AS & A Level Chemistry - 9701 Paper 4 2015 Summer Zone 2

(a) Complete the electronic configurations of the following atoms.

fluorine: $1s^2$........................................
sulfur: $1s^2$.........................................

(b) (i) Write an equation to show the thermal decomposition of HCl.
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(ii) Using all relevant bond energy values from the \textit{Data Booklet}, explain why the thermal stability of HF is \textit{much} more than that of HCl.

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(c) Explain what is meant by the term \textit{electronegativity}, and how it relates to the concept of \textit{bond polarity}.

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(d) Sulfur and fluorine react together to give the covalent compound $SF_4$.

(i) Draw a ‘dot-and-cross’ diagram to show the bonding in $SF_4$. Include \textbf{all} outer shell electrons in your diagram.

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(ii) State whether a molecule of $SF_4$ has a dipole moment. Explain your answer.

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(e) Suggest a reason why sulfur can form both $SF_4$ and $SF_6$ whereas oxygen can only form $OF_2$.

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(f) (i) State a major source of atmospheric sulfur dioxide.

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(ii) State \textbf{one} environmental consequence of atmospheric sulfur dioxide.

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(a) A sample of lead consists of the following isotopes in the percentage abundances stated.

[Table_1]
Use these data to calculate the relative atomic mass of the sample of lead to two decimal places.
$$A_r (\text{Pb}) = \text{.....................}$$ [2]

(b) Tin and lead both form oxides in oxidation states (II) and (IV).

(i) How does the acid-base nature of tin(II) oxide compare to that of tin(IV) oxide?
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(ii) Illustrate your answer to (i) with equations, showing the reaction of each oxide with a suitable acid or base, as appropriate.
SnO ................................................................................................................................................
SnO$_2$ ........................................................................................................................................... [2]

(iii) Describe the reactions, if any, that occur when separate samples of tin(IV) oxide and lead(IV) oxide are heated in air. Include any relevant observations and write equations for any reactions that occur.
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(a) Complete the table with the symbol of the ion that contains the number of protons, electrons and neutrons stated in the following table. The first line has been completed as an example.

[Table_1]

(b) Describe and explain the trend in the solubilities of the sulfates of the Group II elements down the group.
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(c) Calcium sulfate is sparingly soluble in water.

Describe and explain what you would see when a few cm³ of concentrated Na₂SO₄(aq) were added to a saturated solution of CaSO₄(aq).
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(d) When a solution of a chromium salt X is electrolysed, chromium metal is deposited on the cathode, according to the following equation.

$$ \text{Cr}^{n+}(aq) + ne^- \rightarrow \text{Cr}(s) $$

When a current of 1.8A was passed for 40 minutes through a solution of salt X, it was found that 0.776 g of chromium had been deposited.

Calculate the value of n in the above equation. Show your working.

$$ n = \text{.....................} $$
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(a) (i) What is meant by the term buffer solution?
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(ii) Write equations to show how the hydrogencarbonate ion, $\text{HCO}_3^-$, controls the pH of blood.
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(iii) A solution containing both $\text{Na}_2\text{HPO}_4$ and $\text{NaH}_2\text{PO}_4$ is commonly used as a buffer solution. The following equilibrium is present in the solution.

$\text{H}_2\text{PO}_4^- (\text{aq}) \rightleftharpoons \text{HPO}_4^{2-}(\text{aq}) + \text{H}^+(\text{aq}) \hspace{10px} K_a = 6.2 \times 10^{-8} \text{mol dm}^{-3}$

Calculate the pH of a buffer solution made by mixing 100 cm$^3$ of 0.5 mol dm$^{-3}$ $\text{Na}_2\text{HPO}_4$ and 100 cm$^3$ of 0.3 mol dm$^{-3}$ $\text{NaH}_2\text{PO}_4$.

pH = ......................... [2]

(b) Silver phosphate, $\text{Ag}_3\text{PO}_4$, is sparingly soluble in water.

(i) Write an expression for the solubility product, $K_{sp}$, of $\text{Ag}_3\text{PO}_4$, and state its units.

$K_{sp} =$ .................................................... units: ................................. [1]

(ii) The numerical value of $K_{sp}$ is $1.25 \times 10^{-20}$ at 298 K. Use this value to calculate [Ag$^+$(aq)] in a saturated solution of $\text{Ag}_3\text{PO}_4$.

[Ag$^+$(aq)] = ......................................................... mol dm$^{-3}$ [3]

(c) The half-equation for the redox reaction between phosphoric(III) acid and phosphoric(V) acid is shown.

$\text{H}_3\text{PO}_4(\text{aq}) + 2\text{H}^+(\text{aq}) + 2e^- \rightleftharpoons \text{H}_3\text{PO}_3(\text{aq}) + \text{H}_2\text{O}(l) \hspace{10px} \text{E}^\circ = -0.28 \text{V}$

Find suitable data from the Data Booklet to write an equation for the reaction between $\text{H}_3\text{PO}_3$ and $\text{Fe}^{3+}(\text{aq})$ ions, and calculate the $\text{E}^\circ_{cell}$ for the reaction.

equation: .....................................................................................................................

$\text{E}^\circ_{cell}$ = ......................... V [2]

(a) Compound B is a component of several perfumes and flavourings. It can be obtained by the hydrogenation of compound A. During the reaction, the hydrogen atoms all add onto the same side of the benzene ring.



