TOPICAL QUESTIONS

Planning Questions and

Answers

2

AJC JC2 PRELIM 2013 9647/02/H2

1 Planning (P) The enthalpy change of neutralisation of a mixture of acid and base can be determined by

measuring the temperature change when the two reactants are mixed together. A student was asked to investigate the enthalpy change of neutralisation between malonic acid, HO2CCH2CO2H and aqueous sodium hydroxide. She was given two separate solutions, FA 1 and a standard solution of NaOH. FA 1 contains 1.00 mol dm–3 malonic acid To ensure that both acidic protons in the malonic acid were reacted, she mixed 40 cm3 of the 0.20 mol dm–3 solution of NaOH provided with 20 cm3 of diluted FA 1 and recorded the temperature change. The temperature change recorded was approximately 2 °C and the student decided that it was too low and would create significant uncertainty in the results. To overcome the problem, she decided to use the undiluted FA 1 in the next measurement and prepared a more concentrated solution of NaOH, FA 2 from solid anhydrous NaOH. During the preparation, the student found that the process of dissolving NaOH released heat and the resultant FA 2 solution had a significantly higher initial temperature than that of the malonic acid solution. When conducting the experiment, a weighted average of the initial temperatures of the two solutions was found to ensure that the initial temperature of the acid–base mixture is more accurate.

(a) Construct a balanced equation, including the state symbols, for the reaction between

malonic acid and aqueous sodium hydroxide. …………………………………………………………………………………………………………. [1] (b) The enthalpy change of neutralisation between malonic acid and aqueous sodium

hydroxide may be determined by performing an experiment to measure the temperature change and then analysing the results. Write a plan on the following page for such an experiment. In your plan, you should use the same proportions of malonic acid and NaOH solutions as described above. You may assume that you are provided with the following.

x FA 1, having the concentration given above x 12 g solid anhydrous sodium hydroxide x deionised water x the apparatus normally found in a school or college laboratory

Your plan should include the following.

x the intended concentration of FA 2 to be used x details for the preparation of this FA 2 solution from the solid anhydrous sodium

hydroxide provided x a diagram of the assembled apparatus that you would use when measuring the

temperature change x all essential experimental details

[AJC/2013/H2/P2/Q1]

AJC JC2 PRELIM 2013 9647/02/H2 1

H2 Chemistry 9647 2013 AJC2 Prelim P2 Suggested Solutions

1 (a) 2NaOH(aq) + HO2CCH2CO2H(aq) → Na+–O2CCH2CO2–Na+(aq) + 2H2O(l)

correct state symbol given

[1]

(b) Intended [FA 2]

Given that [malonic acid] = 1.00 mol dm-3, and the proportion of solutions are 20 cm3 FA 1 and 40 cm3 aqueous NaOH,

minimum [NaOH] in FA 2 to ensure complete reaction of malonic acid = 1 mol dm-3

n(NaOH) required to prepare 250 cm3 of FA 2 = 1 × 0.25 = 0.250 mol mass of NaOH required = 0.25 × (23.0 + 16.0 + 1.0) = 10.0 g

Therefore, about 11 g of solid anhydrous NaOH (more than 10 g) is used in the preparation of standard solution.

Preparation of FA 2

1. Weigh accurately about 11.0 g of solid anhydrous NaOH in a pre–weighedweighing bottle. Dissolve this solid in a (100 cm3) beaker with 30 cm3 of water.

2. Reweigh the emptied weighing bottle.3. Transfer the solution and washings into a 250 cm3 graduated flask and make up

to the mark dropwise with deionised water.4. Shake the solution to obtain a homogeneous solution and label it FA 2.

Experimental Set-up

Experimental procedure

5. Transfer 20.0 cm3 of FA 1 using a burette into a styrofoam cup. Place athermometer in the solution and record the initial temperature of FA 1.

6. Using a 50 / 100 cm3 measuring cylinder, measure out 40 cm3 of FA 2. Place athermometer in the solution and record the initial temperature of FA 2.

7. Quickly transfer the 40 cm3 of FA 2 into the styrofoam cup containing FA 1. Coverthe cup with a lid and observe the maximum temperature rise on thethermomemter.

