A Level Chemistry 9701 Paper 42, May/June 2025

Calcium nitrate, $\mathrm{C}\mathrm{a}(\mathrm{N}{\mathrm{O}}_3{)}_2$, decomposes on heating. Write an equation for the decomposition of calcium nitrate.

Describe the trend in the decomposition temperature of the Group 2 nitrates. Explain your answer.

A sample of $0.333\mathrm{g}$ of strontium oxide, $\mathrm{S}\mathrm{r}\mathrm{O}$, is completely dissolved in distilled water to form a solution of strontium hydroxide, $\mathrm{S}\mathrm{r}(\mathrm{O}\mathrm{H}{)}_2$. The resulting solution is added to a volumetric flask and made up to $250.0\mathrm{c}{\mathrm{m}}^3$ with distilled water. Calculate the pH of this solution at $298\mathrm{K}$. Give your answer to two decimal places.

When cobalt(II) sulfate, $\mathrm{C}\mathrm{o}\mathrm{S}{\mathrm{O}}_4$, is dissolved in distilled water, solution A is formed. The reaction scheme in Fig. 2.1 shows some reactions of solution A. Complete Table 2.1 to show the formula and colour of each of the cobalt-containing species present in A, B and C. Identify the type of reaction forming each of B and C. Table 2.1

Suggest a suitable reagent for the formation of $[{\mathrm{C}\mathrm{o}\mathrm{C}\mathrm{l}}_4{]}^{2-}$ from solution A.

The complex ion $[{\mathrm{C}\mathrm{o}\mathrm{C}\mathrm{l}}_4{]}^{2-}$ has tetrahedral geometry. The 3d orbitals in an isolated ${\mathrm{C}\mathrm{o}}^{2+}$ ion are degenerate. Complete Fig. 2.2 to show the relative energies of the 3d orbitals in an isolated ${\mathrm{C}\mathrm{o}}^{2+}$ ion and in ${\mathrm{C}\mathrm{o}}^{2+}$ in a tetrahedral complex.

Draw a three-dimensional diagram to show the structure of the complex ion $[{\mathrm{C}\mathrm{o}\mathrm{C}\mathrm{l}}_4{]}^{2-}$.

${\mathrm{H}}_2{\mathrm{O}}_2$ can act as an oxidising agent or a reducing agent when reacting with species that contain manganese. Table 2.2 shows electrode potentials, $E^{\ominus }$, for some electrode reactions. Table 2.2 Use only the species listed in Table 2.2 to suggest: • one reaction in which ${\mathrm{H}}_2{\mathrm{O}}_2$ acts as an oxidising agent and • one reaction in which ${\mathrm{H}}_2{\mathrm{O}}_2$ acts as a reducing agent. Include the value of the standard cell potential, $E_{\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}}^{\ominus }$, and an overall equation for each reaction.

Acidified manganate(VII) ions, $\mathrm{M}\mathrm{n}{\mathrm{O}}_4^{-}$, can be used to analyse the content of iron tablets by titration. Two identical iron tablets are crushed and dissolved in distilled water. The resulting solution is made up to $150.0\mathrm{c}{\mathrm{m}}^3$ with distilled water. $25.0\mathrm{c}{\mathrm{m}}^3$ of this solution requires $18.60\mathrm{c}{\mathrm{m}}^3$ of $0.0500\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{d}{\mathrm{m}}^{-3}$ acidified $\mathrm{M}\mathrm{n}{\mathrm{O}}_4^{-}$ to reach the end-point. All the ${\mathrm{F}\mathrm{e}}^{2+}$ ions are oxidised. The relevant half-equations are shown. $$\mathrm{M}\mathrm{n}{\mathrm{O}}_4^{-}+8{\mathrm{H}}^{+}+5{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{M}\mathrm{n}}^{2+}+4{\mathrm{H}}_2\mathrm{O}$$ $${\mathrm{F}\mathrm{e}}^{2+}\rightleftharpoons {\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{e}}^{-}$$ Describe the colour change observed at the end-point of this titration.

