2024 ASRJC H2 Chem P2 (Ans)

ASRJC JC2 PRELIM 2024 9729/02/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2024 JC 2 PRELIMINARY EXAMINATION CHEMISTRY 9729/02 Paper 2 Structured Questions SUGGESTED SOLUTIONS Answer all the questions. 1 (a) Phosphorus, sulfur and chlorine are Period 3 elements of the Periodic Table. Table 1.1 shows some properties of the three elements. Table 1.1 P S Cl number of electrons in 3p subshell number of unpaired electrons (i) Complete Table 1.1 to show the number of electrons in the 3p subshell and the number of unpaired electrons in an atom of P, S and Cl. [2] P [Ne]3s23p3 S [Ne]3s23p4 Cl [Ne]3s23p5 number of electrons in 3p subshell 3 4 5 number of unpaired electrons 3 2 1 (ii) With reference to the Data Booklet, state and explain the trend of the ionic radius of P3–, S2– and Cl–. [2] • Ionic radius decreases from P3– (0.212 nm), S 2– (0.184 nm) and C l– (0.181 nm) • Nuclear charge increases from P3– to Cl–. • Number of filled quantum (electron) shells and shielding effect remains the same since the anions (P3– to Cl–) are isoelectronic. • Therefore, the stronger (electrostatic) forces of attraction between the nucleus and the outer electrons results in the decreasing ionic radius.

(b) Phosphoryl chloride, POCl3, is a colourless liquid that is used to make phosphate esters. P O Cl Cl Cl POCl3 has similar chemical properties as PC l5. It has a melting point of 1 °C and a boiling point of 106 °C. It also reacts vigorously with water, forming misty fumes and an acidic solution of H3PO4. (i) Explain how the information in (b) suggests that the structure and bonding of POCl3 is simple covalent. [2] POCl3 has low melting and boiling point suggests that it has weak instantaneous dipole-induced dipole attraction / permanent dipole-permanent dipole attraction between molecules that required low amount of energy to overcome. [1] The vigorous reaction with water suggested that hydrolysis had taken place. [1] Thus, the structure and bonding of POCl3 is likely to be simple covalent. (ii) Write a balanced equation for the reaction of POCl3 with water. [1] POCl3 + 3H2O H3PO4 + 3HCl [1] (iii) In H3PO4, there is no hydrogen atom directly bonded to the phosphorus atom. Draw the ‘dot and cross’ diagram of H 3PO4 and state the shape of the molecule with respect to P. [2] P O O O O xx xx xx x x x xx x x x xx xx x x x x x x H H H [1] Shape: tetrahedral [1] (c) Phosphoryl chloride, POC l3, is manufactured industrially from phosphorus trichloride and oxygen as shown in equation 1.1. equation 1.1 2PCl3(g) + O2(g) 2POCl3(g) The standard enthalpy changes of formation for these species are shown in Table 1.2. Table 1.2 Enthalpy change of formation of PCl3(g) −289 kJ mol–1 Enthalpy change of formation