∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞

19. General national chemistry competitionfor high school students

Wednesday February 19, 2020

time: 8-10 (120 min.)

∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞∞

Háskóli Íslands

Tandur hf

19. Chemistry competitionFebruary 19 2020

Name: _____________________________________________________Kennitala: _____________________________________________________

Phone number: _____________________________________________________Email: _____________________________________________________

School: _____________________________________________________Year of study: 1st year 2nd year 3rd year 4th year

General instructions

1. 1 This booklet contains 21 questions on 15 enumerated pages, as well as a formula sheet and aperiodic table. make sure that you have all the pages.The first 10 questions give 3 points each, the next 8 give 5 points each and finally, the last 3questions give 10 points each.

2. 2 Your results and answers must be written in the exam papers (this booklet). Answers on onscratch papers will not be graded.

3. There will be no negative marking for wrong answers.

4. 4 You are only allowed to use a non-programmable calculators and the next two pages, whichinclude formulas, constants and the periodic table. You may tear the formula sheets from theproject.

5. In the multiple choice questions, there is only one correct answer to each question.

6. Some of the questions are in several sections. If any section is answered incorrectly and theanswer is used in subsequent sections there will not be deducted any points in the later sectionsas long as the calculations are correct.

Formulas and constants

∆G = ∆H − T∆S ∆G◦ = ∆H◦ − T∆S ◦ ∆G = ∆G◦ + RT ln Q

∆x =∑

mynd x −∑

hvar f x p =∑

i pi[H3O+] = Ka

2

(−1 +

√1 + 4C0

Ka

)k = Ae−

EaRT ln

(k1k2

)= −Ea

R

(1

T1− 1

T2

)E = E◦ − RT

nF ln Q

∆G◦ = −RT ln K = −nFE◦ q = C∆T q = mc∆T

pH = − log[H3O+] pKa = − log Ka pH = pKa + log [A−]

[HA]

A = εbc PV = nRT E = hcλ

NA = 6.0223 · 1023mól−1 F = 96485 Cmól e− TK = T◦C + 273.15

1atm = 760torr = 101325Pa Kw = 1.00 · 10−14 1bar = 105Pa = 0.9869atm

h = 6.626 · 10−34J · s c = 3 · 108m/s R = 8.3144 JK·mól = 0.08206 L·atm

K·mól

A = A0 · e−kt 1J = 1kg ·m2 · s−2 1 calorie = 4.184J

ax2 + bx + c = 0⇒ x = −b±√

b2−4ac2a

3

lithi

umLi

6,94

1

11

sodi

um

Na

22,9

9

19

pota

ssiu

mK 39,1

0

37

rubi

dium

Rb

85,4

7

55

caes

ium

Cs

132,

9

87

fran

cium

Fr (223

)

4

bery

llium

Be

9,01

2

12

mag

nesi

umM

g24

,31

20

calc

ium

Ca

40,0

8

38

stro

ntiu

m

Sr 87,6

2

56

bari

umBa

137,

3

88

radi

um

Ra

226

21

scan

dium

Sc 44,9

6

39

yttr

iumY 88

,91

71

lute

tium

Lu

175,

0

103

law

renc

ium

Lr

(262

)

22

titan

ium

Ti 47,8

8

40

zirc

oniu

m

Zr

91,2

2

72

hafn

ium

Hf

178,

5

104

ruth

erfo

rdiu

m

Rf

(261

)

23

vana

diumV 50

,94

41

nubi

dium

Nb

92,9

1

73

tant

alum

Ta 180,

9

105

dubn

ium

Db

(262

)

24

chro

miu

m

Cr

52,0

0

42

mol

ybde

num

Mo

95,9

4

74

tung

sten

W 183,

8

106

seab

orgi

umSg (2

66)

25

man

gane

seM

n54

,94

43

tech

netiu

m

Tc

(98) 75

rhen

ium

Re

186,

2

107

bohr

ium

Bh

(264

)

26 ironFe 55,8

5

44

ruth

eniu

m

Ru

101,

1

76

osm

ium

Os

190,

2

108

hass

ium

Hs

(269

)

27 coba

lt

Co

58,9

3

45

rhod

ium

Rh

102,

9

77

irid

ium

Ir 192,

2

109

mei

tner

ium

Mt

(268

)

28 nick

el

Ni

58,6

9

46

palla

dium

Pd 106,

4

78

plat

inum

Pt 195,

1

110

darm

stad

tium

Ds

(281

)

29 copp

erC

u63

,55

47 silv

er

Ag

107,

9

79 gold

Au

197,

0

111

roen

tgen

ium

Rg

(272

)

30 zincZn

65,3

9

48

cadm

ium

Cd

112,

4

80

mer

cury

Hg

200,

6

112

cope

rnic

ium

Cn

(285

)

