A Level Chemistry 9701 Paper 23, Oct/Nov 2025

Cr exists as four stable isotopes. The most common isotope of Cr is chromium-52. Determine the number of protons, neutrons and electrons in an atom of chromium-52. number of protons .................. neutrons .................. electrons .........................................

Describe how an atom of chromium-54 differs from an atom of chromium-52. Refer to numbers of particles in your answer.

The relative isotopic masses of the isotopes of Cr can be determined using mass spectrometry. State what other information is needed to calculate the relative atomic mass, $A_r$, of Cr.

Atoms of ${}^{52}\mathrm{C}\mathrm{r}$, ${}^{53}\mathrm{C}\mathrm{r}$ and ${}^{54}\mathrm{C}\mathrm{r}$ make up more than 95% of naturally occurring chromium atoms. The $A_r$ of naturally occurring Cr is 51.996. Suggest what these statements imply about the relative isotopic mass of the fourth stable isotope of chromium.

The shorthand electronic configuration of chromium is [Ar] $3{\mathrm{d}}^{5}4{\mathrm{s}}^1$. Complete the full electronic configuration of chromium. ........................................................................... $3{\mathrm{d}}^{5}4{\mathrm{s}}^1$

Deduce the total number of unpaired electrons in an atom of chromium.

Acidified dichromate(VI) ions will convert methanal, $\mathrm{C}{\mathrm{H}}_2\mathrm{O}$, to carbon dioxide. The movement of electrons to or from relevant species is shown in the following half-equations. half-equation 1 $\mathrm{C}{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}$ half-equation 2 $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}+14{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to 2{\mathrm{C}\mathrm{r}}^{3+}+7{\mathrm{H}}_2\mathrm{O}$ Identify the species that is reduced in half-equation 2. Explain your answer.

The oxidation state of the carbon atom in methanal is 0. Calculate the oxidation state of carbon in carbon dioxide.

Construct the ionic equation for the reaction of dichromate(VI) ions with methanal in acidic conditions.

Methanal is a liquid at $-30{}^{\circ }\mathrm{C}$ but carbon dioxide is a gas at this temperature. Explain why.

In acidic conditions, a dynamic equilibrium is established between $\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{2-}(\mathrm{a}\mathrm{q})$ and $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}(\mathrm{a}\mathrm{q})$. $$2\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{2-}(\mathrm{a}\mathrm{q})+2{\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})\rightleftharpoons \mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}(\mathrm{a}\mathrm{q})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\mathrm{\Delta }H>0$$ State what is meant by dynamic equilibrium.

Identify the condition necessary to establish dynamic equilibrium.

$\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{2-}(\mathrm{a}\mathrm{q})$ ions are yellow and $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}(\mathrm{a}\mathrm{q})$ ions are orange. State what is observed when the following changes are made to an equilibrium mixture of acidified $\mathrm{C}\mathrm{r}{\mathrm{O}}_4^{2-}(\mathrm{a}\mathrm{q})$ and $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}(\mathrm{a}\mathrm{q})$ ions. Explain your answers. • The equilibrium mixture is warmed gently. observation ................................................................................................................ explanation ................................................................................................................ .................................................................................................................................... .................................................................................................................................... • Dilute $\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})$ is added to the equilibrium mixture. observation ................................................................................................................ explanation ................................................................................................................ .................................................................................................................................... ....................................................................................................................................

Chromium(IV) fluoride, $\mathrm{C}\mathrm{r}{\mathrm{F}}_4$, is a covalent molecule that shows similar chemical properties to $\mathrm{S}\mathrm{i}\mathrm{C}{\mathrm{l}}_4$. Suggest the type of reaction that occurs when $\mathrm{C}\mathrm{r}{\mathrm{F}}_4$ is placed in water. Construct a relevant equation for this reaction. type of reaction ......................................................................................................................... equation ....................................................................................................................................

Fig. 2.1 shows the variation in atomic and ionic radii of the Period 3 elements Na to Cl. The ionic radius of Si is not shown. Explain the trend shown in the atomic radii of the Period 3 elements Na to Cl.

Explain why there is a large difference in the ionic radii of Al and P.

Table 2.1 gives some information about some of the Period 3 oxides. Row B gives the pH of the solution that forms when the Period 3 oxide is added to water. Table 2.1 Complete Table 2.1.

State why there is no data given in row B for $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ and $\mathrm{S}\mathrm{i}{\mathrm{O}}_2$.

