Tjc Chemistry H2 Y1 2009

TEMASEK JUNIOR COLLEGE YEAR 1 JUNE COMMON TEST 2009 CHEMISTRY H2

NAME: ................................................

Time: 2 hr. 30 min.

C.G. ................

Date: 3rd July 2009

Tutor: ............................ This paper consists of 3 Sections A, B and C. Answer all sections. Section A (30 min.) Attempt all the 20 questions on the separate answer sheet provided. Answers to this section have to be handed in 30 minutes after the start of the paper. Section B (1 hour) Answer all the 2 essay questions. Begin the answer to each question on a fresh piece of paper. Section C (1 hour) Answer all the 4 structured questions in the spaces provided on the question paper. FOR EXAMINER'S USE ONLY SECTION A SECTION B

Q1. Q2.

SECTION C

Q1. Q2. Q3. Q4.

TOTAL This Question Paper consists of 21 printed pages including the cover page.

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Section A Part 1: 1

Sarin gas, C4H10FO2P, is an extremely toxic substance that attacks the nervous system of living organisms. In an attack on Allied Forces in World War II, 10 tonnes of Sarin gas was used. How many gaseous atoms are there in the amount of Sarin gas used? (1 tonne = 1000 kg, Relative molecular mass of C4H10FO2P = 140.0) A B C D

2

20 cm3 40 cm3 60 cm3 80 cm3

What is the concentration of the solution that results from mixing 40.0 cm3 of 0.20 mol dm-3 NaOH with 60.0 cm3 of 0.10 mol dm -3 HCl? A B C D

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4.30 x 1025 7.74 x 1026 4.30 x 1028 7.74 x 1029

In an experiment, a sample of 20 cm3 of oxygen, 60 cm3 of hydrogen and 30 cm3 of argon is sparked continuously. What is the maximum theoretical decrease in volume? (All gas volume are recorded at 298K and standard atmospheric pressure) A B C D

3

For each question there are four possible answers, A, B, C and D. Choose the one you consider to be correct.

0.08 mol dm-3 0.15 mol dm-3 0.06 mol dm-3 0.02 mol dm-3

NaCl NaCl NaCl and 0.02 mol dm-3 NaOH NaCl and 0.02 mol dm-3 NaOH

0.50 g of impure ammonium chloride was warmed with an excess of sodium hydroxide solution. The ammonia liberated was absorbed in 22.25 cm3 of 0.20 mol dm-3 sulphuric acid. Calculate the percentage of ammonium chloride in the original sample. NH4Cl + NaOH A B C D

NH3 + NaCl + H2O

95 % 48 % 30 % 15 %

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5

Which of the following graphs of first ionisation energy (IE) of the elements across period 2 and 3 is correct?

A

1st IE / kJ

B 1st IE / kJ

Period 2

C

Proton Number

Period 2

1st IE / kJ

Period 3

Proton Number

D 1st IE / kJ

Period 2

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Period 3

Period 3

Proton Number

Period 2

Period 3

Proton Number

Which of the following has the greatest number of unpaired electrons? A B C D

N Cr Mn Fe

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7

Which of the following correctly describes the forces of attraction present in the figure below?

W

H H

H

O

H

Z

N

H H

C H

H

H

H

H

O

P H

X

H H

Y

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A

W Hydrogen Bond

B

Covalent Bond

C

Hydrogen Bond

D

Van der Waals interaction

Y Hydrogen Bond

Ion dipole interaction Van der Waals interaction

Van der Waals interaction Covalent Bond

Z Ion dipole interaction Van der Waals interaction Hydrogen Bond

Covalent Bond

Hydrogen Bond

Which of the following gives the appropriate shapes of the molecules/ions?

Linear XeF2 XeF2 HOCl HOCl

A B C D 9

X Van der Waals interaction Hydrogen Bond

Bent H2S SO2 SO2 BeCl2

Trigonal Pyramidal NO3PH3 PH3 NO3-

25.0 cm3 of a solution of X2O5 of concentration 0.100 mol dm-3 is reduced by sulphur dioxide to a lower oxidation state. To reoxidise X to its original oxidation number required 50.0 cm3 of 0.0200 mol dm-3 acidified potassium manganate (VII) solution. What is the oxidation number of X after it is being reduced by sulphur dioxide? A B C D

+4 +3 +2 +1

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10

Which of the following is not a disproportionation reaction? A B C D

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12

PH3(g)

In an experiment, 0.10 g of the volatile liquid Z formed 0.025 dm3 of vapour at 100 °C and at atmospheric pressure. By assuming that 1 mol of vapour occupies 22.4 dm3 at standard temperature and pressure, what is the relative molecular mass of Z? A

