2024 Prelims HCI H2 Chem P2 (Ans)

2024 HCI C2 H2 Chemistry Prelims / Paper 2 HWA CHONG INSTITUTION 2024 C2 H2 CHEMISTRY PRELIMINARY EXAMINATION SUGGESTED SOLUTIONS Paper 2 1 (a) 3d ___ ___ ___ ___ ___ 3p ___ ___ ___ 3s ___ [1] Correct number of orbitals for each subshell with labels [1] Correct relative energy gap (bigger gap between 3s and 3p than 3p and 3d) (b) (i) First ionisation energy is the energy required to remove one mole of electrons from one mole of free gaseous atoms to form 1 mole of unipositively charged gaseous ions. (b) (ii) 1st IE of Se is lower than As [1/2] Selenium has a lower 1st IE than arsenic because selenium contains a paired electron in one of its p orbitals, thus it experiences inter-electron repulsion. So, less energy is required to remove the outermost electron in selenium compared to arsenic. [1] Explains that inter -electron repulsion in p -orbital leads to lower 1st IE in Se compared to As. 1st IE of Kr is higher than Br [1/2] Krypton has a higher 1st IE than bromine because krypton has a greater effective nuclear charge than bromine. This is inferred from the fact the krypton has a larger proton number / greater nuclear charge , and electrons are added to the same quantum shell / leading to a relatively constant shielding effect. Therefore, the attraction between the nucleus and the outermost electron is stronger, and more energy is required to remove the outermost electron in krypton than bromine. [1] Uses effective nuclear charge to explain the stronger attraction between the outermost electron in Kr and the nucleus compared to that in Br, leading to higher 1st IE. 2 (a) (i) Trigonal planar [1], 120 [1] energy

1 2 (a) (ii) One 2s orbital is mixed with two 2p orbitals to give 3 hybridised sp2 orbitals. [1/2] correct type of atomic orbitals (2s and 2p) [1/2] correct number of atomic orbitals (1 and 2 respectively) (a) (iii) The three sp2 hybridised orbitals are degenerate and will be equally spaced apart in a trigonal planar arrangement to minimise repulsion. [1/2] Indication of number of hybrid orbitals (state ‘three’ or ‘trigonal planar’) [1/2] Applying VSEPR Theory in the context of hybrid orbitals (i.e. hybrid orbitals will be equally spaced apart / minimise repulsion). (b) PTFE has a greater number of electrons / larger electron cloud than F2C=CF2 [1], thus it is more polarisable and has stronger dispersion forces than F2C=CF2. More energy is required to overcome the stronger dispersion forces in PTFE [1], and so it has a higher boiling point, and is a solid at room temperature while F2C=CF2 is a gas. (c) The C–C and C–F bonds in PFAS are very strong [1] and do not break/hydrolyse in typical environmental conditions. (d) (i) RO• + CHCl=CH2 → RO–CHCl–CH2• [1] RO–CHCl–CH2• + CHCl=CH2 → RO–CHCl–CH2–CHCl–CH2• [1] (d) (ii) The carbon atom is chiral because it has a tetrahedral geometry [1/2] and is bonded