2023 Cambridge IAL Chemistry (9701) Practice Paper with Answers

For a reaction \(2A + B + 2C \rightarrow \text{products}\), initial rate data were obtained at constant temperature:

* Experiment 1: \([A] = 0.10\text{ mol dm}^{-3}\), \([B] = 0.10\text{ mol dm}^{-3}\), \([C] = 0.10\text{ mol dm}^{-3}\); initial rate = \(1.2 \times 10^{-3}\text{ mol dm}^{-3}\text{ s}^{-1}\)
* Experiment 2: \([A] = 0.20\text{ mol dm}^{-3}\), \([B] = 0.10\text{ mol dm}^{-3}\), \([C] = 0.10\text{ mol dm}^{-3}\); initial rate = \(2.4 \times 10^{-3}\text{ mol dm}^{-3}\text{ s}^{-1}\)
* Experiment 3: \([A] = 0.10\text{ mol dm}^{-3}\), \([B] = 0.20\text{ mol dm}^{-3}\), \([C] = 0.10\text{ mol dm}^{-3}\); initial rate = \(4.8 \times 10^{-3}\text{ mol dm}^{-3}\text{ s}^{-1}\)
* Experiment 4: \([A] = 0.10\text{ mol dm}^{-3}\), \([B] = 0.10\text{ mol dm}^{-3}\), \([C] = 0.20\text{ mol dm}^{-3}\); initial rate = \(1.2 \times 10^{-3}\text{ mol dm}^{-3}\text{ s}^{-1}\)

What are the units and the numerical value of the rate constant, \(k\), for this reaction?

The stability constants, \(K_{\text{stab}}\), for three different copper(II) complexes in aqueous solution at \(298\text{ K}\) are given:

1. \([\text{Cu}(\text{H}_2\text{O})_6]^{2+} + 4\text{Cl}^- \rightleftharpoons [\text{CuCl}_4]^{2-} + 6\text{H}_2\text{O} \quad K_{\text{stab}} = 4.2 \times 10^5\text{ dm}^{12}\text{ mol}^{-4}\)
2. \([\text{Cu}(\text{H}_2\text{O})_6]^{2+} + 4\text{NH}_3 \rightleftharpoons [\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+} + 4\text{H}_2\text{O} \quad K_{\text{stab}} = 1.2 \times 10^{13}\text{ dm}^{12}\text{ mol}^{-4}\)
3. \([\text{Cu}(\text{H}_2\text{O})_6]^{2+} + 2\text{en} \rightleftharpoons [\text{Cu}(\text{en})_2(\text{H}_2\text{O})_2]^{2+} + 4\text{H}_2\text{O} \quad K_{\text{stab}} = 1.6 \times 10^{20}\text{ dm}^6\text{ mol}^{-2}\)
(where 'en' represents 1,2-diaminoethane)

Which statement is correct?

The amino acid aspartic acid has the structure \(\text{HOOC-CH}_2\text{-CH(NH}_2\text{)-COOH}\). It has three \(pK_a\) values:

* \(pK_{a1} = 2.0\) (for the \(\alpha\)-carboxyl group)
* \(pK_{a2} = 3.9\) (for the side-chain carboxyl group)
* \(pK_{a3} = 9.8\) (for the \(\alpha\)-ammonium group)

What is the predominant ionic structure of aspartic acid in a buffer solution at \(\text{pH} = 6.0\)?

A sample of \(2.32\text{ g}\) of an anhydrous metal carbonate, \(\text{MCO}_3\), is heated strongly until it completely decomposes according to the following equation:

\[\text{MCO}_3(\text{s}) \rightarrow \text{MO}(\text{s}) + \text{CO}_2(\text{g})\]

The carbon dioxide gas produced occupies a volume of \(480\text{ cm}^3\) at room temperature and pressure (r.t.p.).
[Assume 1 mole of gas occupies \(24.0\text{ dm}^3\) at r.t.p.]

What is the identity of the metal, \(\text{M}\)?

An aqueous solution contains \(4.0\text{ g}\) of an organic compound \(X\) dissolved in \(100\text{ cm}^3\) of water. The partition coefficient, \(K_{pc}\), of compound \(X\) between ether and water is defined as:

\[K_{pc} = \frac{[X]_{\text{ether}}}{[X]_{\text{water}}} = 4.0\]

If the aqueous solution is shaken with a single \(50\text{ cm}^3\) portion of ether, what mass of compound \(X\) is extracted into the ether layer?

Which statement correctly describes and explains the trend in properties of Group 2 elements and their compounds from magnesium to barium?

Aqueous solutions of transition metal complexes are often coloured. Which statement correctly explains the origin of the colour in the complex ion \([\text{Cu}(\text{H}_2\text{O})_6]^{2+}\)?

The decomposition of hydrogen peroxide, \(\text{H}_2\text{O}_2\), is first-order with respect to \(\text{H}_2\text{O}_2\):

\[2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})\]

In an experiment, the concentration of a sample of \(\text{H}_2\text{O}_2\) decreases from \(1.60\text{ mol dm}^{-3}\) to \(0.10\text{ mol dm}^{-3}\) in exactly \(120\text{ minutes}\).

What is the half-life of this reaction under these conditions?

In an investigation of the reaction \(2\text{P} + \text{Q} \rightarrow \text{R} + \text{S}\), the following initial rates were obtained at a constant temperature. Experiment 1: \([\text{P}] = 0.050\text{ mol dm}^{-3}\), \([\text{Q}] = 0.100\text{ mol dm}^{-3}\), Initial rate = \(1.50 \times 10^{-4}\text{ mol dm}^{-3}\text{ s}^{-1}\). Experiment 2: \([\text{P}] = 0.100\text{ mol dm}^{-3}\), \([\text{Q}] = 0.100\text{ mol dm}^{-3}\), Initial rate = \(6.00 \times 10^{-4}\text{ mol dm}^{-3}\text{ s}^{-1}\). Experiment 3: \([\text{P}] = 0.100\text{ mol dm}^{-3}\), \([\text{Q}] = 0.200\text{ mol dm}^{-3}\), Initial rate = \(1.20 \times 10^{-3}\text{ mol dm}^{-3}\text{ s}^{-1}\). What is the correct value and unit of the rate constant, \(k\), for this reaction?

The table below lists some thermodynamic data for a Group 2 metal, \(\text{M}\), and fluorine. Enthalpy change of formation of \(\text{MF}_2\text{(s)}\) = \(-1100\text{ kJ mol}^{-1}\); Enthalpy change of atomisation of \(\text{M}\text{(s)}\) = \(+150\text{ kJ mol}^{-1}\); First ionisation energy of \(\text{M}\text{(g)}\) = \(+550\text{ kJ mol}^{-1}\); Second ionisation energy of \(\text{M}\text{(g)}\) = \(+1150\text{ kJ mol}^{-1}\); Enthalpy change of atomisation of fluorine, \(\Delta H_{at}^\ominus[\text{F}\text{(g)}]\) = \(+80\text{ kJ mol}^{-1}\); First electron affinity of fluorine, \(\text{EA}[\text{F}\text{(g)}]\) = \(-330\text{ kJ mol}^{-1}\). What is the lattice energy, \(\Delta H_{latt}^\ominus\), of \(\text{MF}_2\text{(s)}\)?

