AS & A Level Chemistry - 9701 Paper 4 2021 Summer Zone 2

(a) An aqueous solution of chromium(III) contains the green [Cr(H_2O)_6]^{3+} complex ion.
(i) Complete the electronic configuration of an isolated, gaseous Cr^{3+} ion.
1s^2 ................................................................................................................................. [1]
(ii) Define the term complex ion.
.................................................................................................................................
................................................................................................................................. [1]

(b) [Cr(H_2O)_6]^{3+}(aq) shows some similar chemical properties to [Co(H_2O)_6]^{2+}(aq).
Samples of [Cr(H_2O)_6]^{3+} are reacted separately with either NaOH(aq), H_2O_2(aq), or excess NH_3(aq).
Use this information and the Data Booklet to suggest the formula of the chromium species formed. State the type of reaction taking place in each case.
[Table]
reagent added to [Cr(H_2O)_6]^{3+}(aq) | formula of chromium species formed | type of reaction
NaOH(aq)
H_2O_2(aq)
an excess of NH_3(aq)
[/Table] [5]

(c) [Cr(H_2O)_6]^{2+} and [Cr_2(O_2CCH_3)_4(H_2O)_2] are both complexes of chromium(II) and have different colours.
Explain why the colours of these complexes are different.
.................................................................................................................................
.................................................................................................................................
................................................................................................................................. [2]

(d) The structure of [Cr_2(O_2CCH_3)_4(H_2O)_2] is shown. Ethanoate ions act as ligands in this complex. The ethanoate ligand, CH_3CO_2^-, is shown as O .. O

(i) Water and ethanoate ions behave as different types of ligand in this complex. Suggest an explanation for this statement.
.................................................................................................................................
................................................................................................................................. [1]
(ii) Deduce the coordination number of Cr and the geometry around each Cr atom in this structure.
coordination number ...................................................................................................
geometry around Cr atom .......................................................................................... [1]
(iii) State the type of bond between the two atoms in the Cr–Cr bond.
................................................................................................................................. [1]

(e) The [Cr_2(O_2CCH_3)_4(H_2O)_2] complex reacts with aqueous acid to form Cr^{2+}(aq) ions.
Cr^{2+}(aq) ions react with O_2(aq) under acidic conditions. Cr^{3+}(aq) ions are formed.
Use the Data Booklet to answer the following questions.
(i) Construct an ionic equation for the reaction of Cr^{2+}(aq) with O_2(aq) under acidic conditions.
................................................................................................................................. [2]
(ii) Calculate E_{cell}^{⦵} for the reaction in (e)(i).
E_{cell}^{⦵} = .............................. V [1]

(a) State and explain the trend observed in the thermal stability of the Group 2 nitrates. .......................................................................................................................... .......................................................................................................................... .......................................................................................................................... ......................................................................................................................

(b) (i) Lead(II) nitrate, Pb(NO3)2, decomposes on heating in a similar manner to the Group 2 nitrates.
Write an equation for the decomposition of lead(II) nitrate. .......................................................................................................................... [1]
(ii) Suggest how the ease of decomposition of Pb(NO3)2 would compare to that of Ba(NO3)2. Explain your answer. You may find it useful to refer to the Data Booklet. .......................................................................................................................... .......................................................................................................................... [1]

(c) (i) Barium ethanedioate, BaC2O4, decomposes on heating to produce barium oxide and a mixture of two different gases.
Construct an equation for the decomposition of barium ethanedioate. .......................................................................................................................... [1]
(ii) An impure sample of BaC2O4, of mass 0.500 g, is added to 50.0 cm3 of 0.0200 mol dm−3 acidified MnO4−(aq), an excess. A redox reaction takes place and all the BaC2O4 reacts.
The resulting solution, containing unreacted acidified MnO4−, is titrated with 0.0500 mol dm−3 Fe2+(aq).
The end-point is reached when 30.40 cm3 of 0.0500 mol dm−3 Fe2+(aq) has been added. $$\begin{align} C_2O_4^{2-} &\rightleftharpoons 2CO_2 + 2e^- \\ MnO_4^- + 8H^+ + 5e^- &\rightleftharpoons Mn^{2+} + 4H_2O \\ Fe^{2+} &\rightleftharpoons Fe^{3+} + e^- \\ \end{align}$$ Calculate the percentage by mass of BaC2O4 in the 0.500 g impure sample. Show your working.
[M_r: BaC2O4, 225.3]
percentage by mass of BaC2O4 = ................................. [4]

(d) Barium hydroxide, Ba(OH)2, is completely dissociated in aqueous solution.
Calculate the pH of 0.120 mol dm−3 Ba(OH)2(aq) at 298 K.
pH = ............................ [2]

(a) (i) Define the term standard electrode potential.
.................................................................................................................................
.................................................................................................................................
[2]

Three redox systems, A, B and C, are shown. The ligand 1,2-diaminoethane, $H_{2}NCH_{2}CH_{2}NH_{2}$, is represented by en.

