NJC 2019 H1 Promo Paper 1 solutions

1 NJC Promotional Examination 8873/01/2019 [Turn over NJC 2019 H1 Chemistry Promo Solutions for P1 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. C C D A B C C A D D 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. C B A D A A D B B A 1 Use of the Data Booklet is relevant to this question. What is the number of atoms in 500 cm3 of oxygen under room conditions? A 1.25 × 1022 B 1.34 × 1022 C 2.50 × 1022 D 2.68 × 1022 Amount of O2 = 0.500 24 = 0.02083𝑚𝑜𝑙 No. of O atoms = 0.02082 (2)(6.02 × 1023) = 2.50 × 1022 Ans: C 2 Gallium nitride, GaN, could revolutionise the design of electric light bulbs because only a small length used as a filament gives excellent light at low cost. GaN is an ionic compound containing the Ga3+ ion. Which statements about GaN are correct? 1 The outer electronic configuration of Ga atom is 4s24p1 since Ga is a Group 13 element. 2 The electron arrangement of the nitrogen ion in GaN is 1s 2 2s2 2p3. 3 Ga3+ ion deflects less than Al3+ ion in an electric field. A 1 only B 2 only C 1 and 3 only D 2 and 3 only Statement 1: True Statement 2: N in GaN exists as N3–, electronic configurations 1s2 2s2 2p6 Statement 3: True. Angle of deflection α 𝐶ℎ𝑎𝑟𝑔𝑒 𝑠𝑖𝑧𝑒 . Ga3+ ion has same charge but large Ar than Al3+ ion. Hence Ga3+ ion deflects less Ans: C 3 Oxygen has two isotopes, 16O and 18O. An oxide ion, 𝑂8 𝑥 y–, has the same number of neutrons and electrons. What could be the values of x and y? x y Neutrons Electrons A 16 1 16-8 = 8 8+1 = 9 B 16 2 16-8 = 8 8+2 = 10 C 18 1 18-8 = 10 8+1 = 9 D 18 2 18-8 = 10 8+2 = 10 Ans: D

2 NJC Promotional Examination 8873/01/2019 [Turn over 4 Methyl mercaptan, CH3SH, is a substance often used to impart a smell to natural gas in a pipeline. The chemical equation for the combustion of CH3SH is as shown below. CH3SH + 3O2 → CO2 + SO2 + 2H2O A 10 cm3 sample of CH 3SH is exploded with 60 cm 3 of oxygen and the resultant gas mixture is passed into excess NaOH(aq). What is the percentage of the volume of the resultant gas mixture dissolved in NaOH? [All gas volumes are measured at room temperature and pressure.] A 40.0% B 28.5% C 20.0% D 14.3% CH3SH + 3O2 → CO2 + SO2 + 2H2O Vinitial / cm3 10 60 0 0 Vfinal / cm3 0 30 10 10 Both acidic CO2 + SO2 are absorbed by NaOH(aq)  % vol decrease = 20 50 × 100 = 40.0% Ans: A 5 Which sketch show the correct trend in the stated property for the elements in Period 3 of the Periodic Table? 1 atomic radius This graph shows the trend of the ionic radius instead. Atomic radius decreases across a period due to increase nuclear charges, but insignificant increase in shielding effect. 2 first ionisation energy This graph shows the correct trend of 1st IE’s of period 3 elements. Radius /nm Na Mg Al Si P S Cl First I.E. / kJ mol–1 Na Mg Al Si P S Cl Ar

3 NJC Promotional Examination 8873/01/2019 [Turn over 3 electrical conductivity Si has should have a lower electrical conductivity than Na, Mg & Al. A 1 only B 2 only C 1 and 2 only D 2 and 3 only Ans: B 6 G is an element in period 4 of the Periodic Table. The first thirteen ionisation energies of G are plotted against the order of the removal of electrons as shown below. What is the formula of the compound formed when G reacts with oxygen? A GO B GO2 C G2O D G2O3 1st sharp increase occurs between 1st & 2nd IE, G has 1 valence electron  G is in Group 1 Ans: C 7 In which pair of compounds is the first member less volatile than the second one? A CO2 CS2 Both are non -polar & linear in shape with weak intermolecular td-id interactions. CO 2 has a smaller e cloud & thus weaker td-id interactions; hence a lower bp. B PH3 NH3 PH3 is non -polar with weaker intermolecular td -id interactions while NH3 is polar with stronger intermolecular H bonding. Thus PH3 has a lower bp. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Order of electrons removed from X Log (Ionisation Energy) Order of electrons removed from G Electrical conductivity Na Mg Al Si P S Cl Ar

