AS & A Level Chemistry - 9701 Paper 4 2023 Winter Zone 2

Propanone, CH$_3$COCH$_3$, reacts with iodine, I$_2$, in the presence of an acid catalyst.

CH$_3$COCH$_3$ + I$_2$ → CH$_3$COCH$_2$I + H$^+$ + I$^−$

The rate equation for this reaction is shown.

rate = $k[$CH$_3$COCH$_3$][$H^+$]

(a) Complete Table 1.1 to describe the order of the reaction.

[Table_1]

[2]

(b) An experiment is performed using a large excess of CH$_3$COCH$_3$ and a large excess of H$^+$ (aq). The initial concentration of I$_2$ is 1.00 × 10$^{−5}$ mol dm$^{−3}$. The initial rate of decrease in the I$_2$ concentration is 2.27 × 10$^{−7}$ mol dm$^{−3}$ s$^{−1}$.

(i) Use the axes to draw a graph of [I$_2$] against time for the first 10 seconds of the reaction.



[1]

(ii) State whether it is possible to calculate the numerical value of the rate constant, $k$, for this reaction from your graph. Explain your answer.

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

(c) The experiment is repeated at a different temperature. The initial concentrations of H$^+$ ions, I$_2$ and CH$_3$COCH$_3$ are all 0.200 mol dm$^{-3}$.

The value of $k$ at this temperature is 2.31 × 10$^{-5}$ mol$^{-1}$ dm$^3$ s$^{-1}$.

Calculate the initial rate of this reaction.

rate = ........................................ mol dm$^{-3}$ s$^{-1}$ [1]

(d) The experiment is repeated using an excess of H$^+$ (aq). The new rate equation is shown.

rate = $k_1$[CH$_3$COCH$_3$]

(i) The value of $k_1$ is 1.1 × 10$^{−3}$ s$^{−1}$. Calculate the value of the half-life, $t_{rac{1}{2}}$.

$t_{rac{1}{2}}$ = ..................................................... s [1]

(ii) Use your answer to (i) to draw a graph of [CH$_3$COCH$_3$] against time for this reaction. The initial value of [CH$_3$COCH$_3$] on your graph should be 0.200 mol dm$^{−3}$. The final value of [CH$_3$COCH$_3$] on your graph should be 0.0250 mol dm$^{−3}$.



[1]

(e) A four-step mechanism is suggested for the overall reaction.

CH$_3$COCH$_3$ + I$_2$ → CH$_3$COCH$_2$I + H$^+$ + I$^−$ rate = $k[$CH$_3$COCH$_3$][$H^+$]

Part of this mechanism is shown.

step 1: CH$_3$COCH$_3$ + H$^+$ → CH$_3$C$^+$(OH)CH$_3$
step 2: CH$_3$C$^+$(OH)CH$_3$ → CH$_3$C(OH)=CH$_2$ + H$^+$
step 3: →
step 4: CH$_3$C$^+$(OH)CH$_2$I → CH$_3$COCH$_2$I + H$^+$

(i) Write an equation for step 3.

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

(ii) Suggest the slowest step of the mechanism. Explain your answer.

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(iii) Identify one conjugate acid-conjugate base pair in the mechanism.

conjugate acid ...................................... conjugate base ...................................... [1]

Benzoic acid, $C_6H_5COOH$, is a weak acid. The $K_a$ of benzoic acid is $6.31 \times 10^{-5} \text{mol dm}^{-3}$ at 298K.
A $1.00 \text{dm}^3$ buffer solution is made at 298K containing $1.00 \text{g}$ of $C_6H_5COOH$ and a slightly greater mass of sodium benzoate, $C_6H_5COO^-\text{Na}^+$.
This buffer solution has a pH of 4.15.

(a) Define buffer solution.
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(b) Write equations to show how this solution acts as a buffer solution when the named substances are added to it:
(i) dilute aqueous sodium hydroxide
........................................................................................................................... [1]
(ii) dilute aqueous nitric acid.
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(c) Calculate the $H^+$ concentration and the $C_6H_5COOH$ concentration in the buffer solution described. Use the expression for the $K_a$ of $C_6H_5COOH$ to calculate the concentration of $C_6H_5COO^-\text{Na}^+$ in the buffer solution.
Show your working and give each answer to a minimum of three significant figures.

