A Level Chemistry 9701 Paper 42, May/June 2023

Group 2 carbonates decompose when heated to form the metal oxide and carbon dioxide. Suggest a mechanism for the decomposition of the carbonate ion by adding two curly arrows in Fig. 1.1.

Describe the variation in the thermal stability of Group 2 carbonates. Explain your answer.

Define lattice energy.

The lattice energy of the Group 2 carbonates, $\mathrm{\Delta }{H}_{\text{latt}}^{\ominus }({\text{MCO}}_3)$, becomes less exothermic down the group. The lattice energy of the Group 2 oxides, $\mathrm{\Delta }{H}_{\text{latt}}^{\ominus }(\text{MO})$, also becomes less exothermic down the group. $\mathrm{\Delta }{H}_{\text{latt}}^{\ominus }({\text{MCO}}_3)$ and $\mathrm{\Delta }{H}_{\text{latt}}^{\ominus }(\text{MO})$ change by different amounts going down the group. Suggest how the standard enthalpy change of the decomposition reaction for Group 2 carbonates changes down the group. Explain your reasoning in terms of the relative sizes of the anions and the relative changes in lattice energy down the group.

Potassium sulfite, ${\text{K}}_2{\text{SO}}_3$, is used as a food additive. The concentration of sulfite ions, ${\text{SO}}_3^{2-}$, can be determined by titration using aqueous acidified manganate(VII) ions, ${\text{MnO}}_4^{-}$. • A $250{\text{ cm}}^3$ solution contains $3.40\text{ g}$ of impure ${\text{K}}_2{\text{SO}}_3$. • $25.0{\text{ cm}}^3$ of this solution requires $22.40{\text{ cm}}^3$ of $0.0250\text{ mol}{\text{dm}}^{-3}$ acidified ${\text{MnO}}_4^{-}$ to reach the end-point. All the ${\text{SO}}_3^{2-}$ ions are oxidised. None of the other species in the impure ${\text{K}}_2{\text{SO}}_3$ are oxidised. The reaction occurs as shown by the two half-equations. $${\text{H}}_2\text{O}+{\text{SO}}_3^{2-}\to {\text{SO}}_4^{2-}+2{\text{H}}^{+}+2{\text{e}}^{-}$$ $${\text{MnO}}_4^{-}+8{\text{H}}^{+}+5{\text{e}}^{-}\to {\text{Mn}}^{2+}+4{\text{H}}_2\text{O}$$ Give the ionic equation for the reaction between ${\text{SO}}_3^{2-}$ and acidified ${\text{MnO}}_4^{-}$.

Calculate the percentage purity of the sample of ${\text{K}}_2{\text{SO}}_3$. Show your working.

Potassium disulfite, ${\text{K}}_2{\text{S}}_2{\text{O}}_5$, is another food additive. The disulfite ion, ${\text{S}}_2{\text{O}}_5^{2-}$, has the displayed formula shown in Fig. 1.2. Deduce the geometry (shape) around the $\text{S}(\alpha )$ atom in ${\text{S}}_2{\text{O}}_5^{2-}$.

State two typical chemical properties of a transition element.

Aqueous solutions of cobalt(II) salts contain the complex ion $[\text{Co}({\text{H}}_2\text{O}{)}_6{]}^{2+}$. Define complex ion.

Samples of $[\text{Co}({\text{H}}_2\text{O}{)}_6{]}^{2+}$ are reacted separately with an excess of aqueous ammonia, with an excess of concentrated $\text{HCl}$ and with an excess of aqueous sodium hydroxide, as shown in Fig. 2.1. Complete Table 2.1 about the reactions shown by $[\text{Co}({\text{H}}_2\text{O}{)}_6{]}^{2+}$.

The ethanedioate ion, ${\text{C}}_2{\text{O}}_4^{2-}$, can act as a bidentate ligand. Explain what is meant by a bidentate ligand.

The complex $[\text{Co}({\text{H}}_2\text{O}{)}_2({\text{C}}_2{\text{O}}_4)\text{BrCl}{]}^{-}$ exists as stereoisomers. Complete the three-dimensional diagrams in Fig. 2.2 to show four stereoisomers of $[\text{Co}({\text{H}}_2\text{O}{)}_2({\text{C}}_2{\text{O}}_4)\text{BrCl}{]}^{-}$. The ${\text{C}}_2{\text{O}}_4^{2-}$ ligand is represented using O—O.

State the oxidation state of cobalt in this complex and a type of stereoisomerism shown.

Complete Table 3.1 by placing one tick (✓) in each row to indicate the sign of each type of energy change under standard conditions.

Define standard enthalpy change of atomisation.

