AS & A Level Chemistry - 9701 Paper 4 2024 Winter Zone 3

(a) Disodium phosphate, $(Na^+)_2(HPO_4^{2-})$, reacts with an acid to form monosodium phosphate, $Na^+(H_2PO_4^-)$.

(i) Identify the ions that are a conjugate acid–base pair in this reaction, using the formulae of the species involved.

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

(ii) Define buffer solution.

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

(iii) Write two equations to show how a mixture of $(Na^+)_2(HPO_4^{2-})$ and $Na^+(H_2PO_4^-)$ can act as a buffer solution.

equation 1 ............................................................................................................................................................................................
equation 2 ............................................................................................................................................................................................
[2]

(iv) Identify one inorganic ion that acts as a buffer in blood.

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

(b) Compound $E$ is the hydroxide of a Group 2 element. Compound $E$ is a strong alkali.

2.63 g of $E$ is dissolved in water to make $250\text{ cm}^3$ of solution $F$. Solution $F$ has a pH of 13.09 at 298 K.

(i) Show that the concentration of hydroxide ions in solution $F$ is $0.123 \text{ mol dm}^{-3}$.

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

(ii) Explain why the concentration of compound $E$ in solution $F$ is $0.0615 \text{ mol dm}^{-3}$.

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

(iii) Use the concentration given in (ii) to identify compound $E$.

compound $E$ .....................................................................................................................
[1]

(c) Compound $E$ is much more soluble than magnesium hydroxide.

A saturated solution of magnesium hydroxide in water has a concentration of $1.40 \times 10^{-4} \text{ mol dm}^{-3}$ at 298 K.
Calculate the solubility product, $K_{sp}$, of magnesium hydroxide. Include units.

$K_{sp}$ = ......................................... units ..............................
[3]

(d) Explain why compound $E$ is much more soluble than magnesium hydroxide.

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

(a) Predict and explain the variation in enthalpy change of hydration for the ions $F^-$, $Cl^-$, $Br^-$ and $I^-$. ...................................................................................................................................................................

(b) Fig. 2.1 shows an incomplete energy cycle involving calcium fluoride, CaF$_2$.



(i) Complete line D. Include state symbols. [1]

(ii) The value of the enthalpy change for process 1 can be calculated using the values of five other enthalpy changes which are not referred to in Fig. 2.1.

process 1: Ca(s) + F$_2$(g) → Ca$^{2+}$(g) + 2F$^{-}$(g)
Identify these five other enthalpy changes, using either names or symbols. ...................................................................................................................................................................... [2]

(iii) Define lattice energy, Δ$H_{latt}$. ...................................................................................................................................................................... [2]

(iv) Complete the expression to give the mathematical relationship between Δ$H_{latt}$ of calcium fluoride and the enthalpy changes for processes 1 and 3.
Δ$H_{latt}$ = ................................................................ [1]

(c) Use data from Table 2.1 to calculate a value for the hydration energy, Δ$H_{hyd}$, of fluoride ions, F$^-$(g).

[Table_1]

Δ$H_{hyd}$ F$^-$(g) = ................................. kJmol$^{-1}$ [2]

(d) Define entropy. ...................................................................................................................................................................... [1]

(e) At 298K, the Gibbs free energy change, ΔG, for the solution of compound T is +6.00 kJmol$^{-1}$.
The enthalpy change of solution, Δ$H_{sol}$, of compound T is +30.0 kJmol$^{-1}$ at 298K.
Calculate the value of the entropy change, ΔS, for the solution of compound T at 298K.
ΔS = ................................. JK$^{-1}$mol$^{-1}$ [2]

(f) Predict whether compound T becomes more or less soluble as the water is heated from 298K to 360K. Explain your answer. ...................................................................................................................................................................... [1]

(a) A and B react together to give product AB.

A + B → AB

When the concentrations of A and B are both 0.0100 mol dm⁻³, the rate of formation of AB is 7.62 × 10⁻⁴ mol dm⁻³ s⁻¹. When the concentrations of A and B are both 0.0200 mol dm⁻³, the rate of formation of AB is 3.05 × 10⁻³ mol dm⁻³ s⁻¹.

(i) Complete the three possible rate equations that are consistent with these data.

rate = ..................................................................................................................................................

rate = ..................................................................................................................................................

rate = .................................................................................................................................................. [2]

(ii) Choose one of the rate equations you have written in (i), and calculate the value of the rate constant, k. Include the units of k.

