RAFFLES INSTITUTION 2011 YEAR 5 PROMOTION EXAMINATION

Higher 2

CHEMISTRY 9647

3 October 2011

2 hours 45 min Additional Materials: Data Booklet OMR Answer Sheet Writing Paper Graph Paper

READ THESE INSTRUCTIONS FIRST

DO NOT open this booklet until you are told to do so.

Write your name and civics tutorial group in the spaces provided on the cover sheet on page 7.

This paper consists of three sections A, B and C.

Section A (15 marks) consists of 15 multiple-choice questions. Answers to this section are to be shaded on the OMR answer sheet provided. Section B (35 marks) consists of 4 structured questions. Answers to this section are to be written in the spaces provided in the question paper. Section C (40 marks) consists of 2 free-response questions. Answers to this section are to be written on the writing papers provided. You are to begin each question on a fresh sheet of paper. Attempt ALL questions. INFORMATION FOR CANDIDATES

The number of marks is given in brackets [ ] at the end of each question or part question.

Writing your answers in pencil will be subjected to penalty.

You are reminded of the need for good English and clear presentation in your answers.

This document consists of 17 printed pages.

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Section A (15 marks) For each question, there are four possible answers, A, B, C, and D. Choose the one you consider to be correct and record your choice on the OMR Answer Sheet provided. 1 Thermal decomposition of ammonium dichromate(VI), (NH4)2Cr2O7, produces a grey-

green solid chromium(III) oxide, Cr2O3, with evolution of gas(es).

What are the gas(es) evolved in this decomposition?

A ammonia only B nitrogen and oxygen C nitrogen and steam

D nitrogen dioxide and oxygen 2 A mixture of a gaseous hydrocarbon, C6Hy, and oxygen in stoichoimetric ratio has a

total volume of 1050 cm3. After combustion, the total volume of carbon dioxide and water vapour produced is 1300 cm3, all volumes being measured at the same temperature and pressure.

What is the molecular formula of the hydrocarbon?

A C6H6 B C6H10 C C6H12 D C6H14 3 Ions of the two most common isotopes of the transition metal nickel are shown below:

58Ni2+ 60Ni2+ Which one of the following statements is correct?

A The electron arrangement of both 58Ni2+ and 60Ni2+ is 1s22s22p63s23p63d64s2. B

60Ni2+ has more protons than 58Ni2+. C In a magnetic field, 60Ni2+ will be deflected more than 58Ni2+. D Both 58Ni2+ and 60Ni2+ have the same number of electrons but different number

of neutrons. 4 Trimethylamine, (CH3)3N, reacts with boron trifluoride, BF3, in a 1:1 ratio to form

compound X. Predict the value of the C-N-C bond angle in compound X.

A 120 B 109 C 107 D 105

5 A mixture of sodium and iodine will only react upon heating to form sodium iodide as shown by the equation below.

2Na(s) + I2(s) 2NaI(s)

The melting points of iodine and sodium iodide are 114 C and 662 C respectively. Which of the following statements is incorrect?

A Sodium iodide is an ionic compound with covalent character. B Dissolving sodium iodide in water gives a colourless solution. C The weak covalent bond accounts for the lower melting point of iodine. D Heating is required to overcome the high activation energy barrier of the

reaction.

3

6 Given the reactions,

CH4(g) C(g) + 4H(g) ∆H = +1656 kJ mol–1

HC≡CH(g) 2C(g) + 2H(g) ∆H = +1648 kJ mol–1 What is the C≡C bond energy in kJ mol–1?

A 410 B 420 C 820 D 840 7 Which one of the following sets of conditions would result in a reaction that is

spontaneous at high temperatures but non-spontaneous at low temperatures?

A ∆H>0, ∆S>0 B ∆H>0, ∆S<0 C ∆H<0, ∆S<0 D ∆H<0, ∆S>0

8 Ethene reacts with bromine in tetrachloromethane to form 1,2dibromoethane as shown by the equation,

CH2=CH2 + Br2 CH2BrCH2Br

To find out the orders of reaction, ethene and bromine were first dissolved separately in tetrachloromethane. Various volumes of these solutions and tetrachloromethane were mixed and the time taken for the colour of bromine to disappear was recorded. The results are shown in the table below:

Expt Volume of CH2=CH2

solution / cm3 Volume of Br2 solution / cm3

Volume of CCl4 / cm3

Time taken for colour of Br2 to disappear / s

1 20 20 0 15

2 12 20 8 25

3 20 10 10 15

4 40 20 20 t4

From the above results, which deduction about the reactions is incorrect?

