2026 Cambridge IAL Chemistry (9701) 模擬試題連答案詳解

A \(20\text{ cm}^3\) sample of a gaseous hydrocarbon \(\text{C}_x\text{H}_y\) was completely burnt in \(150\text{ cm}^3\) of oxygen (an excess).

After combustion, the total volume of gas remaining at room temperature and pressure was \(110\text{ cm}^3\).
Passing this remaining gas mixture through aqueous sodium hydroxide caused a decrease in volume of \(80\text{ cm}^3\).

All gas volumes were measured under the same conditions of temperature and pressure.

What is the molecular formula of the hydrocarbon?

The first six successive ionisation energies of element \(X\), in \(\text{kJ mol}^{-1}\), are given:

789, 1577, 3232, 4356, 16091, 19785

Which statement about element \(X\) is correct?

Which pair of species both have the same shape and identical bond angles?

An anhydrous chloride of a Period 3 element, \(E\), is added to excess water.

The reaction is vigorous, releasing steamy acidic fumes, and leaves a white precipitate suspended in an acidic solution of \(\text{pH } \approx 2\).

What is the identity of chloride \(E\)?

Standard enthalpy changes of combustion, \(\Delta H^\circ_\text{c}\), are given in the table.

$$\begin{array}{|c|c|}\hline\text{Substance} & \Delta H^\circ_\text{c} / \text{kJ mol}^{-1} \\hline\text{C(graphite)} & -394 \\hline\text{H}_2\text{(g)} & -286 \\hline\text{CH}_3\text{OCH}_3\text{(g)} & -1460 \\hline\end{array}$$

What is the standard enthalpy change of formation, \(\Delta H^\circ_\text{f}\), of methoxymethane, \(\text{CH}_3\text{OCH}_3\text{(g)}\)?

Equal amounts of four different halogenoalkanes are added separately to warm ethanolic silver nitrate at \(50\ ^\circ\text{C}\).

Which halogenoalkane forms a precipitate in the shortest time?

An unbranched hydrocarbon \(W\), \(\text{C}_6\text{H}_{10}\), contains two carbon–carbon double bonds.

When \(W\) is heated under reflux with hot concentrated acidified \(\text{KMnO}_4\), the only organic products obtained are propanoic acid and ethanedioic acid in a \(1:1\) molar ratio, alongside carbon dioxide and water.

What is the structural formula of \(W\)?

Compound \(Q\) has the molecular formula \(\text{C}_4\text{H}_8\text{O}\).

- \(Q\) gives an orange precipitate when treated with 2,4-dinitrophenylhydrazine.
- \(Q\) does not form a silver mirror when warmed with Tollens' reagent.
- \(Q\) forms a pale yellow precipitate when warmed with alkaline aqueous iodine.

What is the identity of compound \(Q\)?

The first eight successive ionisation energies of an element \(X\) in Period 3 of the Periodic Table are given in \(\text{kJ mol}^{-1}\):

1000, 2251, 3361, 4564, 7013, 8495, 27107, 31719

Which statement about element \(X\) is correct?

A sample of an unknown gaseous hydrocarbon has a mass of \(0.387\text{ g}\). It occupies a volume of \(166.2\text{ cm}^3\) at a temperature of \(300\text{ K}\) and a pressure of \(1.00 \times 10^5\text{ Pa}\).

What is the molecular formula of this hydrocarbon?

Which species contains the smallest bond angle?

Standard enthalpy changes of combustion, \(\Delta H^\circ_c\), are provided:

- \(\Delta H^\circ_c(\text{C(s)}) = -394\text{ kJ mol}^{-1}\)
- \(\Delta H^\circ_c(\text{H}_2\text{(g)}) = -286\text{ kJ mol}^{-1}\)
- \(\Delta H^\circ_c(\text{CH}_3\text{OH(l)}) = -726\text{ kJ mol}^{-1}\)

What is the standard enthalpy change of formation, \(\Delta H^\circ_f\), of methanol, \(\text{CH}_3\text{OH(l)}\)?

A solid chloride of a Period 3 element reacts vigorously with water, releasing steamy white fumes and giving an acidic solution with a pH between 1 and 2.

When excess aqueous sodium hydroxide is added dropwise to this acidic solution, a white precipitate forms initially, which then redissolves to form a colourless solution.

Which element forms this chloride?

Which row correctly describes the trends in properties of the Group 2 elements and their compounds on descending the group from magnesium to barium?

An alkene \(Y\) with molecular formula \(\text{C}_6\text{H}_{12}\) is heated under reflux with hot concentrated acidified potassium manganate(VII).

Propan-2-one, \(\text{CH}_3\text{COCH}_3\), is the only organic product obtained from this reaction.

What is the structure of alkene \(Y\)?

Compound \(Z\) undergoes the following reactions:

- It produces an orange precipitate with 2,4-dinitrophenylhydrazine.
- It does not produce a silver mirror when warmed with Tollens' reagent.
- It forms a pale yellow precipitate when warmed with alkaline aqueous iodine.

Which compound is \(Z\)?

The first seven successive ionisation energies of a Period 3 element, $E$, are shown in the table.

$$\begin{array}{|c|c|c|c|c|c|c|c|}\hline\text{Ionisation energy} & 1^{\text{st}} & 2^{\text{nd}} & 3^{\text{rd}} & 4^{\text{th}} & 5^{\text{th}} & 6^{\text{th}} & 7^{\text{th}} \\hline\text{Value / kJ mol}^{-1} & 1012 & 1903 & 2912 & 4956 & 6274 & 22233 & 25397 \\hline\end{array}$$

Which formula represents the chloride formed when element $E$ is heated in an excess of chlorine gas?

A $20\text{ cm}^3$ sample of a gaseous hydrocarbon, $\text{C}_x\text{H}_y$, was completely burned in $150\text{ cm}^3$ of oxygen gas (an excess).

After cooling to room temperature, the total volume of residual gas was $110\text{ cm}^3$.

When this residual gas was shaken with an excess of aqueous sodium hydroxide, the volume decreased by $80\text{ cm}^3$.

All gas volumes were measured under identical room conditions.

What is the molecular formula of the hydrocarbon?

Which molecule contains exactly two lone pairs of electrons on the central atom and has a non-linear (bent) shape?

The standard enthalpy changes of combustion, $\Delta H^\circ_{\text{c}}$, are given:

$$\begin{aligned}
\text{C}(\text{graphite}) & = -394\text{ kJ mol}^{-1} \\
\text{H}_2(\text{g}) & = -286\text{ kJ mol}^{-1} \\
\text{CH}_3\text{C}\equiv\text{CH}(\text{g}) & = -1938\text{ kJ mol}^{-1}
\end{aligned}$$

What is the standard enthalpy change of formation, $\Delta H^\circ_{\text{f}}$, of propyne, $\text{CH}_3\text{C}\equiv\text{CH}(\text{g})$?

An element $M$ in Period 3 forms an oxide that reacts with aqueous sodium hydroxide but does not react with dilute hydrochloric acid.

The chloride of $M$ reacts vigorously with water to form an acidic solution of $\text{pH } \approx 2$ and a white solid precipitate.

What is the identity of element $M$?

Two separate experiments are carried out using Group 2 compounds.

