AS & A Level Chemistry - 9701 Paper 4 2014 Summer Zone 2

(a) (i) On the diagrams below, show the outer electron arrangements of the atoms and ions indicated. (Use the symbol ↑↓ to represent a pair of electrons in an orbital.)

(ii) Use the above diagrams to explain why $\text{Fe}^{2+}(\text{aq})$ ions are coloured, whereas $\text{Zn}^{2+}(\text{aq})$ ions are colourless.

(b) When concentrated $\text{HCl}$ is added to a solution of $\text{Cu}^{2+}(\text{aq})$ ions, the solution turns yellow.
(i) State the formula of the species responsible for the yellow colour and name the type of
reaction that has occurred.
(ii) Ammonia can react as a base or as a ligand.
Describe the colour changes that occur when $\text{NH}_3(\text{aq})$ is gradually added, with stirring, to the yellow solution, until the $\text{NH}_3(\text{aq})$ is in excess.
Identify the three ions or compounds responsible for the new colours.

(c) When aqueous solutions of KI and $\text{K}_2\text{S}_2\text{O}_8$ are mixed almost no reaction occurs, but when a few drops of $\text{Fe}^{2+}(\text{aq})$ or $\text{Fe}^{3+}(\text{aq})$ are added, iodine, $\text{I}_2(\text{aq})$, is produced at a steady rate.
(i) Write an equation for the overall reaction.
(ii) State the precise role of the iron ions during this reaction.
(iii) By means of equations or otherwise, explain why the presence of either $\text{Fe}^{2+}$ or $\text{Fe}^{3+}$ is able to speed up the reaction.

Lead(II) chloride, PbCl_2, can be used in the manufacture of some types of coloured glass.

PbCl_2 is only sparingly soluble in water. The [Pb^{2+}] in a saturated solution of PbCl_2 can be estimated by measuring the cell potential, E_{cell}, of the following cell.



(a) In the spaces below, identify what the four letters A-D in the above diagram represent.

A ............................................................... B ...............................................................

C ............................................................... D ............................................................... [4]

(b) In a saturated solution of PbCl_2, $[PbCl_2(aq)] = 3.5 \times 10^{-2} \text{ mol dm}^{-3}$.

(i) The $E^o$ for the $Pb^{2+}/Pb$ electrode is –0.13 V. Predict the potential of the right-hand electrode in the diagram above. Indicate this by placing a tick in the appropriate box in the table below.

[Table_1]

Explain your answer.

...................................................................................................................................

(ii) Write an expression for the solubility product, $K_{sp}$, of PbCl_2.

...................................................................................................................................

(iii) Calculate the value of $K_{sp}$, including units.

$K_{sp} = ext{.........................} ext{ units .......................}$ [5]

(c) The behaviours of PbCl_2 and SnCl_2 towards reducing agents are similar, but their behaviours towards oxidising agents are very different.

(i) Illustrate this comparison by quoting and comparing relevant $E^o$ values for the two metals and their ions. Explain what the relative $E^o$ values mean in terms of the ease of oxidation or reduction of these compounds.

...................................................................................................................................

(ii) Writing a balanced molecular or ionic equation in each case, suggest a reagent to carry out each of the following reactions.

the reduction of PbCl_2
...................................................................................................................................

the oxidation of SnCl_2
................................................................................................................................... [5]

(d) (i) Write an equation to represent the lattice energy of PbCl_2. Show state symbols.

...................................................................................................................................

(ii) Use the following data, together with appropriate data from the Data Booklet, to calculate a value for the lattice energy of PbCl_2.

electron affinity of chlorine = –349 kJ mol^{-1}
enthalpy change of atomisation of lead = +195 kJ mol^{-1}
enthalpy change of formation of PbCl_2(s) = –359 kJ mol^{-1}

lattice energy = ..............................kJ mol^{-1}

(iii) How might the lattice energy of PbCl_2 compare to that of PbBr_2? Explain your answer.

................................................................................................................................... [6]

The following four isomeric esters with the molecular formula $C_7H_{14}O_2$ are used as artificial flavours in drinks and sweets to give a pear, banana or plum taste to foodstuffs.

