H2 Chemistry Mock A Level Paper 3

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ABC JUNIOR COLLEGE

Mock A level Paper 2012

Higher 2

CANDIDATE

NAME

CLASS INDEX

NUMBER

CHEMISTRY 9647/03

Paper 3 Free Response Questions 2 hr 00 min

READ THESE INSTRUCTIONS FIRST

Write your name, PDG and register number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. Start each question on a fresh sheet of paper. You may use a pencil for any diagrams, graphs or rough working.

Do not use staples, paper clips, highlighters, glue or correction fluid. Answer ANY FOUR questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together.

FOR EXAMINER’S USE

TOTAL / 80

This document consists of 12 printed pages.

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Answer any four questions.

1 Penicillin is a group of antibiotics derived from Penicillium fungi. It is used to treat many

difference types of infections caused by bacteria, such as ear infections.

Below shows penicillin’s core structure, where “R” is the variable group.

(a) Many difference penicillins have been made and are manufactured for clinical use, since

bacteria vary in their susceptibility to difference penicillins. Also some penicillins are more

effective against penicillin-resistance bacteria than others.

The following table lists some of the most important variations in the R-group in naturally

occurring penicillins.

R-Group Side Chain Code

CH3CH2CH=CHCH2─ pen-F

C6H5CH2─ pen-G

C6H5OCH2─ pen-V

Penicillin-G (pen-G), is noted to possess effectiveness against many types of bacterial

and hence is a widely accepted type of penicillin.

(i) Name two functional groups present in pen-G other than the phenyl group.

(ii) It was discovered that another toxic compound has similar structure as pen-G.

Suggest the number of possible stereoisomers this compound has.

*Note: You can ignore stereoisomerism of the amide group.

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(iii)Suggest a chemical test to distinguish pen-G and the following compound.

[4]

(b) Lactams are cyclic amides formed by intramolecular condensation (addition-elimination)

reactions.

(i) Copy penicillin’s core structure onto your paper and circle the lactam group(s). [1]

(ii) Student A was told to synthesize compound B from compound A. He proposed the

following reaction scheme to synthesize the compound.

1. Describe the mechanism of step II. [3]

2. Identify the step(s) which is inappropriate for the production of compound B in the

above reaction scheme. Explain your answer. [4]

3. Student A was given another chance to synthesize compound B, but this time

from compound C.

Propose a route to synthesize compound B from compound C. Include in your

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answer, the appropriate reactants and conditions. [3]

[11]

(c) Lactones are cyclic esters formed by intramolecular condensation (addition-elimination)

reactions.

The percentage composition of the various types of lactones at equilibrium is shown

below.

Suggest reason(s) for the difference in percentage composition at equilibrium for the

above 3 lactones.

[2]

(d) Like amides and esters, lactams and lactones can also undergo hydrolysis reactions.

Given that basic hydrolysis involves the nucleophilic attack by OH- nucleophile, compare

the rate of (basic) hydrolysis for Ɣ-butyrolactone and compound B. Explain your answer.

[3]

[Total: 20]

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2 An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Acetylene, C2H2, the

simplest alkyne, was once widely used in industry as the starting material for the preparation

of acetaldehyde, acetic acid, vinyl chloride, and other high-volume chemicals. Just like

alkenes, alkynes undergo many similar electrophilic addition reactions.

(a) Suggest the major organic products when 1-Hexyne reacts separately with:

(i) Excess hydrogen bromide gas.

(ii) Hot acidified potassium permanganate.

[2]

(b) The most striking difference between alkenes and alkynes is that terminal alkynes are

weakly acidic. When a terminal alkyne is treated with a strong base, such as sodium

amide NaNH2, the terminal hydrogen is removed and an acetylide anion is formed.

Apart from alkynes, other hydrocarbons are also weakly acidic. The table below shows

the simple hydrocarbons and their pKa values.

Family Hydrocarbon pKa

Alkanes CH4 16

Alkenes CH2=CH2 10

Alkynes CH≡CH 5

(i) Write a balanced equation for the reaction between acetylene and sodium amide.

(ii) With reference to the type of hybridisation, suggest why terminal alkynes are more

acidic than alkenes or alkanes.

(iii)Below shows the titration curve when 10 cm3 of methylacetylene, CH3C≡CH is titrated

against 0.10 mol dm-3 sodium amide.

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1. Calculate the initial pH of methylacetylene. [2]

2. Calculate the pH at the equivalence point. Comment on your answer. [3]

3. Calculate the pH after 15 cm3 of NaNH2 has been added. [2]

[11] (c) Apart from acid base reactions, alkynes also play an important part in organic synthesis.

Describe the mechanism for the formation of the bicyclic product formed in the following

reaction.

[3]

(d) In the analysis of drinking water, the method known as Mohr’s titration is used to

determine the chloride content present. Chloride ions present in a water sample is titrated

against AgNO3 (aq).

As AgNO3 is slowly added, sparingly soluble AgCl precipitate is formed. The indicator

used in the titration is yellow aqueous K2CrO4. The end-point of the titration is obtained

when almost all the chloride present has been precipitated. Addition of AgNO3 beyond this

point causes the formation of a red-brown precipitate of Ag2CrO4.

This method can be used to determine the chloride ion concentration of water samples

from many sources.

However, Mohr’s titration should be carried out under conditions of pH 6.5 – 9. At higher

pH, the AgNO3 titrant added becomes ineffective, while at lower pH, the K2CrO4 indicator

is affected. Chromate (VI) ions, CrO42-, may be removed by an acid-base reaction to form

hydrogen chromate ions (HCrO4-) or dichromate ions (Cr2O7

2-), causing the end-point to

be inaccurate.

