AS & A Level Chemistry - 9701 Paper 4 2020 Spring Zone 2

Iron is a transition element in the fourth period. Iron forms compounds containing the ions $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$.
(a) (i) Define the term \textit{transition element}.
.............................................................................................................................. [1]

(ii) Compare the melting point and density of iron with those of calcium, an s-block element in the fourth period.
melting point ........................................................................................................................
density ...................................................................................................................................... [1]

(iii) Complete the electronic configuration of an isolated gaseous $\text{Fe}^{2+}$ ion.
$1s^2$ ................................................................................................................................. [1]

(iv) Aqueous $\text{Fe}^{3+}$ ions form coloured complexes.
Explain the origin of the colour in transition element complexes.
...................................................................................................................................... [4]

(b) When an excess of CN$^-$ (aq) ions is added to green $\text{[Fe(H}_2\text{O})_6]^{2+}$ (aq) ions, yellow $\text{[Fe(CN)}_6]^{4-}$ complex ions are formed.
Heating $\text{[Fe(CN)}_6]^{4-}$ with dilute nitric acid and then neutralising the product with $\text{Na}_2\text{CO}_3$(aq) produces red crystals, containing the $\text{[Fe(CN)}_5\text{NO]}^{2-}$ complex ion.

NO is a neutral, monodentate ligand.
(i) State the shape of the $\text{[Fe(H}_2\text{O})_6]^{2+}$ (aq) complex ion.
.............................................................................................................................. [1]

(ii) Write the equation for the reaction between $\text{[Fe(H}_2\text{O})_6]^{2+}$ (aq) ions and an excess of CN$^-$ (aq) ions.
............................................................................................................................. [1]

(iii) Deduce the oxidation states of iron in:
$\text{[Fe(CN)}_6]^{4-}$ ....................................................... $\text{[Fe(CN)}_5\text{NO]}^{2-}$ ............................................................. [1]

(iv) Define the term \textit{monodentate ligand}.
.............................................................................................................................. [2]

(v) Complete the diagram to show the three-dimensional structure of the $\text{[Fe(CN)}_5\text{NO]}^{2-}$ complex ion:
[1]

(vi) The two complex ions $\text{[Fe(CN)}_6]^{4-}$ and $\text{[Fe(CN)}_5\text{NO]}^{2-}$ are different colours.
Explain why the colours of the two complex ions are different.
.............................................................................................................................. [2]

(c) E is a complex ion, $\text{[Fe(C}_2\text{O}_4)_2\text{Cl}_2]^{4-}$, containing $\text{Fe}^{2+}$ with a coordination number of 6.
(i) Define the term \textit{coordination number}.
.............................................................................................................................. [1]

(ii) E shows both optical isomerism and cis-trans isomerism.
One isomer of E is shown. The $\text{C}_2\text{O}_4^{2-}$ ion is represented as {} .
In the boxes, draw three-dimensional diagrams to show:
- the trans isomer of E
- the optical isomer of E.
[Images of structures][2]

(iii) $\text{[Fe(C}_2\text{O}_4)_2\text{Cl}_2]^{4-}$ contains ligands which are anions of ethane-dioic acid, $\text{HO}_2\text{CCO}_2\text{H}$.
Complete the table to show any observations for the reactions of $\text{HO}_2\text{CCO}_2\text{H}$ with the named reagents.
Where no change is observed, write 'none'.
[Table_1]
Warm acidified manganate(VII)
2,4-dinitrophenylhydrazine
Warm Tollens’ reagent
.............................................................................................................................. [2]

(a) Group 2 metals form stable carbonates and sulfates.
(i) State and explain the trend in the thermal stability of the Group 2 carbonates down the group.
.............................................................................................................................................. [3]
(ii) The sulfates of Group 2 elements become less soluble down the group.
Explain this trend.
.............................................................................................................................................. [3]

(b) Aluminium is extracted from $Al_2O_3$ by electrolysis. $Al_2O_3$ is dissolved in cryolite in this process.
(i) The half-equation for the reaction at the anode is shown.
$$O^{2-} + C \rightarrow CO + 2e^-$$
Use this half-equation to write the ionic equation for the electrolysis of $Al_2O_3$.
....................................................................................................................................................... [1]
(ii) Aluminium oxide is electrolysed for 3.0 hours using carbon electrodes and a current of $3.5 \times 10^5$ A.
Calculate the mass of aluminium that is formed.
mass of aluminium = .............................. g [3]
(iii) Cryolite can be made from $SiF_4$.
The first step in this conversion is the reaction of $SiF_4$ with $H_2O$, forming $H_2SiF_6$ and $SiO_2$.
Write an equation for this reaction.
....................................................................................................................................................... [1]
[Total: 11]

Gold is an unreactive metal that can only be oxidised under specific conditions.