The stability constant, \(K_{stab}\), for the complex ion \([\text{Fe(SCN)(H}_2\text{O)}_5]^{2+}\) is represented by the equilibrium: \([\text{Fe(H}_2\text{O)}_6]^{3+}\text{(aq)} + \text{SCN}^-\text{(aq)} \rightleftharpoons [\text{Fe(SCN)(H}_2\text{O)}_5]^{2+}\text{(aq)} + \text{H}_2\text{O(l)}\). The value of \(K_{stab}\) is \(8.9 \times 10^2\text{ dm}^3\text{ mol}^{-1}\). In an equilibrium mixture, the concentration of uncomplexed \([\text{Fe(H}_2\text{O)}_6]^{3+}\text{(aq)}\) is \(2.0 \times 10^{-3}\text{ mol dm}^{-3}\) and the concentration of \([\text{Fe(SCN)(H}_2\text{O)}_5]^{2+}\text{(aq)}\) is \(4.0 \times 10^{-3}\text{ mol dm}^{-3}\). What is the equilibrium concentration of \(\text{SCN}^-\text{(aq)}\)?

Lysine is an amino acid with the formula \(\text{H}_2\text{N(CH}_2)_4\text{CH(NH}_2)\text{COOH}\). Which structure represents the predominant ionic species present in a solution of lysine at \(\text{pH} = 1.0\)?

An organic compound, X, is soluble in both water and ether. The partition coefficient, \(K_{pc}\), of X between ether and water is defined as: \(K_{pc} = \frac{[\text{X}]_{\text{ether}}}{[\text{X}]_{\text{water}}} = 4.0\). An aqueous solution contains \(6.0\text{ g}\) of X dissolved in \(100\text{ cm}^3\) of water. This solution is shaken with a single \(50\text{ cm}^3\) portion of ether at a constant temperature. What mass of X remains in the aqueous layer after equilibrium has been established?

When butanone, \(\text{CH}_3\text{COCH}_2\text{CH}_3\), reacts with \(\text{HCN}\) in the presence of \(\text{NaCN}\) catalyst, an organic product is formed. Which statement about this reaction and its product is correct?

A section of a synthetic polymer is shown below: \(\text{...-CO-CH}_2\text{CH}_2\text{-CO-NH-CH}_2\text{CH}_2\text{CH}_2\text{-NH-CO-CH}_2\text{CH}_2\text{-CO-NH-CH}_2\text{CH}_2\text{CH}_2\text{-NH-...}\). Which pair of monomers can be used to prepare this polymer?

Two different Group 2 anhydrous nitrates, \(\text{X(NO}_3)_2\) and \(\text{Y(NO}_3)_2\), are heated under identical conditions. It is observed that \(\text{X(NO}_3)_2\) decomposes at a lower temperature than \(\text{Y(NO}_3)_2\). Which statement about \(\text{X}\) and \(\text{Y}\) is correct?

Which gaseous ion has the greatest number of unpaired d-electrons?

The table shows the initial rate data for the reaction: \(2\text{A} + \text{B} + \text{C} \rightarrow \text{products}\)

* Experiment 1: \([\text{A}] = 0.10 \text{ mol dm}^{-3}\), \([\text{B}] = 0.10 \text{ mol dm}^{-3}\), \([\text{C}] = 0.10 \text{ mol dm}^{-3}\); Initial Rate = \(1.2 \times 10^{-3} \text{ mol dm}^{-3} \text{ s}^{-1}\)
* Experiment 2: \([\text{A}] = 0.20 \text{ mol dm}^{-3}\), \([\text{B}] = 0.10 \text{ mol dm}^{-3}\), \([\text{C}] = 0.10 \text{ mol dm}^{-3}\); Initial Rate = \(4.8 \times 10^{-3} \text{ mol dm}^{-3} \text{ s}^{-1}\)
* Experiment 3: \([\text{A}] = 0.10 \text{ mol dm}^{-3}\), \([\text{B}] = 0.20 \text{ mol dm}^{-3}\), \([\text{C}] = 0.10 \text{ mol dm}^{-3}\); Initial Rate = \(1.2 \times 10^{-3} \text{ mol dm}^{-3} \text{ s}^{-1}\)
* Experiment 4: \([\text{A}] = 0.10 \text{ mol dm}^{-3}\), \([\text{B}] = 0.10 \text{ mol dm}^{-3}\), \([\text{C}] = 0.30 \text{ mol dm}^{-3}\); Initial Rate = \(3.6 \times 10^{-3} \text{ mol dm}^{-3} \text{ s}^{-1}\)

What are the overall order of reaction and the correct units for the rate constant, \(k\)?

A \(10.0 \text{ cm}^3\) sample of a gaseous hydrocarbon was mixed with excess oxygen (\(80.0 \text{ cm}^3\)) and exploded. After cooling to room temperature, the total volume of gaseous mixture remaining was \(65.0 \text{ cm}^3\). When this remaining gas was passed through concentrated aqueous potassium hydroxide (which absorbs carbon dioxide), the volume decreased to \(25.0 \text{ cm}^3\). All gas volumes were measured at room temperature and pressure. What is the molecular formula of the hydrocarbon?

Aspartic acid has the structural formula \(\text{HOOC-CH}_2\text{-CH(NH}_2\text{)-COOH}\). Its three acid-dissociation constants (\(\text{p}K_a\)) are approximately 2.0 (for the \(\alpha\)-carboxyl group), 3.9 (for the side-chain carboxyl group), and 9.8 (for the \(\alpha\)-amino group). Which structure represents the predominant species of aspartic acid present in a buffer solution maintained at \(\text{pH} = 1.0\)?

The table lists thermodynamic data for calcium and bromine.

* Enthalpy of formation of \(\text{CaBr}_2\text{(s)}\) = \(-683 \text{ kJ mol}^{-1}\)
* Enthalpy of atomisation of \(\text{Ca(s)}\) = \(+178 \text{ kJ mol}^{-1}\)
* First ionisation energy of \(\text{Ca(g)}\) = \(+590 \text{ kJ mol}^{-1}\)
* Second ionisation energy of \(\text{Ca(g)}\) = \(+1145 \text{ kJ mol}^{-1}\)
* Enthalpy of atomisation of \(\text{Br}_2\text{(l)}\) = \(+112 \text{ kJ mol}^{-1}\)
* Lattice energy of \(\text{CaBr}_2\text{(s)}\) = \(-2176 \text{ kJ mol}^{-1}\)

Using these data, what is the value of the first electron affinity of bromine?