8. Record the maximum temperature reached.9. Repeat experiment until temperature rise is consistent to 0.2 oC.

[6]

( )

[AJC/2013/H2/P2/Q1]

2

9647/02/CJC JC2 Preliminary Exam 2013

1 Planning (P) Natural water that contains calcium and magnesium ions is said to be 'hard'. Tap water in England is hard water, which is evident from the limescale found in kettles.

The main source of calcium and magnesium ions in hard water is limestones. However, neither calcium carbonate nor magnesium carbonate is appreciably soluble in water. It is said that the solubility is partly the result of carbon dioxide present in rain water.

You are to design an experiment to investigate whether the presence of carbon dioxide in water, indeed, increases the solubility of magnesium carbonate.

In addition to the standard apparatus available in a school laboratory, you are provided with the following materials, x magnesium carbonate x distilled water x distilled water saturated with carbon dioxide x methyl orange indicator x phenolphthalein indicator x standard hydrochloric acid

(a) Write an expression for the solubility product, Ksp, of magnesium carbonate, stating its units.

……………………………………………………………………………………………....[1]

(b) When a solution is saturated, the undissolved solid is in equilibrium with its aqueous

solution. X(s) + aq X(aq)

Describe briefly how you would prepare a saturated solution of magnesium carbonate at room temperature.

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………………………………………………………………………………………………. [3]

[CJC/2013/H2/P2/Q1]

2

9647/02/CJC JC2 Preliminary Exam 2013

1 Planning (P) Natural water that contains calcium and magnesium ions is said to be 'hard'. Tap water in England is hard water, which is evident from the limescale found in kettles.

The main source of calcium and magnesium ions in hard water is limestones. However, neither calcium carbonate nor magnesium carbonate is appreciably soluble in water. It is said that the solubility is partly the result of carbon dioxide present in rain water.

You are to design an experiment to investigate whether the presence of carbon dioxide in water, indeed, increases the solubility of magnesium carbonate.

In addition to the standard apparatus available in a school laboratory, you are provided with the following materials, x magnesium carbonate x distilled water x distilled water saturated with carbon dioxide x methyl orange indicator x phenolphthalein indicator x standard hydrochloric acid

(a) Write an expression for the solubility product, Ksp, of magnesium carbonate, stating its units.

……………………………………………………………………………………………....[1]

(b) When a solution is saturated, the undissolved solid is in equilibrium with its aqueous

solution. X(s) + aq X(aq)

Describe briefly how you would prepare a saturated solution of magnesium carbonate at room temperature.

…………………………………………………………………………………………………..

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………………………………………………………………………………………………. [3]

x Ksp = [Mg2+] [CO32–] mol2 dm–6

x Dissolve solid in water until some solid remains undissolved.

x Allow solution to stand for few hours (to establish equilibrium).

x Filter to remove undissolved solid. The saturated solution is

collected as the filtrate.

[CJC/2013/H2/P2/Q1]

2

© Jurong Junior College 9647/02/PRELIM/2013

1 Planning (P) For

Examiner’s Use

Solubility of a substance is the maximum mass of the substance dissolved in a given volume of a particular solvent to give a saturated solution at a specified temperature.

The units of solubility is grams per hundred grams of water (i.e. g /100 g water).

When solid potassium nitrate is dissolved in water, the temperature of solution decreases.

KNO3(s) K+(aq) + NO3–(aq)

To investigate how the solubility of potassium nitrate varies with temperature, a student demonstrated the experiment, at 30 °C, as follows.

The solid was added into 50 cm3 of water in a small beaker until a saturated solution at 30 °C is obtained. The mixture was filtered and the filtrate was collected in a crucible.

The filtrate in the crucible was heated to evaporate off the water and the mass of potassium nitrate dissolved in 50 cm3 of water was determined.

(a) Predict, in the form of a sketch graph, how the solubility of potassium nitrate changes if the solution temperature is increased. Explain your answer.

Explanation:

……………………………………………………………………………………...……….

……………………………………………………………………………………...……….

……………………………………………………………………………………...……….

……………………………………………………………………………………...………. [2]

(b) State and explain one limitation for heating the solid to dryness.

…………………………………………………………………………………...………….

