Raffles Institution 2011/12

H2 Chemistry Mock Paper 3

Name: _________________

Class: __________________

Duration: 2 hours

Marks : _________/80

Answer any four questions.

1 (a) Unlike Group I and Group II metals, transition metals such as iron tend to form aqueous complex ions which are coloured. The two common ions of iron are Fe2+ and Fe3+.

(i) State the colour of each ion in aqueous solution. (ii) By reference to the aqueous Fe(III) ion, [Fe(H2O)6]

3+, explain why this ion is coloured. [4] (b) The oxidation of Fe3+ ions by sodium chlorate(I), NaClO, in a strongly alkaline solution produces a

purple solution from which a salt with the formula Na2FeO4 can be isolated. Chloride ions are left in the solution.

(i) Determine the oxidation state of iron in Na2FeO4.

(ii) By using the half-equation method, construct an ionic equation for the formation of FeO42− ion

in the above reaction. [3] (c) Sodium chlorate(I), NaClO, can be prepared in the laboratory by bubbling chlorine gas into cold

aqueous NaOH. Cl2 (g) + NaOH (aq) → NaCl (aq) + NaC lO (aq) + H2O (l)

The concentration of NaClO prepared can be determined by reacting it with excess acidified potassium iodide to form chloride ions and iodine as the products. The iodine liberated is then titrated against sodium thiosulphate solution, Na2S2O3 (aq).

It was found that 25.0 cm3 of the NaClO solution liberated enough iodine that required 24.10 cm3

of 0.100 mol dm−3 of sodium thiosulphate solution for complete reaction. (i) State the type of reaction undergone when chlorine gas is bubbled into cold aqueous NaOH. (ii) Write the equation for the reaction between sodium chlorate(I) and iodide ions in acidic

medium. (iii) Hence determine the concentration of sodium chlorate(I) solution. [4] (d) Describe what is observed when sodium chloride and sodium bromide are separately reacted with

concentrated sulphuric acid. In each case, write equations with state symbols where appropriate. Explain any difference in their reactions with concentrated sulphuric acid. [4]

(e) A halogenoalkane, Z, with a labelled carbon, Cα, undergoes the following reaction:

(CH3)2 C

CH3

(CH3)2 CCl CH2

Z (i) State the reagents and conditions for the above reaction.

(ii) State the type of hybridisation and the shape with respect to the carbon atom, Cα, before and after the reaction.

(iii) Describe a chemical test that would distinguish compound Z from chlorobenzene. [5] TJC/2007/P3/1

2 (a) Some data on three nitrogen-containing compounds are given in the table below:

Compound Molecular formula Boiling point/ oC

dinitrogen pentoxide N2O5 decomposes nitric acid HNO3 83

nitrosyl chloride NOCl -6.4

(i) Draw ‘dot and cross’ diagrams for HNO3, N2O5 (a symmetrical molecule) and NOCl (central atom is N). Suggest a value for the bond angle in NOCl.

(ii) Explain the difference in the boiling points of HNO3 and NOCl in terms of structure and bonding. [6]

α α

(b) The dissociation of nitrosyl chloride into nitric oxide and chlorine is an endothermic process which takes place according to the equation:

2NOCl (g) 2NO (g) + Cl2 (g) ∆H > 0 In an evacuated 20 dm3 vessel at 400 K, 0.5 mol of NOCl is injected and the equilibrium

pressure is 101 kPa.

(i) Calculate the total number of moles of gases at equilibrium, assuming the gases behave ideally.

(ii) Hence, calculate the percentage of the nitrosyl chloride that has dissociated.

A mixture of NOCl, NO and Cl2 is injected into a second evacuated vessel of a fixed volume at 400 K. It takes 10 minutes for the system to reach equilibrium and the equilibrium pressure is 200 kPa. The initial and final compositions of the mixture of gases are shown in the table below. Initial number of moles at

400 K Equilibrium number of moles

at 400 K NOCl 0.5 0.3 NO 0.1 0.3 Cl2 0.1 0.2

(iii) Write an expression for Kp for the reaction and calculate Kp, stating its units at 400 K. (iv) The temperature of the system in the second vessel is increased to 500 K at time, t = 25

minutes. Suggest, with an explanation, how the position of equilibrium might change. Give a sketch to show how the number of moles of nitrosyl chloride gas might change with

time from t = 0 min to t = 40 min. [9]

(c) Compound V, C12H14O2 , is insoluble in NaOH(aq) at room temperature and does not give

orange precipitate with Brady’s reagent (2,4−dinitrophenylhydrazine). V reacts with cold, dilute acidified potassium manganate(VII) to form W, C12H16O4 . When reacted with lithium aluminium hydride in dry ether, V gives organic products X and Y. Compound X forms a yellow precipitate when heated with alkaline aqueous iodine. Compound Y decolourises aqueous bromine to give a white precipitate Z, C10H10Br4O2. Compounds V, W, Y and Z are optically active.