[Table_1]
| A | $[Ru(H_{2}O)_{6}]^{3+} + e^{-} \rightleftharpoons [Ru(H_{2}O)_{6}]^{2+}$ |
|---|--------------------------------------------------------|
| B | $[Ru(NH_{3})_{6}]^{3+} + e^{-} \rightleftharpoons [Ru(NH_{3})_{6}]^{2+}$ |
| C | $[Ru(en)_{3}]^{3+} + e^{-} \rightleftharpoons [Ru(en)_{3}]^{2+}$ |

Two electrochemical cells are set up to compare the standard electrode potentials, $E^{\circ}$, of three half-cells. The diagrams show the relative potential of each electrode.


$$\begin{align*}
&+ \quad V \qquad -
&\text{Pt} \quad \text{Pt}
[Ru(en)_{3}^{3+}] \quad [Ru(en)_{3}^{2+}]
\text{(salt bridge)}
&[Ru(NH_{3})_{6}^{3+}] \quad [Ru(NH_{3})_{6}^{2+}]
\end{align*}$$

(ii) Use this information to complete the table by adding the labels A, B and C to deduce the order of $E^{\circ}$ for the three half-cells.

[Table_2]
| $E^{\circ}$ | redox system |
|---|----------------|
| most negative | |
| | |
| least negative | |
[1]

(iii) The complex $[Ru(en)_{3}]^{3+}$ shows stereoisomerism. The ligand en is bidentate.

Draw three-dimensional diagrams to show the two isomers of $[Ru(en)_{3}]^{3+}$. Represent the ligand en by using \(\begin{align}\overset{N}{\curvearrowright} \quad \overset{N}{\curvearrowleft}\end{align}\).
Name the type of stereoisomerism.



type of stereoisomerism .................................................................................................................................
[3]

(b) (i) An electrochemical cell consists of a $Br_{2}/Br^{-}$ half-cell and a $Ag^{+}/Ag$ half-cell, under standard conditions.

Use the Data Booklet to calculate the $E_{cell}^{\circ}$. Deduce the direction of electron flow in the wire through the voltmeter between these two half-cells.

$$E_{cell}^{\circ} = \text{...................} \text{V}$$
direction of electron flow from ................................................ to .............................................
[1]

(ii) Water is added to the $Ag^{+}/Ag$ half-cell in (b)(i).

Suggest the effect of this addition on the $E_{cell}$. Place a tick (✓) in the appropriate box.

[Table_3]
| less positive | no change | more positive |
|---------------|-----------|--------------|
[ ] | [ ] | [✓] |

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

(c) Silver bromide, $AgBr$, dissolves in an aqueous solution of $S_{2}O_{3}^{2-}$ ions to form the complex ion $[Ag(S_{2}O_{3})_{2}]^{3-}$. The $S_{2}O_{3}^{2-}$ ions act as monodentate ligands.

equilibrium 1 $AgBr(s) + 2S_{2}O_{3}^{2-}(aq) \rightleftharpoons [Ag(S_{2}O_{3})_{2}]^{3-}(aq) + Br^{-}(aq)$

(i) Define the term ligand.
.................................................................................................................................
.................................................................................................................................
[1]

(ii) Write an expression for the equilibrium constant, $K_{c}$, for equilibrium 1.
$$K_{c} = \text{........}$$
[1]

(iii) Some additional data are given about the dissolution of $AgBr$ in $S_{2}O_{3}^{2-}(aq)$.

[Table_4]
| equilibrium constant | numerical value |
|--------------------------------|------------------|
| solubility product, $K_{sp}$, of $AgBr$ | $5.4 \times 10^{-13}$ |
| stability constant, $K_{stab}$, of $[Ag(S_{2}O_{3})_{2}]^{3-}$ | $2.9 \times 10^{13}$ |

Use your answer to (c)(ii) and these data to calculate $K_{c}$ for equilibrium 1. Include the units for $K_{c}$.
$$K_{c} = \text{....................} \text{units} \text{....................}$$
[2]

(d) The numerical values for the stability constants, $K_{stab}$, of two other silver(I) complexes are given.

