AS & A Level Chemistry - 9701 Paper 4 2021 Spring Zone 2

(a) The most common oxidation states of cobalt are +2 and +3.

Complete the electronic configurations of the following free ions.

• Co²⁺ [Ar] .................................................................
• Co³⁺ [Ar] .................................................................

(b) Co²⁺ and Co³⁺ both form complexes with edta^4–.

[Table_1]

Use the data in the table to predict what happens, if anything, when separate aqueous solutions of Co³⁺ and [Co(edt)a]^– are left to stand in the air.

aqueous solution of Co³⁺

aqueous solution of [Co(edt)a]^–
................................................................................................

(c) Hydrated cobalt(II) nitrate, Co(NO₃)₂•6H₂O, is a red solid that behaves like hydrated magnesium nitrate, Mg(NO₃)₂•6H₂O, when heated.

Describe in detail what you would expect to observe when crystals of Co(NO₃)₂•6H₂O are heated in a boiling tube, gently at first and then more strongly.
..................................................................................................................

(d) Explain why the thermal stability of the Group 2 nitrates increases down the group.
..................................................................................................................

(a) Iron(II) compounds are generally only stable in neutral, non-oxidising conditions.
It is difficult to determine the lattice energy of FeO experimentally.
(i) Use data from the Data Booklet and this Born–Haber cycle to calculate the lattice energy, $\Delta H_{\text{latt}}$, of FeO(s) in kJ mol$^{-1}$.

$\Delta H_{\text{latt}}$FeO(s) = ......................... kJ mol$^{-1}$ [2]
(ii) Most naturally occurring samples of iron(II) oxide are found as the mineral wüstite.
Wüstite has formula Fe$_{20}$O$_{x}$. It contains both Fe$^{2+}$ and Fe$^{3+}$ ions.
90% of the iron is present as Fe$^{2+}$ and 10% is present as Fe$^{3+}$.
Deduce the value of x.
x = ............................. [1]
(iii) State and explain how the lattice energy of FeO(s) compares to the lattice energy of CaO(s).
........................................................... [2]

(b) Heating of FeO results in the formation of Fe$_{3}$O$_{4}$, as shown.
reaction 1 4FeO $\rightarrow$ Fe + Fe$_{3}$O$_{4}$
Each formula unit of Fe$_{3}$O$_{4}$ contains one Fe$^{2+}$ and two Fe$^{3+}$ ions.
(i) Show how reaction 1 can be described as a disproportionation reaction.
......................................................................................................... [1]
Fe$_{3}$O$_{4}$(l) can be electrolysed using inert electrodes to form Fe.
(ii) Write the half-equation for the reaction that occurs at the anode during the electrolysis of Fe$_{3}$O$_{4}$(l).
......................................................................................................... [1]
(iii) Calculate the maximum mass of iron metal formed when Fe$_{3}$O$_{4}$(l) is electrolysed for six hours using a current of 50A.
Assume the one Fe$^{2+}$ and two Fe$^{3+}$ ions are discharged at the same rate.
mass of iron = .............................. g [3]

(c) LiFePO$_{4}$ can be used in lithium-ion rechargeable batteries.
When the cell is charging, lithium reacts with a graphite electrode to form LiC$_{6}$.
When the cell is discharging, the half-equations for the two processes that occur are as follows.
anode half-equation LiC$_{6}$ $\rightarrow$ 6C + Li$^{+}$ + e$^{-}$
cathode half-equation Li$^{+}$ + FePO$_{4}$ + e$^{-}$ $\rightarrow$ LiFePO$_{4}$
(i) State one possible advantage of developing cells such as lithium-ion rechargeable batteries.
......................................................................................................... [1]
(ii) Use the cathode half-equation to determine the change, if any, in oxidation states of lithium and iron at the cathode during discharging.
[Table_1]
[1]
(iii) Write the equation for the overall reaction that occurs when this cell is discharging.
......................................................................................................... [1]

Iodates are compounds that contain the $\text{IO}_3^{-}$ anion.

(a) The $\text{IO}_3^{-}$ anion is shown.

