AS & A Level Chemistry - 9701 Paper 4 2020 Summer Zone 1

(a) An aqueous solution of cobalt(II) contains the $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ complex ion.

(i) Define the term complex ion.
...............................................................................................................................
...............................................................................................................................

(a) An aqueous solution of cobalt(II) contains the $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ complex ion.

(ii) Samples of $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ are reacted separately with aqueous sodium hydroxide and with an excess of aqueous ammonia.
Give the following information about these reactions.
• the reaction of $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ with aqueous sodium hydroxide
colour and state of the cobalt-containing species ........................................................
ionic equation ................................................................................................
type of reaction .................................................................................................
• the reaction of $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ with an excess of aqueous ammonia
colour and state of the cobalt-containing species ........................................................
ionic equation ................................................................................................
type of reaction .................................................................................................

(b) When concentrated hydrochloric acid is added to a solution containing $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$, a blue solution of $[\text{CoCl}_4]^{2−}$ is formed and the following equilibrium is established.
$$[\text{Co}(\text{H}_2\text{O})_6]^{2+} + 4\text{Cl}^- \rightleftharpoons [\text{CoCl}_4]^{2−} + 6\text{H}_2\text{O}$$
Use Le Chatelier’s principle to suggest the expected observations when silver nitrate solution is added dropwise to the blue solution of $[\text{CoCl}_4]^{2−}$. Explain your answer.
...............................................................................................................................
...............................................................................................................................
...............................................................................................................................

(c) The $[\text{Co}(\text{NH}_3)_3\text{Cl}_3]$ complex shows stereoisomerism.
Complete the three-dimensional diagrams to show the two isomers of $[\text{Co}(\text{NH}_3)_3\text{Cl}_3]$.
Suggest the type of stereoisomerism.

type of stereoisomerism ..............................................

(d) Compound $X$, $\text{C}_6\text{H}_{18}\text{N}_4$, is a tetradentate ligand.
$X$
(i) Suggest why one molecule of $X$ can form four dative bonds.
...............................................................................................................................
...............................................................................................................................
...............................................................................................................................

(d) Compound $X$, $\text{C}_6\text{H}_{18}\text{N}_4$, is a tetradentate ligand.
$X$
(ii) $\text{C}_6\text{H}_{18}\text{N}_4$ reacts with aqueous cobalt(II) ions, $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$, in a 1:1 ratio to form a new complex ion.
Construct an equation for this reaction.
...............................................................................................................................

(a) (i) Describe and explain the trend in the solubility of the Group 2 hydroxides down the group.
.................................................................................................................
................................................................................................................
................................................................................................................
................................................................................................................
............................................................... [4]
Group 2 hydroxides decompose on heating to give the corresponding metal oxide and water vapour.
(ii) Suggest which of Mg(OH)$_{2}$ and Sr(OH)$_{2}$ will decompose at a \textit{lower} temperature.
Explain your answer.
................................................................................................................
................................................................................................................
................................................................................................................
................................................................................................................ [2]

The overall reaction for photosynthesis is shown.

$$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$

Water is oxidised in this process according to the following half-equation.

$$2H_2O \rightarrow O_2 + 4H^+ + 4e^-$$ half-equation 1

(a) (i) Use these equations to deduce the half-equation for the reduction of carbon dioxide in this process. [2]

(ii) Draw a fully labelled diagram of the apparatus that should be used to measure the standard electrode potential, $E^\circ$, of $O_2(g)$ in half-equation 1 under standard conditions. Include all necessary chemicals. [4]

(iii) For the cell drawn in (a)(ii), use the Data Booklet to calculate the $E^\circ_{cell}$ and deduce which electrode is positive.

$E^\circ_{cell}$ = ............................ V
identity of the positive electrode = ............................ [1]

(a) The molecular formulae of three nitrogen-containing compounds are given.

S     $C_6H_5CONH_2$
T     $C_6H_5NH_2$
U     $C_6H_5CH_2NH_2$

Describe and explain the relative basicities of S, T and U.
.................................... $>$ .................................... $>$ ....................................
            most basic             least basic
........................................................................................................................................................
........................................................................................................................................................
........................................................................................................................................................
........................................................................................................................................................
........................................................................................................................................................
[3]

(b) Compound U can be prepared by two different methods as shown.


