2023 TMJC H2 Chem Prelim P2 (Ans)

2023 H2 Chem Prelim P2 (Ans) 1 The Periodic Table shows many interesting and recurring trends. (a) The variation of 1st ionisation energies across Period 4 is similar to that of Period 3. Table 1.1 shows the 1st ionisation energies of six consecutive elements in Period 4. Table 1.1 element Ga Ge As Se Br Kr 1st ionisation energy / kJ mol–1 580 760 950 1140 1350 (i) Explain why the 1 st ionisation energies show a general increase from gallium to krypton. ................................ ................................ ................................ ............................ ................................ ................................ ................................ ............................ ................................ ................................ ................................ ............................ ................................ ................................ ................................ ............................ ................................ ................................ ................................ ............................ ................................ ................................ ................................ ............................ ................................ ................................ ................................ ....................... [2] (ii) Using your knowledge of the variations, c omplete Table 1.1 with a suggested value for the first ionisation of selenium. [1] (b) Electrical conductivity is measured in Siemens per meter, S m –1, where a higher value represents higher electrical conductivity. Calcium is a Group 2 metal and has an electrical conductivity of 2.9 × 107 S m–1 while copper is a transition metal and electrical conductivity of 5.9 × 107 S m–1. (i) State the full electronic configuration of copper. ................................ ................................ ................................ ....................... [1] (ii) Explain the difference in electrical conductivity between calcium and copper. ................................ ................................ ................................ ............................ ................................ ................................ ................................ ............................ ................................ ................................ ................................ ............................ ................................ ................................ ................................ ....................... [2] 1s22s22p63s23p63d104s1 [1] Copper has a higher electrical conductivity [1] than calcium. ▪ Cu has more mobile electrons (per atom) as there are 3d and 4s electrons in Cu are available for delocalisation, compared to only 4s electrons in Ca. [1] Across the period. • the number of protons increases, hence nuclear charge increases. • the number of electrons increases but they are added to the same outermost shell, hence shielding effect remains relatively constant. • Effective nuclear charge increases, resulting in stronger electrostatic attraction between the nucleus and the valence electrons. • Hence, more energy is required to remove the valence electron. 2-3 points – 1m; 4 points -2m 940 [1] accept (760-950) Marker Comments for (b)(i): • Part (b)(i) was generally well done. Marker Comments for (b)(ii): • Some students attempted to argue that Cu having 29 electrons makes it a better electrical conductor. But Krypton has 36 electrons and is an electrical insulator, hence we should realise that not all electrons can contribute to conductivity.

2 2023 H2 Chem Prelim P2 (Ans) Tampines Meridian Junior College 2023 JC2 Preliminary Examination H2 Chemistry (c) Other transition metals such as nickel and platinum are sometimes found in copper ores. To obtain pure copper, the ore was made the anode of an electrolysis cell, with a pure copper cathode and aqueous CuSO4 as electrolyte. Explain, with reference to relevant E⦵ values from the Data Booklet what happens to the platinum impurities during this purification procedure. You should also use the following information: Pt2+ + 2e Pt E⦵= +1.20 V ................................ ................................ ................................ ................................ ..... ................................ ................................ ................................ ................................ ..... ................................ ................................ ................................ ................................ ..... ................................ ................................ ................................ ................................ ..... ................................ ................................ ................................ ................................ ..... ................................ ................................ ................................ ................................ ..... ................................ ................................ ................................ ................................ [2] (d) Propene undergoes reaction with Br 2 to form 1,2 -dibromopropane. This reaction mechanism features a carbocation intermediate. (i) Draw a labelled diagram to show orbital overlap in the C=C double bond of propene. [2] The relevant E⦵ values are: Pt2+ + 2e− ⇌ Pt E⦵ = +1.20 V Cu2+ + 2e− ⇌ Cu E⦵ = +0.34 V Since E⦵Pt2+/Pt is more positive than E⦵Cu2+/Cu [1] with value, Pt does not get oxidised and forms solid deposits below the anode / fall to the base of the cell and form anode sludge. [1] sp2 hybrid orbital of C  bond unhybridised p orbital of C  bond [1] for any 3 points: • label  bond (head-on overlap) • label  bond (side-on overlap • label sp2 orbital • label p orbital • correct shape of sp2 orbital • correct shape of p orbital Marker Comments for (c): • Many students wasted time describing the chemistry of nickel. • Some students attempted to calculate E for a reaction between Pt and Cu2+, which is irrelevant when electrolysis is driven by the external current. • Many students wrongly assessed that Pt 2+ was present, either in the ore or electrolyte, and described reduction of Pt2+, or worse, reduction of Pt. Marker Comments for (d)(i): • For a repeated question, students were surprisingly ill- prepared. Most students drew more than necessary. • Many students labelled the type of overlaps but not the orbitals themselves. • Some students seem to think that the suggested answer is showing 2 π bonds

3 Tampines Meridian Junior College 2023 JC2 Preliminary Examination H2 Chemistry [Turn over (ii) Name and draw the mechanism of the reaction between propene and Br2 to form 1,2-dibromopropane. Show all charges and relevant lone pairs and show the movement of electron pairs by using curly arrows. [3] (iii) Hence, draw the dot -and-cross diagram for the carbocation intermediate in the mechanism drawn in (ii). State the shape and bond angle at the positively charged carbon in the carbocation intermediate. shape: ................................ ................................ ................................ .................. bond angle: ................................ ................................ ................................ .... [3] [Total: 16] Electrophilic addition [1] [1] for every two points: • Correct dipoles shown in first step • Correct Carbocation • Correct curved arrows in first and second step • Both equations are balanced [1] allow ecf • Trigonal planar shape [1] • 120° angle [1] Marker Comments for (d)(ii): • Many students did not respond to the “name