Explain, with reference to the qualitative model of electron-pair repulsion, why the $\text{IO}_3^{-}$ anion has a pyramidal shape.
................................................................................................................................. [1]

(b) The reaction of iodine and hot aqueous sodium hydroxide is similar to that of chlorine and hot aqueous sodium hydroxide. Sodium iodate, $\text{NaIO}_3$, is formed as one of the products.
Suggest an equation for the reaction of iodine and hot aqueous sodium hydroxide.
................................................................................................................................. [1]

(c) The decomposition of hydrogen peroxide, $\text{H}_2\text{O}_2$, is catalysed by acidified $\text{IO}_3^{-}$.
$\text{H}_2\text{O}_2$ reduces acidified $\text{IO}_3^{-}$ as shown.
$$5\text{H}_2\text{O}_2 + 2\text{H}^+ + 2\text{IO}_3^{-} \rightarrow \text{I}_2 + 5\text{O}_2 + 6\text{H}_2\text{O}$$
This reaction is followed by the oxidation of $\text{I}_2$ by $\text{H}_2\text{O}_2$.
[Table_1]
(i) Use the data to show that the separate reactions of $\text{H}_2\text{O}_2$ with $\text{IO}_3^{-}$ and with $\text{I}_2$ are both feasible under standard conditions.
In your answer, give the equation for the reaction of $\text{H}_2\text{O}_2$ with $\text{I}_2$.
................................................................................................................................. [3]

(ii) Write the overall equation for the decomposition of $\text{H}_2\text{O}_2$ catalysed by acidified $\text{IO}_3^{-}$.
................................................................................................................................. [1]

(d) A student collects some data for the reaction of $\text{H}_2\text{O}_2$ with acidified $\text{IO}_3^{-}$, as shown in the table.
[Table_2]
(i) Use the data to determine the order of reaction with respect to $[\text{H}_2\text{O}_2]$, $[\text{IO}_3^{-}]$ and $[\text{H}^+]$.
Show your reasoning.
order with respect to $[\text{H}_2\text{O}_2]$ = ....................................................................
order with respect to $[\text{IO}_3^{-}]$ = ....................................................................
order with respect to $[\text{H}^+]$ = ....................................................................
[3]

(ii) Use your answer to (d)(i) to write the rate equation for this reaction.
rate = ................................................................................................................................. [1]

(iii) Calculate the value of the rate constant, $k$, using data from experiment 4 and your answer to (d)(ii).
Give the units of $k$.
$$k = ............................................$$
units = ............................................
[2]

(e) $\text{Pb(IO}_3)_2$ is only sparingly soluble in water at 25°C.
The solubility product, $K_{sp}$, of $\text{Pb(IO}_3)_2$ is $3.69 \times 10^{-13} \text{mol}^3 \text{dm}^{-9}$ at 25°C.
(i) Write an expression for the solubility product of $\text{Pb(IO}_3)_2$.
$$K_{sp} =$$ ................................................................................................................................. [1]

(ii) Calculate the solubility, in $\text{mol dm}^{-3}$, of $\text{Pb(IO}_3)_2$ at 25°C.
solubility = ............................................ $\text{mol dm}^{-3}$ [2]

(f) $\text{NH}_4\text{IO}_3$ is an unstable compound that readily decomposes when warmed. The decomposition reaction is shown.
$$\text{NH}_4\text{IO}_3(s) \rightarrow \frac{1}{2}\text{N}_2(g) + \frac{1}{2}\text{O}_2(g) + \frac{1}{2}\text{I}_2(g) + 2\text{H}_2\text{O}(l) \quad \Delta H = -154.6\text{kJmol}^{-1}$$
(i) Use the data in the table to calculate the entropy change of reaction, $\Delta S$, of the decomposition of $\text{NH}_4\text{IO}_3(s)$.
[Table_3]
$$\Delta S = ...............................\text{JK}^{-1}\text{mol}^{-1}$$ [2]

(ii) This reaction is feasible at all temperatures.
Explain why, using the data in (f) and your answer to (f)(i).
................................................................................................................................. [1]

The transition elements are able to form stable complexes with a wide range of molecules and ions.

(a) State the meaning of transition element.
................................................................................................................................................................. [1]

(b) The d orbitals in an isolated transition metal ion are degenerate. In complexes, the d orbitals occupy two energy levels.

(i) Complete the diagram to show the arrangement of d orbital energy levels in octahedral and in tetrahedral complexes.
[Image_1: Diagram showing energy levels]
.................................................................. [1]
(ii) Sketch the shape of two d orbitals:
• one d orbital from the lower energy level in an octahedral complex
• one d orbital from the higher energy level in an octahedral complex.
Use the axes below.
[Image_2: Axes for d orbitals]
.................................................................. [2]