Table 3.2 shows some energy changes. Calculate the lattice energy, $\mathrm{\Delta }{H}_{\text{latt}}^{\ominus }$, of ${\text{Ag}}_2\text{O(s)}$ using relevant data from Table 3.2. It may be helpful to draw a labelled energy cycle. Show your working.

Suggest the trend in the magnitude of the lattice energies of the silver compounds ${\text{Ag}}_2\text{S}$, ${\text{Ag}}_2\text{O}$ and ${\text{Ag}}_2\text{Se}$. Explain your answer.

Silver sulfite, ${\text{Ag}}_2{\text{SO}}_3(\text{s})$, is sparingly soluble in water. Give an expression for the solubility product, $K_{\text{sp}}$, of ${\text{Ag}}_2{\text{SO}}_3$.

Calculate the equilibrium concentration of ${\text{Ag}}^{+}$ in a saturated solution of ${\text{Ag}}_2{\text{SO}}_3$ at $298\text{ K}$. [$K_{\text{sp}}$: ${\text{Ag}}_2{\text{SO}}_3$, $1.50\times {10}^{-14}{\text{ mol}}^3{\text{dm}}^{-9}$ at $298\text{ K}$]

The standard enthalpy change of solution, $\mathrm{\Delta }{H}_{\text{sol}}^{\ominus }$, of ${\text{AgNO}}_3(\text{s})$ in water is $+22.6\text{ kJ}{\text{mol}}^{-1}$. Suggest how the feasibility of dissolving ${\text{AgNO}}_3(\text{s})$ in water changes with temperature. Explain your answer.

In aqueous solution, iron(III) ions react with iodide ions, as shown. $$2{\text{Fe}}^{3+}+2{\text{I}}^{-}\to 2{\text{Fe}}^{2+}+{\text{I}}_2$$ A series of experiments is carried out using different concentrations of ${\text{Fe}}^{3+}$ and ${\text{I}}^{-}$, as shown in Table 4.1. Explain what is meant by overall order of reaction.

Use the data in Table 4.1 to deduce the order of reaction with respect to ${\text{Fe}}^{3+}$ and with respect to ${\text{I}}^{-}$. Explain your reasoning.

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

Use your answer to (a)(iii) and the data from experiment 1 to calculate the rate constant, $k$, for this reaction. Include the units of $k$.

Describe qualitatively the effect of an increase in temperature on the rate constant and on the rate of this reaction.

In aqueous solution, iodide ions react with acidified hydrogen peroxide, as shown. $$2{\text{I}}^{-}+{\text{H}}_2{\text{O}}_2+2{\text{H}}^{+}\to {\text{I}}_2+2{\text{H}}_2\text{O}$$ The initial rate of reaction is found to be first order with respect to ${\text{I}}^{-}$, first order with respect to ${\text{H}}_2{\text{O}}_2$ and zero order with respect to ${\text{H}}^{+}$. Fig. 4.1 shows a possible four-step mechanism for this reaction. step 1: ${\text{H}}_2{\text{O}}_2+{\text{I}}^{-}\to {\text{IO}}^{-}+{\text{H}}_2\text{O}$ step 2: ${\text{H}}^{+}+{\text{IO}}^{-}\to \text{HIO}$ step 3: $\text{HIO}+{\text{I}}^{-}\to {\text{I}}_2+{\text{OH}}^{-}$ step 4: ${\text{OH}}^{-}+{\text{H}}^{+}\to {\text{H}}_2\text{O}$ Suggest which of the steps, 1, 2, 3 or 4, in this mechanism is the rate-determining step. Explain your answer.

Identify a step in Fig. 4.1 that involves a redox reaction. Explain your answer in terms of oxidation numbers.

Suggest the role of $\text{HIO}$ in this mechanism. Explain your reasoning.

Methylbenzene can undergo different reactions, as shown in Fig. 5.1. Draw structures in Fig. 5.1 for the possible organic products of the three reactions shown.

Complete Table 5.1.

When methylbenzene reacts with an electrophile, a substitution reaction occurs. No addition reaction takes place under these conditions. Explain why no addition reaction takes place.

The reaction of methylbenzene with thionyl bromide, ${\text{SOBr}}_2$, in the presence of an iron(III) bromide catalyst, ${\text{FeBr}}_3$, is shown in Fig. 5.2. The mechanism of this reaction is similar to that of the bromination of benzene. The first step of the mechanism generates the ${\text{SOBr}}^{+}$ electrophile, as shown. $${\text{SOBr}}_2+{\text{FeBr}}_3\to {\text{SOBr}}^{+}+{\text{FeBr}}_4^{-}$$ The reaction of methylbenzene with ${\text{SOBr}}^{+}$ ions is shown in Fig. 5.3. Complete the mechanism in Fig. 5.3. Include all relevant curly arrows and charges. Draw the structure of the organic intermediate.