(a) The standard electrode potential, $E^{\circ}$, of $\text{Au}^{3+}(aq)/\text{Au}(s)$ is +1.50 V.
(i) Define the term standard electrode potential.
............................................................................................................................................................................... [2]
(ii) Draw a fully labelled diagram of the apparatus that should be used to measure the standard cell potential, $E_{cell}^{\circ}$, of $\text{Au}^{3+}(aq)/\text{Au}(s)$ and $\text{HNO}_3(aq)/\text{NO}(g)$.
Include all necessary chemicals. [4]

Some relevant half-equations and their standard electrode potentials are given.

[Table with half-equations]

(iii) Write an ionic equation to show the spontaneous reaction that occurs when an electric current is drawn from the cell in (a)(ii).
................................................................................................................................................................. [1]
(iv) Calculate the $E_{cell}^{\circ}$ of the reaction in (a)(iii).
$E_{cell}^{\circ} = \text{............................... V}$ [1]

(v) Gold can be oxidised by a mixture of concentrated hydrochloric acid and concentrated nitric acid, known as aqua regia. Concentrated hydrochloric acid is $12 \text{ mol dm}^{-3}$. Concentrated nitric acid is $16 \text{ mol dm}^{-3}$.

Explain why aqua regia is able to dissolve gold.
In your answer, state and explain what effect the use of concentrated hydrochloric acid and concentrated nitric acid have on the $E$ values of half-equations 2 and 3.
................................................................................................................................................................. [3]

(b) Aqueous gold(III) chloride, $\text{AuCl}_3$, reacts with aqueous hydrogen peroxide, $\text{H}_2\text{O}_2$, under certain conditions, forming Au, $\text{O}_2$ and HCl.

A student carries out separate experiments using different initial concentrations of $\text{AuCl}_3$ and $\text{H}_2\text{O}_2$. The initial rate of each reaction is measured.

The table shows the results that are obtained.

[Table with experimental data]

(i) Write an equation for the reaction of $\text{AuCl}_3$ with $\text{H}_2\text{O}_2$.
..................................................................................................................................................... [1]
(ii) Determine the rate equation of the reaction.
Show your reasoning, quoting data from the table.
..................................................................................................................................................... [3]
(iii) Use the results of experiment 2 to calculate the value of the rate constant, $k$, for this reaction.
Include the units of $k$.
rate constant, $k = \text{..............................}$
units = \text{..............................} [2]

(c) $\text{AlF}_3$ is an ionic compound.

The Born–Haber cycle for the formation of $\text{AlF}_3$ is shown.



(i) Name the enthalpy changes labelled $\Delta H_4$ and $\Delta H_6$.
$\Delta H_4 =$ ..............................................................................................................................
$\Delta H_6 =$ .............................................................................................................................. [2]

(ii) Use the data in the table and data from the Data Booklet to calculate the lattice energy of $\text{AlF}_3$.

[Table with process data]

lattice energy of $\text{AlF}_3 = \text{.............................. kJ mol}^{-1}$ [2]

(iii) Scandium fluoride, $\text{ScF}_3$, is an ionic compound.

Use data from the Data Booklet to suggest how the lattice energy of $\text{AlF}_3$ compares with the lattice energy of $\text{ScF}_3$.

Explain your answer.
..................................................................................................................................................... [2]

(d) $\text{AlF}_3$ is sparingly soluble in water. The concentration of its saturated solution at 298 K is $6.5 \times 10^{-2} \text{ mol dm}^{-3}$.

(i) Write an expression for the solubility product, $K_{sp}$, of $\text{AlF}_3$.
$K_{sp} =$ .............................................................................................................................. [1]
(ii) Calculate the numerical value of $K_{sp}$ for $\text{AlF}_3$ at 298 K.
$K_{sp} =$ .............................................................................................................................. [1]

Compound F has been found in small quantities in some cereals and dried fruit.



(a) (i) Give the name of the functional groups labelled A and B.

A .....................................................................................................................................
B ..................................................................................................................................... [2]

(ii) State the number of chiral carbon atoms in one molecule of F.
............................................................................................................................................... [1]

(b) F can be hydrolysed by heating with an excess of dilute hydrochloric acid, as shown.
Three products are formed: G and two others.



Draw the structures of the other products of the reaction in the boxes provided. [3]

(c) Compound H is formed in one step of a different synthesis, as shown.



(i) State the role of FeCl$_3$ in this step.
................................................................................................................................................. [1]

(ii) Use the Data Booklet to suggest two reasons why the chlorine atom in compound H substitutes into the ring at the position shown, instead of the other positions in the ring.
............................................................................................................................................... [2]

(d) Compound J, C$_x$H$_y$O$_2$, is also found in some cereals.
Part of the mass spectrum of J is shown. The M and M+1 peaks are labelled, along with their relative intensities.



(i) Calculate the number of carbon atoms, x, present in J.
x = ............................... [2]

(ii) The mass spectrum has a peak at m/e = 205.
Suggest the identity of the fragment lost from J to form this peak.
............................................................................................................................................... [1]

Gallic acid, $C_7H_6O_5$, is a naturally occurring aromatic molecule.