……………………………………………………………………………………...………. [1]

[JJC/2013/H2/P2/Q1]

3

© Jurong Junior College 9647/02/PRELIM/2013 [Turn over

1 (c) Consider the brief description of the experiment given on Page 2.

The prediction in (a) can be determined by performing a number of experiments.

Write a plan for such a series of experiments. In your plan, you should use the same volume of water to dissolve the solid as described on Page 2.

Your plan should also ensure that the temperature of each solution is between 30 °C and 70 °C.

You may assume that you are provided with the following:

x a bottle of pure anhydrous solid potassium nitrate,

x thermometer covering the temperature range of 0 °C to 100 °C,

x thermostat to maintain the temperature of water–bath,

x crucible,

x deionised water and

x apparatus normally found in a school or college laboratory.

Your plan should include the following:

x the choice of number of experiments to conduct to ensure a wide range of results suitable for analysis;

x the choice of temperature for each experiment;

x all essential experimental details including the apparatus used and measurements recorded.

For Examiner’s

Use

Experimental procedure:

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[JJC/2013/H2/P2/Q1]

© Jurong Junior College Page 1 of 5 2013 Prelim Paper 2 Answers

Suggested Mark Scheme for 2013 JJC Prelim Exam Paper 2 (9647/02) 1. (a)

KNO3(s) K+(aq) + NO3–(aq) 'H > 0 ---(*)

When the temperature is increased, the equilibrium position of (*) shifts right to favour endothermic reaction so as to absorb some heat. Hence, the solubility of KNO3 increases with increasing temperature.

(b) The method does not apply to solid that decomposes on heating as it will result in greater

mass loss. (c) 1. Using a 50 cm3 burette/measuring cylinder, add 50 cm3 of water into a small beaker.

2. Place the small beaker containing water into water–bath with thermostat set at 30 °C. 3. Using a spatula, add KNO3(s) into the water. Stir to dissolve all solid. 4. Repeat step 3 until some solid remains undissolved. 5. Stir the mixture until temperature of solution reaches 30 °C.

Let the mixture stand in the water–bath at 30 °C for some time. 6. Using an electronic weighing balance, measure and record the mass of an empty, dry

crucible. 7. Using a dry filter funnel and filter paper, filter the mixture and collect the filtrate in the

crucible. 8. Using a Bunsen Burner, heat the filtrate to dryness. 9. Using an electronic weighing balance, measure and record the mass of crucible with solid

residue. 10. Repeat step 1 to 9 at 40°C, 50°C, 60°C and 70°C.

(d) � �3

3mass of KNO dissolved in 50 cm of water g �y x

� � � �3solubility of KNO dissolved in 100 g of water g /100g�

ª º u �¬ ¼100 250

y xy x

(e) Use oven/heat resistant gloves or tongs to handle the hot beaker/crucible.

OR Cool hot crucible before handling. 2. (a) (i) It is to quench the reaction by removing H2SO4/H+ in the reaction mixture via acid–

carbonate reaction. (ii) I2(aq) + 2S2O3

2–(aq) o 2I–(aq) + S4O62–(aq)

(iii) Since H2SO4 is a catalyst, it will be regenerated such that [H2SO4] will remain constant

throughout the reaction. Hence, it is not necessary to use H2SO4 in large excess in order to make [H2SO4] constant.

solubility / (g/100g)

temperature / °C (or K)

[JJC/2013/H2/P2/Q1]

PRELIM PAPERS

[Turn over

JURONG JUNIOR COLLEGE 2013 JC 2 PRELIMINARY EXAMINATION Higher 2

CHEMISTRY 9647/01Paper 1 Multiple Choice 19 September 2013

1 hour Candidates answer on separate paper. Additional Materials: Multiple Choice Answer Sheet Data Booklet

READ THESE INSTRUCTIONS FIRST Write in soft pencil. Do not use staples, paper clips, highlighters, glue or correction fluid. Write your name, class and exam index number on the Answer Sheet in the spaces provided unless this has been done for you. There are forty questions on this paper. Answer all questions. For each question there are four possible answers A, B, C or D. Choose the one you consider correct and record your choice in soft pencil on the separate Answer Sheet. Read the instructions on the Answer Sheet very carefully. Each correct answer will score on mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet.