Four separate samples of Period 3 chlorides, \(\text{NaCl}\), \(\text{MgCl}_2\), \(\text{AlCl}_3\), and \(\text{SiCl}_4\), are added to separate beakers of water. Which of the following lists the resulting mixtures in order of decreasing pH (from highest pH to lowest pH)?

An organic compound \(X\) has a partition coefficient, \(K_{pc}\), of 4.0 between ethoxyethane (ether) and water, defined as:

\(K_{pc} = \frac{[X\text{ in ethoxyethane}]}{[X\text{ in water}]}\)

An aqueous solution containing \(5.0\text{ g}\) of \(X\) dissolved in \(100\text{ cm}^3\) of water is shaken with a single \(50\text{ cm}^3\) portion of ethoxyethane. What mass of \(X\) is extracted into the ethoxyethane layer?

Which carbonyl compound reacts with hydrogen cyanide, \(\text{HCN}\), in the presence of a catalytic amount of \(\text{NaCN}\), to produce a reaction mixture that does not rotate the plane of plane-polarised light, even though the reaction product itself is a chiral molecule?

In an aqueous solution, the copper(II) ion forms various complex ions. When concentrated hydrochloric acid is added to aqueous copper(II) sulfate, the hexaaquacopper(II) ion, \([Cu(H_2O)_6]^{2+}\), is converted to the tetrachlorocuprate(II) ion, \([CuCl_4]^{2-}\).

The stability constants, \(K_{stab}\), for these complexes are:
\([Cu(H_2O)_6]^{2+} + 4Cl^- \rightleftharpoons [CuCl_4]^{2-} + 6H_2O\) \(K_{stab} = 4.0 \times 10^5\text{ dm}^{12}\text{ mol}^{-4}\)

If another ligand \(L\) forms a complex \([CuL_4]^{2+}\) with a stability constant of \(2.5 \times 10^{11}\text{ dm}^{12}\text{ mol}^{-4}\), what is the equilibrium constant for the reaction:
\([CuCl_4]^{2-} + 4L \rightleftharpoons [CuL_4]^{2+} + 4Cl^-\text{?}\)

For the reaction \(2A + B \rightarrow C\), the rate equation is determined to be:

\(\text{rate} = k[A]^2[B]\)

If the concentration of \(A\) is halved and the concentration of \(B\) is tripled, by what factor is the initial rate of the reaction multiplied?

A 2.30 g sample of an anhydrous metal carbonate, \(MCO_3\), was dissolved in excess dilute hydrochloric acid. The carbon dioxide gas evolved was collected and measured to be \(480\text{ cm}^3\) at room temperature and pressure (rtp).

[Assume 1 mole of gas occupies \(24.0\text{ dm}^3\) at rtp.]

What is the identity of the metal \(M\)?

Aspartic acid has the structure \(HOOC-CH(NH_2)-CH_2-COOH\).

Which structure represents the predominant ionic species present in a highly alkaline solution of aspartic acid at pH 12?

Consider the following thermodynamic data for magnesium fluoride, \(MgF_2\):

Enthalpy change of formation of \(MgF_2(s) = -1124\text{ kJ mol}^{-1}\)
Enthalpy change of atomisation of \(Mg(s) = +148\text{ kJ mol}^{-1}\)
First ionisation energy of \(Mg(g) = +738\text{ kJ mol}^{-1}\)
Second ionisation energy of \(Mg(g) = +1451\text{ kJ mol}^{-1}\)
Bond dissociation enthalpy of \(F_2(g) = +158\text{ kJ mol}^{-1}\)
First electron affinity of \(F(g) = -328\text{ kJ mol}^{-1}\)

What is the lattice energy, \(\Delta H_{latt}^\ominus\), of \(MgF_2(s)\)?

Which statement best explains why the first ionisation energy of sulfur is lower than that of phosphorus?

An organic compound, \(X\), has a partition coefficient, \(K_{pc}\), of 4.0 between diethyl ether and water:

\(K_{pc} = \frac{[X]_{\text{ether}}}{[X]_{\text{water}}} = 4.0\)

An aqueous solution containing 5.00 g of \(X\) in \(100\text{ cm}^3\) of water is shaken with one \(50\text{ cm}^3\) portion of diethyl ether.

What mass of \(X\) remains in the aqueous layer?

Compound \(Y\) has the molecular formula \(C_4H_8O\). It reacts with 2,4-dinitrophenylhydrazine reagent to form an orange precipitate, but does not form a red precipitate when heated with Fehling's solution.

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

The reaction \(2\text{A} + \text{B} + \text{C} \rightarrow \text{D} + \text{E}\) was investigated at constant temperature. The following initial rate data were obtained:

- Exp 1: \([\text{A}] = 0.10 \text{ mol dm}^{-3}\), \([\text{B}] = 0.10 \text{ mol dm}^{-3}\), \([\text{C}] = 0.10 \text{ mol dm}^{-3}\), Initial rate = \(1.2 \times 10^{-3} \text{ mol dm}^{-3}\text{ s}^{-1}\)
- Exp 2: \([\text{A}] = 0.20 \text{ mol dm}^{-3}\), \([\text{B}] = 0.10 \text{ mol dm}^{-3}\), \([\text{C}] = 0.10 \text{ mol dm}^{-3}\), Initial rate = \(4.8 \times 10^{-3} \text{ mol dm}^{-3}\text{ s}^{-1}\)
- Exp 3: \([\text{A}] = 0.10 \text{ mol dm}^{-3}\), \([\text{B}] = 0.20 \text{ mol dm}^{-3}\), \([\text{C}] = 0.10 \text{ mol dm}^{-3}\), Initial rate = \(2.4 \times 10^{-3} \text{ mol dm}^{-3}\text{ s}^{-1}\)
- Exp 4: \([\text{A}] = 0.10 \text{ mol dm}^{-3}\), \([\text{B}] = 0.10 \text{ mol dm}^{-3}\), \([\text{C}] = 0.20 \text{ mol dm}^{-3}\), Initial rate = \(1.2 \times 10^{-3} \text{ mol dm}^{-3}\text{ s}^{-1}\)

What are the units of the rate constant, \(k\), for this reaction?

When aqueous chromium(III) chloride is treated with excess aqueous ammonia, a violet-purple solution of a complex ion, \([\text{Cr}(\text{NH}_3)_6]^{3+}\), is formed. When treated with excess aqueous sodium hydroxide, a green solution of the complex ion \([\text{Cr}(\text{OH})_6]^{3-}\) is formed instead. Which statement best explains why these two complexes have different colours?