A It took 15 seconds (t4) for the colour of bromine to disappear in experiment 4.

B The rate of this reaction is directly proportional to the volume of bromine used.

C The rate of this reaction is directly proportional to the time taken for the colour of bromine to disappear.

D The volumes of tetrachloromethane were varied to keep the total volume constant for experiments 1 to 3.

9 Equimolar amounts of two gases X and Y are mixed in a 1.00 dm3 container at room temperature and pressure. Gas Y has a molar mass that is twice that of gas X.

Which one of the following statements is incorrect for the mixture of gases X and Y? A On average, molecules of gas X move faster than molecules of gas Y. B The kinetic energy distributions of the two gases are identical. C Gas X makes less of a contribution to the average density of the mixture than

gas Y. D Gas X has a smaller partial pressure than gas Y.

4

10 A pure sample of N2O4(l) is introduced into an evacuated vessel. The vessel, of

constant volume, is heated to a constant temperature such that the equilibrium below is established.

2NO2(g) 2NO(g) + O2(g)

The value of the pressure p is then found to be 20 % greater than if only NO2(g) were present. What is the mole fraction, x, of oxygen in this equilibrium mixture? A 0.17 B 0.20 C 0.33 D 0.40

11 How many isomers (including both structural isomers and stereoisomers) are possible for C4H8?

A 3 B 4 C 5 D 6 12 The drug cortisone has the formula shown.

O

O

CH3

CH3CO

OH

CH2OH

*

**

In addition to those chiral centres marked by an asterisk (*), how many other chiral centres are present in the cortisone molecule?

A 0 B 1 C 2 D 3

5

For each of the following questions, one or more of the three 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 that you consider to be correct). The responses A to D should be selected on the basis of

A B C D

1, 2 and 3

are correct

1 and 2 only are correct

2 and 3 only are correct

1 only

is correct

No other combination of statements is used as a correct response. 13 The rate of the following reaction decreases with increasing temperature.

2NO(g) + O2(g) 2NO2(g)

The proposed mechanism for the reaction is: (1) 2NO N2O2 fast

(2) N2O2 + O2 2NO2 slow What can be deduced from this information only? 1 The overall order of the reaction is 3. 2 The observed rate constant of the reaction decreases with increasing

temperature. 3 In step (1) of the mechanism, the forward reaction is exothermic.

14 Which of the following statements always apply to a reaction, which has reached

equilibrium, at constant pressure?

1 The concentrations of reactants and products remain constant. 2 The ∆G of the reaction is zero. 3 The ∆S of the reaction is zero.

15 Methylpropane was monochlorinated to form two products, 2-chloromethylpropane

and 1-chloromethylpropane. Which of the following statements are correct? 1 Excess chlorine was used in this reaction.

2 2,2,3,3-Tetramethylbutane was formed in trace amounts. 3 The ratio of 2-chloromethylpropane to 1-chloromethylpropane is 1 : 9.

k1

k–1

k2

6

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RAFFLES INSTITUTION 2011 Year 5 Promotion Examination

H2 Chemistry

COVER SHEET

Name: ( ) CT Group:

For Examiners’ Use Only

Section Marks

A / 15

B

Q1 / 11

Q2 / 9

Q3 / 6

Q4 / 9

C

Q1 / 20

Q2 / 20

Total

/ 90

Percentage

/ 100

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Section B (35 marks)

This section consists of four questions. Answer all questions in the spaces provided in the question paper. B1 Barium oxide reacts with carbon dioxide to form barium carbonate as shown in the

following equation.

Reaction 1: BaO(s) + CO2(g) BaCO3(s), H1

It is difficult to directly measure the enthalpy change of the reaction, H1. Instead,

enthalpy changes of the following reactions are found experimentally and H1 is then

calculated from H2 and H3 using Hess’ Law.

Reaction 2: BaO(s) + 2HNO3(aq) Ba(NO3)2(aq) + H2O(l), H2

Reaction 3: BaCO3(s) + 2HNO3(aq) Ba(NO3)2(aq) + CO2(g) + H2O(l), H3 (a) Explain why dilute nitric acid is used rather than dilute sulfuric acid.

[1]

(b) The experimental procedures to determine H2 and H3 are similar. Write a

plan to determine H2. You are given the following reagents:

3 g of barium oxide, FA1

100 cm3 of 0.800 mol dm3 dilute nitric acid, FA2

and apparatus normally found in a school laboratory.

