ACJC 2022 Prelim P2 Answers

© ACJC2022 9729/02/Prelim/2022 [Turn over ACJC solutions for H2 Chemistry Prelim Paper 2 2022 1 (a) Table 1.1 shows the solubility of two organic molecules at 25 °C. Table 1.1 solubility in water / g dm–3 propanone miscible chloromethane 5.04 (i) Identify the type of intermolecular force present between the molecules. propanone: ………… permanent dipole -permanent dipole ……………... chloromethane:…………permanent dipole -permanent dipole ………….[1] Comments:  Some students thought that propanone has hydrogen bonding between its molecules. (ii) Give a reason for the difference in their solubilities in water Propanone is able to form hydrogen bonds with water but chloromethane is unable to. Comments:  Students need to recognise that propanone is able to form hydrogen bonds with the oxygen atom of propanone.  Students need to highlight the differences in solute-solvent interactions when discussing solubility differences.  Some students did not understand that miscible means that propanone mixes very well with water. (b) The boiling point of three ligands are shown in Table 1.2. Table 1.2 ligand formula boiling point / °C water H2O 100 ammonia NH3 –33.3 hydrazine N2H4 114 (i) Explain what is meant by the term ligand. A ligand is a neutral molecule or anion species that has a lone pair of electrons to form a dative bond with a central metal atom or ion.

© ACJC2022 9729/02/Prelim/2022 [Turn over (ii) Explain the difference in the boiling points of the three ligands. Hydrazine has the highest boiling point as it has the largest/most polarisable electron cloud size has strongest instantaneous dipole-induced dipole. Water form more extensive hydrogen bonds than ammonia and hence it has a higher boiling point. Comments:  Students need to select and compare two molecules at each time, using the appropriate concept, rather than make general statements.  Some students were confused about the electronegativity difference between nitrogen and oxygen.  Some students wrongly thought that because hydrazine being a larger molecules will be able to form more extensive hydrogen bonds. Although hydrazine does have more hydrogens atoms that can be used to form hydrogen bonds, the limiting factor is the number of lone pair of electrons. Hydrazine has the same numb er of lone pair of electrons as water (2 x 1 on each N of hydrazine and 1 x 2 on O of water). (c) Polydentate ligands are ligands which form more than one bond with the metal atom or ion. Salicylaldehyde, ethane-1,2-diamine and H2salen are examples of such ligands. H2salen can be synthesised from salicylaldehyde and ethane-1,2-diamine. When the phenolic groups of H 2salen are deprotonated, salen 2– acts as a ligand. It has a high affinity for Co2+ ions and forms a planar complex Co(sale