RI 2019 Prelim P1 Ans

1 2019 Year 6 H2 Chemistry Preliminary Examinations Paper 1 Suggested Solutions Question 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Answer D A B B C A D A C D C D C B B Question 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Answer D C C A D B D D A C A B C A B 1(D) A Incorrect. The number of protons determine the identity of the atom/ion. Since both 65Cu2+ and 65Cu+ are ions of copper, they both have 29 protons. B Incorrect. 63Cu has 29 electrons and has the electronic configuration [Ar]3d 104s1. Hence, 63Cu+ has the electronic configuration [Ar]3d10. C Incorrect. No. of neutrons in 63Cu+ = 63 – 29 = 34 No. of neutrtons in 65Cu3+ = 65 – 29 = 36 D Correct. 63Cu+ : [Ar]3d10. With a fully filled d-subshell, it has no unpaired electrons. 65Cu3+ : [Ar]3d8 i.e. has 2 unpaired electrons. Q2(A) chemical formula structure H2O2 H2S2 N2H4 1 Correct. O, S and N in the respective molecules have 4 regions of electron density and are sp 3 hybridised. Hence, the O –H, S–H and N –H bonds are formed from the overlap between the sp3 hybrid orbitals and the s orbital of H. 2 Correct. Since O is more el ectronegative than S, it draws the O–H bond−pair in H–O–O closer to itself, compared to S drawing the S –H bond−pair in H–S–S to itself. The O–H bond–pair in in H –O–O are thus nearer to the nucleus of the central O, and exert more repulsion than those in H–S–S. 3 Incorrect. S ince H 2S2 has the largest number of electrons, it has the largest and most polarisable electron cloud out of the 3 compounds. Hence, H2S2 has the strongest id-id interactions. 4 Incorrect. N2H4 has 2 lone-pairs of electrons and 4 hydrogens, it forms an average of 2 hydrogen bonds per molecule. H 2O2 has 4 lone -pairs of electrons and 2 hydrogens, it also forms an average of 2 hydrogen bonds per molecule. Q3(B) CH3CH2CO2Na dissociates in water to form CH3CH2CO2– and Na + ions which form strong ion-dipoles interactions with water, allowing it to have high solubility in water. The dissolution is also accompanied by an increase in entropy, favouring the dissolution While CH 3CH2CH2NH2 forms hydrogen bonds with water, CH 3CH2CH2Cl forms pd -pd interactions with water and CH3CH2CH2CH3 forms id-id interactions with water, these are not as strong as the ion -dipole interactions CH3CH2CO2Na form with water. Q4(B) The number of moles of air, n air, in the dented and undented plastic ball are the same. Using data from the dented ball, 6(100000)(27.5 10 ) (8.31)(273 22) 0.001122 mol air pVn RT −== + = At 57 °C when the ball is restored, 6( )(30.0 10 )0.001122 = (8.31)(273 57) 102542 Pa 103 kPa air pn p −= + == Q5(C) 0.010 mol of A lCl3, SiC l4 and PC l5 were added to 1.0 dm3 (i.e. excess) water. AlCl3 + 6H2O → [Al(H2O)6]3+ + 3Cl– • a weakly acidic solution due to [Al(H2O)6]3+. Highest pH of the three resultant solutions. SiCl4 + 2H2O → SiO2 + 4HCl • a strongly acidic solution