(i) Suggest reagents and conditions for reaction 1 and for reaction 2.
reaction 1 ..............................................................................................................................
reaction 2 ..............................................................................................................................
[2]

(ii) State the type of reaction in reaction 1 and name the mechanism in reaction 2.
type of reaction in reaction 1 .............................................................................................
mechanism of reaction 2 .....................................................................................................
[2]

(a) Benzene reacts with bromine in the presence of an aluminium bromide catalyst, $\text{AlBr}_3$, to form bromobenzene. This is a substitution reaction. No addition reaction takes place.
(i) Explain why no addition reaction takes place.
............................................................................................................................................................................................
............................................................................................................................................................................................ [1]
$\text{AlBr}_3$ reacts with bromine to generate an electrophile, $\text{Br}^+$.
(ii) Draw the mechanism of the reaction between benzene and $\text{Br}^+$ ions. Include all relevant arrows and charges.
............................................................................................................................................................................................ [3]
(iii) Write an equation to show how the $\text{AlBr}_3$ catalyst is reformed.
............................................................................................................................................................................................ [1]

(b) Suggest why bromination of phenol occurs more readily than bromination of benzene.
............................................................................................................................................................................................
............................................................................................................................................................................................ [2]

(c) (i) There are four different carbocations with the same formula, $\text{C}_4\text{H}_9^{+}$. One structure is given in the table.
Suggest the structural formulae of the three other carbocations.
[Table]
structure 1 structure 2 structure 3 structure 4
CH$_3$CH$_2$CH$_2$CH$_2^{+}$
............................................................................................................................................................................................ [3]
(ii) Benzene reacts with each of these carbocations in separate Friedel-Crafts alkylation reactions.
In each reaction an organic compound with formula $\text{C}_{10}\text{H}_{14}$ is formed. The number of peaks observed in the carbon-13 NMR spectrum of each compound is given.
Suggest the structures for the three other compounds.

[Table]
number of peaks in carbon-13 NMR = 8
number of peaks in carbon-13 NMR = 6
number of peaks in carbon-13 NMR = 7
number of peaks in carbon-13 NMR = 8
............................................................................................................................................................................................ [4]

(a) Compare and explain the relative acidities of 2-chloropropanoic acid, 3-chloropropanoic acid, and propanoic acid. Explain your answer.
.............................. > .............................. > ..............................
most acidic least acidic
explanation ..................................................................................................... ..................................................................................................... ..................................................................................................... ..................................................................................................... .....................................................................................................

(b)
(i) The numerical values of $K_a$ for methanoic acid, $HCO_2H$, and pyruvic acid, $CH_3COCO_2H$, are given.
[Table_1]
An equilibrium mixture containing the two acid-base pairs is formed.
$HCO_2^- + CH_3COCO_2H \rightleftharpoons HCO_2H + CH_3COCO_2^-$
Use the $K_a$ values to calculate the equilibrium constant, $K_{eq}$, for this equilibrium.
$K_{eq} =$ .............................. [1]
(ii) Use your value of $K_{eq}$ to predict the position of this equilibrium. Indicate this by placing a tick (✓) in the appropriate box in the table. Explain your answer.
[Table_2]
.....................................................................................................
.....................................................................................................

(iii) Ethanedioic acid, $HO_2CCO_2H$, has two dissociation constants, $K_{a1}$ and $K_{a2}$, whose $pK_a$ values are 1.23 and 4.19. Suggest equations to show the two dissociations that give rise to these $pK_a$ values.
$pK_{a1}$ 1.23 ....................................................................
$pK_{a2}$ 4.19 ....................................................................
(iv) State the mathematical relationship between $pK_a$ and the acid dissociation constant $K_a$.
........................................................................................

(c) Three tests were carried out on separate samples of the organic acids shown in the table. The following results were obtained.
$\checkmark$ = observed change
$X$ = no observed reaction
[Table_3]
Complete the table with the reagent(s) and conditions and the observed change for each test. Assume these organic acids all have a similar acid strength.