A \(1.20\text{ g}\) sample of an impure mixture of calcium carbonate (\(\text{CaCO}_3\), \(M_{\text{r}} = 100.1\)) and barium carbonate (\(\text{BaCO}_3\), \(M_{\text{r}} = 197.3\)) was completely dissolved in excess dilute hydrochloric acid. The carbon dioxide gas evolved was collected and measured to be \(192\text{ cm}^3\) at room temperature and pressure (r.t.p.). Assuming the impurities do not react with acid to produce gas, and that one mole of gas occupies \(24.0\text{ dm}^3\) at r.t.p., what is the total number of moles of carbonate ions present in the \(1.20\text{ g}\) sample?

The structural formula of the amino acid glutamic acid is \(\text{HOOC-CH}_2\text{-CH}_2\text{-CH}(\text{NH}_2)\text{-COOH}\). Which species is the major component of a solution of glutamic acid at a very high pH (\(\text{pH} = 13\))?

The Born-Haber cycle for the formation of magnesium oxide, \(\text{MgO}\)(s), involves several energy terms. Which reaction corresponds to the enthalpy change of atomisation of oxygen, \(\Delta H_{\text{at}}^{\ominus}[\text{O}]\)?

An organic compound \(\text{X}\) is distributed between water and an organic solvent, dichloromethane. The partition coefficient, \(K_{\text{pc}}\), is defined as:

\[K_{\text{pc}} = \frac{[\text{X}\text{(dichloromethane)}]}{[\text{X}\text{(aqueous)}]} = 4.00\]

A student has \(100\text{ cm}^3\) of an aqueous solution containing \(2.50\text{ g}\) of \(\text{X}\). Which extraction procedure will remove the greatest mass of \(\text{X}\) from the aqueous solution?

An organic compound \(\text{Y}\) has the molecular formula \(\text{C}_5\text{H}_{10}\text{O}\).

- It gives a yellow precipitate when warmed with alkaline aqueous iodine.
- It does not react with Tollens' reagent or Fehling's solution.
- When reduced with \(\text{NaBH}_4\), it forms a secondary alcohol.

Which structure is consistent with this information?

Which statement correctly describes the trend in properties of the Group 2 elements, magnesium to barium, or their compounds down the group?

A sample of anhydrous Group 2 nitrate, \( \text{M}(\text{NO}_3)_2 \), of mass 1.483 g is heated strongly. It decomposes completely according to the following equation:
\( 2\text{M}(\text{NO}_3)_2(\text{s}) \rightarrow 2\text{MO}(\text{s}) + 4\text{NO}_2(\text{g}) + \text{O}_2(\text{g}) \)

The gaseous products are passed through an excess of concentrated aqueous sodium hydroxide to absorb all the nitrogen dioxide. The remaining oxygen gas is collected and measured at r.t.p. and has a volume of 120 cm\(^3\).

(a) Define thermal decomposition. [1]
(b) (i) Calculate the number of moles of oxygen gas collected at r.t.p. (Assume 1.00 mol of gas occupies 24.0 dm\(^3\) at r.t.p.). [1]
(ii) Use the equation to determine the number of moles of \( \text{M}(\text{NO}_3)_2 \) that decomposed. [2]
(iii) Calculate the relative formula mass of \( \text{M}(\text{NO}_3)_2 \) and identify metal M. [2]
(c) The solid residue of \( \text{MO} \) (where M is the identified metal) is reacted with acid.
(i) Write a balanced equation for the reaction of \( \text{MO} \) with dilute hydrochloric acid. [1]
(ii) The residue of \( \text{MO} \) from the decomposition of 1.483 g of \( \text{M}(\text{NO}_3)_2 \) is dissolved in 50.0 cm\(^3\) of 1.00 mol dm\(^{-3}\) hydrochloric acid (an excess). Calculate the concentration of the unreacted hydrochloric acid remaining in the solution, assuming the volume remains 50.0 cm\(^3\). [3]

The rate of reaction between reactant A and reactant B was studied at constant temperature.
\( 2\text{A} + \text{B} \rightarrow \text{C} + \text{D} \)

The following initial rate data were obtained:
- Experiment 1: \( [\text{A}] = 0.100 \text{ mol dm}^{-3} \), \( [\text{B}] = 0.100 \text{ mol dm}^{-3} \), Initial Rate = \( 1.20 \times 10^{-4} \text{ mol dm}^{-3} \text{ s}^{-1} \)
- Experiment 2: \( [\text{A}] = 0.200 \text{ mol dm}^{-3} \), \( [\text{B}] = 0.100 \text{ mol dm}^{-3} \), Initial Rate = \( 4.80 \times 10^{-4} \text{ mol dm}^{-3} \text{ s}^{-1} \)
- Experiment 3: \( [\text{A}] = 0.200 \text{ mol dm}^{-3} \), \( [\text{B}] = 0.200 \text{ mol dm}^{-3} \), Initial Rate = \( 4.80 \times 10^{-4} \text{ mol dm}^{-3} \text{ s}^{-1} \)

(a) Determine the order of reaction with respect to A and with respect to B, showing your reasoning. [4]
(b) Write the rate equation for this reaction. [1]
(c) Using the data from Experiment 1, calculate the value of the rate constant, k, and state its units. [3]
(d) Suggest a mechanism for this reaction consisting of two steps, indicating which step is the rate-determining step. [2]

Cobalt is a transition element that forms various complex ions with interesting properties.

(a) A pink solution containing hexaaquacobalt(II) ions, \( [\text{Co}(\text{H}_2\text{O})_6]^{2+} \), is mixed with excess concentrated hydrochloric acid to form a blue tetrahedral complex, \( \mathbf{X} \).
(i) State the formula and coordination number of complex \( \mathbf{X} \). [2]
(ii) Write the equilibrium equation for the ligand exchange reaction and describe the color change. [2]
(b) When aqueous ammonia is added dropwise to \( [\text{Co}(\text{H}_2\text{O})_6]^{2+} \), a blue-green precipitate \( \mathbf{Y} \) forms first. Upon adding excess ammonia, \( \mathbf{Y} \) dissolves to give a pale brown solution containing the octahedral complex \( \mathbf{Z} \).
(i) Give the formula of precipitate \( \mathbf{Y} \). [1]
(ii) Write an equation for the conversion of \( [\text{Co}(\text{H}_2\text{O})_6]^{2+} \) to \( \mathbf{Z} \) in excess ammonia. [1]
(iii) When the solution of \( \mathbf{Z} \) is exposed to air, it is oxidised. Write the formula of the cobalt(III) complex ion formed. [1]
(c) Chromium forms a complex with the formula \( [\text{Cr}(\text{H}_2\text{O})_4\text{Cl}_2]^{+} \).
(i) Draw the 3D structures of the two stereoisomers of this complex, labeling them as cis or trans. [3]

Amino acids such as serine, \( \text{HO-CH}_2\text{-CH}(\text{NH}_2)\text{-COOH} \), exhibit both acidic and basic properties.

