AS & A Level Chemistry - 9701 Paper 4 2021 Summer Zone 1

(a) The carbonates and hydroxides of Group 2 elements show similar trends in thermal stability. Suggest and explain the variation in the trend in the thermal stability of the Group 2 hydroxides.
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(b) Calcium hydroxide is slightly soluble in water.
(i) Write an equation to show the dissociation of calcium hydroxide, Ca(OH)_2(s), in aqueous solution. Include state symbols.
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(ii) Calculate the solubility, in mol dm^{-3}, of Ca(OH)_2.
[K_{sp}: Ca(OH)_2, 5.02 \times 10^{-6} mol^3 dm^{-9}]

solubility = .................................... mol dm^{-3} [2]

(iii) Suggest how the solubility of Ca(OH)_2 in aqueous NaOH compares to its solubility in water.
Explain your reasoning.
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(a) Explain why chromium complexes are coloured. .......................................................................................................... .......................................................................................................... .......................................................................................................... .......................................................................................................... ..........................................................................................................

(b) Four different compounds can be obtained when anhydrous chromium(III) chloride reacts with water under various conditions. When samples of each compound are reacted separately with aqueous silver nitrate, different amounts of silver chloride are precipitated. The precipitation leaves the complex ions P, Q, R and S in solution. [Table_1] (i) Draw three-dimensional diagrams for the structures of complex ions P, Q, R and S. Include the charges for each complex ion.

(b) (ii) Suggest why complex ion S is non-polar. ..........................................................................................................

(c) The structure of picolinic acid is shown. The conjugate base of picolinic acid is a bidentate ligand, Z. (i) Define the term bidentate ligand. ..........................................................................................................

(c) (ii) Draw the structure of Z.

(c) (iii) Z reacts with aqueous chromium(III) ions, $[\text{Cr} (\text{H}_2\text{O})_6]^{3+}$, in a 3 : 1 ratio to form a new neutral complex. State the coordination number and the geometry of the chromium(III) centre in the complex. coordination number .............................. geometry ..............................

(d) $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$ decomposes readily on heating to form $\text{Cr}_2\text{O}_3$, steam and an inert colourless gas. (i) Deduce the oxidation numbers of chromium in $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$ and in $\text{Cr}_2\text{O}_3$. $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$ ................................. $\text{Cr}_2\text{O}_3$ .................................

(d) (ii) Construct an equation for the thermal decomposition of $(\text{NH}_4)_2\text{Cr}_2\text{O}_7$. ..........................................................................................................

(a) (i) Define the term transition element.
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(ii) State how the melting point and density of iron compare to those of calcium.
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(b) (i) Define the term standard cell potential, $E_{cell}^{\circ}$.
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(ii) Draw a fully labelled diagram of the apparatus that can be used to measure the cell potential of a cell composed of a Cu(II)/Cu electrode and an Fe(III)/Fe(II) electrode. Include all necessary reactants. [3]

(c) The reaction between $S_2O_8^{2-}$(aq) and $I^{-}$(aq) is catalysed by adding a few drops of $Fe^{3+}$(aq).
(i) Use equations to show the catalytic role of $Fe^{3+}$ in this reaction. [2]
(ii) $Fe^{3+}$(aq) can oxidise $I^{-}$(aq), whereas $[Fe(CN)_6]^{3-}$(aq) cannot oxidise $I^{-}$(aq).
Use $E^{\circ}$ values to explain these observations.
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(d) When aqueous solutions of $S_2O_8^{2-}$ and tartrate ions are mixed the reaction proceeds very slowly. However, this reaction proceeds quickly in the presence of an $Fe^{3+}$(aq) catalyst. The overall equation for this reaction is shown.



(i) Suggest why this reaction is slow without the $Fe^{3+}$ catalyst.
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(ii) Use the overall equation to deduce the half-equation for the oxidation of tartrate ions, $C_4H_4O_6^{2-}$, to carbon dioxide, $CO_2$, and methanoate ions, $HCO_2^{-}$.
$C_4H_4O_6^{2-}$ + ........................ ↔ .......................................................... [1]

(e) (i) Complete the following table to show the structures of the organic products formed when tartaric acid reacts separately with each reagent. Identify each type of reaction.



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(ii) Tartaric acid reacts with the amine 1-phenylethylamine, $C_6H_5CH(NH_2)CH_3$, to form an ionic salt.
Draw the structure of the salt formed in this reaction. Include the charges on the ions. [1]

(a) Samples of $[\text{Cu(H}_2\text{O})_6]^{2+}$ are reacted separately with an excess of aqueous sodium hydroxide or with an excess of aqueous ammonia. Give the following information about these reactions.