The following are relevant Ksp values in Mohr’s titration:

Salt Ksp

AgCl 1.8 x 10-10 mol2dm-6

Ag2CrO4 1.1 x 10-12 mol3dm-9

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(i) Calculate the concentration of Cl- ions at the end-point of the titration.

(ii) Determine the minimum concentration of CrO42- (aq) ions in the titration mixture that is

required to precipitate only Ag2CrO4 immediately after the end-point

(iii)In a 100 cm3 sample of treated tap water, 6.95 cm3 of 0.01 mol dm-3 AgNO3 was

required for Mohr’s titration. Calculate the concentration of Cl- (aq) ions in the treated water.

(iv)Suggest a reason why the AgNO3 titrant becomes ineffective at higher pH.

[4]

[Total: 20]

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3 An Ellingham diagram is a graph showing the temperature dependence of the stability for

compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides

and sulphides.

The Ellingham diagram below shows how changes between 298 K and 2000 K for the

following reactions.

Reaction 1: ( ) ( ) ( )

Reaction 2: ( ) ( ) ( )

Reaction 3: ( ) ( ) ( )

(a) (i) By reference to the diagram, state with a reason, which reaction of carbon is the

better reductant at 500 K and at 2000 K.

(ii) Explain qualitatively the following.

1. Why does for reaction 2 becomes more negative with increasing

temperature?

2. Why is for reaction 1 almost independent of temperature?

[5]

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(b) (i) Use the Ellingham diagram to calculate for the reaction:

( ) ( ) ( )

(ii) Use the diagram to decide if titanium metal could be extracted by heating ( )

with carbon and the conditions required.

(iii)Suggest why this process is not used at the present time.

[5]

(c) (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.

(iii)Suggest, with reason, how the magnitude of the enthalpy change of atomization of

magnesium might compare with that of chromium.

[7]

(d) Use the following data, draw an energy level diagram to show the stability of benzene

relative to the hypothetical cyclohexa-1,3,5-triene.

Give a reason for this difference in stability.

[3]

[Total: 20]

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4 (a) Element X forms a series of coloured ions and complexes. Electrode potentials of X

and its ions in acidic aqueous solution can be represented on a voltage/oxidation state

diagram as shown below.

The electrode potential for a process can be calculated from the formula

Some examples of the ion-electron half equations and electrode potentials, , that

can be derived using the formula and the data obtained from the graph are shown

below.

(i) Explain why the complex ions of element X are coloured.

(ii) Write the ion-electron equation for XO2+/X3+ half-cell. Use the formula and the

graph to calculate the electrode potential for this process.

(iii)Predict whether XO2+ will react with Sn2+. Explain your prediction using the

information derived from the graph as well as relevant data from the Data Booklet.

(iv)Suggest which ion of element X can function as a catalyst for the following reaction:

Explain how it acts as a catalyst with the aid of relevant equations.

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(v) 0.0100 mol sample of an oxochloride of X, XOCly, required 20.00 cm3 of 0.100 mol

dm-3 acidified potassium manganite(VII) for oxidation. Deduce the value of y in the

formula.

[13]

(b) Magnesium and its compounds are very useful in industrial processes. Magnesium is

generally obtained by the electrolysis of magnesium halides.

In the production of magnesium from seawater, magnesium is precipitated as the

hydroxide and converted to the chloride by reaction with hydrochloric acid. A dilute

magnesium chloride solution is obtained at this point. The magnesium chloride is then

recovered by evaporation of the solution, and magnesium metal is obtained by

electrolysis of the molten salt. The chlorine gas by-product is then pumped into

reinforced cylinders each with a capacity of 2.0 dm3. The cylinders are maintained at a

pressure of 1240 kPa and 25 °C.

(i) During the electrolysis of molten magnesium chloride using inert electrodes, a

current of 8.0 A was passed through the cell for 67 hours. Calculate the mass of

magnesium extracted, and the number of cylinders that were filled in this duration.

(ii) By quoting relevant values from the Data Booklet, predict what would be the

products obtained if electrolysis was done on the dilute magnesium chloride

solution instead of the molten salt.

[7]

[Total: 20]

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5 (a) Ethanoic acid has pKa value of 4.7.

Suggest a possible pKa value of trifluoroethanoic acid. Explain your answer.

[2]

(b) Linalool, A, C10HXO is a common constituent of the scent given off by flowers.

A does not react with 2,4-dinitrophenylhydrazine, but reacts with hydrogen in the

presence of platinum catalyst to form B, C10HX+4O. A reacts with ethanoyl chloride to form

C, C12HX+2O2, but does not react with warm acidified K2Cr2O7.

With cold acidified KMnO4, A forms D, C10HX+4O3. A reacts with warm concentrated

sulphuric acid to give a mixture of E and F, both of molecular formula C10HX-2.

E reacts with hot concentrated KMnO4 to give G, C3HYO3, H, C3HYO4, and I, C3HY+2O.

G and I both give a yellow precipitate with alkaline aqueous iodine. Sample of 1 mol of H

requires 1 mol of calcium hydroxide for complete neutralisation.

F reacts with hot concentrated acidified KMnO4 to give I and J, C5HY+2O5. J reacts with

sodium carbonate in the molar ratio of 1:1. J is not optically active and it does not have a

plane of symmetry.

Suggest identities for each lettered compound, explaining your reasoning.

[18]

[Total: 20]

H2 Chemistry Mock A Level Paper 3

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