[Table_5]
| silver(I) complex | numerical value of $K_{stab}$ |
|-----------------|-------------------------|
| $[Ag(CN)_{2}]^{-}$ | $5.3 \times 10^{18}$ |
| $[Ag(NH_{3})_{2}]^{+}$ | $1.6 \times 10^{7}$ |

An aqueous solution containing $Ag^{+}$ is added to a solution containing equal concentrations of $CN^{-}(aq)$, $NH_{3}(aq)$ and $S_{2}O_{3}^{2-}(aq)$. The mixture is left to reach equilibrium.

Deduce the relative concentrations of $[Ag(CN)_{2}]^{-}$, $[Ag(NH_{3})_{2}]^{+}$ and $[Ag(S_{2}O_{3})_{2}]^{3-}$ present in the resulting mixture. Explain your answer.
................................. $>$ ................................. $>$ .................................
highest concentration lowest concentration
.................................................................................................................................
.................................................................................................................................
[2]

(a) (i) Define the term lattice energy.
............................................................................................................................................................
............................................................................................................................................................
............................................................................................................................................................ [2]

(ii) Use the following data to calculate a value for the enthalpy change of solution of copper(II) chloride, CuCl₂(s). You might find it helpful to construct an energy cycle.
enthalpy change of hydration of Cl⁻ = $-378\text{ kJ mol}^{-1}$
enthalpy change of hydration of Cu²⁺ = $-2099\text{ kJ mol}^{-1}$
lattice energy of CuCl₂(s) = $-2824\text{ kJ mol}^{-1}$

enthalpy change of solution of CuCl₂(s) = ............................. kJ mol$^{-1}$ [2]

(iii) The enthalpy change of hydration of Ca²⁺ is $-1579\text{ kJ mol}^{-1}$.
Use the Data Booklet to suggest why there is a big difference in the values of $\Delta H_{hyd}$ for Ca²⁺ and Cu²⁺.
............................................................................................................................................................
............................................................................................................................................................
............................................................................................................................................................ [2]

(b) (i) Identify the substances formed at the anode and at the cathode during the electrolysis of saturated CaCl₂(aq).
at the anode .............................................................................................................................
at the cathode ......................................................................................................................... [1]

(ii) Calcium can be produced by the electrolysis of molten calcium chloride, CaCl₂(l).
Calculate the mass, in g, of Ca formed when a current of 0.75A passes through CaCl₂(l) for 60 minutes.
[A_r: Ca, 40.1]
mass of Ca = ......................................... g [2]

(c) (i) Explain what is meant by the term entropy of a system.
............................................................................................................................................................
............................................................................................................................................................ [1]

(ii) Place one tick (✓) in each row of the table to show the sign of each entropy change, $\Delta S$.
[Table]
process | $\Delta S$ is negative | $\Delta S$ is zero | $\Delta S$ is positive
NaCl dissolving in water | | |
water solidifying to ice | | | [1]

(iii) The evaporation of one mole of water has a standard Gibbs free energy change, $\Delta G^{\circ}$, of $+8.6\text{ kJ}$ at $25^{\circ}\text{C}$.
Sketch a graph on the axes to show how $\Delta G^{\circ}$ changes for this process between $25^{\circ}\text{C}$ and $150^{\circ}\text{C}$ at 101kPa.
[Image of graph]
[2]

(d) The reaction between A and B is feasible at low temperatures but is not feasible at high temperatures.
$\text{A + B} \xrightleftharpoons[\text{C + D}]{\text{}}$
Deduce the signs of $\Delta H$ and $\Delta S$ for this reaction and explain why the feasibility changes with temperature.
sign of $\Delta H =$ ............................. sign of $\Delta S =$ .............................
............................................................................................................................................................
............................................................................................................................................................
............................................................................................................................................................ [2]

(a) Describe and explain the relative basicities of phenylamine, ethylamine and 4-nitrophenylamine.

............................. > ............................. > .............................
most basic          least basic
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
............................................................................................................................. [4]

(b) The dye R can be synthesised from 4-nitrophenylamine in two steps.