Draw the structures of compounds V, W, X, Y and Z. [5]

TJC/2008/P3/1(a),(b) & 5(d)

3 Vanillin and acetylhydroquinone are two commonly used carbonyl compound derivatives in industry.

The largest single use of vanillin is as a flavoring, usually in sweet foods such as ice cream and chocolate while acetylhydroquinone is used in developing photographic emulsions.

Both compounds exists as yellow solids that are soluble in water to form a yellow solution which is perceptible even at minute concentrations. OH

OH C CH3OOH OCH3C OH

acetylhydroquinone vanillin

(a) Both organic compounds react with hydrocyanic acid, HCN at different rates. This reaction requires an amount of a base such as NaOH to be added.

(i) Name and describe the mechanism between HCN and acetylhydroquinone. Explain clearly the role of the base in this reaction.

(ii) In the reaction of vanillin with HCN, only a small amount of a base is required. Suggest why for acetylhydroquinone, more sodium hydroxide is required for any significant reaction to occur. [6]

(b) (i) Both vanillin and acetylhydroquinone reacts with 2,4- dinitrophenylhydrazine. Give the

observation expected and write an equation for the reaction with acetylhydroquinone. (ii) Give one positive chemical test each, for vanillin and acetylhydroquinone, which will allow

the two compounds to be distinguished from one another. State the reagents, conditions and observations expected. [4]

(c) An investigation into the kinetics of the reaction between vanillin of initial concentration 3.00 × 10–2 mol dm–3 and HCN at two different initial concentrations, is conducted at 25 ºC in a pH 9.0 buffer. The results are as shown on the graph below.

time / min

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

[acety

lhyd

roq

uin

on

e]

/ m

ol d

m−− −−

3

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

(i) Suggest one plausible method by which the concentration of vanillin may be followed during the course of the reaction, stating any assumption made.

(ii) Using the graph, determine the order of the reaction with respect to vanillin and HCN. (iii) Hence, write down the rate equation for the reaction between vanillin and HCN. (iv) If the reaction temperature is now lowered to 9 ºC, at a pH of 13.0, with an initial HCN

concentration of 3.00 × 10–2 mol dm–3 and a vanillin concentration of 3.00 × 10–3 mol dm–3, the half-life of vanillin was determined to be 10 seconds.

Predict the value of the half-life of vanillin when the concentration of HCN used is

increased to 12.00 × 10–2 mol dm–3. Explain your answer. (v) Explain how the addition of a small amount of catalyst can cause a large increase in the

rate of the above reaction. [10] MJC/2007/P3/2

[HCN] = 0.100 mol dm–3

[HCN] = 5.00 mol dm–3

[vanillin] /mol dm-3

4 A current research in alternative sources of renewable energy is in the area of Microbial Fuel Cell (MFC). MFCs convert chemical energy directly into electricity using bacteria as catalyst.

An MFC consist of an anode compartment and a cathode compartment separated by a cation exchange membrane which allows only cations to pass through. The bacteria live in the anode compartment and feed on organic substrates, such as ethanoate ion, CH3CO2

− that is continuously introduced. The bacteria will convert the substrate into hydrogen carbonate ions, HCO3

−, protons and electrons. The HCO3− ions and protons will pass into the anode

electrolyte while the electrons will be transferred to the anode. The flow of electrons through the external circuit can be harnessed to do electrical work. At the cathode, oxygen in the cathode electrolyte is reduced to form water.

(a) (i) Write ionic equations for the reactions occurring at the anode and cathode.

(ii) Suggest, with reasoning, the function of the cation exchange membrane. [3] (b) Under standard conditions, the reduction potential for the HCO3

−/CH3CO2− system is +0.19 V

when measured against the standard hydrogen electrode. (i) Draw a labeled diagram to show how the standard electrode potential of the

HCO3−/CH3CO2

− system could be measured. (ii) Calculate the standard cell potential that can be obtained in the MFC. [4]

(c) A current of 0.50 A is produced when the MFC is in operation. Calculate the mass of CH3CO2

− processed by the MFC in one hour. [2]

(d) In actual MFCs, the anode electrolyte contains a mixture of sodium dihydrogen phosphate,

NaH2PO4 and sodium hydrogenphosphate, Na2HPO4 to help maintain the pH at 7. This solution can be obtained by dissolving phosphorus (V) oxide, P4O10 in water followed by careful addition of aqueous sodium hydroxide.