0.10 × 373 × 22.4 0.025 × 273

B

0.025 × 373 × 22.4 0.10 × 273

C

0.10 × 273 × 22.4 0.025 × 373

D

0.025 × 273 × 22.4 0.10 × 373

Samples of F2 gas and Xe gas are mixed in a container of fixed volume. The initial partial pressure of the F2 gas is 8.0 atm and that of the Xe gas is 1.7 atm. When all of the Xe gas reacted, forming a solid compound, the pressure of the unreacted F2 gas was 4.6 atm. The temperature remained constant. What is the formula of the compound? A B C D

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4P(s) + 3NaOH(aq) + 3H2O(l) → 3NaH2PO2(aq) + NH4+NO3- (s) → N2O (g) + 2H2O (l) 2F2 (g) + 2OH- (aq) → F2O (g) + 2F- (aq) + H2O (l) 2H2O2(aq) → 2H2O(l) + O2(g)

XeF XeF2 XeF3 XeF4

The enthalpy changes for three reactions are given below: Na(s) → Na+(g) + e

∆H = +601 kJ mol-1

H+(aq) + OH-(aq) → H2O(l)

∆H = -58 kJ mol-1

Na+(g) + H+(aq) + e → Na+(aq) + ½H2(g)

∆H = -850 kJ mol-1

What is the enthalpy change for the following reaction? Na(s) + H2O(l) → Na+(aq) + ½H2(g) + OH-(aq) A B C D

-191 kJ mol-1 -382 kJ mol-1 -983 kJ mol-1 -1157 kJ mol-1

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14

The conversion of diamond into graphite is exothermic by 2 kJ mol-1. Diamond does not readily change into graphite. Which reaction pathway diagram correctly represents this conversion?

15

Which of the following statements is not correct? A B C D

Entropy increases when sodium in its elemental state is atomised. Entropy increases when water and ethanol are mixed. Organic combustion reactions are non-spontaneous processes because they have high activation energy. Melting of ice is a spontaneous process at 300 K.

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Part 2: For each of the questions in this section, one or more of the 3 numbered statements 1 to 3 may be correct. Decide whether each of the statements is or is not correct. (You may find it helpful to put a tick against the statements which you consider to be correct.) The responses A to D should be selected on the basis of A 1,2 and 3 are correct

B 1 and 2 only are correct

C 2 and 3 only are correct

D 1 only is correct

No other combination of statements is used as a correct response.

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Crude oil is a mixture of hydrocarbons and is separated using fractional distillation based on the difference in their boiling points.

Which of the following statement is true? 1 2 3

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Higher boiling points indicate stronger intermolecular forces of attraction between molecules First fraction of distillation product are collected as gases because they have lower Mr. Larger hydrocarbons have high boiling points as large amounts of energy is required to over come the extensive hydrogen bonding between molecules.

Which of the following pairs of compounds will have the same bond angle? 1 2 3

AlCl3 and BCl3 SO2 and CO2 H2O and NH3

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18

The diagram represents a section of a catalytic converter on the exhaust system of a car. Harmful gases are converted into carbon dioxide, nitrogen and water vapour.

Which of the following pairs will not react with each other in this catalytic converter? 1 2 3 19

Carbon monoxide and hydrocarbons Carbon monoxide and oxides of nitrogen Carbon monoxide and oxygen

Which of the following reactions represent standard enthalpy changes at 298 K and 1 atm? 1 2 3

C(g) + 6H(g) → C2H6(g) C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) NH3(g) + HCl(g) → NH4Cl(s)

20 x

H

H

H

O

H

C

C

C

S

C

H

H

y

Br

H

Which of the following statements about the above molecule are correct? 1 2 3

The bond angle x is equal to the bond angle y. It contains 13 sigma bonds and 2 pi bonds. There are two sp2 hybridised carbon atoms.

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Section B (Essay) Answer all questions. Begin each answer on a separate sheet of paper. 1

Part (a) of this question is on compounds of magnesium. (a)

The mineral hydromagnesite is a hydrated carbonate of magnesium, with the formula Mgx(CO3)y(OH)z.nH2O and a molar mass of 466.0 g mol–1. When 1.000 g of a pure sample of hydromagnesite is heated to constant mass, solid magnesium oxide was obtained. Steam was given off, together with 0.378 g of carbon dioxide. The magnesium oxide obtained was completely dissolved in 50 cm3 of a 1.0 mol dm–3 solution of hydrochloric acid. The resultant solution was transferred into a volumetric flask and diluted to 250 cm3 with deionized water. A 25.0 cm3 aliquot of this diluted solution required 19.00 cm3 of a 0.15 mol dm–3 solution of sodium hydroxide for complete neutralisation. (i)

Calculate the values of x and y.

(ii)

Hence or otherwise, deduce the values of z and n.

(iii)

Explain why magnesium carbonate is insoluble in water.

