Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

21 Starting compounds for synthesis of G

Starting compounds for synthesis of H

Cl Br Na I

J $225{}^{\circ }\text{C}$

K

L

M HBr $45{}^{\circ }\text{C}$

4.5 Draw the starting compounds required for the synthesis of G and H through Diels-

Alder reaction.

(1.5 marks)

(0.5 mark)

4.6 I is an interesting compound. It reacts with metallic sodium to form compound J, which is stable and on heating to $225{}^{\circ }\text{C}$ forms K (C4H6; UV - ${\lambda }_{\text{max}}$ 215nm). One mole of hydrogen bromide reacts with K at $45{}^{\circ }\text{C}$ to give L (C4H7Br) and M (C4H7Br) in the ratio 80:20. Identify $J\to M$

(3 marks) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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2-Aminobenzoic acid on reaction with NaNO2/HCl gives a compound N. In

mass spectrometry, a compound is heated to a high temperature under vacuum and

irradiated with a high energy electron beam to ionize it to form a cation. The cation

undergoes fragmentation. The masses of the fragments are recorded in the spectra.

Alkaline solution of N is subjected to a flash discharge and a specially adapted mass

spectrometer scans the spectrum of the products at rapid intervals. Mass

spectrometric analysis of N gives peaks at masses 28, 44 and 76. 4.7 Structures of the fragments at masses 28, 44 and 76 and N are

Mass: 28

44

76

Compound N

(2.5 marks) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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Topic: Conservation of Energy, Thermodynamics, Astrophysics Metodi: Physical Modeling Competenze: Diagrammatic Reasoning, Physical Reasoning Fonte: Testo (PDF) — p.17

Problema 5

Problem 5

11 Marks Chemical kinetics 5.1 Consider a first order reaction $A\to B$, with initial concentration of A being [A]${}_0$ and that of B being zero.

Which of the following curves correctly describes the variation of [A] for small

values of time (t)?

(0.5 mark) 5.2 Which of the following curves correctly describes the variation of [B] for small

values of time (t)?

(0.5 mark) 5.3 Some chemical reactions are known to follow zero order kinetics. Consider such a

reaction A B with a rate constant k in which concentration [A] reduces with time, t.

a) Write the differential equation for [A].

(1 mark)

Name of Student

Roll no. [A] t t [A] [A] t

[B] [B] [B] t t t Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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b) Assuming the initial concentration of A = [A]${}_0$, write the expression for [A] in

terms of [A]${}_0$, t and k for a zero order reaction

(0.5 mark)

c) Write the expression relating [A]${}_0$, k and the half life of this reaction.

(1 mark) 5.4 Ammonia decomposes on a tungsten wire at 1000 K. The following results have been

obtained in a constant volume system. Find out the order of reaction and the

corresponding rate constant of the reaction by approximate estimation.

a) Order

b) Rate constant with units

(2 marks) 5.5 Decomposition of SO2Cl2(g) is described by the reaction:

SO2Cl2(g) $\to$ SO2(g) + Cl2(g)

The initial concentration and the initial reaction rates are denoted as [SO2Cl2]${}_0$ and R${}_0$, respectively. Determine by approximate calculation the order of the reaction with respect to the following data collected at 298 K. [SO2Cl2]${}_0$ (mol dm${}^{-3}$) 0.10 0.37 0.76 1.22 R${}_0$ (mol dm${}^{-3}$s${}^{-1}$) $2.24\times 10^{-6}$ $8.29\times 10^{-6}$ $1.71\times 10^{-5}$ $2.75\times 10^{-5}$ Determine the order of this reaction and the corresponding rate constant, k (report only 1 significant digit)

a) Order: b) k (with units)

(1.5 marks) P(torr) 310 321 332 355 377 421 476 t(sec) 0 100 200 400 600 1000 1500 Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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5.6 Radioactive decay of an element follows first order kinetics. After how many years

will 25% of the initial quantity of radium remain, if half life of radium is 1620 years?

(0.5 mark)

The Avogadro number can be calculated experimentally. In a classic experiment, Ernest Rutherford observed that a sample of 192 mg left for 83 days produced 6.83 ${\text{mm}}^3$ of He gas measured at 273 K and 1 atm pressure.

(i) Calculate the moles of He produced in the experiment.

