A Level Chemistry 9701 Paper 43, May/June 2026

Write the equation to represent the lattice energy, $\mathrm{\Delta }{H}_{\text{latt}}$, of calcium sulfide, $\mathrm{C}\mathrm{a}\mathrm{S}$. Include state symbols.

Explain why the value of $\mathrm{\Delta }{H}_{\text{latt}}$ of $\mathrm{C}\mathrm{a}\mathrm{S}$ is negative.

State the order of the $\mathrm{\Delta }{H}_{\text{latt}}$ values of $\mathrm{C}\mathrm{a}\mathrm{S}$, $\mathrm{M}\mathrm{g}\mathrm{O}$ and $\mathrm{K}\mathrm{C}\mathrm{l}$, from most negative to least negative. Explain your answer.

The enthalpy change of formation, $\mathrm{\Delta }{H}_{\text{f}}$, of $\mathrm{C}{\mathrm{l}}^{-}(\mathrm{g})$ or ${\mathrm{M}\mathrm{g}}^{2+}(\mathrm{g})$ is the energy change when one mole of gaseous ions forms from the element in its standard state at $298\text{ K}$. enthalpy change of formation of chloride ions, $\mathrm{C}{\mathrm{l}}^{-}(\mathrm{g})=-243\text{ kJ}{\text{mol}}^{-1}$ enthalpy change of formation of magnesium ions, ${\mathrm{M}\mathrm{g}}^{2+}(\mathrm{g})=+2336\text{ kJ}{\text{mol}}^{-1}$ The enthalpy change of formation of $\mathrm{C}{\mathrm{l}}^{-}(\mathrm{g})$ can be calculated using two energy changes only. Identify the two energy changes.

The lattice energy of magnesium chloride, ${\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{l}}_2$, is $-2493\text{ kJ}{\text{mol}}^{-1}$. Use this value, and other data given, to calculate the enthalpy change of formation, in $\text{kJ}{\text{mol}}^{-1}$, of ${\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{l}}_2(\mathrm{s})$.

Magnesium carbonate, ${\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{O}}_3$, decomposes when heated. $${\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{O}}_3\to \mathrm{M}\mathrm{g}\mathrm{O}+{\mathrm{C}\mathrm{O}}_2\mathrm{\Delta }S=+175\text{ J}{\text{K}}^{-1}{\text{mol}}^{-1}$$ $$\mathrm{\Delta }H=+117\text{ kJ}{\text{mol}}^{-1}$$ Predict whether the decomposition of ${\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{O}}_3$ is feasible at $600{}^{\circ }\text{C}$. Use a calculation to explain your answer.

Calcium carbonate, ${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3$, also decomposes when heated. State whether the decomposition of ${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3$ occurs at a higher or lower temperature than that of ${\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{O}}_3$. Explain your answer.

Define pH mathematically.

Define $K_{\text{w}}$ mathematically.

Solution $\mathbf{A}$ is $100{\text{ cm}}^3$ of an aqueous solution of $0.105\text{ mol}{\text{dm}}^{-3}$ butanoic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$. $\mathbf{A}$ has pH $2.90$ at $298\text{ K}$. Calculate the concentration, in $\text{mol}{\text{dm}}^{-3}$, of ${\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})$ in $\mathbf{A}$.

Calculate the value of $K_{\text{a}}$ of $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$ at $298\text{ K}$.

Solution $\mathbf{B}$ is $100{\text{ cm}}^3$ of an aqueous solution containing $0.450\text{ mol}{\text{dm}}^{-3}$ $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$. Solution $\mathbf{C}$ is $200{\text{ cm}}^3$ of an aqueous solution containing sodium butanoate, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{N}\mathrm{a}$. $\mathbf{B}$ is added to $\mathbf{C}$ to produce $300{\text{ cm}}^3$ of solution $\mathbf{D}$. The concentration of ${\mathrm{H}}^{+}$ ions in $\mathbf{D}$ is $6.31\times {10}^{-5}\text{ mol}{\text{dm}}^{-3}$. Calculate the concentration of $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{N}\mathrm{a}$ in $\mathbf{C}$. (If you were unable to calculate an answer in 2(b)(ii), then use $3.00\times {10}^{-5}$ as the value of $K_{\text{a}}$ of $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$. This is not the correct answer.)

Write an equation for the reaction when ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4(\mathrm{a}\mathrm{q})$ is added to $\mathbf{D}$.