Calculate the mass, in mg, of iron in one tablet. Assume that all the iron in the tablets is ${\mathrm{F}\mathrm{e}}^{2+}$. Show your working.

Define entropy.

Fig. 3.1 shows how the entropy, $S$, of a pure substance changes with temperature, $T$. Identify the process occurring at each of the temperatures $T_1$ and $T_2$.

Explain why the entropy change, $\mathrm{\Delta }S$, at $T_2$ is bigger than the entropy change at $T_1$.

The equation for the reduction of iron(III) oxide by carbon monoxide at $450{}^{\circ }\mathrm{C}$ is shown. $$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3(\mathrm{s})+3\mathrm{C}\mathrm{O}(\mathrm{g})\to 2\mathrm{F}\mathrm{e}(\mathrm{s})+3\mathrm{C}{\mathrm{O}}_2(\mathrm{g})\mathrm{\Delta }{G}^{\ominus }=-36.2\mathrm{k}\mathrm{J}\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}$$ Table 3.1 shows the enthalpy of formation, $\mathrm{\Delta }{H}_{\mathrm{f}}^{\ominus }$, and the entropy, $S^{\ominus }$, for some substances. Table 3.1 Use the data in Table 3.1 to calculate the entropy, $S^{\ominus }$, of carbon monoxide at $450{}^{\circ }\mathrm{C}$. Show your working.

Iron(II) oxide can also be reduced to iron by carbon monoxide, as shown. $$\mathrm{F}\mathrm{e}\mathrm{O}(\mathrm{s})+\mathrm{C}\mathrm{O}(\mathrm{g})\to \mathrm{F}\mathrm{e}(\mathrm{s})+\mathrm{C}{\mathrm{O}}_2(\mathrm{g})\mathrm{\Delta }{H}^{\ominus }=-11.1\mathrm{k}\mathrm{J}\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1},\mathrm{\Delta }{S}^{\ominus }=-15.2\mathrm{J}{\mathrm{K}}^{-1}\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}$$ State the effect of increasing temperature on the feasibility of this reaction. Explain your answer.

In aqueous solution, iodide ions react with acidified hydrogen peroxide, as shown in reaction 1. $$\text{reaction 1}2{\mathrm{I}}^{-}+{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{H}}^{+}\to {\mathrm{I}}_2+2{\mathrm{H}}_2\mathrm{O}$$ The rate equation for reaction 1 is shown. $$\text{rate}=k[{\mathrm{I}}^{-}][{\mathrm{H}}_2{\mathrm{O}}_2]$$ Explain what is meant by order of reaction.

Complete Table 4.1. Table 4.1

Sketch a line on Fig. 4.1 to show the relationship between $[{\mathrm{I}}^{-}]$ and time.

Nitrogen dioxide, $\mathrm{N}{\mathrm{O}}_2$, reacts with ozone, ${\mathrm{O}}_3$, as shown in reaction 2. $$\text{reaction 2}2\mathrm{N}{\mathrm{O}}_2+{\mathrm{O}}_3\to {\mathrm{O}}_2+{\mathrm{N}}_2{\mathrm{O}}_5$$ The rate equation for reaction 2 is shown. $$\text{rate}=k[\mathrm{N}{\mathrm{O}}_2][{\mathrm{O}}_3]$$ Two experiments are carried out to measure the rate of reaction 2. In the first experiment, the initial rate is measured starting with known concentrations of $\mathrm{N}{\mathrm{O}}_2$ and ${\mathrm{O}}_3$. In the second experiment, the concentrations of $\mathrm{N}{\mathrm{O}}_2$ and ${\mathrm{O}}_3$ are both increased by a factor of four. Predict how the initial rate for reaction 2 would change.

The reaction mechanism for reaction 2 has two steps. Define rate-determining step.

Suggest equations for the two steps of the reaction mechanism for reaction 2.

