AS & A Level Chemistry - 9701 Paper 4 2020 Summer Zone 3

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

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

• the reaction of $[\text{Co}( ext{H}_2 ext{O})_6]^{2+}$ with an excess of aqueous ammonia
colour and state of the cobalt-containing species ..........................................................
ionic equation .......................................................................................................................
type of reaction ................................................................................................................... [6]

(b) When concentrated hydrochloric acid is added to a solution containing $[\text{Co}( ext{H}_2 ext{O})_{6}]^{2+}$, a blue solution of $[\text{CoCl}_{4}]^{2-}$ is formed and the following equilibrium is established.

$$[\text{Co}( ext{H}_2 ext{O})_{6}]^{2+} + 4\text{Cl}^- \rightleftharpoons [\text{CoCl}_{4}]^{2-} + 6\text{H}_2 ext{O}$$

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

![Image depicting stereo isomers of Cobalt complex Co(NH3)3Cl3]

type of stereoisomerism ............................................... [2]

(d) Compound X, $\text{C}_6\text{H}_{18}\text{N}_4$, is a tetradentate ligand.

X
![Image of Compound X structure]

(i) Suggest why one molecule of X can form four dative bonds.
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(ii) $\text{C}_6\text{H}_{18}\text{N}_4$ reacts with aqueous cobalt(II) ions, $[\text{Co}( ext{H}_2 ext{O})_6]^{2+}$, in a 1:1 ratio to form a new complex ion.
Construct an equation for this reaction.
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(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 $\text{Mg(OH)}_2$ and $\text{Sr(OH)}_2$ 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.

$2H_2O \rightarrow O_2 + 4H^+ + 4e^-$ \text{ 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, $E^*$, 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 \textit{Data Booklet} to calculate the $E^*_\text{cell}$ and deduce which electrode is positive.

$E^*_\text{cell} = \text{......................... V}$
identity of the positive electrode = \text{...............................}

[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
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(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 .................................................................................................................................................................................

(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 ..............................................................................................................................

(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.
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$\text{AlBr}_3$ 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.
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(b) Suggest why bromination of phenol occurs more readily than bromination of benzene.
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(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: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2^+$
structure 2: ............................................
structure 3: ............................................
structure 4: ............................................ [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.
[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 ..................................................................................................................
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(b) (i) The numerical values of $K_a$ for methanoic acid, $\text{HCO}_2\text{H}$, and pyruvic acid, $\text{CH}_3\text{COCO}_2\text{H}$, are given.

[Table_1: acid $|$ $K_a$ \text{HCO}_2\text{H} $|$ $1.78 \times 10^{-4}$ \text{CH}_3\text{COCO}_2\text{H} $|$ $4.07 \times 10^{-3}$]

An equilibrium mixture containing the two acid-base pairs is formed.
$$\text{HCO}_2^- + \text{CH}_3\text{COCO}_2\text{H} \rightleftharpoons \text{HCO}_2\text{H} + \text{CH}_3\text{COCO}_2^-$$
Use the $K_a$ values to calculate the equilibrium constant, $K_{eq}$, for this equilibrium.
$K_{eq} =$ ...............................

(b) (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: equilibrium lies to the left $|$ equilibrium lies in the middle $|$ equilibrium lies to the right]
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(b) (iii) Ethanedioic acid, $\text{HO}_2\text{CCO}_2\text{H}$, has two dissociation constants, $K_{a1}$ and $K_{a2}$, whose $p_{Ka}$ values are 1.23 and 4.19.
Suggest equations to show the two dissociations that give rise to these $p_{Ka}$ values.
$p_{Ka1}$ 1.23 .........................................................................................................
$p_{Ka2}$ 4.19 .........................................................................................................

(b) (iv) State the mathematical relationship between $p_{Ka}$ and the acid dissociation constant $K_a$.
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(c) Three tests were carried out on separate samples of the organic acids shown in the table. The following results were obtained. ✓ = observed change X = no observed reaction
[Table_3: test $|$ reagent(s) and conditions $|$ $\text{HCO}_2\text{H}$ $|$ $\text{CH}_3\text{COCO}_2\text{H}$ $|$ $\text{HO}_2\text{CCO}_2\text{H}$ $|$ observed change]
1 .............. ✓ $|$ X $|$ X
2 .............. X $|$ ✓ $|$ X
3 .............. ✓ $|$ X $|$ ✓
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, $\text{CH}_3\text{COCO}_2\text{H}$, 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: chemical shift ($\delta$) $|$ carbon atom responsible for chemical shift $|$ hybridisation of the circled carbon atom]
27 [Structure Diagrams]
163 [Structure Diagrams]
192 [Structure Diagrams]

(e) An ester of pyruvic acid, F, is dissolved in $\text{CDCl}_3$ 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: chemical shift ($\delta$) $|$ group responsible for the peak $|$ splitting pattern $|$ number of $^1\text{H}$ atoms responsible for the peak]
1.3
2.2
4.0

(f) Deuterium oxide, $\text{D}_2\text{O}$, where D is $^2\text{H}$, can be used as a solvent in proton NMR spectroscopy. The proton NMR spectrum of alanine in $\text{CDCl}_3$ has 4 peaks. The proton NMR spectrum of alanine in $\text{D}_2\text{O}$ has 2 peaks.