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

(iii) Explain why it is **not** possible to calculate a value for the half-life, $t_{\frac{1}{2}}$, of this reaction using the value of the rate constant $k$ calculated in (ii) and the equation $k = \frac{0.693}{t_{\frac{1}{2}}}$.

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

(b) Catalysts may be homogeneous or heterogeneous.

(i) Identify two metals that act as heterogeneous catalysts in the removal of NO₂ from the exhaust gases of car engines.

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

(ii) Iron acts as a heterogeneous catalyst in the Haber process.

Describe the mode of action of this iron catalyst.

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

(iii) $\text{Fe}^{2+}$ ions act as a homogeneous catalyst in the reaction between $\text{I}⁻$(aq) and $\text{S}_2\text{O}_8^{2-}$(aq).

Write equations for the two reactions that occur when $\text{Fe}^{2+}$(aq) is added to a mixture of $\text{I}⁻$(aq) and $\text{S}_2\text{O}_8^{2-}$(aq).

equation 1 $\quad \text{S}_2\text{O}_8^{2-}$ + .................................................................................................................................

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

(iv) Explain the difference between a homogeneous catalyst and a heterogeneous catalyst.

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

(c) Fe²⁺ ions can be oxidised to Fe³⁺ ions under alkaline conditions by suitable oxidising agents.

(i) Iron is a transition element. Explain why iron forms stable compounds in both the +2 and the +3 oxidation states.

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

(ii) The half-equation for the reduction of Fe³⁺ under alkaline conditions, and its $E^{\circ}$ value, are shown.
$$\text{Fe(OH)}_3 + \text{e}^- \rightleftharpoons \text{Fe(OH)}_2 + \text{OH}^- \qquad E^{\circ} = -0.56 \text{ V}$$
Four more half-equations for reactions under alkaline conditions, and their $E^{\circ}$ values, are shown.

$$\text{Al(OH)}_4^- + 3\text{e}^- \rightleftharpoons \text{Al} + 4\text{OH}^- \qquad E^{\circ} = -2.35 \text{ V}$$
$$\text{ClO}^- + \text{H}_2\text{O} + 2\text{e}^- \rightleftharpoons \text{Cl}^- + 2\text{OH}^- \qquad E^{\circ} = +0.89 \text{ V}$$
$$\text{O}_2 + 2\text{H}_2\text{O} + 4\text{e}^- \rightleftharpoons 4\text{OH}^- \qquad E^{\circ} = +0.40 \text{ V}$$
$$\text{Zn(OH)}_4^{2-} + 2\text{e}^- \rightleftharpoons \text{Zn} + 4\text{OH}^- \qquad E^{\circ} = -1.22 \text{ V}$$
Select two oxidising agents that can oxidise $\text{Fe}^{2+}$ ions to $\text{Fe}^{3+}$ ions under alkaline conditions.
Write an equation, and give the $E^{\circ}_{\text{cell}}$ value, for **each** of the two reactions that occur.

oxidising agent 1: ...............................

equation: ...........................................................................................................................

$E^{\circ}_{\text{cell}} = ............................. \text{ V}$

oxidising agent 2: ...............................

equation: ...........................................................................................................................

$E^{\circ}_{\text{cell}} = ............................. \text{ V}$ [4]

(a) Explain why transition elements form complex ions.
............................................................................................................................... [1]

(b) $\text{Co}^{2+}$ ions form complex ion $G$.
Each $G$ ion contains \textbf{two} $\text{Co}^{2+}$ ions, both of which are octahedrally coordinated.
Each $G$ ion contains one $\text{O}_2$ molecule, which donates one pair of electrons to each $\text{Co}^{2+}$ ion, and one $\text{NH}_2^{-}$ ion, which donates one pair of electrons to each $\text{Co}^{2+}$ ion.
The remaining ligands are $\text{NH}_3$ molecules.
(i) Deduce the formula of complex ion $G$. Include its overall charge.
formula of $G$ ........................................................ [2]
(ii) The d-orbitals of the $\text{Co}^{2+}$ ions present in complex ion $G$ are split. State the number of d-orbitals that are at a higher energy level and the number of d-orbitals that are at a lower energy level in each $\text{Co}^{2+}$ ion.

number of d-orbitals at a higher energy level

number of d-orbitals at a lower energy level

[1]
(iii) $\text{Co}^{2+}$ ions form a different complex ion, $M$.
Each $M$ ion contains \textbf{two} $\text{Co}^{2+}$ ions, both of which are octahedrally coordinated, but the ligands are different from the ligands in $G$.
Explain why $G$ and $M$ have different colours.
................................................................................................................................. [2]