This document consists of 14 printed pages and 2 blank pages.

3

© Jurong Junior College 9647/01/PRELIM/2013 [Turn over

4. An experiment was conducted to determine the efficiency of the heating of a can of water using a spirit burner.

The following data were recorded: Mass of ethanol burnt = m g Change in temperature of water = ∆T oC You are also given that: Relative molecular mass of ethanol = 46.0 Enthalpy change of combustion of ethanol = –1371 kJ mol−1

Specific heat capacity of water = c J g−1 K−1

Which expression below gives the efficiency of this heating process?

A

1371 1000 100%300 T 46.0

u uu

u u ' um

c B

T 46.0 100%300 1371 1000

u u ' uu

u um c

C

300 T 46.0 100%1371

u u ' uu

ucm

D 300 T 46.0 100%

1371 1000u u ' u

uu uc

m

5. The radius and charge of six ions are given in the table below:

ion J+ L+ M2+ X– Y– Z2–

radius / nm 0.14 0.18 0.15 0.14 0.18 0.15

Given that the ionic solids, JX, LY and MZ are of the same lattice type, what is the order of their lattice energies (placing the most exothermic first)?

A MZ > JX > LY B MZ > LY > JX

C JX > MZ > LY D LY > MZ > JX 6. Consider the reaction:

2SO2(g) + O2(g) → 2SO3(g) ∆H� = ‒198 kJ mol‒1; ∆S� = ‒188 J K‒1 mol‒1 What conclusion can be drawn about this reaction?

A The products are more disordered than the reactants.

B The reaction is spontaneous under standard conditions.

C As the reaction proceeds, the temperature will decrease.

D As temperature increases, the reaction becomes more spontaneous.

thermometer

can containing 300 g of water

burner containing ethanol

14

© Jurong Junior College 9647/02/PRELIM/2013

6 Diacids can ionise successively in water according to the following equations.

The pKa values of suberic acid and terephthalic acid are shown in the table below.

Compound pKa1 pKa2

suberic acid

4.5 5.5

terephthalic acid

3.5 4.8

For Examiner’s

Use

(a) (i) Explain why the pKa2 value for both acids are higher than their pKa1 value.

………………………………………………………………………………….......

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(ii) Suggest why the pKa1 value of terephthalic acid is lower than the pKa1 value of suberic acid.

………………………………………………………………………………….......

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………………………………………………………………………………….......

…………………………………………………………………………………....... [2]

This document consists of 14 printed pages.

HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2

CANDIDATE NAME CT GROUP 12S

CENTRE NUMBER

INDEX NUMBER

CHEMISTRY Paper 1 Multiple Choice

Additional Materials: Optical Mark Sheet (OMS) Data Booklet

9647/01 25 September 2013

1 hour

READ THESE INSTRUCTIONS FIRST

Write in soft pencil. Complete the information on the optical mark sheet (OMS) as shown below.

There are forty questions on this paper. Answer all questions. For each question, there are four possible answers A, B, C and D. Choose the one you consider correct and record your choice in soft pencil on the OMS.

Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet.

Write your name

Write and shade your

NRICor FIN number

Write your CT group

6

© Hwa Chong Institution 2013 9647 / 01 / C2 Prelim 2013

12 The diagram below shows the presence of a water droplet on a piece of iron, together with an

unknown metal C which is incorporated into the iron piece to prevent it from corrosion.

Which of the following descriptions of the rusting of iron is correct?

A The cathodic site is located at point D in the diagram. B When metal C is zinc, iron behaves as the anode. C When metal C is copper, corrosion of iron will not occur. D The key reaction involving oxygen during rusting is O2 + 2H2O + 4e– o�4OH−.

13 Which of the following chlorides gives a solution of the highest pH when added to water?

A NaCl C CH3NH3Cl

B MgCl2 D SiCl4

14 Magnesium nitrate and barium nitrate were heated strongly in separate experiments.

Which of the following information will distinguish the two compounds?