Dinitrogen pentoxide, ${\mathrm{N}}_2{\mathrm{O}}_5$, can decompose to $\mathrm{N}{\mathrm{O}}_2$ and ${\mathrm{O}}_2$. The rate equation for this decomposition is shown. $$\text{rate}=k[{\mathrm{N}}_2{\mathrm{O}}_5]$$ Fig. 4.2 shows the graph of rate against $[{\mathrm{N}}_2{\mathrm{O}}_5]$ for this decomposition. Use Fig. 4.2 to calculate a value for the rate constant, $k$, for this decomposition.

Use your answer to (c)(i) to calculate the half-life, $t_{\frac{1}{2}}$, in seconds, for this decomposition.

Chloric(I) acid, $\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}$, is a weak Brønsted–Lowry acid. When chloric(I) acid is added to aqueous sodium hydroxide, an acid–base reaction takes place, as shown. $$\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to \mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$ Identify the two conjugate acid–base pairs in this reaction.

The value for the acid dissociation constant, $K_{\mathrm{a}}$, of $\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}(\mathrm{a}\mathrm{q})$ is $3.70\times {10}^{-8}$. Calculate the concentration, in $\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{d}{\mathrm{m}}^{-3}$, of $\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}(\mathrm{a}\mathrm{q})$ at a pH of 4.51.

When a solution of $\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}(\mathrm{a}\mathrm{q})$ is heated, chloric(V) acid and a strong acid not containing oxygen are formed as the only products. Write an equation for this reaction.

Define a buffer solution.

Suggest a substance that could be added to aqueous ethanoic acid to form a buffer solution. Explain your answer.

Some fertilisers contain calcium dihydrogenphosphate, $\mathrm{C}\mathrm{a}({\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4{)}_2$. An aqueous solution containing dihydrogenphosphate ions, ${\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}$, can act as a buffer solution. Write two equations to show how ${\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4^{-}$ ions can act as a buffer.

The solubility of calcium phosphate, $\mathrm{C}{\mathrm{a}}_3(\mathrm{P}{\mathrm{O}}_4{)}_2$, is $1.14\times {10}^{-7}\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{d}{\mathrm{m}}^{-3}$ at $25{}^{\circ }\mathrm{C}$. The expression for the solubility product, $K_{\mathrm{s}\mathrm{p}}$, of $\mathrm{C}{\mathrm{a}}_3(\mathrm{P}{\mathrm{O}}_4{)}_2$ is shown. $$K_{\mathrm{s}\mathrm{p}}=[{\mathrm{C}\mathrm{a}}^{2+}{]}^3[{\mathrm{P}{\mathrm{O}}_4}^{3-}{]}^2$$ Calculate the value of $K_{\mathrm{s}\mathrm{p}}$ for $\mathrm{C}{\mathrm{a}}_3(\mathrm{P}{\mathrm{O}}_4{)}_2$. Include units.

Some solid sodium phosphate is added to a saturated solution of $\mathrm{C}{\mathrm{a}}_3(\mathrm{P}{\mathrm{O}}_4{)}_2$. Predict the effect, if any, on the solubility of $\mathrm{C}{\mathrm{a}}_3(\mathrm{P}{\mathrm{O}}_4{)}_2$. Explain your answer.

Methylbenzene reacts readily with nitronium ions, $\mathrm{N}{\mathrm{O}}_2^{+}$. $\mathrm{N}{\mathrm{O}}_2^{+}$ ions are generated by the reaction between concentrated nitric acid and concentrated sulfuric acid. Write an equation for the formation of the $\mathrm{N}{\mathrm{O}}_2^{+}$ ion.

Complete the mechanism in Fig. 6.1 for the nitration of methylbenzene to form 1-methyl-2-nitrobenzene. Include all relevant curly arrows and charges.

Phenol can be nitrated with dilute nitric acid. Explain why the nitration of phenol occurs under milder conditions than the nitration of benzene.

A sample of 2-nitrophenol is reacted with sodium. Complete the equation in Fig. 6.2 for the reaction of 2-nitrophenol with sodium.

2-nitrophenol can undergo different reactions as shown in Fig. 6.3. Suggest reagents and conditions for reaction 1.

