2019 TMJC H2 Chem Prelim P1 ANS

1 2019 H2 Chem Prelim P1 Work Solutions Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry 1 Some data of the ions of four unknown elements are given in the table below. A+ B– D2– E3– mass number 21 20 20 21 number of protons – 11 9 – number of electrons 9 – – 12 Which of the following correctly shows pairs of isotopic and isoelectronic species? isotopic isoelectronic A D2–, E3– B–, E3– B B–, D2– A+, B– C B–, D2– B–, E3– D D2–, E3– A+, B– Answer: A A+ B– D2– E3– mass number 21 20 20 21 number of protons 10 11 9 9 D2- and E3- are isotopic number of electrons 9 12 11 12 B- and E3- are isoelectronic 2 The first six ionisation energies of element G are plotted in the graph below. number of electrons removed log10 (ionisation energy) TAMPINES MERIDIAN JUNIOR COLLEGE 2019 JC2 H2 Chemistry Prelim Exam Paper 1

2 2019 H2 Chem Prelim P1 Work Solutions Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry Which of the following can be deduced from the graph? A There are 2 quantum shells in element G. B Element G is from Group 2 in the Periodic Table. C The fifth and sixth electrons are removed from the same subshell. D The identity of element G is carbon. Answer: C With limited information from the graph, it is unclear if the jump from the 4th to the 5th I.E. is a change in subshell or quantum shell. Hence, options A, B and D cannot be concluded using the graph. Then only conclusion which can be made is that the 5th and 6th electrons are in the same subshell. 3 For which of the following pairs does the first molecule have a higher polarity than the second molecule? 1 CH3OH, CH3I 2 CF4, CCl4 3 cis-CHF=CHF, trans-CHF=CHF A 1 only B 1 and 3 C 2 and 3 D 1, 2 and 3 Answer: B 1 Although both CH3OH and CH3I are polar, CH3OH has higher polarity because O is more electronegative than I, hence the C –O bond is more polar than the C –I bond. 2 Both CF4 and CCl4 are non-polar as both have net zero dipole moment. 3 trans-CHF=CHF is non-polar as both C–F dipole moment cancels out, hence net zero dipole moment. cis-CHF=CHF is polar.

3 2019 H2 Chem Prelim P1 Work Solutions Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry 4 Three substances H, I and J have physical properties shown in the table below. substance melting point/ °C boiling point/ °C electrical conductivity of solid of liquid H 801 1413 poor good I 2852 3600 poor good J 3500 at very high pressure unable to determine poor unable to determine What are the possible identities for substances H, I and J? H I J A NaCl AlCl3 C (diamond) B MgO AlCl3 C (graphite) C NaCl MgO C (graphite) D NaCl MgO C (diamond) Answer: D  Substance H should have a giant ionic structure , where NaCl or MgO are possible identities.  Substance I should also have a giant ionic structure but with larger lattice energy compared to subst