A Colour of the residual solid B Solubility of the residual solid in water C Colour of the gas evolved D Whether the gas evolved rekindles a glowing splint

15 Which of the following trends concerning halogens and their compounds is correct?

A The boiling point of the hydrogen halides increases in the order

HF < HCl < HBr < HI. B The thermal stability of the hydrogen halides increases in the order

HF < HCl < HBr < HI. C The reducing power of the halide ions increases in the order F− < Cl− < Br− < I−. D The reactivity of the halogens with hydrogen increases in the order

F2 < Cl2 < Br2 < I2.

14

© Hwa Chong Institution 2013 9647 / 02 / C2 Prelim 2013

5 The structure of white phosphorus is given below.

(a) (i) Using the structure above, explain why white phosphorus is highly reactive and unstable.

………………………………………………………………………………………….........

………………………………………………………………………………………….........

………………………………………………………………………………………….........

(ii) Write an equation to illustrate the reaction of white phosphorus with excess oxygen

to give phosphorus oxide. Include state symbols in your equation.

…………………………………………………………………………………………......... [2] (b) Phosphoric(V) acid may be produced by the “thermal” process. The phosphoric(V) acid

produced is essentially free from impurities and is used in food products. The acid is made by burning a spray of molten phosphorus in a mixture of air and steam in a stainless steel combustion chamber.

(i) Write an equation, including state symbols, to represent the conversion of

phosphorus to phosphoric acid using the ‘thermal’ process.

…………………………………………………………………………………………......... (ii) Highly diluted phosphoric(V) acid is used in soft drinks to impart a sour taste.

A 500 cm3 bottle of Coca Cola usually contains a 0.05% solution by mass of phosphoric(V) acid. Assuming that the density of the drink is 1 g cm–3 and taking phosphoric(V) acid to behave as a monoprotic acid, calculate the pH of the drink. [Ka1 (H3PO4) = 7.5 u10–3 mol dm–3; Mr (H3PO4) = 98.0]

[4]

(b) (i)

x The activation energy of a catalysed (Eacat) reaction is lower than that of an uncatalysed

(Eauncat) reaction. x The catalyst provides an alternate pathway with lower activation energy. x This increases the proportion of molecules with KE greater than or equal to Ea and

increases frequency of effective collision and hence increases rate of reaction. [3]

(ii) x C�C consists of 1 V�bond and 2 S bonds

x The V�bond is formed from a head on overlap of the sp hybridised orbitals x The S bonds are formed from a side on overlap of the p orbitals. If a diagram is drawn, diagram must be labeled to show the orbitals involved in each overlap.

[2]

(c) (i) The boron atom is electron deficient and is able to accept an electron pair from the C�C

bond. [1] (ii)

[2]

Kinetic Energy

Number of particles

EA cat. EA

-1-

RAFFLES INSTITUTION

2015 YEAR 6 PRELIMINARY EXAMINATION HIGHER 2

CHEMISTRY 9647/01 Paper 1 Multiple Choice 28 September 2015

1 hour Additional Materials: Multiple Choice Answer Sheet Data Booklet READ THESE INSTRUCTIONS FIRST Do not open this question booklet until you are told to do so. Write in soft pencil. Do not use staples, paper clips, highlighters, glue or correction fluid. Write your name, class and index number in the spaces provided on the Answer Sheet. There are forty questions on this paper. Answer all questions. For each question there are four possible answers A, B, C and D. Choose the one you consider correct and record your choice in soft pencil on the separate Answer Sheet. Read the instructions on the Answer Sheet very carefully. Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet.

This document consists of 16 printed pages.

-4-

8 Use of the Data Booklet is relevant to this question. In a calorimetric experiment, m g of propan-1-ol is burnt. y % of the energy released is absorbed by 200 g of water, which rises in temperature by 50.5 oC.

Given that the enthalpy change of combustion of propan-1-ol is –2020 kJ mol–1, which of the following expressions gives the value of y?

A 200 x 4.18 x 50.5 x 60.0 x 100

2020 x m

B 200 x 4.18 x 50.5 x 60.0 x 100

2020 x 1000 x m

C m x 2020 x 1000

200 x 4.18 x (50.5 + 273) x 60.0 x 100

D m x 4.18 x 50.5 x 100

200 x 2020 x 1000 x 60.0

9 When 36.0 g of water boils at 100 oC, 81.4 kJ of heat is absorbed.

Assuming that ΔHvap and ΔSvap of water do not change with temperature, what is ΔGvap (in kJ mol–1) of water at 50 oC?