[Image_1: Structure of gallic acid]

(a) Gallic acid contains the carboxylic acid and phenol functional groups.
State and explain the relative acid strength of these two functional groups.
........................................................................................................................................................... [2]

(b) A buffer solution was prepared by dissolving 2.04 g of gallic acid in 250 cm$^3$ of a solution containing 0.0600 mol dm$^{-3}$ of gallate ions, $C_7H_5O_5^-$.
$$C_7H_6O_5 \rightleftharpoons C_7H_5O_5^- + H^+ \quad K_a = 3.89 \times 10^{-5} \text{ mol dm}^{-3} \text{ at } 298K$$
(i) Define the term buffer solution.
........................................................................................................................................................ [2]
(ii) Calculate the pH of this buffer solution.
pH = .................................. [3]
(iii) Write two equations to show how a solution containing gallic acid, $C_7H_6O_5$, and gallate ions, $C_7H_5O_5^-$, acts as a buffer.
........................................................................................................................................................ [2]

(c) Compound K is used as the starting material in a synthesis of gallic acid.
A student suggested the first two steps of the synthesis could be as shown.
[Image_2: Reaction involving compounds K, L, and M]

Nitronium ions, $NO_2^+$, are generated by the reaction between concentrated sulfuric acid and concentrated nitric acid.
(i) Construct an equation for the formation of $NO_2^+$ by this method.
........................................................................................................................................................ [1]
(ii) Complete the mechanism and draw the intermediate of step 1.
Include all relevant charges and curly arrows to show the movement of electron pairs.
[Image_3: Reaction mechanism with electron movement]
[2]
(iii) State the name of the mechanism in (c)(ii).
........................................................................................................................................................ [1]

Compound M is converted into compound P as shown.
[Image_4: Reaction showing conversion of M to P]
(iv) State the reagents and conditions for step 4.
........................................................................................................................................................ [2]

P reacts with an excess of sodium nitrite, $NaNO_2$, and dilute $HCl$ at 5°C to form compound Q, $C_9H_7N_6O_2Cl_3$.
Compound Q is then converted into gallic acid. [Image_5: Structure of compound Q]

(v) Suggest the structure of compound Q in the box provided.
[2]
(vi) State the reagents and conditions for step 6.
........................................................................................................................................................ [1]

(d) (i) State the number of peaks that would be observed in the $^{13}C$ NMR spectrum of gallic acid.
[Image_6: Structure of gallic acid]
........................................................................................................................................................ [1]
(ii) The proton NMR spectrum of gallic acid dissolved in $D_2O$ is recorded.
● Predict the number of peaks observed and any expected splitting pattern.
● State the expected chemical shift range ($\delta$) of each peak predicted.
........................................................................................................................................................ [2]

Valinol can be synthesised by the following reactions. Reaction 1 uses valine as the starting material.

(a) (i) Write an equation for reaction 1, using [H] to represent the reducing agent.
.......................................................................................................................... [1]
(ii) Suggest a suitable reagent for reaction 1.
.......................................................................................................................... [1]
(iii) Name the mechanism for reaction 2.
.......................................................................................................................... [1]
(b) Valine and glycine, $\text{H}_2\text{NCH}_2\text{COOH}$, form the tripeptide Gly–Val–Gly.
Draw the structure of this tripeptide. Show the peptide bonds fully displayed. [2]
(c) (i) Valine exists as two stereoisomers.
Draw three-dimensional diagrams to show the two stereoisomers of valine. In your diagrams, the $–\text{CH(CH}_3\text{)}_2$ group can be represented by $–\text{R}$.
State the type of stereoisomerism shown.

type of stereoisomerism .............................................................................. [2]
(ii) Valine is an amino acid.
Draw the zwitterion of valine. [1]
(iii) Valinate, $\text{Val}^-$, is the anion of valine. It takes part in a ligand substitution reaction with hexaaquanickel(II) ions. Complex $Z$ is formed.
[Ni(H$_2$O)$_6$]$^{2+}$(aq) + 2Val$^-$(aq) $\rightleftharpoons$ [Ni(H$_2$O)$_2$(Val)$_2$](aq) + 4H$_2$O(l)
Write an expression for $K_{\text{stab}}$ for this equilibrium.
$K_{\text{stab}}$ = .................................................................................. [1]
(iv) At room temperature, the numerical value of $K_{\text{stab}}$ is $2.34 \times 10^5$.
Explain what this value indicates about the equilibrium and the stability of complex $Z$.
.......................................................................................................................... [1]
(v) $Z$ is an octahedral complex with formula [Ni(H$_2$O)$_2$(Val)$_2$].
Use this information to state the type of ligand that the valinate ion is acting as in this complex.
.......................................................................................................................... [1]