- Experiment 1: Equal molar amounts of solid $\text{Mg(OH)}_2$ and $\text{Ba(OH)}_2$ are added to equal volumes of pure water at $298\text{ K}$.
- Experiment 2: Equal molar amounts of solid $\text{MgCO}_3$ and $\text{BaCO}_3$ are heated strongly under identical conditions with a Bunsen burner.

Which row correctly identifies the compound that forms the solution with the higher pH in Experiment 1 and the compound that decomposes more readily in Experiment 2?

$$\begin{array}{|c|c|c|}\hline & \text{Higher pH (Experiment 1)} & \text{Decomposes more readily (Experiment 2)} \\hline\text{A} & \text{Mg(OH)}_2 & \text{BaCO}_3 \\text{B} & \text{Mg(OH)}_2 & \text{MgCO}_3 \\text{C} & \text{Ba(OH)}_2 & \text{BaCO}_3 \\text{D} & \text{Ba(OH)}_2 & \text{MgCO}_3 \\hline\end{array}$$

An alkene $Q$ is heated under reflux with a hot, concentrated, acidified solution of potassium manganate(VII).

The organic products formed are propanone, $(\text{CH}_3)_2\text{CO}$, and propanoic acid, $\text{CH}_3\text{CH}_2\text{COOH}$, in a $1:1$ molar ratio.

What is the IUPAC name of alkene $Q$?

Compound $W$ has the molecular formula $\text{C}_4\text{H}_8\text{O}$.

Compound $W$:
- forms an orange precipitate when reacted with 2,4-dinitrophenylhydrazine,
- produces no silver mirror when warmed with Tollens' reagent,
- forms a yellow precipitate when warmed with alkaline aqueous iodine.

Which structure represents compound $W$?

An uncharged atom of an element in Period 3 has exactly three unpaired electrons in its ground state.

What is the formula of the chloride formed when this element reacts completely with an excess of chlorine gas?

A gaseous hydrocarbon \(Q\) was completely burned in an excess of oxygen.

\(10\,\text{cm}^3\) of \(Q\) required \(65\,\text{cm}^3\) of oxygen for complete combustion, producing \(40\,\text{cm}^3\) of carbon dioxide. All gas volumes were measured at the same temperature and pressure.

Which statement about compound \(Q\) is correct?

The boiling points of the Group 16 hydrides are shown.

- \(\text{H}_2\text{O}\): \(373\,\text{K}\)
- \(\text{H}_2\text{S}\): \(213\,\text{K}\)
- \(\text{H}_2\text{Se}\): \(232\,\text{K}\)
- \(\text{H}_2\text{Te}\): \(271\,\text{K}\)

Which statement explains why the boiling point of \(\text{H}_2\text{Te}\) is higher than that of \(\text{H}_2\text{S}\)?

Standard enthalpy changes of combustion, \(\Delta H^\circ_c\), are given:

- \(\Delta H^\circ_c[\text{C}_3\text{H}_6\text{(g)}] = -2058\,\text{kJ}\,\text{mol}^{-1}\)
- \(\Delta H^\circ_c[\text{C}_3\text{H}_7\text{OH(l)}] = -2021\,\text{kJ}\,\text{mol}^{-1}\)

What is the standard enthalpy change, \(\Delta H^\circ_r\), for the hydration of propene?

\[\text{C}_3\text{H}_6\text{(g)} + \text{H}_2\text{O(l)} \rightarrow \text{C}_3\text{H}_7\text{OH(l)}\]

Two Period 3 elements, \(X\) and \(Y\), react with chlorine to form chlorides \(X\text{Cl}_n\) and \(Y\text{Cl}_m\).

- Adding solid \(X\text{Cl}_n\) to water gives a neutral solution (\(\text{pH} = 7\)).
- Adding solid \(Y\text{Cl}_m\) to water causes a vigorous reaction, producing an acidic solution and steamy white fumes.
- The oxide of \(X\) is basic.
- The oxide of \(Y\) reacts with both dilute hydrochloric acid and aqueous sodium hydroxide.

Which row correctly identifies elements \(X\) and \(Y\)?

An unlabelled white powder is known to be a Group 2 compound.

- The powder is insoluble in water.
- When dilute nitric acid is added to the powder, vigorous effervescence occurs, and a colourless solution is formed.
- When aqueous sodium sulfate is added to this resulting colourless solution, a dense white precipitate forms immediately.

What is the identity of the unlabelled white powder?

Four separate samples of halogenoalkanes are each mixed with an equal volume of ethanolic silver nitrate and placed in a water bath at \(50\,^\circ\text{C}\).

Which halogenoalkane produces a precipitate in the shortest time?

Compound \(Z\) has the molecular formula \(\text{C}_4\text{H}_8\text{O}\).

- \(Z\) forms an orange precipitate when reacted with 2,4-dinitrophenylhydrazine.
- \(Z\) does not produce a silver mirror when warmed with Tollens' reagent.
- \(Z\) forms a yellow precipitate when warmed with alkaline aqueous iodine.

What is the structure of \(Z\)?

Element \(X\) is in Period 3 of the Periodic Table. The first seven successive ionisation energies of \(X\), in \(\text{kJ mol}^{-1}\), are shown:

1012, 1907, 2914, 4964, 6274, 21267, 25431

Which statement about the oxide of \(X\) in its highest oxidation state is correct?

A \(0.174\text{ g}\) sample of an anhydrous Group 2 metal carbonate, \(M\text{CO}_3\), was added to \(50.0\text{ cm}^3\) of \(0.100\text{ mol dm}^{-3}\) hydrochloric acid, \(\text{HCl}(\text{aq})\) (an excess).

The resulting unreacted acid required \(22.00\text{ cm}^3\) of \(0.100\text{ mol dm}^{-3}\) sodium hydroxide, \(\text{NaOH}(\text{aq})\), for complete neutralisation.

What is the relative atomic mass, \(A_r\), of metal \(M\)?

Which list places the molecules in order of increasing bond angle, with the smallest bond angle first?

The standard enthalpy changes of combustion, \(\Delta H^\circ_c\), for carbon, hydrogen, and propene are given.

\(\text{C}(\text{s}) + \text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g})\quad \Delta H^\circ_c = -394\text{ kJ mol}^{-1}\)

\(\text{H}_2(\text{g}) + \frac{1}{2}\text{O}_2(\text{g}) \rightarrow \text{H}_2\text{O}(\text{l})\quad \Delta H^\circ_c = -286\text{ kJ mol}^{-1}\)

\(\text{C}_3\text{H}_6(\text{g}) + \frac{9}{2}\text{O}_2(\text{g}) \rightarrow 3\text{CO}_2(\text{g}) + 3\text{H}_2\text{O}(\text{l})\quad \Delta H^\circ_c = -2058\text{ kJ mol}^{-1}\)

What is the standard enthalpy change of formation, \(\Delta H^\circ_f\), of propene, \(\text{C}_3\text{H}_6(\text{g})\)?

Two Period 3 chlorides, \(\text{SiCl}_4\) and \(\text{PCl}_5\), are each added separately to an excess of cold water.

Which row correctly describes the observations and the \(\text{pH}\) of the resulting solutions?

Separate samples of solid anhydrous magnesium nitrate, \(\text{Mg}(\text{NO}_3)_2\), and barium nitrate, \(\text{Ba}(\text{NO}_3)_2\), are heated strongly over a Bunsen flame.