(1 mark)

The number of disintegrations undergone by 1g of Ra in a second is $4.6\times 10^{10}{\text{g}}^{-1}{\text{s}}^{-1}$

ii) The number of disintegrations observed in the Rutherford’s

experiment is

(1 mark)

iii) 226 Ra emits $\alpha$ particles to give 214 Pb. Calculate the number of He atoms

produced in the Rutherford experiment.

(1 mark)

iv) Using the above data, calculate the Avogadro number.

(0.5 mark)

Ra 226 88 Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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Curve a Curve b Curve c Curve d

Topic: Nuclear & Particle Physics Metodi: Radioactive Decay Law, Ideal Gas Law, Experimental Data Analysis Competenze: Mathematical Modeling, Experimental Data Analysis, Graph Linearization Fonte: Testo (PDF) — p.23

Problema 6

Problem 6

15 marks A. Kinetic theory of gases and Gas Laws 6.1. Typical Maxwell speed distribution curves for 1 mol of N2 gas at three different temperatures 100 K, 300 K and 700 K are shown below

i. Match the curves with corresponding temperatures

Curve

Temperature

Curve a

Curve b

Curve c

(1 mark)

ii. Theoretically area under the curves is same

True

False

(0.5 mark)

6.2 Plots of PV/RT vs. P for 1 mol of H2, NH3, CH4 and Ar gases are given below.

Name of Student

Roll no. Curve a Curve b Curve c Molecular speed (m/s) No. of molecules PV/RT P(atm) 200 400 600 800 1000 1.5 1 Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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Match the curves with corresponding gases Curve

Gas

Curve a

Curve b

Curve c Curve d

(1.5 marks) 6.3 The van der Waals equation for one mol of a real gas is $(P+a/V^2)(V-b)=RT$. The correct order for the value of ‘a’ for H2, NH3, and CH4 gases is

i) H2 < NH3 < CH4

ii) H2 > NH3 > CH4 iii) H2 < CH4 < NH3

iv) H2 > CH4 > NH3

(0.5 mark) 6.4 A diver dives to 20 m depth of sea water $\rho =1.03\times 10^3{\text{kg m}}^{-3}$ and rises to the surface quickly without breathing. Change of pressure (in atmosphere) in coming to water surface from the depth is

(1 mark) 6.5 The total work done (Joule) on lungs if the same expansion is carried out at the sea surface at constant temperature is (Assume volume of air in the lungs is 2L under normal conditions).

(1.5 marks) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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6.6 Human body functions best when oxygen gas has a partial pressure of 0.2 atm. Calculate the percentage of oxygen by volume in air carried by the diver at the sea depth of 20 m for best body function.

(1 mark) B. Physical and chemical equilibrium

A phase diagram summarizes the conditions at which a substance exists as a solid,

liquid or gas. Each solid line between two phases specifies the conditions of

temperature and pressure under which the two phases can exist in equilibrium. The

phase diagram of a one component system S is shown below. Based on the diagram,

answer the following questions.

6.7 Which phases of S can coexist under atmospheric conditions?

(1 mark)

6.8 Under what conditions all the three phases of S can co-exist?

(0.5 mark) $\to$ T/${}^{\circ }$K 1.0 5.2 72.8 pc $\to$ P (atm) 304(Tc) 298 217 a c d b e -195 Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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6.9 Mark the correct graph showing the variation of molar Gibbs function ($G_m$) vs. temperature (T) along the line a-e of the phase diagram.

(1 mark) 6.10 Choose the correct option/s from the following statements for triple point of system S. a) All the three phases are in equilibrium b) Molar Gibbs energy for the three phases is the same c) Molar volume of the three phases is identical d) Molar entropy of the three phases is the same

(1 mark)

6.11 As a result of increase in external pressure, the melting point of solid S will

a) increase

b) decrease

c) not change

(1 mark) 6.12 How many phases exist at T > 304 K and P > 72.8 atmosphere?

(0.5 mark) $\uparrow G_m$ $\uparrow G_m$ $\uparrow G_m$ $\to$ T $\to$ T $\uparrow G_m$ $\to$ T $\to$ T Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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6.13 For a chemical reaction $A\to B$, the extent of reaction expressed as $ϵ$ is a measure of progress of the reaction. $ϵ$ = 0 corresponds to pure A and $ϵ$ =1 correspond to pure B.