A –2.18 B 0 C +5.46 D +10.91 10 A student carried out an experiment to investigate the decomposition of aqueous H2O2 and it was

determined to be a first-order reaction. 75% of the H2O2 in a solution of concentration 0.10 mol dm−

3 decomposes in 5.0 minutes. The student performed another similar experiment using 0.050 mol dm−

3 aqueous H2O2 solution and with a suitable catalyst added. With all other conditions kept the same, 50% of the H2O2 in this solution decomposes in x minutes. Which of the following is most likely the value of x?

A 2.0 B 2.5 C 4.5 D 5.0 11 If a quantity of 2H2 were added to an equilibrium mixture represented by the equation

1H2(g) + I2(g) ⇌ 2 1HI(g),

how many different types of hydrogen-containing molecules would be present when equilibrium was re-established?

A 3 B 4 C 5 D 6

-13-

Section B

For each of the questions in this section, 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. 31 Use of the Data Booklet is relevant to this question.

In 2011 two new elements were added to the Periodic Table. Both elements had been made artificially and were called ununquadium (Uuq) and ununhexium (Uuh).

Uuq Uuh

proton number 114 116

nucleon number 289 292

Which statements about these elements are correct?

1 One Uuh atom has one more neutron than one Uuq atom.

2 One Uuh+ ion has the same number of electrons as one Uuq− ion.

3 One Uuh atom has the same electronic configuration as one Uuq2– ion.

32 Molecular dimerisation can be described as the process in which two identical molecules combine

to give a single product. Examples of dimers are: (CH3CO2H)2, N2O4 and Al2Cl6 Which of the following descriptions about the above dimers are correct?

1 Hydrogen bonds hold the CH3CO2H molecules together in the dimer.

2 All the nitrogen-oxygen bonds in N2O4 are of equal length.

3 Al2Cl6 is a planar molecule.

[Turn over

10 !

© Raffles Institution 9647/02/S/15 !

(b) Emissions from the combustion of petroleum have devastating results in the urban atmosphere. Nitrogen oxides, emitted by car engines, are components of photochemical smog.

At 298 K, N2 and O2 in air do not react at a significant rate. However, a car engine

produces about 2.5 g of NO per kilometre because the reaction shown below occurs much more quickly at the high temperatures that exist in the engine.

N2(g) + O2(g) 2NO(g)

(i) Give a reason why the reaction takes place slowly at 298 K.

……………………………………………………………………………..…..

……………………………………………………………………………..….. [1]

(ii) At 298 K, the value of Kp for the above reaction is 5.0 x 10–31 while at 1500 K,

its value is 1.0 x 10–5. Use this information to explain whether the formation of NO is endothermic or exothermic.

……………………………………………………………………………..…..

……………………………………………………………………………..….. [1]

(iii) The reaction is only thermodynamically feasible at high temperatures. By

considering the chemical equation and your answer to b(ii), explain clearly what information can be deduced about the ΔS! of the reaction.

……………………………………………………………………………..…..

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……………………………………………………………………………..….. [2]

When exposed to the open atmosphere, NO reacts rapidly with O3 to form NO2.

(iv) Draw a dot-and-cross diagram for NO2.

[1]

For Examiner’s

Use

7

© Raffles Institution 2015 9647/03/S/15 [Turn Over

3 Platinum is an inert transition metal which has numerous applications in catalysis and electrochemical cells.

(a) (i) The reaction of hydrogen with oxygen is extremely exothermic, but takes place only in the presence of a spark or a heterogeneous catalyst, such as platinum.

2H2(g) + O2(g) $$$$$→ 2H2O(g) ΔH < 0

With reference to the stages in a heterogeneous catalysis, explain why a

transition metal, such as Pt, is able to act as a heterogenous catalyst, and how its use speeds up the rate of the above reaction. [3]

(ii) The Döbereiner lighter, first demonstrated in 1823, was popular before the

invention of matchsticks. The design incorporates a piece of platinum gauze as shown in the diagram below.