On the diagram of alanine, circle the protons that show peaks in both NMR spectra. Explain your answer.
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(g) The ionic product, $K_w$, for $\text{D}_2\text{O}$ has a value of $1.35 \times 10^{-15} \text{mol}^2\text{dm}^{-6}$ at 298K.
(i) Write the expression for the $K_w$ of $\text{D}_2\text{O}$.
$K_w =$..................................................................................................................

(g) (ii) Calculate the pH of pure, neutral $\text{D}_2\text{O}$ at 298 K. Assume $[\text{D}^+]$ is equivalent to $[\text{H}^+]$ 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 $25^\circ\text{C}$.

(i) Write an expression for the solubility product, $K_{sp}$, of $\text{Ag}_2\text{CO}_3$, and state its units.
$K_{sp}$ = $\hspace{1cm}$ units = $\text{...........................}$ [2]

(ii) Calculate the equilibrium concentration of Ag⁺ in a saturated solution of $\text{Ag}_2\text{CO}_3$ at $25^\circ\text{C}$.
[Ag⁺] = $\text{...........................}$ $\text{ mol dm}^{-3}$ [1]

(iii) Solid $\text{Ag}_2\text{CO}_3$ is stirred at $25^\circ\text{C}$ with $0.050 \text{ mol dm}^{-3}$ $\text{AgNO}_3$ until no more $\text{Ag}_2\text{CO}_3$ dissolves.
Calculate the concentration of carbonate ions, $\left[\text{CO}_3^{2-}\right]$, in this solution.
$\left[\text{CO}_3^{2-}\right]$ = $\text{...........................}$ $\text{ mol dm}^{-3}$ [1]

(iv) An electrochemical cell is set up to measure the electrode potential, $E$, for the Ag⁺/Ag half-cell using the saturated $\text{Ag}_2\text{CO}_3(\text{aq})$ 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 Ag⁺/Ag half-cell.

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

(b) Silver chloride, $\text{AgCl}$, is sparingly soluble in water. The equation for the enthalpy change of solution is shown.

$\text{AgCl}(s) \rightarrow \text{Ag}^+(\text{aq}) + \text{Cl}^-(\text{aq}) \hspace{1cm} \Delta H^\circ_{\text{sol}} = +65.5 \text{ kJ mol}^{-1}$

Standard entropies are shown in the table.

[Table_1]

(i) Calculate the standard entropy change of solution, $\Delta S^\circ$.
$\Delta S^\circ = \text{...........................}$ $\text{ J K}^{-1} \text{ mol}^{-1}$ [1]

(ii) Explain, with the aid of a calculation, why $\text{AgCl}$ is insoluble in water at $25^\circ\text{C}$.
You should use data from this question and your answer to $(b)(i)$.
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(a) Explain what is meant by the term buffer solution. ...........................................................
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[2]

(b) (i) Write an expression for the acid dissociation constant, $K_a$, for ammonium ions, $NH_4^+$$(aq)$$. $K_a$ = ..............................................
[1]

(ii) Write two equations to describe how a solution containing ammonium ions, $NH_4^+$$(aq)$$, and ammonia, $NH_3$$(aq)$$, can act as a buffer. ...........................................................
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[2]

(iii) The numerical value of $K_a$ for $NH_4^+$$(aq)$$ is $5.6 \times 10^{-10}$ at 298 K. 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 298 K. pH = ...........................
[3]

(a) Manganese(IV) oxide, MnO_2, catalyses the decomposition of hydrogen peroxide, H_2O_2, as shown.



The mechanism involves the formation of the intermediate species, Mn^{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_2 can catalyse this reaction.

equation 1 ........................................................................................................................................
equation 2 ........................................................................................................................................

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



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 $H_2O_2 + I^- \rightarrow IO^- + H_2O$
step 2 $H^+ + IO^- \rightarrow HIO$
step 3 $HIO + I^- \rightarrow I_2 + OH^-$
step 4 $OH^- + H^+ \rightarrow H_2O$

Step 1 is the rate-determining step.

(i) State what is meant by the term rate-determining step.
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(ii) Use this mechanism to construct a balanced equation for this reaction.
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(iii) Deduce the order of reaction with respect to each of the following.
$H_2O_2 = ...................$ $I^- = ...................$ $H^+ = ...................$ [1]

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

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