In an experimental setup, solid P4O10 was dissolved in 50 cm3 of distilled water and added to 50

cm3 of 0.224 mol dm−3 aqueous sodium hydroxide. The resulting solution was used as the anode electrolyte.

(i) Write an equation for the reaction between P4O10 and water. (ii) Explain with the help of equations the buffering roles of NaH2PO4 and Na2HPO4. (iii) The acid dissociation constant, Ka of H2PO4

− is 6.2 x 10−8 mol dm−3. Determine the ratio of the amount of HPO4

2− to H2PO4− which maintains pH at 7 in 100 cm3 of anode electrolyte.

(iv) Using your answer in (d)(iii), calculate the mass of P4O10 dissolved in the 50 cm3 of distilled water. [7]

CH3CO2−

Anode Compartment Cathode Compartment

Bacterium Cation Exchange Membrane

Pt anode Pt cathode

e− flow

O2 + H2O

(e) Chemical mediators are used in MFCs to help in the transfer of electrons. One such mediator is the compound neutral red. Three of the nitrogen present are labelled as Na, Nb and Nc as shown below.

In order to understand the relative basicity of the various nitrogen groups on neutral red, a

comparison is done with similar nitrogeneous bases. The pKb values of these relevant bases are given below. (i) Explain the relative basicity of

• methylamine and trimethylamine,

• pyridine and phenylamine (ii) Rank the three nitrogen labelled Na, Nb and Nc on neutral red in order of increasing basicity.

[4] NYJC/2008/P3/2

5 (a) (i) What do you understand by the term enthalpy change of atomisation of magnesium? (ii) Use the following data together with relevant data from the data booklet, construct an

energy level diagram to calculate the enthalpy change of atomisation of magnesium.

Enthalpy Change ∆H/ kJ mol−1

Lattice energy of magnesium chloride −2500

1st electron affinity for chlorine −349

Standard enthalpy change of formation of magnesium chloride −652

(iii) Suggest, with reason, how the magnitude of the enthalpy change of atomisation of Mg might compare with that of chromium. [7]

(b) Grignard reagent, RMgCl, is useful in organic synthesis as it enables the combination of two

organic molecules into a bigger one by forming a carbon-carbon bond. It is prepared by the reaction of magnesium with a halogenoalkane. One of the most important uses of the Grignard reagent is its reaction with carbonyl compounds to form alcohols as shown below.

R MgCl + C O

R'

H

C

R'

R

H

OH + Mg(OH)ClH2O

Methylamine Trimethylamine Pyridine Phenylamine

CH3NH2

(CH3)3N

3.34 4.19 8.75 9.37

NNH2

Neutral Red

N

N

N

CH3

H3CNH2

CH3

a

b

c

(i) Given the carbon-magnesium bond in Grignard reagent is highly polar, state the type of reaction between R –MgCl and the aldehyde.

(ii) R-MgCl reacts with propanal to give compound A. Compound A gives a yellow precipitate

with I2 in aqueous NaOH. When compound A reacts with excess concentrated H2SO4 at 170 oC, compounds B and C are formed. Compound B upon reaction with hot acidified potassium manganate(VII), produces compound D and effervescence is observed . Compound C exists as a pair of geometric isomers. Deduce the structures of compounds A to D. [7]

(c) The building blocks of proteins are amino acids with the general structure:

C

R

HH2N

COOH

(i) Explain why amino acids are crystalline solids with high melting points. (ii) Mixtures of amino acids can be separated by ion exchange chromatography. This

technique involves pouring the mixture of amino acids dissolved in water at pH 7 down a column containing an ion exchange resin called DEAE-cellulose. The structure of the DEAE-cellulose can be represented as shown in the figure below.

The + signs on the diagram show that the DEAE-cellulose is positively charged at this pH. An aqueous solution containing 3 amino acids with R = (CH2)4NH2, (CH2)2CONH2 and

CH2COOH was poured down the ion exchange column.

���� Predict the order in which the amino acids will be washed off. Explain your answer. ���� Explain why DEAE cellulose is not used to separate mixtures of amino acids such as

alanine and leucine where the R groups are –CH3 and –CH2CH(CH3)2 [6] NJC/2008/P3/1(a),(c) & 5(d)

[End of Paper]