ASR 2020 J2Prelim H2Chem P2 MS

1 ASRJC JC2 PRELIM 2020 9729/02/H2 Anderson Serangoon Junior College 2020 JC2 Preliminary Examinations H2 Chemistry (9729) Suggested Mark Scheme for Paper 2 1 One early nineteenth century Periodic Table had copper placed in the same group as potassium because they both formed +1 ions. (a) State the full electronic configurations of potassium and copper atoms. [1] K: 1s2 2s2 2p6 3s2 3p6 4s1 Cu: 1s2 2s2 2p6 3s2 3p6 3d10 4s1 (b) (i) The mass of an atom is the sum of the masses of all subatomic particles it contains. Calculate the mass, in kg, of one atom of 64 29Cu . Quote relevant values from the Data Booklet and give your answer to three significant figures. [2] No of protons in 64 29Cu = 29 No of electrons in 64 29Cu = 29 No of neutrons in 64 29Cu = 35 Mass of one atom 64 29Cu = (29x1.67x10–27) + (29x9.11x10–31) + (35x1.67x10–27) = 1.07 x 10–25 kg (ii) During the process of ionisation, a Cu atom loses an electron. Cu(g)  Cu+(g) + e− State the electronic configuration of Cu+. Explain your answer. [2] 1s2 2s2 2p6 3s2 3p6 3d10 explanation: It is the highest energy orbital / the electron in this orbital is less attracted to the nucleus.

(iii) On the Cartesian axes given in Fig . 1.1, sketch the shape of the following three 3d orbitals found in copper atom. Fig. 1.1 [2] x y z x y z x y z 3dxz 3dz2 3dx2–y2

ASRJC JC2 PRELIM 2020 9729/02/H2 (c) (i) Define the term standard enthalpy change of formation of a substance. [1] The amount of heat evolved or absorbed when one mole of a substance is formed from its constituent elements, all in their standard states at 298 K and 1 bar. (ii) Table 1.1 enthalpy change value / kJ mol–1 standard enthalpy change for P(s) + 2O2(g) + 3e–  PO43–(aq) –1284 standard enthalpy change for K(s)  K+(aq) + e– –251 standard enthalpy change for K3PO4(s)  3K+(aq) + PO43–(aq) –2 Using a labelled energy cycle, and the enthalpy values given in Table 1.1, determine the standard enthalpy change of formation of solid potassium phosphate, K3PO4. [3] –2 K3PO4 (s) 3K+ (aq) + PO43– (aq) ? 3(–251) –1284 3K (s) + P (s) + 2O2 (g) –2 + Hf (K3PO4) = [3 x (–251) + (–1284)] = –2035 kJ mol–1 (iii) The value of GO at 298 K for K(s)  K+(aq) + e– is –284 kJ mol–1. Calculate SO for the reaction, and explain its sign.