(c) Edds$^{4-}$ and edta$^{4-}$ are polydentate ligands that form octahedral complexes with Fe$^{3+}$(aq).
[Image_3: Structures of edds$^{4-}$ and edta$^{4-}$]
The formulae of the complexes are [Fe(edds)]$^{-}$ and [Fe(edta)]$^{-}$ respectively.
(i) On the diagram of edds$^{4-}$, circle each atom that forms a bond to the Fe$^{3+}$ ion in [Fe(edds)]$^{-}$ [1]
(ii) [Fe(edds)]$^{-}$ is red and [Fe(edta)]$^{-}$ is yellow.
Explain why the two complexes have different colours.
................................................................................................................................................................. [2]
(iii) When edds$^{4-}$(aq) is added to Fe$^{3+}$(aq), the following reaction occurs.
$$\text{[Fe(H}_2\text{O)]}_6^{3+}\text{(aq) + edds}^{4-}\text{(aq)} \rightleftharpoons \text{[Fe(edds)]}^{-}\text{(aq) + 6H}_2\text{O(l)}$$
State the type of reaction that occurs.
............................................................................................ [1]
(iv) Write an expression for the stability constant, $K_{stab}$, of [Fe(edds)]$^{-}$(aq).
$$K_{stab} = \text{...........................................................}$$ [1]
(v) The table shows the values for the stability constants, $K_{stab}$, of both complexes.
[Table_1: Stability constants]
Predict which of the [Fe(edds)]$^{-}$ and [Fe(edta)]$^{-}$ complexes is more stable.
Explain your answer with reference to the $K_{stab}$ value for each complex.
................................................................................................................................................................. [1]
(vi) When an excess of edta$^{4-}$(aq) is added to [Fe(edds)]$^{-}$(aq), the following equilibrium is established.
$$\text{[Fe(edds)]}^{-}\text{(aq) + edta}^{4-}\text{(aq)} \rightleftharpoons \text{[Fe(edta)]}^{-}\text{(aq) + edds}^{4-}\text{(aq)}$$
Calculate the equilibrium constant, $K_c$, for this equilibrium, using the $K_{stab}$ values given in the table in (c)(v).
$$K_c = \text{...........................................................}$$ [1]

(a) Carboplatin and satraplatin are used as anticancer drugs instead of cisplatin.

(i) Describe the action of cisplatin as an anticancer drug. .............................................. [2]
(ii) Suggest the geometry of the platinum centre in the carboplatin complex. .............................................. [1]
(iii) Suggest why carboplatin does not show cis-trans isomerism. .............................................. [1]
(iv) Satraplatin is a neutral complex, containing the ligands $\text{CH}_3\text{CO}_2^-$, $\text{C}_6\text{H}_{11}\text{NH}_2$, $\text{Cl}^-$ and $\text{NH}_3$. Deduce the oxidation state of platinum in satraplatin. .............................................. [1]

(b) Compound M is made from 1,3-dimethylbenzene in a two-step synthesis.

(i) Draw the structure of L. .............................................. [1]
(ii) Suggest reactants and conditions for each step of this synthesis.
step 1 ..............................................
step 2 .............................................. [2]
(iii) Write an equation for step 2. .............................................. [1]
(iv) A student investigates a possible synthesis of M directly from benzene using $\text{COCl}_2$ in the presence of an $\text{AlCl}_3$ catalyst.
Benzene initially reacts with $\text{COCl}_2$ as shown.
`reaction 1` $\text{COCl}_2 + \text{AlCl}_3 \rightarrow \text{AlCl}_4^- + \text{Cl}^+-\text{C} \equiv \text{O}`
`reaction 2` $\text{C}_6\text{H}_6 + \text{Cl}^+-\text{C} \equiv \text{O} \rightarrow \text{C}_6\text{H}_5\text{COCl} + \text{H}^+$
Reaction 2 is the electrophilic substitution of $\text{Cl}^+-\text{C} \equiv \text{O}$ for $\text{H}^+$ in benzene.
Suggest a mechanism for reaction 2. .............................................. [3]
[Total: 12]

Fumaric acid is a naturally occurring dicarboxylic acid.\

\[ \text{fumuric acid} \] \[ \text{HO}_2\text{C}=\text{CH}-\text{CH}=\text{CO}_2\text{H} \]
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(a) Identify the products of the reaction between fumaric acid and an excess of hot, concentrated, acidified manganate (VII).
........................................................................................................................ [1]

(b) Fumaric acid can form addition and condensation polymers.
(i) Draw the repeat unit of the addition polymer poly(fumaric acid). [1]

(ii) Draw the repeat unit of the polyester formed when fumaric acid reacts with ethane-1,2-diol, (CH₂OH)₂. The ester bond should be shown fully displayed [2]

(iii) Explain why polyesters normally biodegrade more readily than polyalkenes.
........................................................................................................................ [1]

(c) Fumaric acid reacts with cold, dilute, acidified manganate (VII) to form compound P.

\[ \text{P} \] \[ \text{OH}\text{O}_2\text{C}-\text{CH}-\text{C} \text{OH} \text{O}_2\text{C}-\]

Only three stereoisomers of P exist. One of the stereoisomers is shown.

Complete the three-dimensional diagrams in the boxes to show the other two stereoisomers of P.