4 NJC Promotional Examination 8873/01/2019 [Turn over r q p s C H H C H N H H C O OH I I OHO I I C  CH3 C C CH3 HH CH3 C C H CH3H Cis-but-2-ene is slightly polar while trans-but-2-ene is non- polar. Thus, cis-but-2-ene has slightly stronger intermolecular forces of attraction & a higher bp. D NH3 H2O More energy is required to overcome the more extensive hydrogen bonding between H 2O molecules than the less extensive hydrogen bonding between NH3 molecules. Ans:C 8 The shapes of three species P, Q, and R are bent, square planar and trigonal pyrimidal respectively. Which of the following can be P, Q and R? Candidates need only to check on those familiar ones to verify that they do not have the stated shape to rule out the choice. E.g. F2O is similar to H2O; CS2 is similar to CO2. P Q R A CCl2 (1lp + 2 bp; bent) BrF4− (2 lp + 4 bp; sq planar) SbF3 (1 lp + 3 bp; trig pyramidal) B CS2 (2 bp only; linear) BH4− (4 bp only; Td) ICl3 (2 lp + 3 bp; T-shape) C F2O (2 lp + 2 bp; bent) ICl4− (2 lp + 4 bp; sq planar) BCl3 (3 bp only; trig planar) D ICl2− (3 lp + 2 bp; linear) XeF4 (2 lp + 4 bp; sq planar) SO32− (1 lp + 3 bp; trig pyramidal) Ans:A 9 The thyroid gland concentrates iodine and uses it to produce thyroxine, which is a hormone that controls the metabolic rate. Thyroxine What are the values of the bond angles p, q, r and s? p q r s A 180 90 180 90 B 105 90 107 180 C 180 90 120 180 D 105 109.5 107 120 p – oxygen has 2 b.p, 2 l.p, bond angle = 105° q – carbon has 4 b.p, no l.p, bond angle = 109.5° r – nitrogen has 3 b.p, 1 l.p, bond angle = 107° s – carbon has 3 b.p, no l.p, bond angle = 120° Ans: (D)

5 NJC Promotional Examination 8873/01/2019 [Turn over 10 Ice is the crystalline form of water. The diagram below shows part of the structure of ice. Which of the following statements is not true about ice? A Ice has a lower density than water at 0 oC due to its open structure. True B The bond angle about oxygen in ice is 109.5o. True C Ice does not conduct electricity. True D The hydrogen bonds are stronger than the O-H covalent bond. False. Ans: (D) 11 In which of the following substances, must covalent bonds break on melting? A Phosphorus(V) chloride, PCl5 Simple covalent molecules, tdid interactions between molecules break on melting B Aluminium chloride, Al2Cl6 Simple covalent molecules, tdid interactions between molecules break on melting C Silicon carbide, SiC Giant covalent structure, covalent bonds between atoms break on melting D Iron(II) hydroxide, Fe(OH)3 Giant ionic structure, metallic bonds between cations and sea of delocalised electrons break on melting Ans: (D) 12 What is the most likely H value for KOH(aq) + HCl (aq) → KCl(aq) + H2O(l)? Given that NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) H = −114 kJ mol−1 A −54 kJ mol−1 B −57 kJ mol−1 C −114 kJ mol−1 D −228 kJ mol−1 when 2 mol of H2O(l) are formed from a strong acid-strong base reaction; −114 kJ are given off. As KOH(aq) + HCl (aq) → KCl(aq) + H2O(l) is also a strong acid-strong base reaction; but for for 1 mole of water. Half amount of heat should be released. Hence H = −(114/ 2) =−57 kJ kJ mol−1 Ans: (B) Hydrogen bond Covalent bond

6 NJC Promotional Examination 8873/01/2019 [Turn over 13 The reaction shown is a first-order reaction. N2O5(g) → 2NO2(g) + ½ O2 (g) Which graph shows how the concentration of N2O5 v