Which statement correctly explains the difference in their thermal stability?

An alkene, \(Y\), has the molecular formula \(\text{C}_6\text{H}_{12}\).

When \(Y\) is heated under reflux with hot concentrated acidified potassium manganate(VII), the only organic products obtained are propanoic acid, \(\text{CH}_3\text{CH}_2\text{COOH}\), and propanone, \((\text{CH}_3)_2\text{CO}\).

What is the systematic IUPAC name of alkene \(Y\)?

Compound \(Z\) has the molecular formula \(\text{C}_4\text{H}_8\text{O}\).

- \(Z\) gives an orange precipitate with 2,4-dinitrophenylhydrazine reagent.
- \(Z\) shows no reaction when warmed with Tollens' reagent.
- \(Z\) forms a yellow precipitate when warmed with alkaline aqueous iodine.

What is the structural formula of \(Z\)?

(a) Define relative atomic mass, \(A_r\). [2]

(b) A naturally occurring sample of element \(\text{M}\) contains four isotopes with the following percentage abundances:
- \(^{84}\text{M}\): 0.56%
- \(^{86}\text{M}\): 9.86%
- \(^{87}\text{M}\): 7.00%
- \(^{88}\text{M}\): 82.58%

Calculate the relative atomic mass, \(A_r\), of \(\text{M}\) to two decimal places. [2]

(c) A volatile organic compound \(\text{Y}\) contains carbon, hydrogen, and oxygen only. Elemental analysis shows that \(\text{Y}\) has a percentage composition of 54.5% carbon, 9.1% hydrogen, and 36.4% oxygen by mass.

(i) Show that the empirical formula of \(\text{Y}\) is \(\text{C}_2\text{H}_4\text{O}\). [2]

(ii) In an experiment, a \(0.220\text{ g}\) sample of liquid \(\text{Y}\) was completely vaporised in a gas syringe at \(373\text{ K}\) and \(101\text{ kPa}\). The volume of vaporised gas collected was \(77.2\text{ cm}^3\).

Use the ideal gas equation, \(pV = nRT\), to calculate the relative molecular mass, \(M_r\), of \(\text{Y}\), and deduce its molecular formula. [\(R = 8.31\text{ J K}^{-1}\text{ mol}^{-1}\)] [4]

(d) State two basic assumptions of the kinetic theory of ideal gases that are not valid for real gases at high pressure. [2]

This question is about Period 3 elements and their compounds.

(a) Write the full electronic configuration of the following ions:
(i) \(\text{Al}^{3+}\) [1]
(ii) \(\text{P}^{3-}\) [1]

(b) Silicon tetrachloride, \(\text{SiCl}_4\), and sodium chloride, \(\text{NaCl}\), are Period 3 chlorides.
(i) Describe the observations when \(\text{SiCl}_4\) is added to water and write a balanced equation for the reaction. [2]
(ii) State the approximate pH of the solution formed when \(\text{NaCl}\) is dissolved in water and when \(\text{AlCl}_3\) is dissolved in water. Explain the difference in pH between these two solutions. [3]

(c) Chlorine gas reacts with cold dilute aqueous sodium hydroxide.
(i) Construct a balanced chemical equation for this reaction. [1]
(ii) State the oxidation state of chlorine in each chlorine-containing product and explain why this reaction is classified as a disproportionation reaction. [2]

(d) Solid potassium bromide reacts with concentrated sulfuric acid.
(i) State two observations made during this reaction. [1]
(ii) Write a balanced chemical equation for the redox reaction in which bromide ions are oxidised by concentrated sulfuric acid. [1]

Sulfur trioxide is manufactured industrially by the oxidation of sulfur dioxide:

\[2\text{SO}_2(\text{g}) + \text{O}_2(\text{g}) \rightleftharpoons 2\text{SO}_3(\text{g}) \quad \Delta H^\circ = -196\text{ kJ mol}^{-1}\]

(a) Define standard enthalpy change of formation, \(\Delta H^\circ_\text{f}\). [2]

(b) The standard enthalpy change of formation of \(\text{SO}_2(\text{g})\) is \(-296.8\text{ kJ mol}^{-1}\).
Calculate the standard enthalpy change of formation of \(\text{SO}_3(\text{g})\). [2]

(c) Use Le Chatelier's principle to predict and explain the effect of increasing the temperature on the equilibrium position and the value of \(K_\text{p}\). [2]

(d) In a closed vessel of fixed volume \(5.00\text{ dm}^3\) at a constant temperature \(T\), an initial mixture of \(2.00\text{ mol}\) of \(\text{SO}_2(\text{g})\) and \(1.00\text{ mol}\) of \(\text{O}_2(\text{g})\) was allowed to reach dynamic equilibrium. At equilibrium, \(1.40\text{ mol}\) of \(\text{SO}_3(\text{g})\) was present.

(i) Calculate the equilibrium amounts, in moles, of \(\text{SO}_2(\text{g})\) and \(\text{O}_2(\text{g})\). [2]

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

(iii) Calculate the value of \(K_\text{c}\) at temperature \(T\) and state its units. [3]

Halogenoalkanes undergo nucleophilic substitution and elimination reactions.

(a) 2-bromobutane, \(\text{CH}_3\text{CH(Br)CH}_2\text{CH}_3\), reacts with warm aqueous sodium hydroxide.
(i) Name the organic product formed and state the type of stereoisomerism it exhibits. [2]
(ii) Outline the mechanism for the \(\text{S}_\text{N}2\) hydrolysis of 2-bromobutane by hydroxide ions. Include relevant dipoles, curly arrows, and the structure of the transition state. [3]
(iii) Explain why tertiary halogenoalkanes, such as 2-bromo-2-methylpropane, react via the \(\text{S}_\text{N}1\) mechanism rather than the \(\text{S}_\text{N}2\) mechanism. [2]

(b) When 2-bromobutane is heated with sodium hydroxide dissolved in ethanol, an elimination reaction occurs to form a mixture of three isomeric alkenes.
(i) Draw the skeletal formulas of all three isomeric alkenes formed and name each one. [3]
(ii) State the type of isomerism shown between the stereoisomers formed in (b)(i). [1]
(iii) Describe a chemical test to distinguish an alkene from an alkane, including the reagent and the observation with the alkene. [1]

Three structural isomers, \(\text{P}\), \(\text{Q}\), and \(\text{R}\), have the molecular formula \(\text{C}_4\text{H}_8\text{O}\).

- Isomer \(\text{P}\) reacts with 2,4-dinitrophenylhydrazine (2,4-DNPH) to give an orange precipitate. It forms a silver mirror with Tollens' reagent, and has a branched carbon chain.
- Isomer \(\text{Q}\) also forms an orange precipitate with 2,4-DNPH. When \(\text{Q}\) is warmed with alkaline aqueous iodine, a yellow precipitate is formed.
- Isomer \(\text{R}\) does not react with 2,4-DNPH. It reacts with sodium metal to release hydrogen gas, and readily decolourises bromine water.