(a) In each of the spaces below, write one or more of the letters A-D, as appropriate.
(i) Which of these compounds can exist as optical isomers?
..................................................................................................................................................
(ii) On hydrolysis, which of these compounds produce(s) a secondary alcohol?
..................................................................................................................................................
[3]
(b) The hydrolysis of all these compounds produces ethanoic acid, $CH_3CO_2H$, as one of the products.
State the reagents and conditions needed for this hydrolysis.
..................................................................................................................................................
[1]

(c) The acid dissociation constant, $K_a$, of ethanoic acid is $1.75 \times 10^{-5} \text{mol dm}^{-3}$.
(i) Explain why this value of $K_a$ is
• much larger than that of ethanol, $CH_3CH_2OH$,
..................................................................................................................................................
..................................................................................................................................................
• smaller than that of chloroethanoic acid, $ClCH_2CO_2H$.
..................................................................................................................................................
..................................................................................................................................................
(ii) Calculate the pH of a $0.100 \text{mol dm}^{-3}$ solution of ethanoic acid.
[4]

(d) $20.0 \text{cm}^3$ of $0.100 \text{mol dm}^{-3}$ NaOH were slowly added to a $10.0 \text{cm}^3$ sample of $0.100 \text{mol dm}^{-3}$ ethanoic acid, and the pH was measured throughout the addition.
(i) Calculate the number of moles of NaOH remaining at the end of the addition.
..........................................................................................
(ii) Calculate the $[OH^-]$ at the end of the addition.
..........................................................................................
(iii) Using the expression $K_w = [H^+][OH^-]$ and your value in (ii), calculate $[H^+]$ and the pH of the solution at the end of the addition.
..........................................................................................
(iv) On the following axes, sketch how the pH will change during the addition of a total of $20.0 \text{cm}^3$ of $0.100 \text{mol dm}^{-3}$ NaOH. Mark clearly where the end point occurs.

(v) From the following list of indicators, put a tick in the box by the side of the indicator you consider most suitable for this titration.
[Table_1]
[7]

Both ethene and benzene react with bromine.

(a) What type of reaction is the reaction of bromine with
(i) ethene,
............................................................................................................................
(ii) benzene?
............................................................................................................................
[1]
(b) Write an equation to show the formation of the electrophile during the reaction between bromine and benzene.
............................................................................................................................
[1]
(c) Each of these reactions involves an intermediate.
(i) Draw the structure of the intermediate in each reaction.

(ii) Suggest why the product of the reaction between bromine and benzene, bromobenzene, is still unsaturated.
............................................................................................................................
[3]

(d) When methylbenzene is nitrated, 4-nitromethylbenzene is formed, but when benzoic acid is nitrated, 3-nitrobenzoic acid is produced.
Consider the following synthesis of 3-chlorobenzoic acid, F, from methylbenzene. Use the information given above to suggest
• the structure of the intermediate E,
• the reagents and conditions needed for reactions 1 and 2.

reagents and conditions for reaction 1
............................................................................................................................
reagents and conditions for reaction 2
............................................................................................................................
[3]

(e) Consider the following synthesis of 3-chlorophenylmethylamine, H, from F. Suggest
• the structure of the intermediate G,
• the reagents for reactions 3 and 4.

reagents for reaction 3
............................................................................................................................
reagents for reaction 4
............................................................................................................................
[3]

Although now remembered for his music, the Russian composer Alexander Borodin was a chemist. He is credited with the discovery of the aldol reaction, a product of which is compound J. J shows the following properties:

- its molecular formula is $C_4H_8O_2$,
- it is neutral,
- it reacts with sodium metal,
- it reacts with Fehling's solution,
- it does not react with aqueous bromine.

(a) Suggest which functional groups are responsible for the reactions with

(i) sodium,
...............................................................................................................................

(ii) Fehling's solution.
............................................................................................................................... [2]

(b) The result of the bromine test shows a functional group is absent from compound J. Suggest the identity of this functional group.
............................................................................................................................... [1]

(c) In the boxes below, draw three possible straight-chain structures for J that fit the above results, and that are structural isomers of each other.
[3]

(d) Compound J reacts with alkaline aqueous iodine to give a pale yellow precipitate.

(i) Which functional group does this reaction show that J contains?
...............................................................................................................................

(ii) Which of your three structures K, L or M contains this group and is therefore J?
............................................................................................................................... [2]

(e) Compound J exists as stereoisomers.

(i) Name the type of stereoisomerism shown by J.
...............................................................................................................................

