2023_DHS_H2_Chem_Prelim_Paper_1_Worked_Solutions

2023 Y6 Preliminary Examination H2 Chemistry 9729 Paper 1 Suggested Solutions  DHS Chemistry Unit Page 1 of 6 Answer Key 1 2 3 4 5 6 7 8 9 10 C A B C D C C D A B 11 12 13 14 15 16 17 18 19 20 D B A C C D D B D A 21 22 23 24 25 26 27 28 29 30 B A D C B A A D B C 1 C Since atomic radius of X is larger than that of sulfur, X is the element below sulfur in Group 16 , selenium. Since atomic radius of Y is larger than that of X, Y precedes X in the same period and is arsenic. Its proton number is 33. 2 A Se: [Ar]3d104s24p4 Br: [Ar]3d104s24p5 Kr: [Ar]3d104s24p6 The 7th electron removed from Se is from an inner shell (n = 3) as compared to those removed from Br and Kr (n = 4). The inner shell electron is more strongly attracted to the nucleus and requires more energy for its removal. Kr has one more proton and a higher nuclear charge than Br. Since the additional electron in Kr is added to the same valence shell (n = 4), the valence electrons in Kr experience a shielding effect similar to those in Br. Thus, Kr has a higher effective nuclear charge than Br and more energy is required to remove the 7 th electron from Kr. FYI: element Se Br Kr 7th IE / kJ mol−1 14990 9940 10710 3 B Hexane molecule is non -polar and has id -id interactions between molecules. W ater molecule is polar and has hydrogen bonds between molecules. Hence statement 2 is correct. Hexane and water molecules form weak id -id interactions which do not release sufficient energy to overcome the stronger hydrogen bonds between water molecules. Hence statement 4 is correct. As a result , hexane and water molecules do not mix well and hexane is immiscible with water.  1 Hexane has a larger, more polarisable electron cloud than water, leading to stronger id -id interactions between hexane molecules. This statement is true but it does not help to explain the observation.  3 O–H bond has a larger bond energy than C–H bond so it is a true statement but it does not help to explain the observation as these covalent bonds are not broken or formed when hexane and water are mixed. 4 C  A Ice has a simple molecular structure with hydrogen bonds between water molecules. There are covalent bonds between atoms in the water molecules.  B Iodine, I2, has a simple molecular structure and is a non-polar molecule. There exists id -id interactions between molecules and covalent bond between iodine atoms in a molecule.  C Sodium sulfate, Na 2SO4, has a giant ionic structure with ionic bonds between oppositely charged Na + and SO 4 2− ions. Within the SO 4 2− ion, there are covalent bonds between S and O atoms.  D Copper has a giant metallic structure with metallic bonds between the positively charged metal cations and sea of delocalised electrons. 5 D pV = nRT  Mr = mRT/(pV) Mr If