Write an equation for the reaction when $\mathrm{K}\mathrm{O}\mathrm{H}(\mathrm{a}\mathrm{q})$ is added to $\mathbf{D}$.

Predict the relative acidities of 2,2-dichlorobutanoic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{C}{\mathrm{l}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$, and butanoic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$. Explain your answer.

Complete Table 3.1. Table 3.1

In experiment 1, the rate of the reaction is $2.80\times {10}^{-10}\text{ mol}{\text{dm}}^{-3}{\text{s}}^{-1}$ when $[\mathrm{N}\mathrm{O}]$ is $5.00\times {10}^{-2}\text{ mol}{\text{dm}}^{-3}$ and $[{\mathrm{H}}_2]$ is $8.00\times {10}^{-2}\text{ mol}{\text{dm}}^{-3}$. Calculate the value of the rate constant, $k$, in experiment 1. State the units of $k$.

Experiment 2 is carried out under different conditions from experiment 1. The value of $k$ for experiment 2 is $2.13\times {10}^{-5}$. In experiment 2, a large excess of $\mathrm{N}\mathrm{O}$ is used. The initial value of $[{\mathrm{H}}_2]$ is $1.20\times {10}^{-3}\text{ mol}{\text{dm}}^{-3}$. A graph of $[{\mathrm{H}}_2]$ against time for experiment 2 shows that the reaction has a constant half-life. Explain why the graph shows a constant half-life, referring to: • the overall order of the reaction under these conditions • the reason why this is the overall order under these conditions.

Calculate the half-life, in s, of the reaction in experiment 2 and state the time taken, in s, for the value of $[{\mathrm{H}}_2]$ to fall to $7.50\times {10}^{-5}\text{ mol}{\text{dm}}^{-3}$.

The overall reaction of nitrogen monoxide with hydrogen has a three-step mechanism. $$2\mathrm{N}\mathrm{O}+2{\mathrm{H}}_2\to {\mathrm{N}}_2+2{\mathrm{H}}_2\mathrm{O}$$ Part of this mechanism is shown.

Complete the mechanism by writing an equation for step 2.

It can be deduced that step 1 is not the rate-determining step of the reaction. Explain why.

Complete the equation that can be used to calculate the standard Gibbs free energy change, $\mathrm{\Delta }{G}^{\ominus }$, for a cell reaction using the standard cell potential, $E_{\text{cell}}^{\ominus }$. $$\mathrm{\Delta }{G}^{\ominus }=$$

An electrochemical cell is set up using a ${\mathrm{V}}^{3+}\mathrm{/}{\mathrm{V}}^{2+}$ electrode and a ${\mathrm{C}\mathrm{u}}^{2+}\mathrm{/}\mathrm{C}\mathrm{u}$ electrode, both under standard conditions. Write an equation for the cell reaction that occurs and calculate the value for $E_{\text{cell}}^{\ominus }$.

Identify two atoms or ions from Table 4.1 that will reduce ${\mathrm{F}\mathrm{e}}^{3+}$ to ${\mathrm{F}\mathrm{e}}^{2+}$ under neutral conditions, but will not reduce $\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}{)}_3$ to $\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}{)}_2$ under alkaline conditions.

Identify the strongest oxidising agent and the strongest reducing agent in Table 4.1.

A dilute solution of $\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}(\mathrm{a}\mathrm{q})$ is electrolysed using inert electrodes. A steady current of $2.40\text{ A}$ is passed for $2.00\text{ hours}$. Calculate the mass, in g, of oxygen that is released at the cathode.

Cobalt is a transition element. It forms a wide range of coloured compounds that contain ${\mathrm{C}\mathrm{o}}^{2+}$ ions or ${\mathrm{C}\mathrm{o}}^{3+}$ ions. Explain why a transition element, such as cobalt, has variable oxidation states in its compounds.

Complete the electronic configurations of a $\mathrm{C}\mathrm{o}$ atom and a ${\mathrm{C}\mathrm{o}}^{2+}$ ion.

Explain why compounds containing ${\mathrm{C}\mathrm{o}}^{2+}$ ions are coloured.

A few drops of water are added to a piece of dry cobalt(II) chloride paper. Describe the colour change that is seen. Write an equation for the reaction that causes this colour change.