(a) Identify isomers \(\text{P}\) and \(\text{Q}\) by giving their systematic names and structural formulas. [4]

(b) (i) Identify the yellow precipitate formed when \(\text{Q}\) reacts with alkaline aqueous iodine. [1]
(ii) State the specific structural feature in \(\text{Q}\) responsible for the positive tri-iodomethane test. [1]

(c) Isomer \(\text{Q}\) can be reduced to an alcohol, \(\text{S}\), using sodium borohydride, \(\text{NaBH}_4\).
(i) Write an equation for the reduction of \(\text{Q}\) to form \(\text{S}\), using \([\text{H}]\) to represent the reducing agent. [1]
(ii) Describe how the infrared spectra of \(\text{Q}\) and \(\text{S}\) differ by stating the bonds and characteristic absorption wave numbers involved. [2]

(d) Draw one possible displayed formula for isomer \(\text{R}\). [1]

(e) Alcohol \(\text{S}\) is heated under reflux with ethanoic acid in the presence of a few drops of concentrated sulfuric acid.
(i) Draw the skeletal formula of the organic ester produced. [1]
(ii) State the role of concentrated sulfuric acid in this reaction. [1]

FA 1 is a solution containing \(6.15\text{ g dm}^{-3}\) of an impure sample of hydrated ethanedioic acid, \(\text{H}_2\text{C}_2\text{O}_4\cdot 2\text{H}_2\text{O}\).
FA 2 is aqueous potassium manganate(VII), \(\text{KMnO}_4\), of concentration \(0.0200\text{ mol dm}^{-3}\).
FA 3 is \(1.00\text{ mol dm}^{-3}\) sulfuric acid, \(\text{H}_2\text{SO}_4\).

You are to determine the percentage purity of the hydrated ethanedioic acid in the sample by titrating FA 1 with FA 2 in the presence of dilute sulfuric acid.

Method
1. Fill the burette with FA 2.
2. Pipette \(25.0\text{ cm}^3\) of FA 1 into a conical flask.
3. Using a measuring cylinder, add \(20.0\text{ cm}^3\) of FA 3 to the conical flask.
4. Heat the contents of the conical flask to approximately \(65^{\circ}\text{C}\).
5. Titrate the hot solution with FA 2 until a permanent pale pink colour is obtained.
6. Repeat the titration until concordant results are achieved.

(a) Record your titration results in an appropriate table in the space below.

| Titration number | 1 | 2 | 3 |
| :--- | :---: | :---: | :---: |
| Final burette reading / \(\text{cm}^3\) | 24.50 | 48.65 | 24.15 |
| Initial burette reading / \(\text{cm}^3\) | 0.00 | 24.50 | 0.00 |
| Titre / \(\text{cm}^3\) | 24.50 | 24.15 | 24.15 |

From your titration results, obtain a suitable value for the volume of FA 2 to be used in your calculations. Show clearly how this value is obtained.

(b)
(i) Calculate the amount, in \(\text{mol}\), of \(\text{MnO}_4^-\) present in the mean titre volume of FA 2.

(ii) The equation for the reaction is:
\[2\text{MnO}_4^-(aq) + 5\text{H}_2\text{C}_2\text{O}_4(aq) + 6\text{H}^+(aq) \rightarrow 2\text{Mn}^{2+}(aq) + 10\text{CO}_2(g) + 8\text{H}_2\text{O}(l)\]
Calculate the amount, in \(\text{mol}\), of \(\text{H}_2\text{C}_2\text{O}_4\) in \(25.0\text{ cm}^3\) of FA 1.

(iii) Calculate the concentration, in \(\text{mol dm}^{-3}\), of \(\text{H}_2\text{C}_2\text{O}_4\) in FA 1.

(iv) Calculate the mass of pure hydrated ethanedioic acid, \(\text{H}_2\text{C}_2\text{O}_4\cdot 2\text{H}_2\text{O}\), present in \(1.00\text{ dm}^3\) of FA 1. \([M_r: \text{H}_2\text{C}_2\text{O}_4\cdot 2\text{H}_2\text{O} = 126.0]\)

(v) Calculate the percentage purity by mass of the hydrated ethanedioic acid sample in FA 1.

(c) The maximum error associated with each reading of the \(50.0\text{ cm}^3\) burette is \(\pm 0.05\text{ cm}^3\). Calculate the maximum percentage error in the titre volume for Titration 2.

FA 6, FA 7, and FA 8 are aqueous solutions. Each contains one cation and one anion from the Qualitative Analysis Notes.

Perform the tests described in the table below. Record your observations and deduce the identities of the ions present.

(a) Complete the observations table:

| Test | Observations with FA 6 | Observations with FA 7 | Observations with FA 8 |
| :--- | :--- | :--- | :--- |
| 1. To a \(1\text{ cm}\) depth of solution in a test-tube, add aqueous sodium hydroxide dropwise, then in excess. | Green precipitate formed, insoluble in excess; turns brown at the surface on standing. | Grey-green precipitate formed, dissolves in excess to give a dark green solution. | White precipitate formed, soluble in excess to give a colourless solution. |
| 2. To a \(1\text{ cm}\) depth of solution in a test-tube, add aqueous ammonia dropwise, then in excess. | Green precipitate formed, insoluble in excess; turns brown at the surface. | Grey-green precipitate formed, insoluble in excess. | White precipitate formed, insoluble in excess. |
| 3. To a \(1\text{ cm}\) depth of solution in a test-tube, add a few drops of aqueous barium nitrate followed by dilute nitric acid. | No precipitate / no change. | White precipitate formed, insoluble in dilute nitric acid. | White precipitate formed, dissolves with effervescence in dilute nitric acid; gas turns limewater milky. |
| 4. To a \(1\text{ cm}\) depth of solution in a test-tube, add acidified potassium manganate(VII), \(\text{KMnO}_4\), dropwise. | Purple solution is decolourised / turns pale yellow-brown. | No change / solution remains purple. | No change / solution remains purple. |

(b) Deduce the chemical formula for each solution:

(i) Cation in FA 6:
Anion in FA 6:
Formula of FA 6:

(ii) Cation in FA 7:
Anion in FA 7:
Formula of FA 7:

(iii) Cation in FA 8:
Anion in FA 8:
Formula of FA 8:

(c) Write an ionic equation for the reaction observed in Test 4 with FA 6.

In this experiment, you will determine the concentration of potassium manganate(VII) in solution FA 1 by titrating it against a standard solution of hydrated iron(II) sulfate, \(\text{FeSO}_4 \cdot 7\text{H}_2\text{O}\).

FA 1 is aqueous potassium manganate(VII), \(\text{KMnO}_4\).
FA 2 is dilute sulfuric acid, \(1.00\text{ mol dm}^{-3}\text{ H}_2\text{SO}_4\).
Solid \(\text{FeSO}_4 \cdot 7\text{H}_2\text{O}\) has a molar mass of \(278.0\text{ g mol}^{-1}\).

(a) A student weighs out \(6.950\text{ g}\) of \(\text{FeSO}_4 \cdot 7\text{H}_2\text{O}\), dissolves it in FA 2 and distilled water, and makes the solution up to exactly \(250.0\text{ cm}^3\) in a volumetric flask to prepare solution FA 3.

Calculate the concentration, in \(\text{mol dm}^{-3}\), of \(\text{Fe}^{2+}\text{(aq)}\) in solution FA 3.