When the stopper is removed, sulfuric acid in the outer chamber flows into the inner chamber. A sequence of events occurs resulting in a flame at the outlet. With the aid of information from (a)(i), describe the sequence of events which leads to the formation of the flame. Your answer should include relevant chemical equation(s). [3]

(b) Use of the Data Booklet is relevant for this part. A quinhydrone half-cell comprises a platinum electrode immersed in a solution which is prepared by dissolving a mixture of quinone and hydroquinone in acid. The standard reduction potential, E!, of the quinone-hydroquinone system is +0.70 V.

(i) By considering the oxidation states of the appropriate carbon atoms in quinone and hydroquinone, show that the conversion of quinone to hydroquinone is a reduction. [2]

remove stopper

Zn metal

inner chamber

stopper

H2SO4(aq)

Pt gauze outlet

outer chamber

Pt catalyst

Question 1 (a) Finely powdered Ca metal has a large surface area which increases its rate of reaction with

water vapour in the air to produce hydrogen gas.

Ca + H2O → CaO + H2

The heat produced causes hydrogen gas to combust spontaneously with oxygen gas in the air, causing explosions.

2H2 + O2 → 2H2O

This generates more water which spark off further explosive reactions with finely powdered Ca metal.

(b)(i) Q = I x t = ne x F

ne = 1×105×3×60×6096500

=1.12×104mol Ca2+ (l) + 2e−

→ Ca(s) Mass of Ca formed = !.!"×!"

!

! ×40.1 = 2.24×10!! = 0.224!tonnes (b)(ii) From Data Booklet,

2H2O + 2e− → 2H2 + 2OH− E!=−0.83V Ca2+ + 2e−

→ Ca E!=−2.87 V Since E!(H2O/H2) is less negative than E!(Ca2+/Ca), H2O will be preferentially reduced at

the cathode instead of Ca2+. No Ca metal will be obtained. (c)(i) Ca(CH3CO2)2 → CaCO3 + CH3COCH3 solid M (c)(ii) White residue contains CaCO3 and CaO. Method 1

Amount of CaCO3 formed at 450 oC = Amount of Ca(CH3CO2)2 = !.!!"#.! = 0.01265 mol

Let the amount of CO2 evolved = y mol.

Amount/mol CaCO3 ###→ CaO + CO2 Initial 0.01265 - -

Change -y +y +y Final 0.01265 - y y y

Mass of CaCO3 in the residue = (0.01265 – y)(100.1) = (1.266 – 100.1y) g Mass of CaO in the residue = 56.1y g

!"##!!"!!"!!!!"!!"#!!"#$%&"!"##!!"!!"#!!"!!"#!!"#$%&" = !"!"

⇒ !.!""!!"".!!!".!!! = !!

1.266 – 100.1y = 130.9y y = 0.005483 Amount of CO2 evolved = 0.005483 mol Mass of CO2 evolved = (0.005483)(44.0) = 0.241 g

Question 5 (a)(i)

(a)(ii) Compound A is more acidic than C. While both conjugate bases of A and C are resonance stabilized, the negative charge in A is

delocalized/dispersed over more atoms as compared to C. (a)(iii) Step III : LiAlH4 in dry ether Step IV: excess conc. H2SO4, 170°C (a)(iv)

(b)(i)

(b)(ii) Using bond energy data from data booklet,

ΔHatom [A(g)] = BE(C=C) + 3 BE(C–C) + 2 BE(C=O) + 2BE(C–O) + 2 BE(O–H) = 610 + (350x3) + (740x2) + (360x2) + (460x2) = +4780 kJ mol–1

(b)(iii)

Applying Hess’ Law, ΔHsub = −1210 + (40.7) + 8(805) + 2(460) − 2.5(496) – 4780 = +170.7 kJ mol−1

+ 2.5 O2(g) –1210 4 CO2(g) + H2O (l)

ΔHsub

+ 2.5 O2(g)

4C(g) + 2H(g) + 4O(g) + 2.5O2(g)

4780 2.5(496)

4C(g) + 2H(g) + 9O(g)

4 CO2(g) + H2O (g)

40.7

8(805) + 2(460)