(d) A sample of pyruvic acid, $CH_3COCO_2H$, is analysed by carbon-13 NMR spectroscopy. Three peaks are observed. Complete the table by:
• circling the carbon atom responsible for the chemical shift
• stating the hybridisation of the circled carbon atom.
[Table_4]

(e) An ester of pyruvic acid, $F$, is dissolved in $CDCl_3$ and analysed by proton NMR spectroscopy.

The proton NMR spectrum of $F$ is shown.

Use the proton NMR spectrum of $F$ to complete the table.
[Table_5]

(f) Deuterium oxide, $D_2O$, where $D$ is $^2H$, can be used as a solvent in proton NMR spectroscopy. The proton NMR spectrum of alanine in $CDCl_3$ has 4 peaks. The proton NMR spectrum of alanine in $D_2O$ has 2 peaks.
[Diagram_1]
On the diagram of alanine, circle the protons that show peaks in both NMR spectra. Explain your answer.
.....................................................................................................
.....................................................................................................

(g) The ionic product, $K_w$, for $D_2O$ has a value of $1.35 \times 10^{-15}$ mol$^2$ dm$^{-6}$ at 298K.
(i) Write the expression for the $K_w$ of $D_2O$.
$K_w =$ ................................................................................ [1]
(ii) Calculate the pH of pure, neutral $D_2O$ at 298K. Assume $[D^+]$ is equivalent to $[H^+]$ for pH calculations.
pH = ..........................................................

(a) Silver carbonate, $\text{Ag}_2\text{CO}_3$, is sparingly soluble in water. The numerical value of the solubility product, $K_{sp}$, for silver carbonate is $6.3 \times 10^{-12}$ at 25°C.

(i) Write an expression for the solubility product, $K_{sp}$, of $\text{Ag}_2\text{CO}_3$, and state its units.

$K_{sp} = $
units = ............................. [2]

(ii) Calculate the equilibrium concentration of $\text{Ag}^+$ in a saturated solution of $\text{Ag}_2\text{CO}_3$ at 25°C.

$[\text{Ag}^+] = $ .............................. mol\text{dm}^{-3} [1]

(iii) Solid $\text{Ag}_2\text{CO}_3$ is stirred at 25°C with $0.050$ mol\text{dm}^{-3} $\text{AgNO}_3$ until no more $\text{Ag}_2\text{CO}_3$ dissolves.
Calculate the concentration of carbonate ions, $[\text{CO}_3^{2-}]$, in this solution.

$[\text{CO}_3^{2-}] = $ .............................. mol\text{dm}^{-3} [1]

(iv) An electrochemical cell is set up to measure the electrode potential, $E$, for the $\text{Ag}^+\text{/Ag}$ half-cell using the saturated $\text{Ag}_2\text{CO}_3$(aq) with a standard hydrogen electrode.
Use the Data Booklet, your answer to (a)(ii), and the Nernst equation to calculate the electrode potential, $E$, for this $\text{Ag}^+\text{/Ag}$ half-cell.

$E$ for $\text{Ag}^+\text{/Ag}$ half-cell = .............................. V [2]

(b) Silver chloride, $\text{AgCl}$, is sparingly soluble in water. The equation for the enthalpy change of solution is shown.
$$\text{AgCl}(s) \rightarrow \text{Ag}^+(aq) + \text{Cl}^-(aq) \quad \Delta H^\circ_{sol} = +65.5 \text{kJ mol}^{-1}$$
Standard entropies are shown in the table.

[Table_1]
\begin{tabular}{|c|c|c|c|}\hline \text{species} & \text{AgCl}(s) & \text{Ag}^+(aq) & \text{Cl}^-(aq) \\ \hline \text{S}^\circ / \text{J K}^{-1} \text{mol}^{-1} & +96.2 & +72.7 & +56.5 \\ \hline \end{tabular}

(i) Calculate the standard entropy change of solution, $\Delta S^\circ$.

$\Delta S^\circ = $ .............................. $\text{J K}^{-1} \text{mol}^{-1} [1]$

(ii) Explain, with the aid of a calculation, why $\text{AgCl}$ is insoluble in water at 25°C.
You should use data from this question and your answer to (b)(i).