[2]\

(d) The enzyme fumarase catalyses the reaction of fumarate ions, \( ext{C}_4 ext{H}_2 ext{O}_4^{2-} \), with water to form malate ions, \( ext{C}_4 ext{H}_4 ext{O}_5^{2-} \).

\[ ext{C}_4 ext{H}_2 ext{O}_4^{2-} + ext{H}_2 ext{O} \rightleftharpoons ext{C}_4 ext{H}_4 ext{O}_5^{2-} \]

Describe, with the aid of a suitably labelled diagram, how an enzyme such as fumarase can catalyse a reaction.
..........................................................................................................................
[3]


\text{[Total: 10]}

Proline (Pro) is a naturally occurring amino acid.


(a) Proline is often found bonded to glycine (Gly) in a protein.

(i) Draw the dipeptide Pro-Gly. The peptide bond must be shown fully displayed.

(ii) Name the type of reaction that forms a dipeptide from two amino acids.
............................................................................................................................

(iii) Proline is able to form a poly(proline) peptide chain.
A section of a poly(proline) chain is shown.

Suggest why the secondary structure of poly(proline) cannot be stabilised by hydrogen bonding.
............................................................................................................................

(b) The reaction scheme shows several reactions of proline.

(i) Write an equation for the reaction of proline with NaOH(aq) in reaction 1.
$C_4H_7NHCO_2H +$ ...........................................................................................

(ii) Proline has a secondary amine functional group.
Secondary amines react with acyl chlorides. For example, dimethylamine reacts with RCOCl according to the following equation.

Suggest the skeletal structure of \(R, C_7H_{11}NO_3\), the product of reaction 2.

(iii) Suggest the reagent required for reaction 3.
............................................................................................................................

(c) Proline was first synthesised in the laboratory using a multi-stage synthetic route.
In stage 1, \(CH_2(CO_2C_2H_5)_2\) and \(CH_2=CHCN\) react to form a single product \(U\).

(i) Name all the functional groups present in the reactants of stage 1.
\(CH_2(CO_2C_2H_5)_2\) .........................................................................
\(CH_2=CHCN\) .................................................................................

(ii) Suggest the type of reaction that occurs in stage 1.
............................................................................................................................

In stage 2, \(U\) reacts with reagent \(V\) to form \(W\).

(iii) Suggest a suitable reagent \(V\).
............................................................................................................................

Stage 3 takes place in the presence of an acid catalyst.
\(X\) and \(Y\) are the only products of the reaction.

(iv) Suggest the type of reaction that occurs in stage 3.
............................................................................................................................

(v) Deduce the identity of \(Y\).
............................................................................................................................

After several further stages, \(Z\) is produced.

In the final stage of the synthesis, \(Z\) reacts via a nucleophilic substitution mechanism to form proline.
(vi) Complete the diagram to describe the reaction mechanism of the final stage. Draw curly arrows, ions and charges, partial charges and lone pairs of electrons, as appropriate.
Draw the structure of any organic intermediate ion.


(vii) Identify with an asterisk (*) the chiral centre in proline.


(d) Part of the structure of gelatin is shown.

Identify the number of amino acid units in the structure shown.
............................................................................................................................

(e) (i) At pH 6.5, proline exists in aqueous solution as a zwitterion.
Draw the structure of the zwitterion of proline.
Explain how the zwitterion of proline forms.
............................................................................................................................

(ii) The isoelectric point of an amino acid is the pH at which it exists as a zwitterion.
Three of the amino acids in gelatin are proline, alanine and glutamic acid. Their isoelectric points are shown.
[Table_1]
A mixture of these amino acids was analysed by electrophoresis using a buffer solution at pH 4.0.
Draw and label three spots on the diagram of the electropherogram to indicate the likely position of each of these three species after electrophoresis.
Explain your answer.


(f) The weak acid \textbf{ACES} is a compound that can be used to make a buffer solution for electrophoresis experiments.

The anion of the sodium salt of \textbf{ACES}, \(C_4H_9N_2O_4SNa\), is a strong base.
A buffer solution is prepared by the following steps.
- 3.50 g of \(C_4H_9N_2O_4SNa\) is dissolved in 100 cm\(^3\) of distilled water.
- 50.0 cm\(^3\) of 0.200 mol dm\(^{-3}\) dilute hydrochloric acid is added to the solution.
- The resulting mixture is transferred to a 250.0 cm\(^3\) volumetric flask, and the solution made up to the mark.
\(C_4H_9N_2O_4SNa\) reacts with \(HCl\) with a 1:1 stoichiometry.
The \(pK_a\) of \textbf{ACES} is 6.88 at 298 K.
Calculate the pH of the buffer solution formed at 298 K.
[Mr: \(C_4H_9N_2O_4SNa\), 204.1]
pH = ........................................