(a) Draw the structural formula of serine in:
(i) strongly acidic solution (pH 1) [1]
(ii) strongly alkaline solution (pH 13) [1]
(iii) its zwitterionic form at its isoelectric point. [1]
(b) Serine can be esterified by reacting it with methanol in the presence of concentrated sulfuric acid catalyst.
(i) Draw the structural formula of the protonated organic product formed. [1]
(ii) State the name of the new organic functional group formed. [1]
(c) Two different amino acids, alanine (Ala) and serine (Ser), react to form two different dipeptides.
(i) Draw the structural formulas of the two dipeptides, Ala-Ser and Ser-Ala. Circle the peptide bond in one of your drawn structures. [3]
(d) Explain, in terms of structure and bonding, why amino acids have relatively high melting points compared to other organic compounds of similar molecular mass. [2]

The Born-Haber cycle is used to determine lattice energies of ionic compounds.

(a) Define the term *lattice energy*. [2]
(b) Consider the determination of the lattice energy of calcium chloride, \( \text{CaCl}_2(\text{s}) \), using the following data:
- Enthalpy change of formation of \( \text{CaCl}_2(\text{s}) = -796 \text{ kJ mol}^{-1} \)
- Enthalpy change of atomisation of calcium = \( +178 \text{ kJ mol}^{-1} \)
- First ionisation energy of calcium = \( +590 \text{ kJ mol}^{-1} \)
- Second ionisation energy of calcium = \( +1145 \text{ kJ mol}^{-1} \)
- Enthalpy change of atomisation of chlorine = \( +121 \text{ kJ mol}^{-1} \)
- Electron affinity of chlorine = \( -349 \text{ kJ mol}^{-1} \)

(i) Write the chemical equation, including state symbols, that represents the lattice energy of calcium chloride. [1]
(ii) Use the data above to calculate the lattice energy of \( \text{CaCl}_2(\text{s}) \). Show your working clearly. [4]
(c) Explain how the lattice energy of calcium chloride compares with:
(i) the lattice energy of calcium bromide, \( \text{CaBr}_2 \). [1]
(ii) the lattice energy of magnesium chloride, \( \text{MgCl}_2 \). [2]

An organic acid, \( \mathbf{W} \), partitions between ethoxyethane (ether) and water. The partition coefficient, \( K_{\text{pc}} \), is defined as:
\( K_{\text{pc}} = \frac{[\mathbf{W}]_{\text{ether}}}{[\mathbf{W}]_{\text{water}}} = 4.0 \)

(a) Define the term *partition coefficient*. [2]
(b) A solution is prepared containing 3.00 g of \( \mathbf{W} \) in 100 cm\(^3\) of water.
(i) Calculate the mass of \( \mathbf{W} \) extracted when the solution is shaken with a single 50 cm\(^3\) portion of ether. [3]
(ii) Calculate the total mass of \( \mathbf{W} \) extracted when the original solution (3.00 g of \( \mathbf{W} \) in 100 cm\(^3\) of water) is shaken with two successive 25 cm\(^3\) portions of ether. [4]
(c) State, with a reason, which extraction method is more efficient. [1]

You are required to determine the water of crystallisation, \( x \), in hydrated copper(II) sulfate, \( \text{CuSO}_4 \cdot x\text{H}_2\text{O} \), using iodometric titration.

**Procedure**
1. Weigh out accurately \( 6.20\text{ g} \) of the hydrated copper(II) sulfate sample and dissolve it in distilled water. Transfer the solution quantitatively to a \( 250.0\text{ cm}^3 \) volumetric flask and make it up to the mark with distilled water. Label this solution **FA 1**.
2. Pipette a \( 25.00\text{ cm}^3 \) aliquot of **FA 1** into a conical flask.
3. Add \( 10\text{ cm}^3 \) of \( 1.0\text{ mol dm}^{-3} \) potassium iodide, \( \text{KI} \) (an excess), to the flask. A brown mixture containing a precipitate of copper(I) iodide and dissolved iodine is formed.
4. Titrate the liberated iodine with \( 0.100\text{ mol dm}^{-3} \) sodium thiosulfate solution, **FA 2**, until the brown colour fades to a pale yellow.
5. Add a few drops of starch indicator. The mixture will turn blue-black.
6. Continue titrating dropwise with **FA 2** until the blue-black colour completely disappears, leaving a white/off-white precipitate.
7. Record your results in a suitable table showing initial and final burette readings, and the volume of **FA 2** added for a rough titration and two suitable concordant titrations.

**Data and Results**
Assume the average concordant titre of **FA 2** obtained is \( 24.85\text{ cm}^3 \).

(a) Calculate the number of moles of thiosulfate ions, \( \text{S}_2\text{O}_3^{2-} \), used in your average titre.
(b) Using the equations below, determine the number of moles of \( \text{Cu}^{2+} \) ions present in the \( 25.00\text{ cm}^3 \) sample of **FA 1**:
\( 2\text{Cu}^{2+}(\text{aq}) + 4\text{I}^-(\text{aq}) \rightarrow 2\text{CuI}(\text{s}) + \text{I}_2(\text{aq}) \)
\( \text{I}_2(\text{aq}) + 2\text{S}_2\text{O}_3^{2-}(\text{aq}) \rightarrow 2\text{I}^-(\text{aq}) + \text{S}_4\text{O}_6^{2-}(\text{aq}) \)
(c) Calculate the number of moles of \( \text{Cu}^{2+} \) in the original \( 250.0\text{ cm}^3 \) of solution **FA 1**.
(d) Calculate the molar mass of \( \text{CuSO}_4 \cdot x\text{H}_2\text{O} \).
(e) Determine the value of \( x \), showing your working. (\( A_r \): \( \text{Cu} = 63.5 \), \( \text{S} = 32.1 \), \( \text{O} = 16.0 \), \( \text{H} = 1.0 \))

You are investigating the reaction between peroxodisulfate ions, \( \text{S}_2\text{O}_8^{2-} \), and iodide ions, \( \text{I}^- \):
\( \text{S}_2\text{O}_8^{2-}(\text{aq}) + 2\text{I}^-(\text{aq}) \rightarrow 2\text{SO}_4^{2-}(\text{aq}) + \text{I}_2(\text{aq}) \)

The reaction rate is studied using an iodine clock method. A small volume of sodium thiosulfate solution, \( \text{Na}_2\text{S}_2\text{O}_3 \), and starch indicator are added to the reaction mixture. The liberated iodine reacts instantly with thiosulfate:
\( \text{I}_2(\text{aq}) + 2\text{S}_2\text{O}_3^{2-}(\text{aq}) \rightarrow 2\text{I}^-(\text{aq}) + \text{S}_4\text{O}_6^{2-}(\text{aq}) \)

Once all the thiosulfate is consumed, any further iodine produced reacts with starch to form a blue-black complex. The time taken, \( t \), for the blue-black colour to appear is measured.