In your plan, you should: calculate suitable quantities of barium oxide and dilute nitric acid to be

used. include essential details of the experimental procedure.

For Examiner’s

Use

9

[5] (c) In an experiment which uses x g of FA1 and y cm3 of FA2 (in excess), the

temperature change is found to be T. Show how these results could be used

to determine H2.

(Assume the specific heat capacity of all solutions = 4.18 J g–1 K

–1.)

[2]

(d) To determine H3, the same experimental procedure in (b) is carried out for reaction 3. Explain why heat loss is a more significant problem in this experiment compared to reaction 2.

[1]

For Examiner’s

Use

10

(e) With the help of an energy cycle, show how H1 may be calculated from H2

and H3. [2]

[Total: 11]

B2 Phosphine, PH3, is a colourless poisonous toxic and flammable gas. It can react with

oxygen to form the tribasic phosphoric acid, H3PO4, as shown in the equation below. PH3 + 2O2 H3PO4

(a) State the oxidation numbers for phosphorus in phosphine and phosphoric acid.

Oxidation number of P in phosphine : ________ Oxidation number of P in phosphoric acid : _______

[1] (b) Describe the shape of phosphine; explain its shape in terms of the numbers

and types of electron pairs it contains. State the bond angle.

[3]

For Examiner’s

Use

11

(c) By considering the structure and bonding present in phosphine and phosphoric acid, explain why phosphine is a gas while phosphoric acid is a liquid at room temperature.

[3] B2 (d) The solubility of phosphine in water is low but high in most organic solvents.

Suggest possible reasons to account for phosphine’s low solubility in water and yet high solubility in organic solvents.

[2] [Total: 9]

B3 Metal phosphides react with acids to yield phosphine, PH3, and metal salts.

However, in water, aluminium phosphide, AlP, and magnesium phosphide, Mg3P2,

yield phosphine and insoluble compounds.

(a) Draw the dot-and-cross diagram for magnesium phosphide.

[1]

For Examiner’s

Use

12

(b) Predict, with reasoning, if the melting point of magnesium phosphide is higher or lower than that of aluminium phosphide.

[3]

(c) Write a balanced equation for the reaction between aluminium phosphide and

water.

[1]

(d) Suggest a reason to account for the formation of phosphine when metal phosphides react with water.

[1] [Total: 6]

For Examiner’s

Use

13

B4 The compound C5H12 exists as several isomeric forms. One such isomer is pentane, CH3CH2CH2CH2CH3.

(a) Bromine reacts with pentane in the presence of sunlight to give a mixture of

monobrominated products with the molecular formula C5H11Br. Among these is A, which exists as a pair of stereoisomers. (i) Give the IUPAC name of A. State the type of stereoisomerism

exhibited by A and draw a suitable diagram to illustrate this isomerism.

(ii) Describe the mechanism of the formation of A, labelling each step in

the mechanism appropriately.

[7]

For Examiner’s

Use

14

(b) A group of students conducted experiments to determine the standard

enthalpy change of combustion, ∆Hc, of three isomers of C5H12. Their results

are given below.

Isomer ∆Hc / kJ mol−1

CH3CH2CH2CH2CH3(l) 3503 (CH3)2CHCH2CH3(l) 3509

(CH3)4C(l) 3517

The variation in ∆Hc

values is due to the differences in the enthalpy change of vaporisation of the isomers. Account for the differences in the enthalpy change of vaporisation.

[2] [Total: 9]

For Examiner’s

Use

15

Section C (40 marks)

This section consists of two questions. Answer all questions from this section on the writing paper provided. Begin each question on a fresh sheet of paper. C1 Nitryl chloride, NO2Cl, was found by Steinkopf and Kühnel to be an effective agent in

the nitration and chlorination of aromatic hydrocarbons.

At 700 K, nitryl chloride decomposes according to the following equation,

2NO2Cl(g) 2NO2(g) + Cl2(g)

74.15 g of NO2Cl is placed in a 1 m3 sealed vessel at 700 K. Decomposition occurs and the rate of reaction is followed by measuring the partial pressure of Cl2 at various times. In this experiment, the partial pressure of Cl2 increases as follows.

