Home / AS & A Level Chemistry 9.1 Periodicity of physical properties of the elements in Period 3: Exam Style Questions Paper 2

AS & A Level Chemistry 9.1 Periodicity of physical properties of the elements in Period 3: Exam Style Questions Paper 2

A Level Chemistry Paper 2 - All Chapters
Question

Cobalt, rhodium and iridium are metals in the same group of the Periodic Table.
(a) The shorthand electronic configuration of cobalt is \([Ar]3d^74s^2\).
(i) Identify what is meant by [Ar] by giving its full electronic configuration.
(ii) The lowest-energy electrons in cobalt are in the 1s orbital. Draw the shape of a 1s orbital.

(iii) Deduce the number of unpaired electrons in a cobalt atom.
(b) Table 1.1 gives some details of the stable naturally occurring isotopes of rhodium and iridium.

Complete Table 1.1.

(c) Table 1.2 shows the relative abundances of isotopes in a sample of an alloy containing rhodium and iridium only.

(i) Define relative isotopic mass.
(ii) Use Table 1.2 to calculate the relative atomic mass, Ar, of iridium in the alloy. Give your answer to two decimal places.

(d) Hydrated rhodium(III) chloride, RhCl₃•xH₂O, catalyses the conversion of ethene to but-2-ene. Both stereoisomers of but-2-ene are formed in the reaction.
(i) Hydrated rhodium(III) chloride contains 20.5% by mass of water of crystallisation. Deduce the integer value of x in RhCl₃•xH₂O. Show your working.

(ii) Define stereoisomers.
(iii) Explain how the conversion of ethene to but-2-ene can be described as an addition reaction.
(iv) Draw the two stereoisomers of but-2-ene.

▶️ Answer/Explanation
Solution

(a)(i) \([Ar]\) represents the electronic configuration of argon: \(1s^22s^22p^63s^23p^6\).

Explanation: The shorthand notation \([Ar]\) stands for the noble gas argon’s full configuration, which serves as the core electrons for cobalt.

(a)(ii) The shape of a 1s orbital is a sphere.

Explanation: The 1s orbital is spherically symmetric, representing the region where the electron is most likely to be found.

(a)(iii) Cobalt has 3 unpaired electrons.

Explanation: The \(3d^7\) configuration has 3 unpaired electrons (Hund’s rule: electrons occupy degenerate orbitals singly before pairing).

(b)

Explanation: The table is completed using the given isotopic data, calculating missing values such as nucleon numbers and percentages where necessary.

(c)(i) Relative isotopic mass is the mass of an isotope relative to 1/12th the mass of a carbon-12 atom.

Explanation: It is a dimensionless quantity comparing the mass of an isotope to the standard carbon-12 scale.

(c)(ii) The relative atomic mass of iridium is 192.22.

Explanation: Calculated as \((191 \times 0.373) + (193 \times 0.627) = 192.22\) (weighted average of isotopic masses).

(d)(i) \(x = 3\).

Explanation: Given 20.5% water by mass, solving \(\frac{18x}{209.5 + 18x} = 0.205\) yields \(x \approx 3\).

(d)(ii) Stereoisomers are compounds with the same structural formula but different spatial arrangements of atoms.

Explanation: They differ in the 3D orientation of groups around a double bond or ring.

(d)(iii) The reaction combines two ethene molecules (\(C_2H_4\)) to form but-2-ene (\(C_4H_8\)), adding atoms without losing any.

Explanation: Addition reactions involve the combination of molecules to form a single product.

(d)(iv)

Explanation: The two stereoisomers of but-2-ene are the cis (same side) and trans (opposite sides) forms.

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