2012 JJC H1 Prelim P2 answers

1 Jurong Junior College 8872/02/Prelim 2012 Suggested Answers Suggested MARK SCHEME for 2012 H1 Chemistry Preliminary Examination Paper 2 Section A Minus 1m overall for 3 s.f. mistake General comments: Most students did well for the sketches for the trends of first ionisation energy and ionic radii for the selected elements. However, they still are not careful in their explanation s, often missing out key words in their answers, especially on the reasons for the trend observed in ionic radii from Si4+ to Cl−. 1. (a) (i) Common mistakes: 1. Students plotted the ionic radius of Si4+ as the highest point. (ii) the difference in first ionisation energy between phosphorus and sulfur. the trend in ionic radius from silicon to chlorine. [5] Ionic radius Si P S Cl first ionisation energy Si P S Cl [1m] • Point for P is higher than Si and S but lower than Cl • Point for Si lower than S [1m] • Decreasing trend from P to Cl • Ionic radius for Si4+ is lower than those of P, S and Cl S has a lower first ionisation energy than P due to mutual repulsion between the paired 3p electrons in S which makes it easier to remove one of the paired 3p electrons in S than the unpaired 3p electron in N. [1m] Si4+ has one less quantum shell of electrons than P3−, S2− and Cl−/ the rest of the ions. Hence Si4+ has the smallest ionic radius. [1m] Across the isoelectronic series* (P3−, S2−, Cl−), the ionic radii decreases due to increasing nuclear charge and constant shielding effect by the same number of inner shells’ electrons. [1m]

2 Jurong Junior College 8872/02/Prelim 2012 Suggested Answers 1. (a) (ii) Common mistakes: 1. For 1 st IE: Some students gave the factors resulting in the general increase in first ionisation energies across a period. 2. For 1 st IE: Some did not state the trend (but these students still gained one mark for correct reasoning). 3. For ionic radius: Some students g ave reasons to explain why the ions have different radii from their respective atoms (this is an irrelevant answer). 4. Many students did not quote the identities of the ions in their explanation. For instance, the ionic radius of Si is smaller because it has one less quantum shell. In answers where the subject is not clear, marks were not awarded. 5. Many did not state why shielding effect by inner shell electrons remains relatively constant for P3−, S2− and Cl−. 6. Small of them were uncertain of how to use the term, ‘nuclear charge’. Wrong term like ‘atomic charge’ was seen. (b) (i) (ii) P Cl % by mass 30.4 69.6 Ar 31.0 35.5 Amount/mol 0.981 1.96 Ratio 1 2 Empirical formula is PCl2. [1m] with relevant working Let the molecular formula be (PCl2)n n  31.0 + 2n  35.5 = 200 n  200 102 = 2 Molecular formula is P2Cl 4. [1m] with relevant working involving the use of Mr value of 200 given White fumes of HCl. [1m] PCl3 + 3H2O → H3PO3 +