(iv)

Both beryllium oxide and magnesium oxide are ionic compounds with partial covalent character. Which oxide has more covalent character? Give a reason for your answer.

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(b)

The first ionisation energies of nine successive elements A to I in two consecutive Periods 2 and 3 of the Periodic Table are shown below:

First

I

(i)

Which of the elements A to I is a noble gas?

(ii)

A and I are in the same group. Explain why the first ionisation energy of I is found to be lower than that of A.

(iii)

Explain the discontinuity in first ionization energies between A and B, and between F and G.

(iv)

The bonds within the chlorides of E and F are the same. State the type of bonds present. Compare the strength of the bonds in the chlorides of E and F. Suggest a reason for your answer.

(v)

The chloride of G reacts with the chloride of A in a molar ratio 1:1 to form an addition product K. Draw the shape of the product K formed, clearly indicating the type of bonds present.

(v)

Explain why the boiling point of chloride of G (178oC; it sublimes) is higher than that of chloride of H (58oC).

[10]

[Total: 20]

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2

(a)

A household bleach contains sodium chlorate(I), NaOCl. A 25.0 cm3 sample of household bleach is diluted to 250 cm3. A 25.0 cm3 portion of the diluted solution is added to an excess of acidified potassium iodide solution. Sodium chlorate(I) is reduced to chloride ions and the liberated iodine formed is titrated against 0.010 mol dm -3 sodium thiosulphate(VI) solution. The volume required for the titration is 18.50 cm3. The equation for the titration is as follows:

I2(aq) + 2S2O32-(aq)

(b)

(c)

→

2I-(aq) + S4O62-(aq)

(i)

Write the equation for the reaction that occur between sodium chlorate(I) and acidified potassium iodide solution.

(ii)

What is the concentration of sodium chlorate(I) in the household bleach ?

[4]

In the Raschiq process, hydrazine, N2H4, a colourless oily liquid is prepared by reacting sodium chlorate(I) with aqueous ammonia. (i)

Draw a dot-cross-diagram to show the electron arrangement of a molecule of N2H4.

(ii)

Which compound has a higher boiling point, ammonia or hydrazine? [3] Explain.

Hydrazine is often used as rocket fuel. It undergoes combustion according to the following equation: N2H4(l)

+

O2(g) →

N2(g)

+

2H2O(l)

A chemist conducted an experiment to determine the standard enthalpy change of combustion of hydrazine. In the experiment, 0.210g of hydrazine was burnt as fuel to heat up a beaker containing 200 cm3 of water. The temperature of water rose by 4oC. You may assume the process has 80 % efficiency. Given: The specific heat capacity and density of water are 4.18 J g-1 K-1 and 1.0 g cm-3 respectively. (i)

Explain what is meant by standard enthalpy change of combustion of hydrazine.

(ii)

Calculate the standard enthalpy change of combustion of hydrazine.

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[4]

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(d)

Given the following data: Standard enthalpy change of formation of steam = - 242 kJ mol-1 Standard enthalpy change of vapourisation of water = + 44 kJ mol-1 and using the value you have calculated in c(ii), draw an appropriate energy cycle to determine the standard enthalpy change of formation of liquid hydrazine. [3]

(e)

Given that gaseous hydrazine and fluorine react according to the following equation: N2H4(g) + 2 F2(g)

(f)

→ N2(g) + 4 HF(g)

θ

∆H = -1206 kJ mol-1

(i)

Use relevant data from the data booklet to calculate the average N-H bond energy.

(ii)

State with reason, the expected sign of the ∆S for this reaction. Hence [4] explain why the above reaction is always spontaneous.

Explain the following observation as fully as you can. Standard enthalpy change of neutralisation of nitric acid, HNO3, and hydrochloric acid, HCl, are both equal to -57.3 kJ mol-1. However, the standard enthalpy change of neutralisation of nitrous acid, HNO2 is less exothermic than -57.3 kJ [2] mol-1. [Total: 20]

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Section C (Structured Questions) Answer all questions in the spaces provided. 1

(a)

(i)

State and explain the conditions for a gas to behave ideally.

(ii) A flask of volume, 2v dm3, containing argon at a pressure of 4p atm, is connected to another flask containing argon at a pressure of p atm and having a volume of v dm3. What is the final pressure in the flasks if the gas tap between the two flasks is opened and the pressure is allowed to equilibrate, assuming that there is no change in temperature and the volume of the connector pipe is negligible?

Argon

Argon

[4]

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1

(b)

The compressibility graphs for hydrogen chloride and hydrogen fluoride are shown in the graph below. Explain why hydrogen fluoride exhibits greater negative deviation at low pressure but smaller positive deviation at high pressure as compared to hydrogen chloride.