Three experiments were conducted at a constant temperature of \( 298\text{ K} \) with varying initial concentrations of the reactants. The total volume of each reaction mixture was kept constant at \( 50.0\text{ cm}^3 \) by adding distilled water.

The experimental data is shown below:

- **Experiment 1**:
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{KI}(\text{aq}) \): \( 20.0\text{ cm}^3 \)
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{Na}_2\text{S}_2\text{O}_3(\text{aq}) \): \( 5.0\text{ cm}^3 \)
- Volume of distilled water: \( 15.0\text{ cm}^3 \)
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{K}_2\text{S}_2\text{O}_8(\text{aq}) \): \( 10.0\text{ cm}^3 \)
- Time taken for blue-black colour to appear, \( t_1 \): \( 44\text{ s} \)

- **Experiment 2**:
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{KI}(\text{aq}) \): \( 10.0\text{ cm}^3 \)
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{Na}_2\text{S}_2\text{O}_3(\text{aq}) \): \( 5.0\text{ cm}^3 \)
- Volume of distilled water: \( 25.0\text{ cm}^3 \)
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{K}_2\text{S}_2\text{O}_8(\text{aq}) \): \( 10.0\text{ cm}^3 \)
- Time taken for blue-black colour to appear, \( t_2 \): \( 88\text{ s} \)

- **Experiment 3**:
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{KI}(\text{aq}) \): \( 20.0\text{ cm}^3 \)
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{Na}_2\text{S}_2\text{O}_3(\text{aq}) \): \( 5.0\text{ cm}^3 \)
- Volume of distilled water: \( 20.0\text{ cm}^3 \)
- Volume of \( 0.100\text{ mol dm}^{-3}\ \text{K}_2\text{S}_2\text{O}_8(\text{aq}) \): \( 5.0\text{ cm}^3 \)
- Time taken for blue-black colour to appear, \( t_3 \): \( 88\text{ s} \)

(a) Deduce the order of reaction with respect to:
(i) \( \text{I}^- \) ions. Show your reasoning by comparing appropriate experiments.
(ii) \( \text{S}_2\text{O}_8^{2-} \) ions. Show your reasoning by comparing appropriate experiments.

(b) Construct the rate equation for this reaction.

(c) Calculate the initial rate of reaction for Experiment 1 in \( \text{mol dm}^{-3}\text{ s}^{-1} \).
*(Hint: First calculate the concentration of \( \text{S}_2\text{O}_8^{2-} \) that has reacted during the time \( t_1 \), which is stoichiometrically related to the moles of \( \text{S}_2\text{O}_3^{2-} \) consumed.)*

(d) Calculate the rate constant, \( k \), for the reaction, stating its units.

You are provided with two solutions: **FA 3** (containing one transition metal cation and one halide anion) and **FA 4** (containing one transition metal cation and one Group 16 oxyanion).

You are required to perform the tests described in the table below and record all of your observations. From these observations, you will identify the ions present and write balanced ionic equations.

| Test | Observation |
|---|---|
| **Test 1**: To a \( 1.0\text{ cm}^3 \) portion of **FA 3** in a test-tube, add aqueous sodium hydroxide dropwise until in excess. | A red-brown precipitate is formed, which is insoluble in excess sodium hydroxide. |
| **Test 2**: To a \( 1.0\text{ cm}^3 \) portion of **FA 3** in a test-tube, add \( 1\text{ cm}^3 \) of dilute nitric acid followed by a few drops of aqueous silver nitrate. | A thick white precipitate is formed. |
| **Test 3**: To a \( 1.0\text{ cm}^3 \) portion of **FA 4** in a test-tube, add aqueous sodium hydroxide dropwise until in excess. | A grey-green precipitate is formed. The precipitate dissolves in excess sodium hydroxide to give a dark green solution. |
| **Test 4**: To a \( 1.0\text{ cm}^3 \) portion of **FA 4** in a test-tube, add aqueous ammonia dropwise until in excess. | A grey-green precipitate is formed, which is insoluble in excess ammonia. |
| **Test 5**: To a \( 1.0\text{ cm}^3 \) portion of **FA 4** in a test-tube, add a few drops of aqueous barium nitrate, followed by dilute nitric acid. | A white precipitate is formed, which does not dissolve when dilute nitric acid is added. |

(a) Identify the ions present in **FA 3** and **FA 4**:
- Cation in **FA 3**
- Anion in **FA 3**
- Cation in **FA 4**
- Anion in **FA 4**

(b) Write a balanced ionic equation for the reaction that occurs when aqueous sodium hydroxide is added to **FA 3** in Test 1. Include state symbols.

(c) Write a balanced ionic equation showing the dissolution of the precipitate formed in Test 3 when excess sodium hydroxide is added.

Cobalt(III) complexes with bidentate ligands show significant thermodynamic stability and interesting stereoisomerism.

(a) Define the term *bidentate ligand*.

(b) An aqueous solution of cobalt(II) ions is oxidised in the presence of 1,2-diaminoethane (en) to form the octahedral complex ion \([Co(en)_3]^{3+}\).
(i) Draw the 3D stereoisomers of \([Co(en)_3]^{3+}\). State the type of isomerism shown.
(ii) Suggest why the coordination of three bidentate 'en' ligands to a single metal ion results in a highly thermodynamically stable complex compared to coordination with monodentate ammonia ligands, referring to entropy.

(c) The stability constant, \(K_{stab}\), for the equilibrium:
\[[Co(H_2O)_6]^{2+} + 3en \rightleftharpoons [Co(en)_3]^{2+} + 6H_2O\]
is \(1.5 \times 10^{14}\ \text{dm}^9\ \text{mol}^{-3}\).
(i) Write the expression for \(K_{stab}\).
(ii) Calculate the concentration of free \([Co(H_2O)_6]^{2+}\) remaining in a solution where the equilibrium concentration of \([Co(en)_3]^{2+}\) is \(0.050\ \text{mol\ dm}^{-3}\) and the equilibrium concentration of free \(en\) ligand is \(2.0 \times 10^{-4}\ \text{mol\ dm}^{-3}\).

