2019 SAJC H2 Chem Prelim P2 ANS

Prelims Paper 2 Answers 1. (a) P, Q, R and S are consecutive elements in Period 4. The table below shows the first four ionisation energies (in kJ mol–1) of the elements. Element 1st I.E. 2nd I.E. 3rd I.E. 4th I.E. 5th I.E. P 908 1730 3828 5980 7970 Q 577 1980 2960 6190 8284 R 762 1540 3300 4390 9020 S 947 1798 2735 4837 6043 (i) Explain why the second ionisation energy of Q is higher than that of R. Q is in group 13, there is a great jump in IE from 3rd IE to 4th IE Q: 4s24p1 Q+: 4s2 R: 4s24p2 R+ : 4s24p1 Q: the second electron is removed from the 4s orbital, whereas for R, the second electron is removed from the 4p orbital. The 4p orbital is further away from the nucleus than the 4s and experiences additional shielding effect by the two 4s electrons. These factors outweigh the effect of increase in nuclear charge from Q to R, resulting in a weaker attraction by nucleus . Less energy is required to remove an electron from 4p than the 4s orbital. [2] (ii) Explain why P is not considered to be a transition metal like most of the d -block metals. They do not form stable ions with a partially filled d subshell. [1] [1] (b) The idea of covalent bonding was first described in 1916 by an American physical chemist Gilbert Newton Lewis. HCOCl is an example of a polar covalent molecule. (i) Explain what is covalent bonding. Covalent bonding is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. [1] (ii) State what is meant by the term polar when applied to a covalent bond. [1]

A polar covalent bond is one in which the electron density is unequally shared due to the difference in electronegativity of atoms bonded, resulting in ∂+ and ∂ - across bond. (iii) State and explain with reference to the Valence Shell Electron Pair Repulsion theory, the shape of HCOCl molecule. 3 bond pair no lone pair so trigonal planar . The 3 electrons bond pairs arrange themselves to maximise stability and minimise electronic repulsion. [2] (iv) The molecule of HCOCl contains both σ (sigma) and π (pi) bonds. Draw labelled diagrams to show how orbitals overlap to form  a σ (sigma) bond  a π (pi) bond σ (sigma) bond is formed from head-on overlap of s orbital of H and sp2 of C or sp2 of O and sp2 of C or π (pi) bond is formed from side-on overlap of unhybridised p orbitals. [2] [Total:9] Unhybridised p orbitals of C Unhybridised p orbitals of O

2 Below is a labelled diagram of a hydrogen-oxygen fuel cell in an alkaline electrolyte. (i) Write the half equations for both electrodes and hence the overall equation. Cathode: O2(g) + 2H2O(l) + 4e- → 4OH-(aq) Anode: H2(g) + 2OH-(aq) → 2H2