(c) Cadmium forms complex ion $X$, $[\text{Cd}(\text{NH}_3)_4]^{2+}$.
When a solution containing $\text{CN}^-$ ions is added to an aqueous solution of $X$, a ligand exchange reaction takes place, forming complex ion $Y$. $Y$ contains no $\text{NH}_3$ ligands and no $\text{H}_2\text{O}$ ligands.
$Y$ is in a much higher concentration in the mixture than $X$.
The oxidation state and coordination number of cadmium do not change in this reaction.
(i) Write an ionic equation for this reaction, using the formulae of the complex ions.
............................................................................................................................ [2]
(ii) Cadmium forms complex ion $Z$ in the same oxidation state and with the same coordination number as in $X$. All the ligands in $Z$ are $\text{Cl}^-$ ions.
When $\text{NaCl}(aq)$ is added to a solution of $X$, very little $Z$ forms.
Write the \textbf{three} cadmium complexes, $X$, $Y$ and $Z$, in order of increasing stability constant, $K_{\text{stab}}$.
........................................... ........................................... ...........................................
smallest value of $K_{\text{stab}}$ largest value of $K_{\text{stab}}$ [1]

(d) Ethanedioate ions, $\text{C}_2\text{O}_4^{2-}$, form complexes with transition element ions.
The concentration of $\text{C}_2\text{O}_4^{2-}$ ions can be found by reaction with acidified $\text{Cr}_2\text{O}_7^{2-}$ ions.
$\text{C}_2\text{O}_4^{2-}$ ions are protonated and form HOOC-COOH molecules which are oxidised by $\text{Cr}_2\text{O}_7^{2-}$.
The half-equations are shown.
$$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{e}^- \rightleftharpoons 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$$
$$2\text{CO}_2 + 2\text{H}^+ + 2\text{e}^- \rightleftharpoons \text{HOOC-COOH}$$
(i) Construct an equation for the reaction between acidified $\text{Cr}_2\text{O}_7^{2-}$ and HOOC-COOH.
............................................................................................................................... [1]
(ii) A 25.0 cm³ sample of a solution of $\text{Na}_2\text{C}_2\text{O}_4$ reacts with exactly 16.20 cm³ of an acidified solution of 0.0500 mol dm^-3 $\text{K}_2\text{Cr}_2\text{O}_7$.
Calculate the concentration of the solution of $\text{Na}_2\text{C}_2\text{O}_4$.
$[\text{Na}_2\text{C}_2\text{O}_4] = ................................................$ mol dm^-3 [2]
[Total: 12]

The shapes of four different complexes, P, Q, R and S, are shown in Table 5.1.
The symbol J represents an atom or ion of a transition element.
The symbol L is used to represent a monodentate ligand.

[Table_1]

(a) Label one bond angle on each of complexes P, Q, R and S, and identify the size of the angle in degrees.
[2]

(b) Identify the shapes of complexes P, Q, R and S.

P ............................................................................................................................
Q ............................................................................................................................
R ............................................................................................................................
S ............................................................................................................................
[2]

(c) Two L ligands are exchanged with two different monodentate ligands X and Y in each of complexes P, Q, R and S.

Identify all the complexes which form new complexes that show geometrical isomerism.
............................................................................................................................
[1]

(d) Three L ligands are exchanged with three different monodentate ligands X, Y and Z in each of complexes P, Q and R.

Identify all the complexes which form new complexes that show optical isomerism.
............................................................................................................................
[1]

Benzene, C_6H_6, reacts with chloroethane, C_2H_5Cl, in the presence of a suitable catalyst to form ethylbenzene, C_6H_5C_2H_5. In the presence of the catalyst, the ion C_2H_5^+ is formed. This ion reacts with benzene.

(a) Complete the equation for the reaction of C_2H_5Cl with this catalyst to form C_2H_5^+ as one product.

$$C_2H_5Cl + ext{...............................} \rightarrow C_2H_5^+ + ext{...............................}$$ [1]

(b) Ethylbenzene reacts with more C_2H_5Cl, forming a mixture containing 1,2-diethylbenzene and 1,4-diethylbenzene.

(i) Draw the structures of 1,2-diethylbenzene and 1,4-diethylbenzene.


[1]

(ii) Explain why there is very little 1,3-diethylbenzene in the product mixture.
...................................................................................................................................... [1]

(c) 1,2-diethylbenzene can be oxidised to benzene-1,2-dioic acid, C_6H_4(COOH)_2.