Name the type of reaction for reaction 1.

Reaction 2 is carried out at room temperature. Draw the structure of the organic product from reaction 2 in Fig. 6.3.

Name the mechanism for reaction 2.

The $\mathrm{N}{\mathrm{O}}_2$ group in 2-nitrophenol is electron withdrawing. Suggest the relative acidities of ethanol, 2-nitrophenol, phenol and water. Explain your answer.

Salbutamol is a pharmaceutical drug that contains a phenol functional group. Name and classify the three other functional groups in salbutamol in Table 6.1. Table 6.1

Salbutamol reacts with $\mathrm{B}{\mathrm{r}}_2(\mathrm{a}\mathrm{q})$ to form organic product X. Draw the structure of X.

Salbutamol reacts with an excess of $\mathrm{S}\mathrm{O}\mathrm{C}{\mathrm{l}}_2$ to form organic product Y. The molecular formula of Y is ${\mathrm{C}}_{13}{\mathrm{H}}_{19}\mathrm{C}{\mathrm{l}}_2\mathrm{N}\mathrm{O}$. Draw the structure of Y.

2-aminobutane, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}(\mathrm{N}{\mathrm{H}}_2)\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$, exists as a mixture of two enantiomers. Define enantiomers.

Explain why an aqueous solution of 2-aminobutane has a pH greater than 7. Include an equation in your answer.

A $0.10\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{d}{\mathrm{m}}^{-3}$ solution of diethylamine, $(\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2{)}_2\mathrm{N}\mathrm{H}$, has a higher pH than a $0.10\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{d}{\mathrm{m}}^{-3}$ solution of 2-aminobutane, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}(\mathrm{N}{\mathrm{H}}_2)\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$. Suggest why.

$(\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2{)}_2\mathrm{N}\mathrm{H}$ reacts with ethanoic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$. Complete the equation for this reaction. $$(\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2{)}_2\mathrm{N}\mathrm{H}+\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\to \text{..................................................................................}$$

$(\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2{)}_2\mathrm{N}\mathrm{H}$ reacts with ethanoyl chloride, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{l}$. Complete the equation for this reaction. $$(\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2{)}_2\mathrm{N}\mathrm{H}+\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{l}\to \text{....................................................................................}$$

Table 7.1 shows monomers that can undergo polymerisation. Complete Table 7.1. Table 7.1

Ethanedioic acid, $\mathrm{H}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$, can react with propane-1,3-diamine, ${\mathrm{H}}_2\mathrm{N}\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{N}{\mathrm{H}}_2$, to form polymer W. Draw a section of polymer W showing only one repeat unit. The new functional group formed should be displayed.

Poly(alkenes) biodegrade very slowly. Explain why.

Compound A is analysed by carbon-13 NMR and proton (${}^1\mathrm{H}$) NMR spectroscopy. State the reference substance and a solvent that can be used in NMR spectroscopy.

Predict the number of peaks in the carbon-13 NMR spectrum of A.

The proton (${}^1\mathrm{H}$) NMR spectrum of A shows peaks in four different chemical environments. Complete Table 8.1 for the proton (${}^1\mathrm{H}$) NMR spectrum of A. Table 8.1

The structures of the amino acids serine and lysine are shown in Fig. 9.1. Draw the structure for the dipeptide, ser–lys, with molecular formula ${\mathrm{C}}_9{\mathrm{H}}_{19}{\mathrm{N}}_3{\mathrm{O}}_4$. The peptide functional group formed should be displayed.

The isoelectric point of serine is 5.7 and of lysine is 9.7. State what is meant by isoelectric point.

A mixture of serine, lysine and ser–lys is analysed by electrophoresis using a buffer at pH 5.7. Draw and label three spots on Fig. 9.2 to indicate the predicted position of each of these three species, serine, lysine and ser–lys, after electrophoresis. Explain your answer.

Fig. 9.3 shows the synthesis of compound R from compound P. Draw the structure of Q in Fig. 9.3.

State the reagents and conditions for steps 1 and 2 in Fig. 9.3.