(i) reaction 1: $[\text{Cu(H}_2\text{O})_6]^{2+}$ with an excess of aqueous of sodium hydroxide

colour and state of the copper-containing species ..............................................................
ionic equation ............................................................................................................................
type of reaction ..........................................................................................................................
[3]

(ii) reaction 2: $[\text{Cu(H}_2\text{O})_6]^{2+}$ with an excess of aqueous ammonia

colour and state of the copper-containing species ..............................................................
ionic equation ............................................................................................................................
type of reaction ..........................................................................................................................
[3]

(b) Copper(I) oxide is added to hot dilute sulfuric acid. A blue solution, X, and a red-brown solid, Y, form. Suggest the identities of X and Y. Name the type of reaction.

X ................................................................................................................................................
Y ................................................................................................................................................
type of reaction ..........................................................................................................................[2]

Dinitrogen pentoxide, N\(_2\)O\(_5\), is dissolved in an inert solvent (solv) and the rate of decomposition of N\(_2\)O\(_5\) is investigated. This reaction produces nitrogen dioxide, which remains in solution, and oxygen gas.

$$\text{N}_2\text{O}_5(\text{solv}) \rightarrow 2\text{NO}_2(\text{solv}) + \frac{1}{2}\text{O}_2(\text{g})$$

(a) Suggest what measurements could be used to follow the rate of this reaction from the given information.
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(b) In a separate experiment, the rate of the decomposition of N\(_2\)O\(_5\)(g) is investigated.

$$\text{N}_2\text{O}_5(\text{g}) \rightarrow 2\text{NO}_2(\text{g}) + \frac{1}{2}\text{O}_2(\text{g})$$

The graph shows the results obtained.



The reaction is first order with respect to N\(_2\)O\(_5\). This can be confirmed from the graph using half-lives.

(i) Explain the term half-life of a reaction.
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(ii) Determine the half-life of this reaction. Show your working on the graph.
half-life = ............................. s [1]

(iii) Suggest the effect on the half-life of this reaction if the initial concentration of N\(_2\)O\(_5\) is halved.
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(c) (i) Use the graph in 5(b) to determine the rate of reaction at 200 s. Show your working.
rate = .............................
units = ............................. [2]

The rate equation for this reaction is shown.

$$\text{rate} = k[\text{N}_2\text{O}_5]$$

(ii) Use your answer to (c)(i) to calculate the value of the rate constant, k, for this reaction and state its units.

$$k = ............................. \text{ units } .............................$$ [1]

(d) Nitrogen dioxide reacts with ozone, O\(_3\), as shown.

$$2\text{NO}_2 + \text{O}_3 \rightarrow \text{N}_2\text{O}_5 + \text{O}_2$$

The rate equation for this reaction is rate = k[NO\(_2\)][O\(_3\)].
Suggest a possible two-step mechanism for this reaction.
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(a) Compare and explain the relative acidities of butanoic acid, ethanol, ethanoic acid and water.
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most acidic                                                least acidic
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(b) Three carboxylic acids, methanoic acid, $HCO_2H$, ethanedioic acid, $HO_2CCO_2H$, and butanedioic acid, $HO_2CCH_2CH_2CO_2H$, are compared. Two tests were carried out on separate samples of each organic acid, as shown.
The following results were obtained. ✓ = observed change x = no observed reaction

| test | reagents and conditions | $HCO_2H$ | $HO_2CCO_2H$ | $HO_2CCH_2CH_2CO_2H$ | observed change |
|------|-------------------------|---------|--------------|--------------------|----------------|
| 1 | | ✓ | x | x | |
| 2 | | ✓ | ✓ | x | |

(i) Complete the table with the reagents and conditions and the observed change for a positive test.
Assume these organic acids all have a similar acid strength. [3]

(ii) Each compound, $HCO_2H$, $HO_2CCO_2H$ and $HO_2CCH_2CH_2CO_2H$, is dissolved separately in $CDCl_3$. Proton $(^1H)$ NMR and carbon-13 $(^{13}C)$ NMR spectra are then obtained.
Complete the table.

| compound | number of peaks in proton NMR | number of peaks in carbon-13 NMR |
|------------------|-----------------------------|--------------------------------|
| $HCO_2H$ | | |
| $HO_2CCO_2H$ | | |
| $HO_2CCH_2CH_2CO_2H$ | | |
[2]

(iii) The proton NMR spectrum of $HCO_2H$ in $D_2O$ is obtained.
Describe and explain the difference observed between this spectrum and the proton NMR spectrum of $HCO_2H$ in (b)(ii).
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(c) 1,4-dibromobutane, $Br(CH_2)_4Br$, is used in the synthesis of the dicarboxylic acid J and diamine K as shown.