(b) The student pipettes \(25.0\text{ cm}^3\) of FA 3 into a conical flask and adds \(20\text{ cm}^3\) of FA 2 using a measuring cylinder. FA 1 is placed in the burette. The titration results are recorded below:

- Rough Titre: Initial = \(0.00\text{ cm}^3\), Final = \(22.80\text{ cm}^3\), Titre = \(22.80\text{ cm}^3\)
- Titre 1: Initial = \(0.50\text{ cm}^3\), Final = \(22.90\text{ cm}^3\), Titre = \(22.40\text{ cm}^3\)
- Titre 2: Initial = \(22.90\text{ cm}^3\), Final = \(45.40\text{ cm}^3\), Titre = \(22.50\text{ cm}^3\)

(i) Calculate the suitable average titre of FA 1 to use in your calculations.
(ii) In acidic solution, manganate(VII) ions react with iron(II) ions according to the equation:
$$\text{MnO}_4^-(\text{aq}) + 5\text{Fe}^{2+}(\text{aq}) + 8\text{H}^+(\text{aq}) \rightarrow \text{Mn}^{2+}(\text{aq}) + 5\text{Fe}^{3+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})$$
Calculate the concentration, in \(\text{mol dm}^{-3}\), of \(\text{KMnO}_4\) in FA 1. Give your answer to 3 significant figures.

(c) State the colour change observed at the end-point of this titration.

(d) The student used a balance with an uncertainty of \(\pm 0.005\text{ g}\) to weigh the \(6.950\text{ g}\) of \(\text{FeSO}_4 \cdot 7\text{H}_2\text{O}\) by difference (two weighings). Calculate the percentage uncertainty in this mass.

Cobalt forms a wide variety of coordination complexes in the +2 and +3 oxidation states.

(a) An aqueous solution of cobalt(II) chloride contains the hexaaquacobalt(II) ion, \([\text{Co}(\text{H}_2\text{O})_6]^{2+}\), which is pink.
(i) State the electronic configuration of the \(\text{Co}^{2+}\) ion in terms of \(s\), \(p\), and \(d\) orbitals.
(ii) Explain why transition element complexes, such as \([\text{Co}(\text{H}_2\text{O})_6]^{2+}\), are coloured.

(b) When concentrated hydrochloric acid is added to an aqueous solution containing \([\text{Co}(\text{H}_2\text{O})_6]^{2+}\), a deep blue solution containing \([\text{CoCl}_4]^{2-}\) is formed.
(i) Write a balanced ionic equation for this ligand exchange reaction.
(ii) State the geometry and coordination number of the cobalt ion in \([\text{CoCl}_4]^{2-}\).

(c) The complex \([\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+\) exhibits stereoisomerism.
(i) Name the type of stereoisomerism shown by \([\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+\).
(ii) Draw three-dimensional diagrams to show the structures of the stereoisomers of \([\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+\).

(d) The stability constant, \(K_{\text{stab}}\), for the formation of the complex \([\text{Co}(\text{NH}_3)_6]^{2+}\) from \([\text{Co}(\text{H}_2\text{O})_6]^{2+}\) is \(7.7 \times 10^4\text{ dm}^{18}\text{ mol}^{-6}\), whereas \(K_{\text{stab}}\) for \([\text{Co}(\text{en})_3]^{2+}\) (where \(\text{en} = \text{ethane-1,2-diamine}\)) is \(3.8 \times 10^{14}\text{ dm}^9\text{ mol}^{-3}\).
Explain, with reference to entropy changes, why \([\text{Co}(\text{en})_3]^{2+}\) has a significantly larger stability constant than \([\text{Co}(\text{NH}_3)_6]^{2+}\).

Lactic acid, \(\text{CH}_3\text{CH(OH)COOH}\), is a weak monobasic acid that plays an essential role in biological processes. It can be represented as \(\text{HLac}\).

(a) (i) Write the expression for the acid dissociation constant, \(K_a\), of lactic acid.
(ii) A \(0.120\text{ mol dm}^{-3}\) aqueous solution of lactic acid has a \(\text{pH}\) of \(2.39\) at \(298\text{ K}\).
Calculate the value of \(K_a\) for lactic acid at \(298\text{ K}\). Include units in your answer.

(b) A buffer solution is prepared by mixing \(60.0\text{ cm}^3\) of \(0.120\text{ mol dm}^{-3}\text{ HLac}\) with \(40.0\text{ cm}^3\) of \(0.0800\text{ mol dm}^{-3}\text{ NaOH}\).
(i) Define what is meant by a buffer solution.
(ii) Calculate the \(\text{pH}\) of this buffer solution at \(298\text{ K}\). Use your value of \(K_a\) from (a)(ii).

(c) Explain, with the aid of an ionic equation, how this buffer solution resists a change in \(\text{pH}\) when a small quantity of aqueous hydrochloric acid is added.

(d) Solid calcium lactate, \(\text{Ca(Lac)}_2\), is sparingly soluble in water. At \(298\text{ K}\), its solubility product, \(K_{\text{sp}}\), is \(2.40 \times 10^{-6}\text{ mol}^3\text{ dm}^{-9}\).
Calculate the solubility of calcium lactate in water at \(298\text{ K}\) in \(\text{g dm}^{-3}\).
[\(M_r(\text{Ca(Lac)}_2) = 218.2\)]

This question is about lattice energy, enthalpy changes, and Gibbs free energy.

(a) Define the term standard lattice energy (\(\Delta H^\circ_{\text{latt}}\)).

(b) Use the following data to calculate the standard lattice energy of strontium chloride, \(\text{SrCl}_2(\text{s})\).
- Standard enthalpy change of formation of \(\text{SrCl}_2(\text{s}) = -829\text{ kJ mol}^{-1}\)
- Standard enthalpy change of atomisation of \(\text{Sr}(\text{s}) = +164\text{ kJ mol}^{-1}\)
- First ionisation energy of \(\text{Sr}(\text{g}) = +550\text{ kJ mol}^{-1}\)
- Second ionisation energy of \(\text{Sr}(\text{g}) = +1064\text{ kJ mol}^{-1}\)
- Enthalpy change of atomisation of chlorine, \(\Delta H^\circ_{\text{at}}(\text{Cl}_2) = +122\text{ kJ mol}^{-1}\)
- Electron affinity of chlorine, \(\text{EA}_1(\text{Cl}) = -349\text{ kJ mol}^{-1}\)

(c) The lattice energy of barium chloride, \(\text{BaCl}_2\), is \(-2056\text{ kJ mol}^{-1}\).
Explain why the lattice energy of \(\text{SrCl}_2\) is more exothermic than that of \(\text{BaCl}_2\).

(d) The thermal decomposition of strontium carbonate is given by:
\[\text{SrCO}_3(\text{s}) \rightarrow \text{SrO}(\text{s}) + \text{CO}_2(\text{g}) \quad \Delta H^\circ = +235\text{ kJ mol}^{-1}\]
Standard molar entropies, \(S^\circ\), are given in the table:
| Substance | \(S^\circ / \text{J K}^{-1}\text{ mol}^{-1}\) |
|---|---|
| \(\text{SrCO}_3(\text{s})\) | 97.0 |
| \(\text{SrO}(\text{s})\) | 54.4 |
| \(\text{CO}_2(\text{g})\) | 213.6 |

(i) Calculate the standard entropy change of the system, \(\Delta S^\circ_{\text{sys}}\), for this reaction. Include sign and units.
(ii) Calculate the minimum temperature, in \(\text{K}\), at which this decomposition becomes thermodynamically feasible.