$[H^+] = ......................... \text{mol dm}^{-3}$
$[C_6H_5COOH] = ......................... \text{mol dm}^{-3}$
$[C_6H_5COO^-\text{Na}^+] = ......................... \text{mol dm}^{-3}$ [3]

(d) A $10.0 \text{cm}^3$ sample of the buffer solution is mixed with $10.0 \text{cm}^3$ of $1.00 \text{mol dm}^{-3}$ KOH. Both solutions are at 298K. A reaction is allowed to occur without stirring.
Two observations are recorded:
• the temperature, after the reaction is complete, is fractionally above 298K
• the pH, after the reaction, is greater than 13.
Explain these two observations.
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........................................................................................................................... [2]

(e) Magnesium benzoate, Mg($C_6H_5COO$)_2, has a solubility in water of less than $1.00 \text{g dm}^{-3}$ at 298K.
$K_{sp} = [\text{Mg}^{2+}][C_6H_5COO^-]^2 = 1.76 \times 10^{-7}$ at 298K

(i) Calculate the solubility of Mg($C_6H_5COO$)_2 in water at 298K. Give your answer in $\text{g dm}^{-3}$.
Show your working.
$[M_r : \text{Mg}(C_6H_5COO)_2 , 266.3]$
solubility = ............................ $\text{g dm}^{-3}$ [2]

(ii) An excess of Mg($C_6H_5COO$)_2 is added to a sample of $0.50 \text{mol dm}^{-3}$ MgSO_4 at 298K.
State whether the equilibrium concentration of Mg($C_6H_5COO$)_2 is higher than, the same as, or lower than your answer to (i). Explain your answer.
The concentration is .................................................................. the concentration in (i).
explanation ..............................................................................
................................................................................................. [1]

(a) (i) Complete the diagram to show a standard hydrogen electrode. Label your diagram. Identify all substances. You do not need to state standard conditions.
[1]
(ii) An electrochemical cell is set up using an $\text{Fe}^{3+}/\text{Fe}^{2+}$ electrode and a standard hydrogen electrode.
Identify the positive electrode in the electrochemical cell and the direction of electron flow in the external circuit.
positive electrode ............................
Electrons flow from the ....................... electrode to the ....................... electrode.
[1]

(b) The vanadium-containing species in the electrode reactions given in Table 3.1 are $\text{V}$, $\text{V}^{2+}$, $\text{V}^{3+}$, $\text{VO}^{2+}$ and $\text{VO}_{2}^{+}$.
(i) Identify one vanadium-containing species that does not react with $\text{Fe}^{2+}$ ions under standard conditions. Use data from Table 3.1 to explain your answer.
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[1]
(ii) Identify all the vanadium-containing species that will react with $\text{Fe}^{2+}$ ions under standard conditions.
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[1]
(iii) Write an equation for one of the possible reactions identified in (ii).
..................................................
[1]

(c) Another electrochemical cell is set up using an $\text{Fe}^{3+}/\text{Fe}^{2+}$ electrode and an alkaline $\text{ClO}^{-}/\text{Cl}^-$ electrode.
The concentration of $\text{Fe}^{3+}$ is 1000 times greater than the concentration of $\text{Fe}^{2+}$ in the $\text{Fe}^{3+}/\text{Fe}^{2+}$ electrode. All other conditions are standard.
(i) Use the Nernst equation to calculate the $E$ value of the $\text{Fe}^{3+}/\text{Fe}^{2+}$ electrode.
Show your working.
$E = .............................. \text{V}$
[2]
(ii) Write an equation for the reaction that occurs in the cell, under these conditions.
.................................................
[1]

(d) Another electrochemical cell is set up using an $\text{Fe}^{2+}/\text{Fe}$ electrode and an alkaline $\text{ClO}^{-}/\text{Cl}^-$ electrode under standard conditions.
Calculate the value of $\Delta G^\circ$ for the cell.
$\Delta G^\circ = .............................. \text{kJ mol}^{-1}$
[3]

(e) A solution of iron(II) sulfate, $\text{FeSO}_4(\text{aq})$, is electrolyzed with iron electrodes. Under the conditions used, no gas is evolved at the cathode.
A current of 0.640 A is passed for 17.0 minutes. The mass of the cathode increases by 0.185 g.
Use these results to calculate an experimental value for the Avogadro constant, $L$.
Show your working.
$L = ...................... \text{mol}^{-1}$
[3]

(f) Iron(II) chloride, $\text{FeCl}_2$, is oxidized by chlorine to form iron(III) chloride, $\text{FeCl}_3$, under standard conditions.
$2\text{FeCl}_2(\text{s}) + \text{Cl}_2(\text{g}) \rightarrow 2\text{FeCl}_3(\text{s})$
$\Delta H^\circ = -128 \text{kJ mol}^{-1}$
Table 3.2
[Table_1]
(i) Use Table 3.2 and other data to calculate the Gibbs free energy change, $\Delta G^\circ$, for this reaction.
Show your working.
$\Delta G^\circ = .......................... \text{kJ mol}^{-1}$
[3]
(ii) Predict whether this reaction becomes more or less feasible at a higher temperature.
Explain your answer.
The reaction becomes ................... feasible.
explanation ..................................................
[1]