13

alum

inum

Al

26,9

8

31

galli

umG

a69

,72

49

indi

umIn 114,

8

81

thal

lium

Tl

204,

4

113

unun

triu

m

Uut

50 tinSn 118,

7

82 leadPb 207,

2

114

flero

vium

Fl (289

)

83

bism

uth

Bi

209,

0

115

unun

pent

ium

Uup

57

lant

hanu

m

La

138,

9

58 ceri

umCe

140,

1

59

pras

eody

miu

mPr 14

0,9

60

neod

ymiu

mN

d14

4,2

61

prom

ethi

umPm (1

45)

62

sam

ariu

m

Sm 150,

4

63

euro

pium

Eu

152,

0

64

gado

liniu

mG

d15

7,3

65

terb

ium

Tb

158,

9

66

dysp

rosi

umD

y16

2,5

67

holm

ium

Ho

164,

9

68

erbi

umEr

167,

3

69

thul

ium

Tm

168,

9

70

ytte

rbiu

mY

b17

3,0

89

actin

ium

Ac

(227

)

90

thor

ium

Th

232,

0

91

prot

actin

ium

Pa 231,

0

92

uran

ium

U 238,

0

93

nept

uniu

mN

p(2

37)

94

plut

oniu

mPu (2

44)

95

amer

iciu

m

Am

(243

)

96

curi

um

Cm

(247

)

97

berk

eliu

m

Bk

(247

)

98

calif

orni

um

Cf

(251

)

99

eins

tein

ium

Es

(252

)

100

ferm

ium

Fm (257

)

101

men

dele

vium

Md

(258

)

102

nobe

lium

No

(259

)

5

boro

nB10

,81

14 silic

onSi 28,0

9

32

germ

aniu

mG

e72

,61

33

arse

nic

As

74,9

2

51

antim

ony

Sb 121,

8

52

tellu

rium

Te 127,

6

84

polo

nium

Po (209

)

85

asta

tine

At

(210

)

116

liver

mor

ium

Lv

(293

)

117

unun

sept

ium

Uus

1

hydr

ogen

H 1,00

8

6

carb

onC 12,0

1

7

nitr

ogen

N 14,0

1

15

phos

phor

usP

30,9

7

8

oxyg

enO 16,0

0

16 sulf

urS32

,07

34

sele

nium

Se 78,9

6

9

fluor

ine

F19

,00

17

chlo

rine

Cl

35,4

5

35

brom

ine

Br

79,9

0

53 iodi

neI12

6,9

2

heliu

m

He

4,00

3

10 neonNe

20,1

8

18 argo

nA

r39

,95

36

kryp

ton

Kr

83,8

0

54 xeno

nX

e13

1,3

86 rado

n

Rn

(222

)

118

unun

octiu

m

Uuo

Part I - 3 point questions

Question 1

Which one of the following elements is an alkaline earth metal?

Zink (Zn)

Silver (Ag)

Magnesium (Mg)

Lithium (Li)

Nickel (Ni)

Question 2

Valgerður the scientist must pour 20.0 mL of strong acid into her solution. The acid is stored in a largeglass bottle. How should Valgerður measure the volume she needs?

Pour from the bottle into a beaker and read from the scale.

Pipette from the bottle into her solution.

Pour from the bottle into a beaker and from the beaker into a graduated cylinder.

Pour from the bottle straight into the solution.

Pour from the bottle into a volumetric flask and fill up to the line with deionised water.

Question 3

Aluminum in the Earth is mostly on the form of alumina. Another name for alumina is aluminumoxide. What is the chemical formula for aluminum oxide?

Al2O

Al3O2

AlO3

Al2O3

Al2O

1

Question 4

Guttormur the bull is probably the most loved Icelandic bull of all time. Icelanders loved it so muchthat some argued that it was worth its weight in gold. When Guttormur was at his heaviest he weighed942 kg. How many moles of gold (Au) was Guttormur worth?

4780 moles

1.86 · 108 moles

1.86 · 105 moles

4.78 moles

2.09 · 10−4 moles

Question 5

Mercury (Hg) is the only metal that is in liquid phase at room temperature. When this metal reactswith chlorine it produced HgCl2 which is, on the other hand, a solid at room temperature.What is the number of protons (p), neutrons (n) and electrons (e) in the mercury ion 200Hg2+ in theHgCl2?

80 p, 120 n, 80 e

80 p, 120 n, 82 e

78 p, 200 n, 78 e

80 p, 120 n, 78 e

82 p, 200 n, 78 e

Question 6

Gunnar wants to prepare a buffer solution with pH around 4.7. What two substances could he mixtogether to achieve such a solution?