Electrochemical cells provide valuable insights into redox thermodynamic tendencies.

(a) An electrochemical cell is set up under standard conditions consisting of a \(\text{Cr}^{3+}(\text{aq})/\text{Cr}(\text{s})\) half-cell and an \(\text{Fe}^{3+}(\text{aq})/\text{Fe}^{2+}(\text{aq})\) half-cell.
Standard electrode potentials are given:
\[\text{Cr}^{3+}(\text{aq}) + 3e^- \rightleftharpoons \text{Cr}(\text{s}) \quad E^\circ = -0.74\text{ V}\]
\[\text{Fe}^{3+}(\text{aq}) + e^- \rightleftharpoons \text{Fe}^{2+}(\text{aq}) \quad E^\circ = +0.77\text{ V}\]
(i) State the material used as the electrode in the \(\text{Fe}^{3+}(\text{aq})/\text{Fe}^{2+}(\text{aq})\) half-cell.
(ii) Calculate the standard cell potential, \(E^\circ_{\text{cell}}\), for this electrochemical cell.
(iii) Write the overall balanced ionic equation for the cell reaction that occurs spontaneously.

(b) The Nernst equation for a metal/metal-ion electrode can be expressed as:
\[E = E^\circ + \frac{0.059}{z} \log_{10}[\text{M}^{z+}]\]
(i) Calculate the electrode potential, \(E\), of the \(\text{Cr}^{3+}/\text{Cr}\) half-cell when \([\text{Cr}^{3+}] = 0.0200\text{ mol dm}^{-3}\) at \(298\text{ K}\).
(ii) Predict and explain how the cell potential, \(E_{\text{cell}}\), changes when the concentration of \(\text{Cr}^{3+}\) is decreased from \(1.00\text{ mol dm}^{-3}\) to \(0.0200\text{ mol dm}^{-3}\), keeping all other concentrations standard.

(c) An aqueous solution of chromium(III) sulfate, \(\text{Cr}_2(\text{SO}_4)_3\), is electrolysed using inert electrodes for \(45.0\text{ minutes}\) with a constant current of \(2.50\text{ A}\).
Calculate the mass of chromium deposited at the cathode.
[\(F = 9.65 \times 10^4\text{ C mol}^{-1}\); \(A_r(\text{Cr}) = 52.0\)]

The reaction between persulfate ions, \(\text{S}_2\text{O}_8^{2-}\), and iodide ions, \(\text{I}^-\), in aqueous solution is represented by:
\[\text{S}_2\text{O}_8^{2-}(\text{aq}) + 2\text{I}^-(\text{aq}) \rightarrow 2\text{SO}_4^{2-}(\text{aq}) + \text{I}_2(\text{aq})\]

(a) Initial rate experiments were carried out at \(298\text{ K}\) to determine the kinetics of the reaction. The results obtained are shown:
| Experiment | \([\text{S}_2\text{O}_8^{2-}] / \text{mol dm}^{-3}\) | \([\text{I}^-] / \text{mol dm}^{-3}\) | Initial rate / \(\text{mol dm}^{-3}\text{ s}^{-1}\) |
|---|---|---|---|
| 1 | 0.040 | 0.030 | \(4.80 \times 10^{-5}\) |
| 2 | 0.080 | 0.030 | \(9.60 \times 10^{-5}\) |
| 3 | 0.040 | 0.060 | \(9.60 \times 10^{-5}\) |

(i) Deduce the order of reaction with respect to \(\text{S}_2\text{O}_8^{2-}\) and with respect to \(\text{I}^-\). Explain your reasoning.
(ii) Write the rate equation for this reaction.
(iii) Calculate the value of the rate constant, \(k\), at \(298\text{ K}\). Include the units of \(k\).

(b) This reaction is catalysed by aqueous iron(III) ions, \(\text{Fe}^{3+}(\text{aq})\).
(i) Explain why uncatalysed reaction between \(\text{S}_2\text{O}_8^{2-}\) and \(\text{I}^-\) has a high activation energy.
(ii) Construct two balanced equations to show how \(\text{Fe}^{3+}(\text{aq})\) acts as a homogeneous catalyst for this reaction.

(c) In a separate experiment, the decomposition of hydrogen peroxide is first order with a half-life of \(420\text{ s}\) at \(298\text{ K}\).
Calculate the time taken, in seconds, for the concentration of \(\text{H}_2\text{O}_2\) to decrease from \(0.800\text{ mol dm}^{-3}\) to \(0.0500\text{ mol dm}^{-3}\).

Benzene and its derivatives undergo characteristic electrophilic substitution reactions.

(a) Methylbenzene can be converted into 4-nitromethylbenzene.
(i) State the reagents and conditions needed to convert methylbenzene into 4-nitromethylbenzene.
(ii) Outline the mechanism for the mononitration of methylbenzene. Include all relevant curly arrows, dipoles, charges, and the generation of the electrophile.

(b) 4-Nitromethylbenzene can be converted into 4-aminobenzoic acid via a two-stage synthetic route:
- Stage 1: Oxidation of the methyl group to a carboxylic acid using hot alkaline \(\text{KMnO}_4\) followed by acidification.
- Stage 2: Reduction of the nitro group to an amine group.
(i) State the reagents and conditions required for Stage 2.
(ii) 4-Aminobenzoic acid can exist as a zwitterion in aqueous solution. Draw the displayed formula of the zwitterion formed by 4-aminobenzoic acid.

(c) Phenylamine reacts with nitrous acid, \(\text{HNO}_2\), at \(5\,^\circ\text{C}\) to form a diazonium salt, which is subsequently coupled with naphthalen-2-ol in alkaline solution.
(i) State how nitrous acid is prepared in situ.
(ii) Explain why the temperature of the reaction mixture must be kept below \(10\,^\circ\text{C}\).
(iii) Draw the structure of the azo dye formed when the benzenediazonium ion reacts with naphthalen-2-ol.

Phenol, \(\text{C}_6\text{H}_5\text{OH}\), and ethanol, \(\text{CH}_3\text{CH}_2\text{OH}\), both possess a hydroxyl group but show distinctly different chemical properties.

(a) By considering the electronic structures of the conjugate bases, explain why phenol is significantly more acidic than ethanol.

(b) Describe what would be observed, and write a balanced equation, when aqueous bromine is added to an aqueous solution of phenol at room temperature.

(c) Phenol reacts with ethanoyl chloride, \(\text{CH}_3\text{COCl}\), in the presence of a base to form an ester, \(\mathbf{E}\).
(i) Name the type of reaction occurring and identify ester \(\mathbf{E}\).
(ii) Contrast the reactivity of ethanoyl chloride, chloroethane, and chlorobenzene towards hydrolysis by water at room temperature. Explain these differences in reactivity in terms of their structures and bonding.

(d) When phenol is treated with dilute nitric acid at room temperature, a mixture of two mononitrated structural isomers is formed.
(i) Draw the structures of these two isomers.
(ii) Explain why phenol reacts with dilute nitric acid at room temperature, whereas benzene requires concentrated nitric acid and concentrated sulfuric acid with heating to undergo nitration.

