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 a