HWA CHONG INSTITUTION

College 1 Promotional Examination

Higher 2

CANDIDATE NAME

CT GROUP 12S

CHEMISTRYPaper 2 Structured Questions

Additional Materials: Data Booklet

9647/02September 2012

1 hour 20 minutes

INSTRUCTIONS TO CANDIDATES

1) Write your name, CT group, centre number and index number clearly in the

spaces at the top of this page.

2) Answer all questions in the spaces provided in this Question Booklet.

INFORMATION FOR CANDIDATES

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

A Data Booklet is provided.

Do not use staples, paper clips, highlighters, glue or correction fluid.

You may use a calculator.

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

For Examiner’s Use

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This document consists of 15 printed pages, including this cover page.

Hwa Chong Institution C1 H2 Chemistry 20121 (a) On heating gaseous pentachloride, the following equilibrium is set up:

PCl5(g) PCl3(g) + Cl2(g)

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Hwa Chong Institution C1 H2 Chemistry 2012

(i) Use the Data Booklet to calculate the relative molecular masses (Mr) of PCl5, PCl3 and Cl2.

Mr of PCl5 ………………………………

Mr of PCl3 ………………………………

Mr of Cl2 ………………………………

When 15.0 g of phosphorus pentachloride was put into a sealed evacuated vessel of capacity 1 L, and heated to 473 K, the pressure increased to 3.10 x 105 Pa.

(ii) Use the above data and the ideal gas equation to calculate the average Mr of the gaseous mixture.

(iv) Go and Kc are related by the following equation

Go = –2.303 RT lg Kc

where Go is the standard free energy change in J mol–1, R is the gas constant and T is the temperature at which the equilibrium is established.

Calculate the standard free energy change of the above equilibrium.

(v) With the help of an energy cycle, calculate the enthalpy change of the above equilibrium using the following enthalpy changes of formation.

[Hf (PCl5) = –443 kJ mol–1; Hf (PCl3) = –319 kJ mol–1]

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Hwa Chong Institution C1 H2 Chemistry 2012

(vi) Hence calculate the entropy change of the above equilibrium.

[10]

(b) Titanium is strong, of low density and resistant to chemical attack. Although it is expensive to produce, it is highly in demand in the aerospace industry and manufacture of implants in medicine.

Extraction of titanium from its ore cannot be carried out using carbon as a reducing agent. Given that Ho and So for the reduction of titanium dioxide to titanium using carbon is +720 kJ mol –1 and +365 J mol–1 K–1 respectively, calculate the temperature at which the reaction is feasible.

Hence, suggest a reason why this method of reduction is not carried out in the industry.

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Hwa Chong Institution C1 H2 Chemistry 2012

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

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

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200 400 600 800 1000 1200 1400 1600 1800 2000

–300

–400

–600

–500

G / kJ mol–1

T / K

–374

–345

–353

–326

–593

–475

Hwa Chong Institution C1 H2 Chemistry 2012(c) At present, titanium is more commonly produced by reducing titanium tetrachloride with magnesium.

Titanium tetrachloride is a colourless liquid that boils at 400 K. To optimise this reduction process, it is necessary to know how G of the reaction changes with temperature. Graphs of G against T are known as Ellingham diagrams. The Ellingham diagram below is for the following reactions.

reaction 1: ½Ti + Cl2 ½TiCl4

reaction 2: Mg + Cl2 MgCl2

(i) Explain, with the aid of equations, why the graph for reaction 1 becomes more positive between 300 K and 400 K but becomes almost independent of temperature from 400 K to 2000 K.

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

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

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

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

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

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Industrially, the reduction of titanium tetrachloride by magnesium is carried out at 1100 K in an atmosphere of argon.

(ii) Suggest a reason for the reduction to be carried out in an atmosphere of argon.

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

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Mg + Cl2 MgCl2

½Ti + Cl2 ½TiCl4

Hwa Chong Institution C1 H2 Chemistry 2012……………………………………………………………………………………………………...

(iii) Write a balanced equation for the reaction at this temperature.

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

Hence, by reference to the diagram,

(iv) evaluate G at 1100 K.

(v) state the temperature above which it is not feasible to reduce TiCl4 with Mg.

……………………………………………………………………………………………………...[8]

[Total: 20]

2 (a) Pseudohalogen is a class of compounds that show a chemical reactivity similar to halogens. An example is thiocyanogen, (SCN)2.

(i) Draw the shape of the hybrid orbitals around one of the carbon atoms in thiocyanogen.

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Hwa Chong Institution C1 H2 Chemistry 2012

(ii) Draw a diagram to illustrate the shape of thiocyanogen about the two sulfur atoms.

(iii) Thiocyanogen can be synthesized by reacting sodium thiocyanate, NaSCN, with chlorine gas. Write an equation for the formation of thiocyanogen.

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

(b) Thiocyanogen can react with hydrocarbons such as methylbenzene via free radical substitution.

(i) Write equations that describe the following steps.

Initiation

………………………………………………………………………………………………

Propagation

………………………………………………………………………………………………

………………………………………………………………………………………………

Termination

………………………………………………………………………………………………

(ii) An isomer of the product, shown below, was also isolated. Suggest a reason for the formation of this isomer.

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

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Hwa Chong Institution C1 H2 Chemistry 2012…………………………………………………………………………………………………….

[5]

(c) In the determination of the mass percentage of ascorbic acid (C6H8O6) present in vitamin C tablets, the following reaction involving iodine takes place.