Copper is a transition element. A piece of copper is added to ${\mathrm{H}\mathrm{N}\mathrm{O}}_3(\mathrm{a}\mathrm{q})$. All the copper forms $\mathrm{C}\mathrm{u}({\mathrm{N}\mathrm{O}}_3{)}_2(\mathrm{a}\mathrm{q})$. An excess of $\mathrm{K}\mathrm{I}(\mathrm{a}\mathrm{q})$ is added to the $\mathrm{C}\mathrm{u}({\mathrm{N}\mathrm{O}}_3{)}_2(\mathrm{a}\mathrm{q})$ in a conical flask. All the copper present forms $\mathrm{C}\mathrm{u}\mathrm{I}$. The equation for the reaction is shown. $$\text{reaction 1}2\mathrm{C}\mathrm{u}({\mathrm{N}\mathrm{O}}_3{)}_2+4{\mathrm{I}}^{-}\to 2\mathrm{C}\mathrm{u}\mathrm{I}+{\mathrm{I}}_2+4{\mathrm{N}\mathrm{O}}_3^{-}$$ The ${\mathrm{I}}_2$ formed in this reaction is titrated with $0.200\text{ mol}{\text{dm}}^{-3}$ ${\mathrm{N}\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3(\mathrm{a}\mathrm{q})$. The end-point of the titration is reached when $37.60{\text{ cm}}^3$ of ${\mathrm{N}\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3(\mathrm{a}\mathrm{q})$ has been added. The equation for the reaction is shown. $$\text{reaction 2}2{\mathrm{S}}_2{\mathrm{O}}_3^{2-}+{\mathrm{I}}_2\to {\mathrm{S}}_4{\mathrm{O}}_6^{2-}+2{\mathrm{I}}^{-}$$ Describe two observations that can be made about the contents of the conical flask after $\mathrm{K}\mathrm{I}(\mathrm{a}\mathrm{q})$ has been added to $\mathrm{C}\mathrm{u}({\mathrm{N}\mathrm{O}}_3{)}_2(\mathrm{a}\mathrm{q})$.

Name a suitable indicator for this titration and describe the colour change at the end-point of the titration.

Name the type of reaction that occurs in both reactions 1 and 2.

Calculate the mass, in g, of the piece of copper added to ${\mathrm{H}\mathrm{N}\mathrm{O}}_3(\mathrm{a}\mathrm{q})$.

Explain what is meant by bidentate ligand.

One ${\mathrm{F}\mathrm{e}}^{3+}$ ion forms an octahedral complex with three ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}$ ions. State the formula and charge of this complex.

This complex shows stereoisomerism. Draw three-dimensional diagrams to show all the stereoisomers of this complex. The ${\mathrm{C}}_2{\mathrm{O}}_4^{2-}$ ion can be represented using O${\displaystyle ⌢}$O.

State the type or types of stereoisomerism shown by this complex.

${\mathrm{H}\mathrm{g}}^{2+}$ ions and iodide ions, ${\mathrm{I}}^{-}$, form a complex with the formula $[{\mathrm{H}\mathrm{g}\mathrm{I}}_4{]}^{2-}$. Write the expression for $K_{\text{stab}}$ of $[{\mathrm{H}\mathrm{g}\mathrm{I}}_4{]}^{2-}$. State its units.

Phenol, ${\mathrm{C}}_6{\mathrm{H}}_6\mathrm{O}$, can be made from benzene, ${\mathrm{C}}_6{\mathrm{H}}_6$, in a four-step process, as shown in Figure 7.1. $${\mathrm{C}}_6{\mathrm{H}}_6\stackrel{\text{step 1}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{O}}_2\stackrel{\text{step 2}}{\to }{\mathrm{C}}_6{\mathrm{H}}_7\mathrm{N}\stackrel{\text{step 3}}{\to }{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}_2\mathrm{C}\mathrm{l}\stackrel{\text{step 4}}{\to }{\mathrm{C}}_6{\mathrm{H}}_6\mathrm{O}$$ Figure 7.1 State the reagents and conditions for step 1.

State the reagents and conditions for step 3.

Name the compound ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{N}}_2\mathrm{C}\mathrm{l}$ formed in step 3.

Phenol reacts with propanone in the presence of an acid catalyst to form compound $\mathbf{G}$, as shown in Figure 7.2. Complete Table 7.1 to show the number of $\text{sp}$, ${\text{sp}}^2$ and ${\text{sp}}^3$ hybridised carbon atoms in propanone and in $\mathbf{G}$. Table 7.1

State the number of peaks in the carbon-13 NMR spectrum of $\mathbf{G}$.