HCl and NaOH

Acetic acid (CH3COOH) and HCl

NH3 and NaOH

NaCl and NaOH

Acetic acid (CH3COOH) and NaOH

2

Question 7

Electromagnetic rays that have energies between 6.63·10−17 J and 6.63·10−15 J are classified as X-rays.What is the lowest and highest possible wavelenght of an X-ray?

0.03 and 3.00 nm

1.00 · 1017 and 1.00 · 1019 nm

0.03 and 3.00 m

450 and 700 nm

3.00 and 300 nm

Question 8

Fire extinguishers contain carbon dioxide (CO2) under high pressure which causes it to be on a liquidstate, but when it is sprayed out of the extinguisher it turns into gas. 5 L fire extinguisher contains2.00 kg af CO2. What will the volume of the gas be at 25 ◦C and 1.00 atm once all the substance inthe extinguisher has been sprayed out?

93.3 L

9.50 · 103 L

1.12 · 105 L

1.11 · 103 L

473 L

Question 9

The following reaction is exothermic. Which change will shift the equilibrium to the right?

2SO2(g) + O2(g) 2SO3(g)

Raising the temperature

Adding SO2

Removing O2

All of the above

None of the above

3

Question 10

Iceland spar is a mineral called "silver rock" in Icelandic. Despite the name, Iceland spar does notcontain any silver. It is pure crystallized calcium carbonate (CaCO3). In order to be classified as acrystal, the crystal must have at least 1 ·1014 CaCO3 units. What is the lowest possible mass of Icelandspar so that it can be classified as a crystal?

11.31 ng

16.62 ng

11.31 g

6.809 · 1015 g

1.001 · 1016 g

4

Part II - 5 point questions

Question 11

A buffer solution is 0.100 M CH3COONa and 0.100 M CH3COOH. 12.36 mL of 0.134 M HCl wasadded to 25.00 mL of the solution. What is the pH after the addition? pKa(CH3COOH) = 4.76.

4.07

4.29

4.76

5.07

5.23

Question 12

Kinetic measurements were done on the transformation of cyclopropane to propene. The values of therate constant were collected at different temperatures. Two measurements were used to determine theactivation energy of the reaction. At 750 K the rate constant was k = 0.00018 s−1 and at 850 K therate constant was k = 0.030 s−1. What is the activation energy for the reaction?

6.7 · 10−3 J/mól

17 kJ/mól

33 kJ/mól

1.2 · 102 kJ/mól

2.7 · 102 kJ/mól

5

Question 13

The mass percent of oxygen (O) in the human body is 65%, but the atomic percent is 24%. What isthe total number of all atoms in a human body which weighs 62 kg?

6.32 · 1030 atoms

1.62 · 1030 atoms

6.32 · 1027 atoms

1.62 · 1027 atoms

1.52 · 1027 atoms

Question 14

Consider the following reaction

Al2S3(s) + 6H2O(l)→ 2Al(OH)3(s) + H2S(g)

What amount of Al2S3 remains after the reaction of 0.133 mol Al2S3 and 0.111 mol H2O?

17.2 g

28.3 g

14.0 g

8.33 g

19.8 g

6

Question 15

When sulfite reacts with permanganate the following oxidation-reduction reaction takes place:

SO 2−3 + MnO −

4 → SO 2−4 + MnO2

Write the balanced equation for the raction when it takes place in a basic solution.

Balanced chemical equation:________________________________________________________

Question 16Aspartame is a sweetener which is 200 times sweeter than sugar. There are five functional groups inan aspartame molecule. Draw circle around these functional groups and write their names.

7

Question 17

The concentration of solutions can be determined by using spectroscopy. Fanney chemistry studentmeasured the absorbance of a CuCl2 solution, in order to determine the concentration of Cu2+ in thesolution. The graph below shows the absorbance (Abs) of the solution as a function of wavelength. Atthe maximum absorbance the extinction coefficient was ε = 12.39 L

mol·cm .

a) Use Beer’s law (A = εbc) to calculate the molarity of Cu2+ in the solution.

CCu2+ = ___________________ M

b) What is the electron configuration of the Cu2+ ion?