Time, t / s 0 300 900 1500 2000 3000

∞

Partial pressure of Cl2 x 10–2 / atm

0 0.48 1.18 1.68 1.94

2.30 2.62

(a) Explain what is meant by the rate of reaction. [1] (b) Show that the final partial pressure of Cl2 upon completion of the

decomposition at 700 K is approximately 2.62 x 10–2 atm. [2]

(c) (i) Plot a graph to show how the partial pressure of Cl2 changes during

the first 3000 s of the reaction and determine the order of reaction with respect to NO2Cl.

(ii) Write the rate equation for the reaction. (iii) Calculate the rate constant for the reaction, stating its units. [6] (d) The decomposition reaction was found to follow a two-step mechanism as

shown: Step 1: NO2Cl NO2 + Cl Step 2: NO2Cl + Cl NO2 + Cl2 (i) Deduce whether Step 1 or 2 is the rate determining step.

(ii) An experimental study reported that the enthalpy change for the

overall decomposition process is +205 kJ mol–1. For the first step, the activation energy for the forward reaction is 240 kJ mol–1 and that of the reverse reaction is 30 kJ mol–1. For the second step, the activation energy for the reverse reaction is 20 kJ mol–1. Draw a labelled energy profile diagram for the decomposition reaction.

[5]

16

The N–Cl bond in nitryl chloride can undergo homolytic fission to produce NO2 and Cl

radicals. At 295 K, NO2 dimerises to form N2O4 according to the following equation,

2NO2(g) N2O4(g) Hr = –57.2 kJ mol–1

(e) (i) Draw the dot-and-cross diagram for NO2.

(ii) The bond angle in NO2 was found to be 134o. Explain why the bond angle is greater than 120o.

(iii) The bond lengths of both the nitrogen–oxygen bonds in NO2 molecule

were experimentally found to be 1.20 x 10–10 m. With reference to your answer in (e) (i), explain why both the nitrogen–oxygen bonds in NO2 are of the same length.

(iv) Construct an energy cycle to determine the enthalpy change of

formation of NO2(g) given that the Hf [N2O4(g)] = +9.2 kJ mol–1. [6]

C2 Phosgene, COCl2, is a colourless and toxic gas which was used during World War I

as a chemical warfare agent that was responsible for the death of many thousands of people.

(a) Phosgene reacts readily with water to produce carbon dioxide gas and

another gas, W, which is responsible for the toxicity of phosgene during the hydrolysis reaction. By constructing an equation, suggest the identity of the gas, W. [1]

(b) A student attempted to prepare phosgene gas for use in the synthesis of an

organic compound. The student placed 5.05 atm of carbon monoxide gas and 4.63 atm of chlorine gas in a reaction vessel at 423 K.

CO(g) + Cl2(g) COCl2(g)

The equilibrium partial pressure of Cl2 inside the cylinder was found to be

3.73 atm at 423 K. (i) Calculate the value of Kp for the reaction at 423 K, stating its units. (ii) Calculate the apparent molecular mass of the equilibrium mixture.

[5]

(c) The student carried out the synthesis of phosgene under variable conditions and obtained some data as shown in the table below:

Experiment Initial partial pressure of CO / atm

Initial partial pressure of

Cl2 / atm

Temperature / K

Kp / atm1

I 3.00 4.00 300 3.25 x 101

II 3.00 4.00 500 5.50 x 102 III 6.00 4.00 500 Y

(i) From the data given, deduce whether the enthalpy change for the

synthesis of phosgene below is exothermic or endothermic.

CO(g) + Cl2(g) COCl2(g)

17

(ii) Hence, state how the yield of phosgene can be increased by varying the temperature.

(iii) Suggest the value of Y for experiment III. Explain your answer.

[5]

(d) The rate of synthesis of phosgene can be increased by adding a catalyst. With the aid of a sketch of the Boltzmann distribution, explain how the presence of a catalyst increases the rate of the reaction. [3]

(e) At a particular temperature and pressure, phosgene, a non-ideal gas, was observed to have a smaller molar volume than that of an ideal gas. Suggest why this is so. [1]

(f) 1.20 g of an unknown noble gas, X, occupies 1.05 x 104 cm3 at a constant

pressure of 14.5 kPa at 25 oC.

(i) Deduce the identity of the noble gas, X.

(ii) From the ideal gas equation, derive an expression to show the relationship between density and pressure for gas X at a fixed temperature.

(iii) Hence, on the same axes, sketch the graphs of density against

pressure for a fixed mass of gas X at 25 oC and at 100 oC. Assume that gas X behaves ideally.

[5]

E N D O F P A P E R

2