The structure of the polydentate ligand, EDTA^{4-}, is shown in Fig. 4.1.
[Image_1: Fig. 4.1 showing the structure of EDTA^{4-}]

The stability constants, at 298K, of five octahedral complexes are given in Table 4.1.
[Table_1: Table 4.1 for stability constants]

(a) Define stability constant.
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(b) Calculate the oxidation states of Cu in [Cu(EDTA)]^{2-} and Cr in [Cr(EDTA)]^{-}.
Cu ...............................
Cr ............................... [1]

(c) Deduce the number of lone pairs donated by each EDTA^{4-} ligand in a single [Fe(EDTA)]^{2–} complex ion.
......................................................................................................................................................................................... [1]

(d) Identify the most stable complex in Table 4.1. Explain your choice.
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(e) In a solution at equilibrium at 298K, [[Cu(H_2O)_6]^{2+}] = 3.00 \times 10^{-10} \text{mol dm}^{-3} and [EDTA^{4-}] = 5.00 \times 10^{-12} \text{mol dm}^{-3}.

Use the expression for K_{stab} to calculate the concentration of [Cu(EDTA)]^{2–} in this solution.
Show your working.

[[Cu(EDTA)]^{2–}] = ................................... \text{mol dm}^{-3} [2]

(f) A solution of [Cu(EDTA)]^{2–} ions is pale blue while a solution of [Cu(NH_3)_4(H_2O)_2]^{2+} ions is deep blue.

Explain this difference in colour.
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Some of the ionic compounds of Group 2 elements undergo thermal decomposition.

Thermal decomposition of solid anhydrous magnesium ethanedioate, MgC₂O₄, occurs above 650 °C. The products are magnesium oxide and a mixture of two different gases, one of which gives a white precipitate with saturated calcium hydroxide solution.

(a) Complete the equation for the thermal decomposition of MgC₂O₄.

MgC₂O₄ → ...................................... [1]

(b) Suggest which of MgC₂O₄ or CaC₂O₄ undergoes thermal decomposition at a lower temperature. Explain your answer.

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

(c) The ethanedioate ion is oxidised by acidified KMnO₄:

5C₂O₄^2- + 2MnO₄^- + 16H⁺ → 10CO₂ + 2Mn²⁺ + 8H₂O

An experiment is performed to find the solubility of MgC₂O₄ in water.

A 40.0 cm³ sample of saturated aqueous MgC₂O₄ requires 27.05 cm³ of 0.00200 mol dm^-3 acidified KMnO₄ to oxidise all the C₂O₄^2- ions.

Calculate the solubility, in mol dm^-3, of MgC₂O₄ in water. Show your working.

solubility = .............................. mol dm^-3 [3]

(a) Phosphine, :PH₃, and carbon monoxide, :CO, are monodentate ligands found in some transition element complexes.

(i) Define monodentate ligand.
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(ii) Define transition element complex.
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(iii) Explain why transition elements form complexes.
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(b) The formulae of six complexes are given in Table 6.1.

The abbreviation \( en \) is used for 1,2-diaminoethane.
The abbreviation \( dien \) is used for the tridentate ligand \( H_2NCH_2CH_2NHCH_2CH_2NH_2 \).
The \( dien \) ligand forms three bonds to the gold ion in \([Au(dien)(H_2O)_2C]^{2+}\) and \( Au(dien)Cl_3 \).
These three bonds all lie in the same plane.
The CO ligand coordinates through the carbon atom in \([Rh(CO)_2Cl_2]^{+}\).



(i) Complete Table 6.1 to state the geometry of the first three complexes. Each complex is either square planar, tetrahedral or octahedral. [1]

(ii) Use complexes \([Au(dien)(H_2O)_2Cl_2]^{2+}\) and \( Au(dien)Cl_3 \) to write an equation showing ligand exchange.
.......................................................................................................................... [1]

(iii) Draw the three-dimensional structure of \( Au(dien)Cl_3 \) in the box. The \( dien \) ligand can be drawn as:
\[ \begin{array}{c} N \quad \quad \quad \quad \quad N \end{array} \]
[1]

(iv) Draw the three-dimensional structure of \( Ni(PH_3)_2Cl_2 \) in the box.
[1]

(v) One of the complexes, \([Rh(en)_2Cl_2]^{+}\) or \([Rh(CO)_2Cl_2]^{+}\), can exist in three isomeric forms.
Identify this complex and the types of isomerism shown.
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(vi) Draw the three-dimensional structures of the two isomers of \([Ni(H_2O)_2(NH_3)_4]^{2+}\) in the boxes and identify the type of isomerism shown.

type of isomerism shown ......................................... [2]

Benzene can be used to make benzoic acid in the two-step process shown in Fig. 7.1.


(a) Give the reagents and conditions for step 1 and step 2.
step 1 ..................................................................................................................................................
step 2 ..................................................................................................................................................