Condensation polymers are widely used in synthetic fibres and biomaterials.

(a) Kevlar is a tough polyamide prepared from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine.
(i) Draw the structure of the repeat unit of Kevlar, showing all bonds in the amide link.
(ii) State the type of intermolecular forces between adjacent polymer chains of Kevlar that account for its exceptional tensile strength.
(iii) Name the small molecule eliminated during the industrial polymerization reaction.

(b) Poly(lactic-co-glycolic acid), PLGA, is a biodegradable polyester synthesized from 2-hydroxypropanoic acid (lactic acid) and 2-hydroxyethanoic acid (glycolic acid).
(i) Draw the repeat unit of the polymer formed when one molecule of lactic acid alternates with one molecule of glycolic acid.
(ii) Explain why polyesters such as PLGA are biodegradable, whereas addition polymers like poly(propene) are non-biodegradable.

(c) A tripeptide, \(\mathbf{T}\), was hydrolysed using concentrated hydrochloric acid under reflux, yielding three amino acids:
- Glycine, \(\text{H}_2\text{NCH}_2\text{COOH}\)
- Alanine, \(\text{H}_2\text{NCH(CH}_3)\text{COOH}\)
- Cysteine, \(\text{H}_2\text{NCH(CH}_2\text{SH})\text{COOH}\)
(i) State how thin-layer chromatography (TLC) can be used to separate and identify these amino acids in the hydrolysate.
(ii) Draw the displayed structure of one possible tripeptide \(\mathbf{T}\) containing these three amino acids in a 1:1:1 ratio.

Compound \(\mathbf{X}\) is an aromatic organic compound with the molecular formula \(\text{C}_9\text{H}_{10}\text{O}_2\).

(a) Compound \(\mathbf{X}\) gives the following analytical results:
- It effervesces with aqueous sodium carbonate, \(\text{Na}_2\text{CO}_3\), releasing a gas that turns limewater cloudy.
- When reacted with alkaline aqueous iodine, no yellow precipitate is formed.
- The \(^{13}\text{C}\) NMR spectrum of \(\mathbf{X}\) exhibits 7 distinct peaks.
- The \(^1\text{H}\) NMR spectrum of \(\mathbf{X}\) recorded in \(\text{CDCl}_3\) is summarized in the table:

| Chemical shift, \(\delta / \text{ppm}\) | Splitting pattern | Relative peak area |
|---|---|---|
| 1.25 | triplet | 3 |
| 2.68 | quartet | 2 |
| 7.30 | doublet | 2 |
| 8.02 | doublet | 2 |
| 11.80 | singlet | 1 |

(i) Identify the functional group responsible for the effervescence with \(\text{Na}_2\text{CO}_3\).
(ii) State what happens to the signal at \(\delta = 11.80\text{ ppm}\) when a drop of \(\text{D}_2\text{O}\) is added to the NMR tube. Write an equation to support your answer.
(iii) Deduce the structural fragment responsible for the peaks at \(\delta = 1.25\) and \(\delta = 2.68\text{ ppm}\). Explain the splitting patterns using the \(n+1\) rule.
(iv) Deduce the substitution pattern of the benzene ring from the signals at \(\delta = 7.30\) and \(\delta = 8.02\text{ ppm}\).
(v) Deduce the identity of compound \(\mathbf{X}\) and draw its full structural formula.

(b) An isomer of \(\mathbf{X}\), compound \(\mathbf{Y}\), does not react with \(\text{Na}_2\text{CO}_3\) but reacts with 2,4-dinitrophenylhydrazine to give an orange precipitate. \(\mathbf{Y}\) gives a positive tri-iodomethane test and its \(^1\text{H}\) NMR spectrum shows a singlet corresponding to 3 protons at \(\delta = 2.15\text{ ppm}\).
Suggest a structure for compound \(\mathbf{Y}\).

Cobalt is a transition element that forms a wide variety of coordination complexes with different coordination numbers, geometries, and colours.

(a) (i) Complete the electronic configuration of the \(\text{Co}^{2+}\) ion.

\(1\text{s}^2 2\text{s}^2 2\text{p}^6 \dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\) [1]

(ii) Explain why aqueous transition metal complex ions are typically coloured. [3]

(b) An aqueous solution of cobalt(II) chloride contains the pink complex ion \([\text{Co}(\text{H}_2\text{O})_6]^{2+}\).
When concentrated hydrochloric acid is added to this solution, a reversible reaction occurs to form a deep blue solution containing \([\text{CoCl}_4]^{2-}\).

(i) Construct an equation for this equilibrium reaction. [1]

(ii) State the shape and the coordination number of the complex ion present in the deep blue solution.

Shape: \(\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\)
Coordination number: \(\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\) [1]

(c) When 1,2-diaminoethane, \(\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2\) (abbreviated as \(\text{en}\)), is added to an aqueous solution of cobalt(II) ions followed by reaction with a suitable oxidising agent and dilute hydrochloric acid, the octahedral complex \([\text{Co}(\text{en})_2\text{Cl}_2]^+\) is formed.

(i) State the type of ligand represented by 1,2-diaminoethane. [1]

(ii) The complex \([\text{Co}(\text{en})_2\text{Cl}_2]^+\) exhibits stereoisomerism.
- State the two types of stereoisomerism shown by \([\text{Co}(\text{en})_2\text{Cl}_2]^+\).
- Draw three-dimensional diagrams to show the two optical isomers of \([\text{Co}(\text{en})_2\text{Cl}_2]^+\). You may use the abbreviation \(\overset{\frown}{\text{N}\phantom{-}\text{N}}\) to represent 1,2-diaminoethane. [3]

(d) The stability constant, \(K_{\text{stab}}\), for the equilibrium between \([\text{Co}(\text{H}_2\text{O})_6]^{2+}\) and 1,2-diaminoethane is given by the reaction:

\[[\text{Co}(\text{H}_2\text{O})_6]^{2+}(\text{aq}) + 3\text{en}(\text{aq}) \rightleftharpoons [\text{Co}(\text{en})_3]^{2+}(\text{aq}) + 6\text{H}_2\text{O}(\text{l})\]

Write an expression for \(K_{\text{stab}}\) for this equilibrium and deduce its units. [1]

A student plans an experiment to determine the stoichiometric formula of a complex formed between aqueous nickel(II) ions, \(\text{Ni}^{2+}(\text{aq})\), and a bidentate ligand, 1,2-diaminoethane (abbreviated as \(\text{en}\)), using the method of continuous variation (Job's method) and a visible spectrophotometer (colorimeter).

The reaction can be represented by the general equation:
\[\text{Ni}^{2+}(\text{aq}) + n\,\text{en}(\text{aq}) \rightarrow [\text{Ni}(\text{en})_n]^{2+}(\text{aq})\]

Neither \(\text{Ni}^{2+}(\text{aq})\) nor \(\text{en}(\text{aq})\) absorbs significantly at the wavelength \(\lambda_{\text{max}} = 545\text{ nm}\), whereas the complex \([\text{Ni}(\text{en})_n]^{2+}\) exhibits strong absorbance at this wavelength.