The reaction shown in Fig. 5.2 produces a small amount of a by-product, P, with the molecular formula ${\text{C}}_{14}{\text{H}}_{14}\text{OS}$. Suggest a structure for by-product P.

Acyl bromides, $\text{RCOBr}$, can be synthesised by the reaction of a carboxylic acid and ${\text{SOBr}}_2$. This is a similar reaction to the synthesis of acyl chlorides using ${\text{SOCl}}_2$. Give an equation for the reaction between ethanoic acid and ${\text{SOBr}}_2$.

Suggest the relative ease of hydrolysis of acyl bromides, $\text{RCOBr}$, acyl chlorides, $\text{RCOCl}$, and alkyl chlorides, $\text{RCl}$. Explain your answer.

Perindopril is a drug used to treat heart disease. State the number of chiral carbon atoms present in one molecule of perindopril.

Suggest one benefit and one disadvantage of producing a drug such as perindopril as a single pure optical isomer.

Name all the functional groups in perindopril.

A sample of perindopril is hydrolysed with hot aqueous acid. Draw the structures of the three organic products of the complete acid hydrolysis of perindopril.

Explain why phenol is brominated much more easily than benzene is brominated.

Iodine monobromide, $\text{I-Br}$, reacts with benzene in the presence of an ${\text{AlBr}}_3$ catalyst. Predict whether the organic product will be bromobenzene or iodobenzene. Explain your answer.

Fig. 7.1 shows some reactions of phenol. Give an equation for the reaction of phenol with $\text{Na(s)}$.

Draw the structure of the organic product, R, formed when phenol reacts with an excess of ${\text{Br}}_2\text{(aq)}$.

State the reagents and conditions for reaction 1 and reaction 2 in Fig. 7.1.

Describe the difference in reactivity between chloroethane and chlorobenzene with ${\text{OH}}^{-}\text{(aq)}$. Explain your answer.

Compound T, ${\text{C}}_5{\text{H}}_9{\text{O}}_2\text{Cl}$, is a useful synthetic intermediate. Fig. 8.1 shows some reactions of T. Give the systematic name for T.

Draw the structures of W, X, Y and Z in Fig. 8.1.

State the reagents and conditions for steps 1 and 2 in Fig. 8.1.

The proton (${}^1\text{H}$) NMR spectrum of compound T, ${\text{C}}_5{\text{H}}_9{\text{O}}_2\text{Cl}$, in ${\text{CDCl}}_3$ is shown in Fig. 8.2. Suggest why ${\text{CDCl}}_3$ is used as a solvent instead of ${\text{CHCl}}_3$ for the proton (${}^1\text{H}$) NMR spectrum.

Complete Table 8.2 for the proton (${}^1\text{H}$) NMR spectrum of T.

Explain the splitting pattern of the peak at $\delta 3.9\text{ ppm}$.

Define standard cell potential, $E_{\text{cell}}^{\ominus }$.

An electrochemical cell is set up to measure $E_{\text{cell}}^{\ominus }$ of a cell consisting of an ${\text{Fe}}^{3+}\mathrm{/}{\text{Fe}}^{2+}$ half-cell and a ${\text{Cl}}_2\mathrm{/}{\text{Cl}}^{-}$ half-cell. Draw a labelled diagram of this electrochemical cell. Include all necessary substances. It is not necessary to state conditions used.

The cell reaction for the electrochemical cell in (b) is shown. $${\text{Cl}}_2+2{\text{Fe}}^{2+}\to 2{\text{Fe}}^{3+}+2{\text{Cl}}^{-}{E}_{\text{cell}}^{\ominus }=+0.59\text{ V}$$ Calculate $\mathrm{\Delta }{G}^{\ominus }$, in $\text{kJ}{\text{mol}}^{-1}$, for this cell reaction.

Another experiment is set up using the same electrochemical cell. In this experiment the ${\text{Fe}}^{2+}$ concentration is $0.15\text{ mol}{\text{dm}}^{-3}$. All other concentrations remain at their standard values. The Nernst equation is shown. $$E=E^{\ominus }+(0.059\mathrm{/}z)\log \frac{[\text{oxidised species}]}{[\text{reduced species}]}$$ Use the Nernst equation to calculate the electrode potential, $E$, for the ${\text{Fe}}^{3+}\mathrm{/}{\text{Fe}}^{2+}$ half-cell in this experiment. [$E^{\ominus }$: ${\text{Fe}}^{3+}\mathrm{/}{\text{Fe}}^{2+}=+0.77\text{ V}$]

Use your answer to (d)(i) to calculate $E_{\text{cell}}$ for this electrochemical cell.