Write an equation for the reaction of $\mathrm{N}{\mathrm{a}}_2\mathrm{O}$ with dilute hydrochloric acid.

Construct an equation for the reaction of $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ with a base to form $\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_2$.

Group 2 nitrates decompose on heating to form oxides. State the trend in thermal stability of the Group 2 nitrates down the group.

Identify the other products of the thermal decomposition of Group 2 nitrates.

Define empirical formula.

Cyclopentane, ${\mathrm{C}}_5{\mathrm{H}}_{10}$, has four cyclic structural isomers. One of these isomers is C, shown in Fig. 3.1. Complete Fig. 3.1 to show two other cyclic structural isomers of ${\mathrm{C}}_5{\mathrm{H}}_{10}$.

Cyclopentane reacts with $\mathrm{C}{\mathrm{l}}_2$ in the presence of ultraviolet light to form ${\mathrm{C}}_5{\mathrm{H}}_9\mathrm{C}\mathrm{l}$. The reaction is initiated by the bond fission of $\mathrm{C}{\mathrm{l}}_2$. State the type of bond fission shown in the initiation step.

Complete the equations to show the two propagation steps that follow the initiation step. propagation 1 ${\mathrm{C}}_5{\mathrm{H}}_{10}+$ ........................................ $\to {\mathrm{C}}_5{\mathrm{H}}_9^{\bullet }+$ ........................................ propagation 2 ${\mathrm{C}}_5{\mathrm{H}}_9^{\bullet }+$ ........................................ $\to$ ......................................................

The final step is shown. $${\mathrm{C}}_5{\mathrm{H}}_9^{\bullet }+\mathrm{C}{\mathrm{l}}^{\bullet }\to {\mathrm{C}}_5{\mathrm{H}}_9\mathrm{C}\mathrm{l}$$ Give the name for this step in the reaction.

Fig. 3.2 shows a reaction cycle involving cyclopentane, cyclopentene and ${\mathrm{C}}_5{\mathrm{H}}_9\mathrm{C}\mathrm{l}$. Identify a suitable reagent for reaction 3.

Use the data in Fig. 3.2 and in Table 3.1 to calculate the enthalpy change of reaction 2, $\mathrm{\Delta }{H}_2$. Table 3.1 $\mathrm{\Delta }{H}_2=$ ........................................... $\mathrm{k}\mathrm{J}\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}$

Cyclopentene, ${\mathrm{C}}_5{\mathrm{H}}_8$, reacts with hot concentrated acidified $\mathrm{K}\mathrm{M}\mathrm{n}{\mathrm{O}}_4$ to form compound W, ${\mathrm{C}}_5{\mathrm{H}}_8{\mathrm{O}}_4$. Draw the structure of W.

The infrared spectrum of W is shown in Fig. 3.3. Identify two absorptions in the infrared spectrum of W that would not be present in the infrared spectrum of cyclopentene. • Write 1 or 2 on Fig. 3.3 against each of these two absorptions. • Complete Table 3.2 to show which bond is responsible for each absorption that you have identified in Fig. 3.3. Table 3.2

Identify the type of reaction that occurs in reaction 1.

Suggest a suitable reagent for reaction 2.

Reaction 3 is an elimination reaction. Write an equation for this reaction and identify the solvent and conditions used. equation ............................................................................................................................ solvent and conditions .......................................................................................................

${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{H}$ can be directly converted to ${\mathrm{C}}_3{\mathrm{H}}_6$. Suggest the reagent and conditions for this conversion.

Under suitable conditions, propene polymerises to form poly(propene). Poly(propene) exhibits stereoisomerism. Identify the type of polymerisation that forms poly(propene) from propene.

Define stereoisomerism.

Draw a section of poly(propene), showing two repeat units. Use your diagram to identify the type of stereoisomerism shown by poly(propene). Explain your answer. section of poly(propene)

State two difficulties associated with the disposal of poly(propene). 1 ........................................................................................................................................ 2 ........................................................................................................................................

Under different conditions, two molecules of propene can combine to form compounds X and Y. Name X.

Fig. 4.2 shows the mass spectrum of either compound X or compound Y. Identify which of X and Y gives this mass spectrum. Give one reason for your answer, referring to the fragmentation pattern.

The molecular ion peak in the spectrum in Fig. 4.2 has relative abundance 34.7. Calculate the relative abundance of the [M+1]${}^{+}$ peak in this spectrum. relative abundance of [M+1]${}^{+}$ peak = .........................................................