(i) State the reagent and conditions used for this reaction.
................................................................................................................................................... [1]

(ii) Complete the overall equation for this reaction.
An atom of oxygen from the oxidising agent is represented as [O].
All of the atoms in the two ethyl groups are fully oxidised in this reaction.

$$ ext{............................} + ext{..................} [O] \rightarrow C_6H_4(COOH)_2 + ext{............................} + ext{............................} \] $$ [2]

(iii) Predict the number of peaks in the carbon-13 NMR spectrum of benzene-1,2-dioic acid.
.................................................................... [1]

(d) The proton ($^1$H) NMR spectra of ethylbenzene, C_6H_5C_2H_5, in CDCl_3 and of benzene-1,2-dioic acid, C_6H_4(COOH)_2, in CDCl_3 are shown. They have not been identified.


[Figure 6.1]


[Figure 6.2]

(i) Explain the use of CDCl_3, instead of CHCl_3, as the solvent when obtaining these spectra.
.................................................................................................................................................. [1]

(ii) Identify the substance shown by the spectrum in Fig. 6.1, and complete Table 6.1.
substance .................................................................................................................................

[Table_1]

| peak at $ \delta = 1.2$ | peak at $ \delta = 2.6$ |
| ---------------------- | ---------------------- |
| name of splitting pattern | |
| group responsible for peak | |
| explanation of splitting pattern | |

[3]

(iii) Identify the substance shown by the spectrum in Fig. 6.2, and complete Table 6.2.
substance .................................................................................................................................

[Table_2]

| peak at $ \delta = 7.8$ | peak at $ \delta = 13.1$ |
| ---------------------- | ----------------------- |
| group responsible for peak |
| |

[1]

(iv) When D_2O is used as a solvent, the spectrum obtained is different from the spectrum in Fig. 6.2.
Describe this difference and explain your answer.
................................................................................................................................. [1]

(e) Benzene-1,2-dioic acid can be used to produce K.




Suggest the name of this type of reaction.
....................................................................................................................................... [1]

A reaction scheme is shown in Fig. 7.1.
The reagents needed for reaction 2 and reaction 3 are stated.
Reaction 5 takes place when $C_2H_5NH_2$ is mixed with compound V. No special conditions are required.



(a) Identify compound U which contains only three elements.
................................................................................................................................. [1]

(b) Describe the reagents and conditions for reaction 1.
................................................................................................................................. [1]

(c) Identify compound V.
................................................................................................................................. [1]

(d) Complete the equation for reaction 3.
$CH_3COOH + SOCl_2 \rightarrow$ ................................................................................................................................ [1]

(e) Identify compound W.
................................................................................................................................. [1]

(f) Describe the conditions for reaction 4.
................................................................................................................................. [1]

(g) Suggest the reagent needed for reaction 6.
................................................................................................................................. [1]

(h) Complete Table 7.1 by adding the reaction numbers, 1, 2, 3, 4, 5 and 6, to the right-hand column. Use the reaction numbers given in Fig. 7.1.
Each of the numbers 1, 2, 3, 4, 5 and 6 should be used once only.

[Table_1]

(i) Compare the basicities of $C_2H_5NHCOCH_3$, $C_2H_5NHC_2H_5$ and $NH_3$.
Explain your answer.
................................................. ................................................. .................................................
most basic                                                           least basic
................................................................................................................................. [4]

(a) An aqueous solution of phenol, $C_6H_5OH$, is acidic at 298 K.
Explain why phenol is more acidic than water.
......................................................................................................................................................... [2]

(b) (i) Name the \textbf{two} products formed when phenol reacts with an excess of $Br_2(aq)$.
........................................................... and .......................................................... [1]

(ii) Draw the structures of the \textbf{two} isomeric organic products, with $M_r = 139$, that are formed when phenol reacts with $HNO_3(aq)$ at room temperature.


[1]

(iii) Write the equation for the reaction between phenol, $C_6H_5OH$, and sodium metal.
......................................................................................................................................................... [1]

(c) Phenol can be produced from phenylamine in a two-step synthesis.
\begin{align*} \text{phenylamine} & \xrightarrow[\text{step one}]{\text{}} \text{intermediate compound} \xrightarrow[\text{step two}]{\text{}} \text{phenol} \end{align*}
Describe the reagents and conditions needed in each step.
step one:
reagents ................................................................................................................................................
conditions ...........................................................................................................................................
step two:
reagents ................................................................................................................................................
conditions ...........................................................................................................................................
[2]

[Total: 7]