The kinetics of the reaction between three reactants, A, B, and C, were investigated at 298 K:
\[2A + B + C \rightarrow D + E\]
The initial rate data obtained from four experiments are shown below:

| Experiment | Initial [A] / \(\text{mol
dm}^{-3}\) | Initial [B] / \(\text{mol
dm}^{-3}\) | Initial [C] / \(\text{mol
dm}^{-3}\) | Initial Rate / \(\text{mol
dm}^{-3}\text{s}^{-1}\) |
| :--- | :--- | :--- | :--- | :--- |
| 1 | 0.10 | 0.10 | 0.10 | \(1.2 \times 10^{-3}\) |
| 2 | 0.20 | 0.10 | 0.10 | \(2.4 \times 10^{-3}\) |
| 3 | 0.10 | 0.30 | 0.10 | \(1.08 \times 10^{-2}\) |
| 4 | 0.10 | 0.10 | 0.20 | \(1.2 \times 10^{-3}\) |

(a) Deduce the order of reaction with respect to A, B, and C. Explain your reasoning clearly.

(b) Write the rate equation for this reaction.

(c) Calculate the rate constant, \(k\), using the data from Experiment 1, and state its units.

(d) Describe and explain how the rate constant, \(k\), changes, if at all, when:
(i) the temperature is increased.
(ii) a catalyst is added at constant temperature.

Lattice energy measurements provide crucial insight into ionic bonding and structure.

(a) Define the term *lattice energy*.

(b) Construct a fully labelled Born-Haber cycle for the formation of solid calcium chloride, \(CaCl_2(s)\), from its constituent elements in their standard states. Include state symbols and enthalpy labels.

(c) Use the data below to calculate the lattice energy (\(\Delta H^\ominus_{latt}\)) of \(CaCl_2(s)\).
- Standard enthalpy of formation of \(CaCl_2(s) = -796\ \text{kJ\ mol}^{-1}\)
- Enthalpy change of atomisation of \(Ca(s) = +178\ \text{kJ\ mol}^{-1}\)
- First ionisation energy of \(Ca(g) = +590\ \text{kJ\ mol}^{-1}\)
- Second ionisation energy of \(Ca(g) = +1145\ \text{kJ\ mol}^{-1}\)
- Bond enthalpy of \(Cl_2(g) = +242\ \text{kJ\ mol}^{-1}\)
- First electron affinity of \(Cl(g) = -349\ \text{kJ\ mol}^{-1}\)

(d) Explain why the lattice energy of calcium fluoride, \(CaF_2\), is significantly more exothermic than that of calcium chloride, \(CaCl_2\).

The partition coefficient of an organic carboxylic acid, X, between ethoxyethane and water is 8.0 (meaning X is more soluble in ethoxyethane).
\[K_{partition} = \frac{[X]_{ethoxyethane}}{[X]_{water}} = 8.0\]
An aqueous solution contains \(4.50\ \text{g}\) of solute X dissolved in \(100\ \text{cm}^3\) of water.

(a) Define the term *partition coefficient*.

(b) Calculate the mass of X extracted if the \(100\ \text{cm}^3\) aqueous solution is shaken with a single \(50.0\ \text{cm}^3\) portion of ethoxyethane.

(c) Alternatively, the same aqueous solution of X is extracted using two successive \(25.0\ \text{cm}^3\) portions of ethoxyethane.
(i) Calculate the total mass of X extracted after these two successive extractions.
(ii) Compare the efficiency of a single extraction versus successive extractions based on your calculations.

The behavior of amino acids in aqueous solution depends significantly on pH.

(a) Draw the structural formula of aspartic acid, \(\text{H}_2\text{N-CH(CH}_2\text{COOH)-COOH}\), in:
(i) its zwitterionic form at its isoelectric point.
(ii) strongly acidic conditions (\(\text{pH} = 1\)).
(iii) strongly alkaline conditions (\(\text{pH} = 13\)).

(b) A mixture containing aspartic acid (isoelectric point = 2.8), alanine (isoelectric point = 6.0), and lysine (isoelectric point = 9.7) is subjected to electrophoresis in a buffer solution maintained at \(\text{pH} = 6.0\).
(i) State which amino acid(s) will migrate towards the anode (+), which will migrate towards the cathode (-), and which will remain stationary at the origin.
(ii) Explain your answer to (b)(i) by describing the net charge on each amino acid at \(\text{pH} = 6.0\).

(c) Draw the structure of the dipeptide formed when the carboxyl group of alanine reacts with the amino group of aspartic acid.

Propanone undergoes nucleophilic addition with hydrogen cyanide.

(a) Propanone reacts with hydrogen cyanide, \(HCN\), in the presence of a catalyst, \(NaCN\), to form 2-hydroxy-2-methylpropanenitrile.
(i) Write the chemical equation for this reaction, showing the structural formulas of both the reactant and the organic product.
(ii) Outline the detailed, step-by-step mechanism for this reaction. Include curly arrows to show the movement of electron pairs, and show any relevant dipoles and lone pairs.
(iii) Explain why \(NaCN\) (or a base like \(NaOH\)) is added to initiate the reaction, rather than using pure \(HCN\) alone.

(b) Describe a chemical test (reagents and observations) that can be used to distinguish between:
(i) Propanal and propanone.
(ii) Propanone and propan-2-ol.

Electrode potentials provide quantitative predictions regarding redox reactions, particularly for transition metals.

(a) An electrochemical cell is set up using the standard \(Fe^{3+}/Fe^{2+}\) half-cell and the standard \(Cr^{3+}/Cr^{2+}\) half-cell.
Given standard electrode potentials:
\[Fe^{3+}(aq) + e^- \rightleftharpoons Fe^{2+}(aq) \quad E^\ominus = +0.77\ \text{V}\]
\[Cr^{3+}(aq) + e^- \rightleftharpoons Cr^{2+}(aq) \quad E^\ominus = -0.41\ \text{V}\]
(i) Write the overall ionic equation for the spontaneous cell reaction.
(ii) Calculate the standard cell potential, \(E^\ominus_{cell}\).
(iii) Deduce which species is the strongest reducing agent under standard conditions.

(b) The concentration of ions in the chromium half-cell is changed.
The Nernst equation is given by:
\[E = E^\ominus + \frac{0.059}{z} \log_{10} \frac{[\text{oxidised species}]}{[\text{reduced species}]}\]
Calculate the electrode potential, \(E\), of the chromium half-cell at \(298\ \text{K}\) when \([Cr^{3+}(aq)] = 0.015\ \text{mol\ dm}^{-3}\) and \([Cr^{2+}(aq)] = 0.35\ \text{mol\ dm}^{-3}\).

(c) Explain, with reference to the \(d\)-orbital splitting, why transition metal complexes are typically coloured.

Polymers can be synthesized by different mechanisms, yielding distinct physical properties and environmental lifetimes.

(a) Kevlar is a highly strong synthetic polyamide. It is synthesized from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine.
(i) Draw the repeat unit of Kevlar, clearly showing the amide linkage.
(ii) Explain, in terms of intermolecular forces, why Kevlar has such high mechanical strength and thermal stability.