(i) Suggest reagents and conditions for this reaction. ................................................................................................................................. [1]

(ii) Circle all the chiral atoms on the structure of B above. [1]

(iii) How many possible optical isomers are there with the same structural formula as B? ................................................................................................................................. [1]

(iv) Complete the following part-structure to show the structure of one of the isomers of B that would be formed during the above reaction.



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(b) Compound A can be obtained from propan-2-ylbenzene by the following route.



(i) Suggest the structure of the intermediate cation C and draw it in the box above. [1]

(ii) Suggest reagents and conditions for the following steps.
step 1 ...................................................................................................................

step 2 ...................................................................................................................

step 3 ................................................................................................................... [4]

(c) Suggest the structures of the organic products of the reactions between each of the compounds A and B and the following reagents. If no reaction occurs write 'no reaction' in the relevant box.

[Table_1] [5]

(a) Carboxylic acids can be converted into primary amines by the following sequence of reactions.

RCO$_2$H → (step 1) → D → (step 2) → RCONH$_2$ → (step 3) → RCH$_2$NH$_2$

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(i) Suggest the identity of intermediate D and write its structure in the box above. [1]
(ii) Suggest the reagents for
step 1 ............................................................
step 2 ............................................................
step 3 ............................................................ [2]

(b) Four compounds, E, F, G and H, are isomers of each other. Each compound contains an aromatic ring and two functional groups from the following list.

• alcohol
• amide
• amine
• carboxylic acid
• ester
• phenol

(i) Which of these functional groups react readily with cold HCl(aq)? ................................................ [1]
(ii) Which of these functional groups react readily with cold NaOH(aq)? ............................................. [1]

The molecular formula of the four isomers, E, F, G and H, is C$_8$H$_9$NO$_2$. All four compounds are insoluble in water. Table 1 shows their solubilities in acid or alkali.
![Table_1]

(iii) Use this information to suggest the two functional groups, taken from the list on page 10,
that each compound contains.
![Table_2]

(iv) Suggest a structure for each compound.
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(a) Study the structures of the three molecules below. One of the molecules could be a building block for a protein while the other two could be building blocks for other biological polymers.



Which of the three could be a building block for a protein? Explain your answer.
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(b) Outline the different roles played by mRNA and tRNA in producing a protein with a specific primary structure.

mRNA ..............................................................................................................................
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tRNA ............................................................................................................................
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(c) Sickle cell anaemia is a genetic-based disease in which one of the glutamic acid residues is replaced by a valine residue.



Suggest and explain how this change in the primary structure of the protein would affect the overall structure and function of the protein.
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(a) NMR spectroscopy and X-ray crystallography can both be used to examine the structure of organic compounds.
NMR is very useful at examining hydrogen atoms in compounds, but hydrogen atoms are invisible to X-rays.
(i) Explain why NMR spectroscopy can detect hydrogen atoms in molecules.
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(ii) Explain why hydrogen atoms are invisible to X-rays.
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(iii) The molecular formula of the amino acid cysteine is $\text{C}_3\text{H}_7\text{O}_2\text{NS}$.
Explain which of the atoms present would show the greatest absorption on exposure to X-rays.
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(b) The NMR spectrum below was obtained from an organic liquid, P, which contains five carbon atoms per molecule.

(i) How many protons are present in one molecule of P? Explain your answer.
number of protons ..............................
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(ii) When a little $\text{D}_2\text{O}$ is added to P, the absorption at $\delta\,2.0$ disappears.
Explain what this tells you about the group responsible for this absorption and why.
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(iii) What does the absorption at $\delta\,0.9$ tell you about the adjacent carbon atom?
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(iv) What group(s) is/are responsible for the absorption at $\delta\,0.9$?
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(v) Suggest a structure for P.
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(c) When an isomer of P is heated with concentrated $\text{H}_2\text{SO}_4$, it forms a new compound, Q. This new compound Q reacts with bromine to give a dibromide, R.
(i) A mass spectrum was obtained of R. The ratio of the heights of the M:M+1 peaks was 9.3:0.5
Show that there are five carbon atoms present in one molecule of R.
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(ii) Predict the ratio of the heights of the M:M+2:M+4 peaks as a result of the two bromine atoms in the dibromide R. Show your working.
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ratio ................................................ [1]
(iii) What is the molecular formula of R?
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Polymers consist of monomers joined either by addition or condensation reactions.

(a) Complete the table by placing a tick (✓) in the correct column to indicate the type of reaction that would polymerise each of the monomers.

[Image_1: Table with monomers and columns for addition, condensation, both]
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(b) Poly(ethene) bags pollute the environment for a long time because they are non-biodegradable.

Suggest why.

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(c) There has been considerable research into making biodegradable plastic bags. The repeat unit for one of the polymers used, polylactic acid (PLA), is shown.

[Image_2: Structure of PLA]

(i) Draw the structure of the monomer for PLA.

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(ii) Suggest why PLA breaks down more easily in the environment than poly(ethene).

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(d) The table shows the melting points of three polymers.

[Table_1: Melting points of polyethene, polychloroethene (PVC), nylon 6,6]

Explain the differences in melting point of these three polymers in terms of the intermolecular forces between the chains.

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