(b) Methylbenzene and benzoic acid each have five different peaks in the carbon (^{13}C) NMR spectrum.

[Table_1]
Use Table 7.1 to complete the two sentences to suggest descriptions of these two spectra.
The carbon (^{13}C) NMR spectrum of methylbenzene:
• has .................. peak(s) in the chemical shift range of ....................... and
• has .................. peak(s) in the chemical shift range of ....................... .

The carbon (^{13}C) NMR spectrum of benzoic acid:
• has .................. peak(s) in the chemical shift range of ....................... and
• has .................. peak(s) in the chemical shift range of ....................... .

(c) (i) When treated with Cl_2 under suitable conditions, methylbenzene forms compound J.
When treated with Cl_2 under \textit{different} conditions with \textit{different} reagents, methylbenzene forms compound K.
Suggest and draw structures of compounds J and K in the boxes. The molecular formula of each compound is given.


State the reagents and conditions required to form each product.
to form compound J ............................................................................................................................
to form compound K ............................................................................................................................

(ii) When treated with a chlorine-containing reagent under suitable conditions, benzoic acid forms compound L.
When treated with a \textit{different} chlorine-containing reagent under \textit{different} conditions, benzoic acid forms compound M.
Suggest and draw structures of compounds L and M in the boxes. The molecular formula of each product is given.


State the reagents and conditions to form compound M from benzoic acid.
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(a) (i) Draw a short length of the PLA polymer chain, including a minimum of two monomer residues. The methyl groups may be written as \( - CH_3 \) but all other bonds should be shown fully displayed.
Label one repeat unit of polylactic acid on your diagram.

(a) (ii) Give the name of the type of polymerisation involved in the formation of PLA and the name of the functional group that forms between the monomers.
type of polymerisation ..........................................................................................................................
functional group ..................................................................................................................................

(a) (iii) Predict whether PLA is readily biodegradable. Explain your answer.
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(b) (i) Use Fig. 8.1 and Table 8.1 to complete Table 8.2.



[Table_1]

Table 8.2

| proton environment | chemical shift (\( \delta \)) | name of splitting pattern |
|--------------------|----------------------------|----------------------------|
| \(-COOH\) | | |
| \(>CH\) | | |
| \(-OH\) | | |
| \(-CH_3\) | | |

(b) (ii) Name the substance responsible for the peak at \( \delta = 0.0 \).
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(b) (iii) Explain why \( CDCl_3 \) is a better solvent than \( CHCl_3 \) for use in proton NMR.
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(c) (i) An impure sample of \( CH_3CH(OH)COOH \) contains pentan-3-one as the only contaminant. The mixture is analysed using gas/liquid chromatography. The pentan-3-one is found to have a longer retention time than the lactic acid.
Explain what is meant by retention time.
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(c) (ii) Suggest suitable substances, or types of substances, that could be used as the mobile and stationary phases.
mobile phase ................................................................................................................................................
stationary phase .............................................................................................................................................

(c) (iii) Describe how the percentage composition of the mixture can be determined from the gas/liquid chromatogram.
.............................................................................................................................................................................
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(a) State the reactants and conditions for two different types of reactions that both produce diethylamine, $\text{CH}_3\text{CH}_2\text{NHCH}_2\text{CH}_3$.

reaction one .............................................................................................................................
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reaction two ............................................................................................................................
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[4]

(b) Describe the relative basicities of diethylamine, phenylamine and ammonia in aqueous solution.

Explain your answer in terms of structure.

........................................... ........................................... ...........................................
least basic                                                 most basic
explanation ..............................................................................................................................
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[3]

(c) Phenylamine reacts with $\text{HNO}_2(\text{aq})$ at 4°C to form compound P. Compound P reacts with phenol under alkaline conditions at 4°C. The product of this reaction is acidified, forming azo compound Q.

Draw the structure of compound Q.
Circle the azo group on your structure.

State one use of an azo compound such as Q.

compound Q:

An azo compound can be used ......................................................................................... . [2]

(d) $\text{CH}_3\text{CH}_2\text{NHCH}_2\text{CH}_3$ reacts with ethanoyl chloride, $\text{CH}_3\text{COCl}$, to give the amide $\text{N,N-diethylethanamide, CH}_3\text{CON(C}_2\text{H}_5)_2$.

An incomplete description of the mechanism of this reaction is shown in Fig. 9.1.

[Image: Fig. 9.1]

(i) Complete the mechanism in Fig. 9.1. You should include:
• all relevant dipoles ($\delta^+$ and $\delta^−$) and full electric charges (+ and –) on the species in box one and in box two
• all relevant lone pairs on the species in box one and in box two
• all relevant curly arrows to show the movement of electron pairs in box one and in box two
• the formula of the second product in box three.

[4]

(ii) Name this mechanism.
................................................................................................................................ [1]