.............................................................................................................................................
............................................................................................................................................. [3]

(a) Explain what is meant by the term buffer solution.
.........................................................................................................................................
.........................................................................................................................................
.........................................................................................................................................
......................................................................................................................................... [2]

(b) (i) Write an expression for the acid dissociation constant, $K_a$, for ammonium ions, $\text{NH}_4^+(aq)$.

$K_a = $

........................................................................ [1]

(ii) Write two equations to describe how a solution containing ammonium ions, $\text{NH}_4^+(aq)$, and ammonia, $\text{NH}_3(aq)$, can act as a buffer.

.........................................................................................................................................
......................................................................................................................................... [2]

(iii) The numerical value of $K_a$ for $\text{NH}_4^+(aq)$ is $5.6 \times 10^{-10}$ at 298K.
A buffer solution was prepared by adding $0.80 \text{dm}^3$ of $0.25 \text{mol dm}^{-3}$ ammonia, an excess, to $0.20 \text{dm}^3$ of $0.20 \text{mol dm}^{-3}$ hydrochloric acid.

Calculate the pH of the buffer solution formed at 298K.

$\text{pH} = $ .............................. [3]

(a) Manganese(IV) oxide, MnO\textsubscript{2}, catalyses the decomposition of hydrogen peroxide, H\textsubscript{2}O\textsubscript{2}, as shown.

$$\text{2H}_2\text{O}_2\text{(aq)} \xrightarrow{\text{MnO}_2} \text{2H}_2\text{O(l)} + \text{O}_2\text{(g)}$$

The mechanism involves the formation of the intermediate species, Mn\textsuperscript{2+}, in the first step which is subsequently used up in the second step.

State and use relevant electrode potentials, $E^o$, to construct two equations to show how MnO\textsubscript{2} can catalyse this reaction.

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

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

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

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

equation 1 ......................................................................................................................................

equation 2 ......................................................................................................................................

(b) The equation for the decomposition of hydrogen peroxide without a catalyst is shown.

$$\text{2H}_2\text{O}_2\text{(aq)} \rightarrow \text{2H}_2\text{O(l)} + \text{O}_2\text{(g)}$$

Under certain conditions this reaction is found to be first order with respect to hydrogen peroxide, with a rate constant, $k$, of $2.0 \times 10^{-6} \, \text{s}^{-1}$ at 298 K.

Calculate the initial rate of decomposition of a $0.75 \, \text{mol dm}^{-3}$ hydrogen peroxide solution at 298 K.

initial rate = ................................. $\text{mol dm}^{-3} \text{s}^{-1}$

(c) A four-step mechanism is suggested for the reaction between hydrogen peroxide and iodide ions in an acidic solution.

step 1 $\text{H}_2\text{O}_2 + \text{I}^{-} \rightarrow \text{IO}^{-} + \text{H}_2\text{O}$
step 2 $\text{H}^+ + \text{IO}^{-} \rightarrow \text{HIO}$
step 3 $\text{HIO} + \text{I}^{-} \rightarrow \text{I}_2 + \text{OH}^{-}$
step 4 $\text{OH}^{-} + \text{H}^{+} \rightarrow \text{H}_2\text{O}$

Step 1 is the rate-determining step.

(i) State what is meant by the term rate-determining step.
.......................................................................................................................................
....................................................................................................................................... [1]

(ii) Use this mechanism to construct a balanced equation for this reaction.
....................................................................................................................................... [1]

(iii) Deduce the order of reaction with respect to each of the following.
$$\text{H}_2\text{O}_2 = ....................... \quad \text{I}^{-} = ....................... \quad \text{H}^{+} = .......................$$

(a) The electronic configuration of transition element Q is $[\text{Ar}] \, 3d^2 \, 4s^2$.

Predict the likely oxidation states of element Q in compounds.
.................................................................................................................................... [1]

(b) Suggest why transition elements often show variable oxidation states in their compounds, but typical s-block elements such as calcium do not.
....................................................................................................................................
.................................................................................................................................... [1]

(c) Many enzymes contain transition element complexes.

Describe, with the aid of a suitably labelled diagram, how an enzyme catalyses the breakdown of a substrate molecule.

....................................................................................................................................
....................................................................................................................................
.................................................................................................................................... [3]