HCI 2021 Prelim Paper 1 Solutions

2021 HCI C2 H2 Chemistry Preliminary Exam / Paper 1 ANSWERS: Comments 1 B In order to find the number of valence electrons in the G+ ion, we take the difference between all the successive ionisation energies to find the one with the greatest difference. No of electrons removed 1 2 3 4 5 6 Ionisation energy/ kJ mol –1 1907 2914 4964 6274 21 267 25 431 Difference in IE 1007 2050 1310 14993 4164 After analysing the table, we can see that the greatest jump in ionisation energy is when the 5th electron is removed. This means that the G+ ion has 4 electrons in its valence shell. Therefore, G would have 5 electrons in the valence shell, and belong to group 15. 2 A 1 HCHO, CH3OH HCHO CH3OH Intermolecular force: Permanent dipole-permanent dipole interactions Intermolecular Force: Hydrogen bonding CH3OH has the higher boiling point. (strength of dispersion forces similar since number of electrons for each molecule is similar.) HWA CHONG INSTITUTION 2021 C2 H2 CHEMISTRY PRELIMINARY EXAM SUGGESTED SOLUTIONS 1 2 3 4 5 6 7 8 9 10 B A A C C D B C D D 11 12 13 14 15 16 17 18 19 20 A D C A A D B B C D 21 22 23 24 25 26 27 28 29 30 C C D C B B A D B B

2 BF3: Simple covalent compound. Dispersion forces present between the non-polar molecules of BF3. AlF3: Ionic compound. Ionic bonds present between oppositely charged ions. AlF3 has the higher boiling point. 3 CH3CH2CH2CH2CH3, C(CH3)4 Both compounds are simple non-polar covalent molecules, with same number of electrons. However, C(CH3)4 is a branched chain hydrocarbon while CH3CH2CH2CH2CH3 is a straight chain hydrocarbon. CH3CH2CH2CH2CH3 has linear shape and greater surface area of contact between molecules, hence more extensive dispersion forces. CH3CH2CH2CH2CH3 has the higher boiling point. 3 A Option A: Both NH3 and HF can form only one hydrogen bond on average per molecule. Hence while it is true that HF has a higher boiling point than NH 3, it is because the hydrogen bonds between HF molecules are stronger than the hydrogen bonds between NH3 molecules. F more electronegative than N, so there is higher partial positive charge on the H of HF, hence the hydrogen bonds between HF molecules are stronger. Option B: In the solid state, H2O molecules are held at fixed positions and arranged in an orderly manner to form a regular lattice such that hydrogen bonding is maximised to four per molecule. The hydrogen bonding between water molecules in ice are positioned in a roughly tetrahedral shape around each O atom. This produces an open lattice, with empty spaces between the H2O molecules. The more random arrangement of hydrogen bonding in liquid water results in H2O molecules packing much more closely and, together, take up less space. So the lattice structure of ice occupies a larger volume for the same mass of liquid water, hence ice has a lower density than liquid water. Option C: CH3CO2H d