    4-nitrophenylamine            Q
                          [Image showing reaction step 1]


                           R
                          [Image showing reaction step 2]


(i) Deduce and draw the structure of the organic salt Q in the box. [1]

(ii) Suggest reagents and conditions for step 1 and 2 in (b).
step 1 .............................................................................................................................
step 2 ............................................................................................................................. [2]

(c) Compound G can be synthesised from methylbenzene in three steps.

    methylbenzene            E
                       [Image showing reaction step 1]


                       F
                       [Image showing reaction step 2]


                         G

(i) Give the systematic name of compound G.
............................................................................................................................. [1]

(ii) Deduce the identities of E and F and draw their structures in the boxes. [2]

(iii) Suggest reagents and conditions for each of steps 1 to 3 in (c).
step 1 .............................................................................................................................
step 2 .............................................................................................................................
step 3 ............................................................................................................................. [3]

(a) There are four possible structural isomers of $C_8H_{10}$ that contain a benzene ring.

Draw the \textit{skeletal} formulae of the four structural isomers in the appropriate boxes. The number of peaks observed in the carbon-13 ($^{13}C$) NMR spectrum of each compound is given.



(b) A three-step synthesis of X $(C_{10}H_{10}O)$ from benzene is suggested as shown.

(i) Step 1 is the alkylation of benzene by electrophilic substitution.
Use R–Cl to represent $Cl(CH_2)_3CO_2H$.
Write an equation for the formation of an electrophile from R–Cl and AlCl$_3$.
.................................................................................................................... [1]

(ii) Deduce and draw the structures of W and X in the boxes. [2]

(iii) Suggest the reagents and conditions for step 2.
.................................................................................................................... [1]

(iv) Complete the mechanism for the reaction of benzene with the electrophile formed in (b)(i). Include all relevant charges and curly arrows showing the movement of electron pairs.
Draw the structure of the intermediate.
[3]

(a) In aqueous solution, chlorine dioxide, $\text{ClO}_2$, reacts with hydroxide ions as shown.

$$2\text{ClO}_2 + 2\text{OH}^- \rightarrow \text{ClO}_3^- + \text{ClO}_2^- + \text{H}_2\text{O}$$

A series of experiments is carried out using different concentrations of $\text{ClO}_2$ and $\text{OH}^-$. The table shows the results obtained.

\begin{array}{|c|c|c|c|}\hline \text{experiment} & [\text{ClO}_2]/\text{mol dm}^{-3} & [\text{OH}^-]/\text{mol dm}^{-3} & \text{initial rate}/\text{mol dm}^{-3}\text{min}^{-1} \\ \hline 1 & 0.020 & 0.030 & 7.20 \times 10^{-4} \\ 2 & 0.020 & 0.120 & 2.88 \times 10^{-3} \\ 3 & 0.050 & 0.030 & 4.50 \times 10^{-3} \\ \hline \end{array}

(i) Explain the term order of reaction.

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

(ii) Use the data in the table to determine the order of reaction with respect to each reactant, $\text{ClO}_2$ and $\text{OH}^-$.

Explain your reasoning.

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

(iii) Use your answer to (a)(ii) to construct the rate equation for this reaction.

rate = ..................................................... [1]

(iv) Use your rate equation and the data from experiment 1 to calculate the rate constant, $k$, for this reaction.

Include the units of $k$.

$k = ........................................$ units ...................................... [2]

(b) The decomposition of benzenediazonium ions, $\text{C}_6\text{H}_5\text{N}_2^+$, using a large excess of water, is a first-order reaction.

The graph shows the results obtained.

(i) Draw the structure of the organic product formed in this reaction.

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

(ii) Use the graph to determine the rate of reaction at 100 s. Show your working.

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

(c) Sketch a concentration–time graph for a zero-order reaction.

Use your graph to suggest how successive half-lives for a zero-order reaction vary as the concentration of a reactant decreases. Indicate this by placing a tick (✓) in the appropriate box in the table.



successive half-lives decrease ...................
no change in successive half-lives ....................
successive half-lives increase .................... [1]

(a) State and explain the relative rate of hydrolysis of acyl chlorides, alkyl chlorides and aryl chlorides.
............................. > ............................. > .............................
       fastest                              slowest
.................................................................................
.................................................................................
.................................................................................
.................................................................................
.................................................................................

(b) The drug remifentanil is shown.

Remifentanil is \textit{completely} hydrolysed under acidic conditions. Three different organic compounds are formed.
Draw the structures for these organic compounds in the boxes.


(c) Compound Y, C$_5$H$_{10}$O$_2$, reacts with Na$_2$CO$_3$(aq) to evolve bubbles of gas. The proton ($^1$H) NMR spectrum of compound Y in D$_2$O is shown.

(i) Use this information to suggest a structure for Y.
[1]
(ii) Use the \textit{Data Booklet}, the proton ($^1$H) NMR spectrum and your answer to (c)(i) to complete the table.

[3]