The first stage of the mechanism for the reaction in Figure 7.2 involves the formation of ion $\mathbf{J}$. Complete the mechanism in Figure 7.3 to show the formation of $\mathbf{J}$ by adding dipoles and curly arrows, as appropriate.

The next stage of the mechanism involves the reaction between $\mathbf{J}$ and phenol to form intermediate $\mathbf{K}$. A proton is then lost from $\mathbf{K}$ to produce $\mathbf{G}$. Complete Figure 7.4 to show these stages of the mechanism. Include relevant curly arrows and charges.

Benzene is mixed with propanone in the presence of ${\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})$. Predict whether benzene reacts with propanone more readily, as readily or less readily than phenol does under the same conditions. Explain your answer.

$\mathbf{G}$ reacts with $\mathrm{B}{\mathrm{r}}_2(\mathrm{a}\mathrm{q})$ in the dark. The organic product of this reaction is compound $\mathbf{H}$. Complete the equation for this reaction in Figure 7.5.

Name the mechanism for this reaction.

Write an equation for the reaction of $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$ with $\mathrm{S}\mathrm{O}\mathrm{C}{\mathrm{l}}_2$ in step 1.

State an alternative reagent that reacts with $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$ to produce $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{l}$ in step 1.

Compound $\mathbf{L}$ is an amide. Draw the structure of $\mathbf{L}$ in Figure 8.1.

Draw the structure of the reagent added to $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{C}\mathrm{l}$ in step 2.

Name the mechanism of the reaction forming $\mathbf{L}$ in step 2.

Suggest a reagent for step 3.

The proton (${}^1\mathrm{H}$) NMR spectrum of $\mathbf{M}$ dissolved in ${\mathrm{D}}_2\mathrm{O}$ is obtained. Four of the proton environments, $\mathbf{r}$, $\mathbf{s}$, $\mathbf{t}$ and $\mathbf{u}$, are labelled on the structure of $\mathbf{M}$ in Figure 8.2. Complete Table 8.1 to identify the splitting patterns for proton environments $\mathbf{r}$, $\mathbf{s}$, $\mathbf{t}$ and $\mathbf{u}$. Table 8.1

The proton (${}^1\mathrm{H}$) NMR spectrum of $\mathbf{M}$ dissolved in ${\mathrm{C}\mathrm{D}\mathrm{C}\mathrm{l}}_3$ is obtained. State and explain how this spectrum differs from the proton (${}^1\mathrm{H}$) NMR spectrum of $\mathbf{M}$ dissolved in ${\mathrm{D}}_2\mathrm{O}$.

The structures of three amino acids, $\mathbf{P}$, $\mathbf{Q}$ and $\mathbf{R}$, are shown in Figure 9.1.

Figure 9.1 Two of the amino acids in Figure 9.1 show stereoisomerism. Draw the two stereoisomers of each of these amino acids in Figure 9.2. Name the type of stereoisomerism shown for each pair of stereoisomers. Use three-dimensional bonds where appropriate.

The structures of amino acids $\mathbf{T}$ and $\mathbf{U}$ and their isoelectric points are shown in Table 9.1. Explain what is meant by isoelectric point.

Complete Table 9.2 to show the predominant species formed by amino acids $\mathbf{T}$ and $\mathbf{U}$ at the pH values shown. Table 9.2

Amino acids $\mathbf{P}$, $\mathrm{N}{\mathrm{H}}_2\mathrm{C}\mathrm{H}(\mathrm{C}{\mathrm{H}}_3)\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$, and $\mathbf{R}$, $\mathrm{N}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$, can form tripeptide $\mathbf{W}$, ${\mathrm{C}}_8{\mathrm{H}}_{15}{\mathrm{N}}_3{\mathrm{O}}_4$. The structure of tripeptide $\mathbf{W}$ can be represented as $\mathbf{P}\text{–}\mathbf{R}\text{–}\mathbf{P}$. Draw the structure of $\mathbf{W}$. The peptide functional groups formed should be displayed.

$\mathbf{W}$ reacts under suitable conditions to form two molecules of $\mathbf{P}$ and one molecule of $\mathbf{R}$. Name the type of reaction.

Amino acids $\mathbf{P}$ and $\mathbf{R}$ can also form a number of polymers. One of these polymers, $\mathbf{X}$, has a structure that can be represented as —[ $\mathbf{P}\text{–}\mathbf{R}$ ]—${}_n$. Name the type of polymerisation involved in the formation of $\mathbf{X}$.