C6H8O6 + I2 → C6H6O6 + 2I– + 2H+

Due to the low solubility of iodine in water, it is generated in situ using potassium iodate(V), potassium iodide and sulfuric acid. In this reaction, iodine is the only product.

(i) Write a balanced equation for the reaction between IO3– and I– to obtain I2.

A 3.0 g sample of the tablet is crushed and dissolved in 100.0 cm3 of dilute sulfuric acid. To this solution, 1.08 g of potassium iodate(V) and 4.32 g of potassium iodide is added. A 25.0 cm3 aliquot is withdrawn and 21.60 cm3 of 0.1 mol dm–3 thiosulfate is required to titrate with the excess iodine, using starch as an indicator.

(ii) Calculate the total amount of iodine produced from reacting potassium iodate(V) with potassium iodide.

(iii) Determine the total amount of excess iodine in the 100 cm3 solution.

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Hwa Chong Institution C1 H2 Chemistry 2012

(iv) Hence or otherwise, obtain the mass percentage of ascorbic acid in the sample.

[5][Total: 13]

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Hwa Chong Institution C1 H2 Chemistry 2012[RJC 2011, edited]3 The rate of reaction of bromine with bromopropanone was studied in acid solution at

250C. The table gives some results (repeated on page 7) obtained for the reaction.

H+CH3COCH2Br + Br2 -----> CH3COCHBr2 + HBr

initial concentration / mol dm–3 [Br2] at specified times / mol dm–3

Expt bromopropanone hydrochloric acid 0 s 1500 s 3000 s 4500 s1 0.050 0.50 0.050 0.036 0.025 0.0192 0.050 0.50 0.100 0.086 0.076 0.0683 0.020 0.50 0.050 0.044 0.038 0.034

The reaction may be followed by either chemical or physical methods of analysis.

Sampling & TitrationIn each of the above experiments, the reagents were mixed and then three samples were successively withdrawn, at appropriate times, as the reaction proceeded. At 1500 s, ice-cold water was added to the first sample. To this, excess aqueous potassium iodide was added. The resultant solution was then titrated against standard sodium thiosulfate solution. This procedure was repeated with the second and third samples, with ice-cold water being added at 3000 s and 4500 s respectively.

(a) (i) Why was ice-cold water added?

______________________________________________________________________________________________________________________________________.

(ii) Suggest why an indicator was not reacted for the titration.

______________________________________________________________________________________________________________________________________.

(iii) Suggest how the above reaction could be followed by a physical method of analysis.

______________________________________________________________________________________________________________________________________.

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Hwa Chong Institution C1 H2 Chemistry 2012The rate equation for the reaction is

Rate = k[Br2]a[CH3COCH2Br]b[H+]

(b) (i) The orders of reaction with respect to bromine and bromopropanone are to be determined. Use the data below (repeated from page 6) to plot suitable graphs on the grid given below.

[4]

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Hwa Chong Institution C1 H2 Chemistry 2012(ii) Determine the values of a and b, showing clearly how you arrived at your

answer.

order of reaction with respect to Br2, a = …………

order of reaction with respect to CH3COCH2Br, b = …………

[4]

(iii) What are the units of the rate constant for this reaction?

___________________________________________________________________.

[1]

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Hwa Chong Institution C1 H2 Chemistry 2012(c) The bromination of propanone in the absence of acid or base was studied over

a temperature range of 516 K to 618 K. This system is an example of a homogeneous equilibrium.

CH3COCH3(g) + Br2(g) ⇌ CH3COCH2Br(g) + HBr(g)

(i) What do you understand by the term homogeneous equilibrium?

________________________________________________________________________________________________________________________________

(ii) State and explain the effect which an increase in the total pressure of the system will have on:

(1) the rate of bromination;

________________________________________________________________________________________________________________________________________________________________________________________________

(2) the position of equilibrium.

________________________________________________________________________________________________________________________________

[5]

[HCI 2012, adapted]4 The Haber Process is a crucial industrial process that allows us to produce

ammonia. The equation of the reaction process is found below.

N2 (g) + 3H2 (g) ⇌ 2NH3 (g) ∆H<0

A molar ratio of 1:3 of nitrogen to hydrogen gas at 50atm, 618K were added to a reaction vessel and was allowed to reach equilibrium. There was 15.3% NH3 in the reaction vessel eventually.

(a) (i) Write an expression for Kp, indicating clearly the units.

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Hwa Chong Institution C1 H2 Chemistry 2012(ii) Hence calculate the Kp of the Haber Process at 618K.

[4]

(b) Ammonia can easily auto-ionize to form ions in water, according to the equation below:

2NH3 ⇌ NH4+ + NH2

-

Draw the structures of the two ions formed, clearly indicating the shape and bond angle of the ions. In each of these, suggest the hybridization of the N atoms.

[4]

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Hwa Chong Institution C1 H2 Chemistry 2012

(c) FeO/Al2K2O4 can be used as catalysts for this reaction.The table below also shows the different equilibriums under different pressures and temperatures.

(i) Explain why the yield of NH3 increases with pressure.

________________________________________________________________________________________________________________________________________________________________________.

(ii) State and explain which temperature listed above has the highest value.

________________________________________________________________________________________________________________________________________________________________________.

(iii) With the aid of a diagram, explain the role of FeO and Al2K2O4 in the reaction mixture.

________________________________________________________________________________________________________________________________________________________________________________________________________________________________.

[5][Total: 13]

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