(b) Poly(lactic acid), PLA, is a biodegradable polyester made from 2-hydroxypropanoic acid (lactic acid), \(\text{CH}_3\text{CH(OH)COOH}\).
(i) Draw the repeat unit of PLA.
(ii) Explain why PLA is biodegradable whereas addition polymers like poly(ethene) are not.
(iii) State the type of reaction that breaks down PLA in the environment.

A student plans to determine the partition coefficient, \(K_{pc}\), of butanoic acid between water and trichloromethane at \(298\text{ K}\). In each of four experiments, a known mass of butanoic acid was dissolved in \(50.0\text{ cm}^3\) of water. This solution was shaken thoroughly with \(50.0\text{ cm}^3\) of trichloromethane in a separating funnel and allowed to reach equilibrium. A \(10.0\text{ cm}^3\) sample of the aqueous layer was then titrated against \(0.100\text{ mol dm}^{-3}\) aqueous sodium hydroxide.

Experimental Data:
- Experiment 1: Mass of butanoic acid initially dissolved = \(1.20\text{ g}\); Volume of \(0.100\text{ mol dm}^{-3}\) \(NaOH\) required = \(9.10\text{ cm}^3\).
- Experiment 2: Mass of butanoic acid initially dissolved = \(1.80\text{ g}\); Volume of \(0.100\text{ mol dm}^{-3}\) \(NaOH\) required = \(13.60\text{ cm}^3\).

[Molar mass of butanoic acid = \(88.1\text{ g mol}^{-1}\)]

(a) For Experiment 1, calculate:
(i) the concentration of butanoic acid remaining in the aqueous layer, \([\text{butanoic acid}]_{\text{aq}}\), in \\text{mol dm}^{-3}\\.
(ii) the concentration of butanoic acid extracted into the trichloromethane layer, \([\text{butanoic acid}]_{\text{org}}\), in \\text{mol dm}^{-3}\\.

(b) Calculate the partition coefficient, \(K_{pc} = \frac{[\text{butanoic acid}]_{\text{org}}}{[\text{butanoic acid}]_{\text{aq}}}\), for Experiment 1 and Experiment 2. Show that these values are consistent.

(c) Using an average \(K_{pc}\) value of \(2.00\), calculate the mass of butanoic acid that would remain in the aqueous layer if \(1.20\text{ g}\) of butanoic acid dissolved in \(50.0\text{ cm}^3\) of water was equilibrated with \(100\text{ cm}^3\) of trichloromethane.

(d) State and explain how the value of \(K_{pc}\) would change if the experiment was conducted at a higher temperature, given that the transfer of butanoic acid from water to trichloromethane is an exothermic process.

The kinetics of the reaction between peroxodisulfate ions and iodide ions was investigated using an iodine clock reaction:

\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})

In each experiment, the reaction was timed at \(298\text{ K}\) until a blue-black colour suddenly appeared. This occurs when all the added sodium thiosulfate, \text{Na}_2\text{S}_2\text{O}_3, has reacted with the iodine produced.

Experimental Data (total volume of mixture in each run is exactly \(100.0\text{ cm}^3\), containing \(10.0\text{ cm}^3\) of \(0.0100\text{ mol dm}^{-3}\) \text{Na}_2\text{S}_2\text{O}_3 and starch indicator):

- Experiment 1: \(20.0\text{ cm}^3\) of \(0.100\text{ mol dm}^{-3}\) \text{K}_2\text{S}_2\text{O}_8, \(20.0\text{ cm}^3\) of \(0.200\text{ mol dm}^{-3}\) \text{KI}; time, \(t = 45.0\text{ s}\).
- Experiment 2: \(10.0\text{ cm}^3\) of \(0.100\text{ mol dm}^{-3}\) \text{K}_2\text{S}_2\text{O}_8, \(20.0\text{ cm}^3\) of \(0.200\text{ mol dm}^{-3}\) \text{KI}; time, \(t = 90.0\text{ s}\).
- Experiment 3: \(20.0\text{ cm}^3\) of \(0.100\text{ mol dm}^{-3}\) \text{K}_2\text{S}_2\text{O}_8, \(10.0\text{ cm}^3\) of \(0.200\text{ mol dm}^{-3}\) \text{KI}; time, \(t = 90.0\text{ s}\).

(a) Explain the chemical basis of this clock reaction, explaining why the blue-black colour only appears after a specific time interval.
(b) Deduce the order of reaction with respect to both \text{S}_2\text{O}_8^{2-} and \text{I}^-, showing your reasoning.
(c) For Experiment 1, calculate:
(i) the initial concentration of \text{S}_2\text{O}_8^{2-}(\text{aq}) in the mixture.
(ii) the initial rate of reaction, expressed as the rate of formation of \text{I}_2 in \\text{mol dm}^{-3}\\text{ s}^{-1}.
(d) Write the rate equation for the reaction, and calculate the value of the rate constant, \(k\), for Experiment 1, including its units.

The formula of the purple complex formed between iron(III) ions and salicylate ligands is investigated using Job's method of continuous variation. In a series of 9 mixtures, the total volume of \(0.00200\text{ mol dm}^{-3}\) \text{Fe}^{3+}(\text{aq}) and \(0.00200\text{ mol dm}^{-3}\) salicylate, \text{Sal}^-(\text{aq}), was kept constant at \(10.0\text{ cm}^3\). The absorbance of each mixture was measured at \(525\text{ nm}\).

Data:
- Mixture 2: \(V_{\text{sal}} = 2.0\text{ cm}^3\), Absorbance = \(0.30\)
- Mixture 4: \(V_{\text{sal}} = 4.0\text{ cm}^3\), Absorbance = \(0.60\)
- Mixture 5: \(V_{\text{sal}} = 5.0\text{ cm}^3\), Absorbance = \(0.68\)
- Mixture 6: \(V_{\text{sal}} = 6.0\text{ cm}^3\), Absorbance = \(0.60\)
- Mixture 8: \(V_{\text{sal}} = 8.0\text{ cm}^3\), Absorbance = \(0.30\)

(a) Calculate the mole fraction of salicylate, \(x_{\text{sal}}\), for Mixtures 2, 4, 6, and 8.
(b) Explain how the formula of the complex can be determined by plotting absorbance against \(x_{\text{sal}}\), drawing two best-fit intersecting straight lines, and finding the stoichiometric intersection point. State the value of \(x_{\text{sal}}\) at this intersection.
(c) Use your answer to (b) to deduce the formula of the complex, including its overall charge (assume salicylate has a charge of -1 and iron is in the +3 oxidation state).
(d) Explain why the absorbance measured for Mixture 5 (the stoichiometric point) is lower than the value predicted by the intersection of the two straight lines.
(e) Suggest why a wavelength of \(525\text{ nm}\) (green light) was selected for this colorimetric measurement.