$BrCH_2CH_2CH_2CH_2Br$
                           step 1| step 3
                                       |
                                       G    H
step 2 ↓                                      ↓ step 4
                                            J             K
       $HO_2CCH_2CH_2CO_2H$             $H_2NCH_2CH_2CH_2CH_2CH_2NH_2$

(i) Draw the structures of G and H in the boxes. [2]

(ii) Suggest reagents and conditions for each of steps 1 to 4.
step 1 ........................................................................................................................................
step 2 ........................................................................................................................................
step 3 ........................................................................................................................................
step 4 ........................................................................................................................................ [4]

(d) Polyamide L can be synthesised from dicarboxylic acid J, $HO_2C(CH_2)_2CO_2H$, and diamine K, $H_2N(CH_2)_6NH_2$.
Draw the repeat unit of the polymer formed in the box. Any functional groups should be shown displayed.

(a) 3-aminobenzoic acid can be synthesised from methylbenzene in three steps.

methylbenzene → M → N → 3-aminobenzoic acid

(i) Draw the structures of M and N in the boxes. [2]

(ii) Suggest reagents and conditions for each step of the synthesis.
step 1 ...............................................................................................................................
step 2 ...............................................................................................................................
step 3 ............................................................................................................................... [3]

(b) A mixture of serine, HOCH$_2$CH(NH$_2$)CO$_2$H, and lysine, H$_2$N(CH$_2$)$_4$CH(NH$_2$)CO$_2$H, reacts to form several different products.

(i) Draw the structures of the two structural isomers with the molecular formula C$_6$H$_{12}$N$_2$O$_5$ that could be present in the product mixture.
The functional group formed in each case should be displayed. [3]

(ii) Predict the number of different structural isomers with the molecular formula C$_9$H$_{19}$N$_3$O$_4$ that could be present in the product mixture.

$$\begin{array}{|c|c|}\hline \text{molecular formula} & \text{number of structural isomers formed} \\ \hline C_9H_{19}N_3O_4 & \text{..................} \\ \hline\end{array}$$ [1]

(c) Glutathione is a naturally occurring compound found in plants.

glutathione

(i) On the diagram of glutathione, label each chiral centre with an asterisk (*). [1]

(ii) Draw the structures of the three products formed after complete acid hydrolysis of glutathione. Assume the thiol group, –SH, does not react. [2]

(iii) Glutathione is soluble in water.
By referring to the structure of glutathione, explain why glutathione is soluble in water.
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(a) The sketch graph for the titration of ethanoic acid, CH$_3$CO$_2$H, with sodium hydroxide is shown.

(i) In the region circled on the graph, identify the two organic species that are present in the solution. Explain why the pH of the mixture only changes slowly and gradually in this region when sodium hydroxide is being added.
two species present .................................................. .........................................
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(ii) The equivalence point in this acid-base titration is where the two solutions have been mixed in exactly equal molar proportion.
Suggest why the pH is greater than 7 at the equivalence point in this titration.
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(b) An impure sample of ammonium vanadate(V), NH$_4$VO$_3$, with mass 0.150 g, is dissolved in an excess of dilute acid.
In this solution all vanadium is present as VO$_2^+$ ions. An excess of zinc powder is added to the solution and all the VO$_2^+$ ions are reduced to V$^{2+}$ ions. The mixture is filtered to remove any remaining zinc powder.
$$VO_2^+ + 4H^+ + 3e^- \rightarrow V^{2+} + 2H_2O$$
When the resulting solution is titrated, 20.10 cm$^3$ of 0.0250 mol dm$^{-3}$ acidified MnO$_4^-$ oxidises all V$^{2+}$ ions back to VO$_2^+$ ions.
$$MnO_4^- + 8H^+ + 5e^- \rightleftharpoons Mn^{2+} + 4H_2O$$
Calculate the percentage by mass of NH$_4$VO$_3$ in the 0.150 g impure sample of NH$_4$VO$_3$.
Give your answer to three significant figures.
[M$_r$: NH$_4$VO$_3$, 116.9]
percentage by mass of NH$_4$VO$_3$ = ................................... % [3]

The carbon-13 ($^{13}C$) NMR spectrum of compound A, $C_8H_8O_2$, contains six peaks.
Compound A reacts with an excess of bromine water to give compound B, $C_8H_6Br_2O_2$.
Compound A reacts with alkaline aqueous iodine to form a yellow precipitate C and compound D.
Compound D reacts with $PCl_5$ to form compound E, $C_7H_5O_2Cl$.
Compound E reacts with propan-2-ol to form compound F.
Draw the structures of compounds A, B, C, D, E and F in the boxes.