2023 TMJC H2 Chem Prelim P3 (Ans)

2023 H2 Chem Prelim P3 (Ans) Section A Answer all the questions in this section. 1 The Group 17 elements and its halides can react with many transition metals. (a) (i) Use the Data Booklet to describe the relative reactivity of the Group 17 elements as oxidising agents and relate this reactivity to relevant E⦵ values. [2] (ii) The following reagents are contained in three bottles labelled A, B and C. NaBr(aq) Cl2(aq) I2(aq) Table 1.1 shows the results of tests that were carried out using the reagents in the bottles. In each test, equal volumes of the reagents were mixed and the resultant colour of the solution was recorded. Table 1.1 test reagents mixed colour of resultant solution 1 A and B brown 2 C and B orange 3 A and C brown State the identity of the reagent in bottle A and write equations for any reactions that occur. [2] ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... (i) Down the group, E⦵ becomes less positive[1] . Hence, the oxidising power of the halogens decreases down the group. (or lower tendency for the halogen (X2) to gain electrons/be reduced.) [1] for both trend and explanation (ii) The reagent in bottle A is I2(aq). [1] Cl2(aq) + 2Br−(aq) ⎯→ 2Cl−(aq) + Br2(aq) [1] B C A Marker Comments for (a): • For (a)(i), some candidates included unnecessary details such as the trend of electronegativity, bond strengths or effective nuclear charge. This question merely required candidates to use the relevant E⦵ values and relate them to the oxidising power. • For (a)(ii), it should be noted that only test 2 has a reaction taking place. Some candidates wrote equations such as I2 + Br− ⎯→ 2I− + Br2 which is incorrect as I2 is a weaker oxidising agent than Br2 and hence will not be able to oxidise Br– to Br2.

2 2023 H2 Chem Prelim P3 (Ans) Tampines Meridian Junior College 2023 JC2 Preliminary Examination H2 Chemistry (b) Cobalt forms many complexes with ligands such as H 2O, NH 3 and Cl−. The most common oxidation states of cobalt are +2 and +3. Table 1.2 gives information about two octahedral cobalt(III) complexes D and E. Table 1.2 complex colour of aqueous ion D [CoCl2(NH3)4]+ violet E [Co(NH3)6]3+ yellow (i) Define the term complex. [1] (ii) The five d orbitals in a gas-phase transition metal ion are degenerate, but are split into two levels when the ion is in an octahedral complex. Explain why the d orbitals are split into two levels in an octahedral comple