8

Question 18

Gunnar the chemist is interested in kinetics and decided to study the following reaction:

2HgCl2(aq) + C2O 2−4 (aq)→ 2Cl−(aq) + 2CO2(g) + Hg2Cl2(s)

He measured the initial rate of the reaction with different initial concentrations of HgCl2 and C2O 2 –4

and gathered the following results:

Measurement [HgCl2] [M] [C2O 2−4 ] [M] Initial rate [mol L−1min−1]

1 0.105 0.15 1.8 · 10−5

2 0.052 0.30 7.1 · 10−5

3 0.052 0.15 8.9 · 10−6

The rate law of the reaction has the form

rate = k[HgCl2]a[C2O 2−4 ]b

Determine the values of a and b in the kinetic equation.

a = ___________________

b = ___________________

9

Part III - 10 point questions

Question 19 Methanol

Methanol (CH3OH) is used as a substitute for gasoline in some vehicles. To design engines that willrun on methanol, we must understand its thermochemistry

Standard enthalpy of formation for a few substances are given in the following table

Substance ∆Hof [kJ/mol]

CH3OH(l) −238.66CH3OH(g) −200.66

CO2(g) −393.51CO2(aq) −413.80H2O(l) −285.83H2O(g) −241.82

a) The methanol in an automobile engine must be in the gas phase before it can react. Calcula-te the heat (in kJ) that must be added to 1.00 kg liquid methanol to increase its temperaturefrom 25.0 ◦C to its boiling point, 65.0 ◦C. The molar heat capacity of liquid methanol is81.6 J K−1 mól−1.

q = ____________________ kJ

b) Calculate the heat that must be added to vaporize 1.00 kg of methanol. The molar enthalpy ofvaporization for methanol is 38 kJ mól−1.

q = ____________________ kJ

10

c) Once it is in the vapor phase, the methanol can react with oxygen in the air according to

CH3OH(g) +32

O2(g)→ CO2(g) + 2H2O(g)

Calculate the heat of cumbustion of methanol (in kJ/mol), that is, calculate ∆Ho for the reaction.

∆Ho = ____________________ kJmol

d) Calculate the heat released when 1.00 kg of gaseous methanol is burned in air at constantpressure.

q = ____________________ kJ

e) Suppose that the methanol is burned inside the cylinder of an automobile. Taking the radius ofthe cylinder to be 4.0 cm and the distance moved by the piston during one stroke to be 12 cm.The work done by the gas is called work (w) and is given by w = −P∆V , where P is the externalpressure and ∆V is the volume change. Calculate the work done (in J) per stroke as the gasexpands against an external pressure of 1.00 atm. (1 L · atm = 101.3 J and the volume of acylinder is πr2h).

w = ____________________ J

11

Question 20 Equilibrium constant

Air pollution contains, among other substances, NO, NO2 and SO2. These substances are released tothe atmosphere when oil is burned. The equilibrium constants for the following reactions are knownat 25 ◦C:

SO2(g) +12

O2(g) SO3(g) Kc1 = 2,2 (1)

NO(g) +12

O2(g) NO2(g) Kc2 = 0,25 (2)

The equilibrium constant, Kc, for the reaction of SO2 with NO2, is unknown.

SO2(g) + NO2(g) SO3(g) + NO(g) Kc =? (3)

a) Use the information given for equations (1) and (2) to determine the value of the equilibriumconstant for reaction (3) at 25 ◦C.

Kc = ____________________

b) Kp and Kc have a known relation of Kp = Kc(RT )∆n, where ∆n is the change in number ofgaseous molecules in the chemical equation. Calculate the equilibrium constant Kp for reaction(3) at 25 ◦C. If you could not solve part a), then use the value Kc = 10.

Kp = ____________________

12

c) SO2 and NO2 was put into an empty reaction vessel at 25 ◦C and allowed to reach equilibrium.At equilibrium the partial pressures of the substances were measured as follows:

Substance Partial pressure, Pi

SO2 0.2 atmNO2 0.15 atmSO3 0.3 atm

Calculate the partial pressure of NO at equilibrium. If you could not solve part b), then use thevalue Kp = 12.

PNO = ___________________ atm

d) Again, SO2 and NO2, was put into an empty reaction vessel at 25 ◦C. The initial pressure ofSO2 was 0.20 atm and the initial pressure of NO2 was 0.10 atm. Calculate the partial pressureof NO when the reaction has reached equilibrium. If you could not solve part b), then use thevalue Kp = 12.

PNO = ___________________ atm

13

Question 21 Organic Chemistry

In organic chemistry the Diels-Alder reaction is an important way to synthesize ring structures. Oneof the starting materials in the reaction is a diene, a chemical that has conjugated double bonds. Thediene reacts with another double bond to form a ring. The mechanism for the reaction is shown below.The curved arrows show the movement of electrons during the reaction.

If the diene or the other alkene have any side groups (R) they stay relative to each other as shownbelow.

As you can see, the alkene can react with the diene in two possible ways, depending on how the alkeneis flipped.

14

a) What two products can be formed when the following molecules react? Draw the products.

Compound A was synthesized with the Diels-Alder reaction.

b) Draw the two compounds that were used to synthesize compound A.

c) What is the name of compound A according to the IUPAC naming system?

15