(a) The student is provided with solid nickel(II) sulfate hexahydrate, \(\text{NiSO}_4\cdot 6\text{H}_2\text{O}\) (\(M_{\text{r}} = 262.8\)), and an aqueous solution of 1,2-diaminoethane of concentration \(0.100\text{ mol}\,\text{dm}^{-3}\).

(i) Calculate the mass of \(\text{NiSO}_4\cdot 6\text{H}_2\text{O}\) required to prepare \(250.0\text{ cm}^3\) of a \(0.100\text{ mol}\,\text{dm}^{-3}\) standard solution of \(\text{Ni}^{2+}(\text{aq})\). [1]

(ii) Describe step-by-step how the student should prepare this \(250.0\text{ cm}^3\) standard solution using standard laboratory volumetric apparatus. [3]

(b) In the continuous variation method, mixtures of the \(0.100\text{ mol}\,\text{dm}^{-3}\) \(\text{Ni}^{2+}(\text{aq})\) solution and \(0.100\text{ mol}\,\text{dm}^{-3}\) \(\text{en}(\text{aq})\) solution are prepared such that the total volume of each mixture is constantly maintained at \(20.00\text{ cm}^3\).

(i) Name a suitable piece of apparatus to measure the volumes of both solutions accurately. [1]

(ii) State the independent variable and the dependent variable in this investigation. [2]

(iii) Before measuring the absorbance of each mixture in the colorimeter, describe what the student must do to calibrate the colorimeter at \(545\text{ nm}\). [1]

(c) The results obtained by the student are shown in the table:

\[\begin{array}{|c|c|c|c|} \hline \text{Mixture} & \text{Volume of } 0.100\text{ M Ni}^{2+}\text{ / cm}^3 & \text{Volume of } 0.100\text{ M en / cm}^3 & \text{Absorbance at } 545\text{ nm} \\ \hline 1 & 18.00 & 2.00 & 0.09 \\ 2 & 15.00 & 5.00 & 0.23 \\ 3 & 12.00 & 8.00 & 0.36 \\ 4 & 10.00 & 10.00 & 0.45 \\ 5 & 7.00 & 13.00 & 0.59 \\ 6 & 5.00 & 15.00 & 0.68 \\ 7 & 4.00 & 16.00 & 0.54 \\ 8 & 2.00 & 18.00 & 0.27 \\ \hline \end{array}\]

(i) Explain why the maximum absorbance corresponds to the stoichiometric ratio of reactants in the complex. [1]

(ii) Determine the mole fraction of \(\text{en}\), \(x_{\text{en}} = \frac{V_{\text{en}}}{V_{\text{Ni}^{2+}} + V_{\text{en}}}\), at maximum absorbance. Hence, deduce the integer value of \(n\) in the formula \([\text{Ni}(\text{en})_n]^{2+}\). [2]

(iii) Draw the 3D stereochemical structure of one optical isomer of \([\text{Ni}(\text{en})_n]^{2+}\), clearly showing the coordination geometry around the nickel ion (represent the bidentate ligand as \(\text{N}\!\!\frown\!\!\text{N}\) or \(\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2\)). [2]

(d) 1,2-diaminoethane is corrosive and harmful by inhalation. State one specific safety precaution, other than wearing eye protection and gloves, that the student must take when handling concentrated 1,2-diaminoethane. [1]

(e) Suggest why the experiment must be carried out at a constant temperature. [1]

A student investigates the kinetics of the reaction between persulfate ions, \(\text{S}_2\text{O}_8^{2-}(\text{aq})\), and iodide ions, \(\text{I}^-(\text{aq})\), at different temperatures:
\[\text{S}_2\text{O}_8^{2-}(\text{aq}) + 2\text{I}^-(\text{aq}) \rightarrow 2\text{SO}_4^{2-}(\text{aq}) + \text{I}_2(\text{aq})\]

The rate constant \(k\) varies with absolute temperature \(T\) according to the Arrhenius equation:
\[\ln k = -\frac{E_{\text{a}}}{R}\left(\frac{1}{T}\right) + \ln A\]
where \(R = 8.31\text{ J}\,\text{K}^{-1}\,\text{mol}^{-1}\), \(E_{\text{a}}\) is the activation energy in \(\text{J}\,\text{mol}^{-1}\), and \(A\) is the pre-exponential factor.

(a) The student measures the rate constant \(k\) across a range of temperatures. The experimental data obtained are given in the table below.

(i) Complete the table by calculating the values of \(\frac{1}{T} \times 10^3\) and \(\ln k\). Record \(\frac{1}{T} \times 10^3\) to 3 decimal places and \(\ln k\) to 2 decimal places. [2]

\[\begin{array}{|c|c|c|c|c|} \hline \text{Experiment} & \text{Temperature, } \theta\text{ / }^\circ\text{C} & T\text{ / K} & \frac{1}{T} \times 10^3\text{ / K}^{-1} & k\text{ / }\text{dm}^3\,\text{mol}^{-1}\,\text{s}^{-1} & \ln k \\ \hline 1 & 15.0 & 288.1 & 3.471 & 1.10 \times 10^{-3} & -6.81 \\ 2 & 22.5 & 295.6 & 3.383 & 1.85 \times 10^{-3} & -6.29 \\ 3 & 30.0 & 303.1 & 3.299 & 3.15 \times 10^{-3} & \text{...............} \\ 4 & 37.0 & 310.1 & \text{...............} & 5.12 \times 10^{-3} & -5.27 \\ 5 & 45.0 & 318.1 & 3.144 & 8.75 \times 10^{-3} & -4.74 \\ 6 & 52.5 & 325.6 & 3.071 & 1.18 \times 10^{-2} & -4.44 \\ 7 & 60.0 & 333.1 & 3.002 & 2.12 \times 10^{-2} & -3.85 \\ \hline \end{array}\]

(ii) Plotting \(\ln k\) on the \(y\)-axis against \(\frac{1}{T} \times 10^3\text{ / K}^{-1}\) on the \(x\)-axis yields a straight line.

Identify the experiment that contains an anomalous result. Suggest one possible experimental error that could have led to this specific anomaly. [2]

(iii) Using the non-anomalous points from the table, two coordinates on the best-fit line are \((3.000, -3.82)\) and \((3.450, -6.65)\).

Calculate the gradient of the line of best fit. State the units of your gradient. [2]

(b) Use your gradient from (a)(iii) and the Arrhenius equation to calculate a value for the activation energy, \(E_{\text{a}}\), in \(\text{kJ}\,\text{mol}^{-1}\). Include the appropriate sign. [2]

(c) The reaction is monitored using an iodine clock method, in which a small, fixed amount of sodium thiosulfate and starch indicator are added to the initial mixture.

(i) State the observation that indicates the end-point of the clock reaction. [1]

(ii) Explain the role of the thiosulfate ions in this clock reaction. [2]

(iii) Explain why the amount of sodium thiosulfate added must be very small relative to the amounts of persulfate and iodide ions. [1]

(d) In a further experiment, a student adds a few drops of aqueous iron(II) sulfate, \(\text{FeSO}_4(\text{aq})\), to the mixture.

(i) State the effect of adding \(\text{FeSO}_4(\text{aq})\) on the gradient of the Arrhenius plot of \(\ln k\) against \(\frac{1}{T}\). [1]

(ii) Explain your answer to (d)(i) in terms of transition metal chemistry and reaction pathways. [2]