Draw a graph showing the variation of Gibbs free energy ($G_m$) vs. $ϵ$ for the spontaneous reaction A $\to$ B as well as for B $\to$ A.

Mark the following points on your graph i) P1 corresponding to $\partial G_m/\partial ϵ<0$,

ii) P2 corresponding to $\partial G_m/\partial ϵ=0$ and iii) P3 corresponding to $\partial G_m/\partial ϵ>0$.

(2 marks) 6.14 van’t Hoff’s equation for a chemical reaction under equilibrium is given by

is standard reaction enthalpy at temperature T and K is the equilibrium constant. Predict how K will vary with temperature for an exothermic reaction. a) K decreases as the temperature rises

b) K increases as the temperature rise c) K remains unchanged.

(1 mark)

H RT H dT K d r r $\Delta$ $\Delta$

where ln 2

Gm $\uparrow$ $ϵ\to$ 0 1 Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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Topic: Kinetic Theory, Thermodynamics, Fluid Mechanics Metodi: Kinetic Theory of Gases, Ideal Gas Law, First Law of Thermodynamics, Hydrostatic Equilibrium Competenze: Mathematical Modeling, Physical Reasoning, Graph Linearization Fonte: Testo (PDF) — p.26

Problema 7

Problem 7

11 marks Acid-Base chemistry Part A 7.1 Consider a pH titration of 50 mL 0.1 M benzoic acid vs 0.1M NaOH at $25^{\circ }C$. The

experimentally observed curve is shown below, on which points A, B, $C\dots \dots$ F are

marked.

Benzoic acid vs NaOH

a) Which point corresponds to approximately 0.07 M

benzoic acid solution?

b) At which point is the pH changing very rapidly?

c) Which point represents the smallest concentration of [H+]?

(2 marks) Name of Student

Roll no. 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 Volume of NaOH (mL) pH • B • C • F • • D E • A Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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7.2 The pH at half equivalence point may be employed to estimate the pKa value of a

weak acid. Estimate the pKa value from the graph.

(1 mark) 7.3 Suppose you are carrying out a conventional titration of benzoic acid and NaOH.

Using the answer obtained in 7.2, choose an appropriate indicator from the

table below.

(0.5 mark)

Part B

The percentage of NaHCO3 and Na2CO3 in a sample can be determined measuring

the changes in pH during the titration of the sample with HCl. In a typical experiment,

0.500 g of the sample was dissolved in deionised water to make 50.0 mL solution.

This solution was titrated with 0.115 M HCl. The pH titration curve showed two

breaks at pH = 9.0 and pH = 5.0. Indicator pH range Colour change Methyl yellow 2.9 - 4.0 red-yellow Methyl orange 3.1 – 4.4 red-orange Phenolphthalein 8.0 - 9.8 colorless-red Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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7.4 Write the balanced equation corresponding to these breaks.

(1 mark) 7.5 Calculate the total number of moles of acid used

a) to reach equivalence point at pH = 9.0 if the volume of acid required is 9.6 mL.

b) to reach equivalence point at pH = 5.0 if the volume of acid required is 34.3 mL.

(1 mark) 7.6 Determine the number of grams of Na2CO3 and NaHCO3 and their percentage in the

mixture.

(2.5 marks) Indian National Chemistry Olympiad Theory 2009 HBCSE, 31st January 2009

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7.7 30 mL of 0.1 M Na2CO3 solution was titrated with 0.1 M HCl. (The initial pH of the

Na2CO3 solution is around 11.8). Qualitatively sketch the pH vs $V_{\text{HCl}}$ curve for this

titration.

(1 mark) 7.8 Choose the correct statement/s for the titration curve of pH vs $V_{\text{HCl}}$ for Na2CO3 drawn

by you in 7.7

i) The total volume of HCl required to reach the 2nd end point is twice that of the first one ii) Number of moles of CO3${}^{2-}$ is equal to the number of moles of HCO3${}^{-}$ at some point on this curve iii) Number of moles of HCO3${}^{-}$ is equal to twice the number of moles of CO3${}^{-2}$ at some point on this curve iv) The total volume of HCl required to reach the 2nd end point is half that of the first one

(2 marks)

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 Volume of HCl (mL) pH

Metodi: Experimental Data Analysis, Physical Modeling Competenze: Experimental Data Analysis, Graph Linearization, Physical Reasoning Fonte: Testo (PDF) — p.31