PV/nRT HCl HF

1.0

P/atm

[4] [Total: 8]

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2

The principal source of phosphorus is phosphate rock, which contains mainly calcium phosphate. Phosphorus is commercially produced by heating calcium phosphate, graphite and silicon dioxide in an electric furnace. The reaction is represented by the equation shown below. 2 Ca3(PO4)2 + 10 C(graphite) + 6 SiO2 → 6 CaSiO3 + 10 CO + P4 (a)

Arrange C(graphite), CO and P4 in order of increasing boiling point. Explain your answer with reference to their structure and bonding.

[4]

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(b)

The phosphorus produced from the furnace is collected and condensed to a solid. As phosphorus will ignite spontaneously in air, it has to be stored under a liquid. (i)

Discuss the solubility of phosphorus in water and in liquid hexane.

(ii)

Hence, determine which is a more suitable liquid for storing of phosphorus.

[3]

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(c)

Phosphorus can react with chlorine gas to give phosphorus trichloride, PCl3 and phosphorus pentachloride PCl5 as the products. Solid phosphorus pentachloride has electrostatic interactions between PCl4+ and PCl6- ions. (i)

Draw and name the shapes of PCl3 and PCl4+.

(ii)

Using the VSEPR theory, compare the bond angles of PCl3 and PCl4+. Explain your answer.

[3] [Total: 10]

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3

(a)

Sausage contains sodium sulphite which acts as a preservative. The amount of sulphite present in sausage can be determined by boiling the sausage with a strong acid, and measuring the amount of sulphur dioxide gas evolved. 50 g of sausage together with 200 cm3 of water and 20 cm3 of 10 mol dm-3 HCl are boiled in a distillation flask. The sulphur dioxide gas liberated is dissolved in water in a receiving flask. The resultant solution in the receiving flask is found to require 7.00 cm3 of 0.0250 mol dm-3 iodine solution in order to completely oxidize the sulphur dioxide to sulphate(VI) ion. As a check on the result of the titration, excess BaCl2 solution is then added and the resulting precipitate collected and weighed.

(i)

Give an ion-electron half equation for the oxidation reaction of sulphur dioxide gas to sulphate (VI) ion and hence write a balanced equation for the reaction taking place between sulphur dioxide and iodine solution.

(ii)

Calculate the mass of sulphur dioxide gas evolved from 50 g of sausage.

(iii) Express the mass of sulphur dioxide in sausage in parts per million (ppm).

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(iv) Identify and calculate the mass of the precipitate formed when BaCl2 solution is added to the solution at the end of iodine titration?

[7]

(b) Sulphur dioxide is also a major air pollutant. One common source is from the flue gas produced during the combustion of fossil fuels which contain sulphur impurities. One method which has been suggested for purification of contaminated flue gases is to pass them through a solution of acidified dichromate(VI) ions. The sulphur dioxide is converted to sulphate(VI) ion.

(i)

How many moles of sulphur dioxide will react with one mole of dichromate(VI) ions.

(ii)

Approximately 45 million tonne of coal, having an average sulphur content of 1%, are used in electricity generation each year in Australia. If all the sulphur present is converted to sulphur dioxide, what would be the cost of potassium dichromate needed to purify the flue gases from power stations? The cost of potassium dichromate(VI) is $500 per tonne. [Given: 1 tonne = 1000 kg].

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(iii) What volume of sulphur dioxide, measured at 300C and 1.12 x 105 Pa, could be extracted from flue gases by 1.00 g of potassium dichromate (VI) in acidic solution?

[6] [Total: 13]

4

One of the major uses of chloroethane is to produce the anti-knock agent, tetraethyl lead (TEL). However, due to the growing awareness of air pollution, TEL is being phased out in most of the industrialized countries. Chloroethane can be formed by the free-radical chlorination of ethane.

(a)

(i)

Explain what is meant by a free radical.

(ii)

With the aid of equations, briefly outline the mechanism of the formation of chloroethane, labelling each step in the mechanism appropriately.

[4]

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A sample of ethane was reacted with excess chlorine in the presence of ultraviolet light. The reaction mixture was analysed using gas chromatography-mass spectrometry (GCMS). Compounds of the following relative molecular masses were found to be present in the mixture. (b)

Identify the compounds by writing the structural formulae of the compounds in the table below. Relative Molecular Mass

Structural Formula of Compound

58.0 133.5 (c)

[2]

Explain why free-radical chlorination of ethane is not a good method for preparing chloroethane.

[1]

Chloroethane can react with chlorine in the presence of ultraviolet light to form 1,1dichloroethane and 1,2-dichloroethane as shown in the equations below. CH3CH2Cl + Cl2 → CH3CHCl2 + HCl CH3CH2Cl + Cl2 → CH2ClCH2Cl + HCl (d)

Suggest, with a reason, the appropriate ratio in which 1,1-dichloroethane and 1,2dichloroethane are formed.

[2] [Total: 9]

END OF PAPER

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