(ii) Draw two structures of J to illustrate this stereoisomerism.
[2]

This question looks at the formation and breakdown of protein chains in the body.
(a) Proteins are formed from chains of amino acid monomers joined together. The structures of two amino acids, valine and serine are shown.
[Image_1: Structures of valine and serine]
(i) Draw the structure of the dipeptide val-ser, showing the peptide bond in displayed form.

(ii) What type of reaction has taken place in order to form this dipeptide?

(iii) Identify the other molecule produced in this reaction. [4]
(b) Both DNA and RNA are involved in protein synthesis.
Complete the table to show three differences between the structures of DNA and RNA.
[Table_1: Differences between DNA and RNA]
[3]

(c) In protein synthesis, sections of the DNA are copied by mRNA and this, in turn, is read by the ribosome in order to assemble the amino acids for the new protein chain. Each group of three bases codes for one amino acid, with some amino acids having several codes. The codes are summarised in the table.
[Table_2: Amino acid codes]
In general the amino acid chains start with the code AUG, and end with one of the three 'stop' codes shown in the table.
(i) Use the abbreviations to show the sequence of amino acids in the peptide for the base sequence shown.
– AUGCUAACACCCGGAGUAA –

(ii) Sometimes an error can occur in the base sequence. What are these errors called?

(iii) This type of error can lead to the formation of a protein with a different structure from the original, as in sickle cell anaemia. In this case the amino acid glutamic acid (glu) is replaced by valine (val) in the protein as a result of one base being changed in a three base code.
Use the table to suggest the change of base that causes this. [3]

Modern methods of chemical analysis often rely on the interpretation of data gathered from instrumental techniques.

(a) Electrophoresis and paper chromatography can both be used to separate amino acids from a mixture obtained from polypeptides.



In each case, give one property of the amino acids that causes their separation.
electrophoresis ..........................................................
..........................................................
paper chromatography ..........................................................
.......................................................... [2]

(b) Amino acids are colourless.
How are the positions of the different amino acids made visible so that measurements can be made?
..........................................................
.......................................................... [1]

(c) Which measurements need to be made in order to identify individual amino acids in paper chromatography?
..........................................................
.......................................................... [1]

(d) The diagram shows the results of electrophoresis on a mixture of the amino acids glycine, lysine and glutamic acid at pH 7.0. The structures of the amino acids at pH 7.0 are shown.

glycine: $\text{H}_3\text{N}^+\text{CH}_2\text{CO}_2^-$
lysine: $\text{H}_3\text{N}^+\text{CH(CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{NH}_3^+)\text{CO}_2^-$
glutamic acid: $\text{H}_3\text{N}^+\text{CH(CH}_2\text{CH}_2\text{CO}_2)\text{CO}_2^-$



Identify the amino acids responsible for the spots labelled R, S and T.
R ..........................................................
S ..........................................................
T .......................................................... [3]

(e) This diagram shows the results of two-way paper chromatography of a mixture of amino acids.



To answer these questions you need to indicate clearly on the diagram above as directed in the questions.
(i) Put a U next to the amino acid that travelled furthest in solvent 2.
(ii) Put a ring around the two amino acids that were not separated in solvent 1.
(iii) Put a W next to the amino acid that was very soluble in both solvents. [3]

(a) Name an example of a synthetic addition polymer and a synthetic condensation polymer.
Addition polymer ................................................
Condensation polymer ..........................................

(b) Addition polymers are long-term pollutants in the environment but condensation polymers are often biodegradable.
(i) What type of reaction occurs when condensation polymers biodegrade?
........................................................
(ii) Identify two functional groups that could undergo this type of reaction.
........................................................

(c) Petroleum is a non-renewable resource from which a wide range of useful polymers is currently produced. Current polymer research is looking at renewable plant material as a potential source of monomers.
Two monomers obtained from plants are shown.
$\text{CH}_3\text{CH(OH)COOH}$ $\text{HOCH}_2\text{COOH}$
Draw the displayed formula of the repeat unit of a polymer using both monomers.

(d) Monomers obtained from plant sources do not usually form addition polymers. Suggest why this is.
................................................

(e) The diagrams show sections of two polymers Y and Z.

(i) What would be the main force between the chains in each polymer?
Y ........................................................
Z ........................................................
(ii) Which is likely to be the more hydrophilic of these two polymers? Explain your answer.
................................................