2025 ASRJC Prelim H2Chem P1_Solution

9729/01/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2025 JC2 PRELIMINARY EXAMINATION Paper 1 Solutions 1 Sodium azide, NaN3 is an explosive used to inflate airbags in cars when they crash. It consists of positive sodium ions and negative azide ions. What are the number of electrons in the sodium ion and the azide ion? sodium ion azide ion A 10 20 B 10 22 C 12 20 D 12 22 Ans: B Na+ (11 – 1 = 10) and N3- (7 x 3 + 1 = 22) 2 The graph shows the variation of the first ioni sation energy with proton number for some elements. The letters used are not the actual symbols for the elements. Which statement about the elements is correct? A P and X are in the same period in the Periodic Table. B The general increase from Q to X is due to increasing atomic radius. C The small decrease from R to S is due to decreased shielding. D The small decrease from U to V is due to repulsion between paired electrons. Ans: D A – same Group B – increasing nuclear charge (and relatively constant shielding effect) C – higher energy level (ns2 vs np1)

2 9729/01/H2 3 The table identifies the shape and polarity of four molecules. Which row is correct? molecule molecular shape polarity A boron trichloride trigonal pyramidal polar B nitrogen trichloride trigonal planar non-polar C sulfur dioxide bent polar D trichloromethane tetrahedral non-polar Ans: C BCl3 NCl3 SO2 CHCl3 No. of bp, lp on central atom 3 bp, 0 lp 3 bp , 1 lp 2 bp, 1 lp 4 bp, 0 lp Molecular shape trigonal planar trigonal pyramidal bent tetrahedral B Cl Cl Cl N Cl Cl Cl S O O C Cl Cl Cl H polarity non–polar polar polar polar 4 The element tin exists in two forms, grey tin and white tin. Some properties of grey tin and white tin are shown. grey tin white tin boiling point 2543 oC 2533 oC electricial conductivity none in solid or liquid good in solid and liquid malleability brittle malleable Which structural change might take place when grey tin changes to white tin? A giant covalent to giant ionic B giant covalent to giant metallic C giant ionic to giant covalent D giant ionic to giant metallic Ans: B White tin must have giant metallic structure as it conducts electricity in solid and liquid state and its malleable. Grey tin cannot have giant ionic structure as it does not conduct electricity when in liquid state, thus, it has to be giant covalent structure.

3 9729/01/H2 [Turn over 5 0.01 mol of K IOn reacts with 0.05 mol of K I stoichiometrically to produce I2 under acidic conditions. In this reaction, all the iodine containing reactants were converted to I2. What is the value of n? A 1 B 2 C 3 D 4 Ans: C Let the oxidation state of iodine in KIOn be In+ (this approach allow you to focus on the species that is undergoing Redox.) Reduction: (2n) e + 2 In+ → I2 x 2 Oxida