2024 TMJC H2 Chem Prelim P2 (Ans)

1 Some main group elements are similar to transition elements as they are capable of showing different oxidation states. (a) Fig. 1.1 shows a sketch of the nine successive ionisation energies of element A, a main group element. Fig. 1.1 (i) Write the equation for the third ionisation energy of element A. [1] (ii) Explain the general trend in successive ionisation energy shown in the graph of Fig. 1.1. [2] 1 2 3 4 5 6 7 8 9 log (IE) number of electrons removed Marker’s Comments • This question was generally well attempted, errors include balancing the electron wrongly, not remembering the definition of ionisation energy and not including the state symbols. A2+ (g) → A3+ (g) + e− [1] for correct equation with state symbols There is a general increase in successive ionisation energy. This is due to increasing amount of energy required to remove successive electrons from an increasingly positive ion [1] due to an increasing strength of electrostatic forces of attraction between the nucleus and valence electrons. [1] Marker’s Comments • Candidates need to understand the question was referring to the general trend of successive ionisation energies, not trend across the period. • A significant number also tried to explain about the large increase between 6th and 7th IE, not realising that later part in (iii) was asking about similar concept. • Some errors included not understanding that the nuclear charge did not increase (same number of protons) in A, and trying to explain with effective nuclear charge as an approach. • Strong answers included the increasing positive ion that is formed and trying to remove an electron from this would require increasing energy.

2024 H2 Chem Paper 2 (Suggested Ans) 2 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry (iii) Suggest, with reasoning, the group number of element A and state its identity, given that it belongs to Period 4. [2] 4 ✓ [2] 2✓ [1] Transition elements form complexes that are responsible for the colour of many gemstones such sapphires and rubies. (b) Define the term transition element. [1] (c) In an isolated gas phase transition metal ion, the d-orbitals have the same energy. However, in the octahedral complex ion, [Cr(H2O)6]3+, the d subshell is spilt into two energy levels. (i) Complete the electronic configuration of Cr and Cr3+. Cr : 1s2 Cr3+ : 1s2 [1] The biggest increase between 6th and 7th ionisation energy is due to the fact the 6th electron is removed from the inner quantum shell ✓ which is closer to the nucleus. Hence, there is stronger electrostatic force of attraction between the 6th electron and nucleus and a lot more energy ✓is required to remove it. Element A belongs to Group 16 ✓ and is selenium ✓. A transition element is a d-block element that forms at least one stable ion with partially-filled d subshell. [1] 2s2 2p6 3s2 3p6 3d5 4s1 2s2 2p6 3s2 3p6 3d3 [1] for both Marker’s Comments • Common errors included not answering either the group number or identity of A. Candidates need to be more careful in their answering of questions and not miss out parts of the question requirements. • Emphasis of this question was on the large increase between 6 th and 7 th IE which enable the determination of the group number. • Candidates also need to be updated with writing down the correct Group Number of the Periodic Table, it is Group 16 and not Group 6. Marker’s Comments • Candidates need to realise that there is additional stability for half-filled d-orbitals for Cr, hence electronic configuration should be [Ar]3d54s1 instead of [Ar]3d44s2.

2024 H2 Chem Paper 2 (Suggested Ans) 3 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry (ii) Explain why the splitting of d-orbitals occurs in the presence of ligands. [1] (iii) Draw and name the shape of one of the d orbitals in the lower energy level and one of the d orbitals in the upper energy level for the octahedral complex ion. Indicate the axes labels clearly. Name of lower energy d orbital: __________________ Name of upper energy d orbital: __________________ [2] Electron pairs in d orbitals that are near / pointing towards lone pairs of the ligands are repelled to higher energy level. [1] OR different extent of repulsion towards the ligands OR words to that effect. dxy (dxz OR dyz are also acceptable) correct shape + label [1] lobes in between the axes lobes along the axes dz2 OR dx2 – y2 correct shape + label [1] Marker’s Comments • Emphasis of this question was about the different extent of repulsion between the electrons in d orbitals and lone pairs of electrons in ligands due to orientation of the ligands and orbitals. • A significant number of candidates tried to answer the question by rephrasing the question without addressing the ‘why’. Marker’s Comments • Quite a number of candidates did not obtain the marks for this bookwork question about the label and drawing of d orbitals. They need to realise the name of the d orbitals will need to be in sync with the axis’s labels.

2024 H2 Chem Paper 2 (Suggested Ans) 4 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry (iv) Cr3+ ions forms a complex that gives rise to the red colour of rubies. Explain how the colour arises. [2] (d) Chromium(III) picolinate, Cr(C 5H4N(COO))3, another bright red complex , is used to treat Type 2 diabetes. Picolinate is bidentate and forms an octahedral complex ion with chromium(III). The nitrogen atom in the benzene ring of picolinate has a lone pair of electrons available for coordination. picolinate ligand Using the information provided, draw the octahedral structure of chromium( III) picolinate in the space below showing the coordinate bonds clearly. [1] [1] [Total: 13] When an electron in lower energy d-orbital moves to upper energy level, it absorbs light energy/ energy from visible spectrum/ a photon. [1] Since energy absorbed corresponds mainly to the green region of the visible spectrum, the complex appears red (the complementary colour of the colour absorbed). [1] Marker’s Comments • One major misconception is a significant portion of candidates mentioned that d-orbitals absorb energy and got promoted instead of electrons. Marker’s Comments • The lone pair of electrons on N (indicated in question stem) and the lone pair of electrons on the –O– of the carboxylate anion are more available for coordination (dative bonding). Since picolinate ligand is bidentate, 3 moles of picolinate ligands are needed to form octahedral structure of chromium(III) picolinate. • Candidates need to take note of the distribution of charges and be accurate in their presentation of their structures, showing the shape accurately. • The coordinate bonds should also be reflected as arrows without the inclusion of the l one pair of electrons.

2024 H2 Chem Paper 2 (Suggested Ans) 5 Tampines Meridian Junior College 2024 JC2 Preliminary Examination H2 Chemistry 2(a) Magnesium oxide reacts reversibly with chlorine according to the following equation. equation 2.1 2MgO(s) + 2Cl2(g) 2MgCl2(s) + O2(g) Under certain conditions, a dynamic equilibrium is established. (i) Explain what is meant by the term dynamic equilibrium. [1] (ii) At 1.00  105 Pa and 500 K, 70% of the initial amount of Cl2(g) has reacted with MgO. Calculate the equilibrium constant, Kp for the reaction. [2] Dynamic equilibrium is a state in a reversible reaction where the rate of forward reaction is equal t