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 complex, including which of the d orbitals are in each level. [2] (iii) Draw the shapes of the d orbitals that occupy the higher energy level. [2] (iv) The colour of the complexes is due to the absorption of light at specific wavelengths. The colour observed is the complement of the colour absorbed. Suggest why the two complexes D and E have different colours. [1] ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... (i) A complex contains a central metal atom/ion attached to ligands through dative / coordinate bonds. [1] (ii) The dx2−y2 and dz2 orbitals have their lobes pointing at the ligands along the axes. Hence, they experience greater repulsion from the ligands. On the other hand, the dxy, dxz and dyz orbitals experience less repulsion as their lobes are in between the coordinate axes. [1] As a result, the five d orbitals are split into 2 energy levels, with the dx2−y2 and dz2 orbitals having the higher energy level and the dxy, dxz and dyz orbitals having the lower energy level. [1] (iii) dx2−y2 dz2 x y z x y z

3 Tampines Meridian Junior College 2023 JC2 Preliminary Examination H2 Chemistry [Turn over ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… (c) An electrochemical cell is set up using a Co3+(aq)/Co2+(aq) half -cell and a [Ag(NH3)2]+(aq)/Ag(s) half-cell under standard conditions. The electrode potential of the [Ag(NH3)2]+(aq)/Ag(s) half-cell is –0.37 V. When the cell discharges, the following reaction occurs. Co3+(aq) + Ag(s) + 2NH3(aq) ⎯→ Co2+(aq) + [Ag(NH3)2]+(aq) (i) Write a half-equation for the oxidation of Ag to [Ag(NH3)2]+. [1] (ii) With the aid of the Data Booklet , calculate the Ecell ⦵ and ∆ G⦵ for the above electrochemical reaction. [2] (iii) Explain the effect on Ecell ⦵ when H + is added to the [Ag(NH 3)2]+(aq)/Ag(s) half-cell. [2] (iv) Cl− and NH3 have different ligand (field strength), resulting in different size of energy gap. [1] Marker Comments for (b): • For (b)(i), it was disappointing that candidates were unable to give the definition of a complex when this is the basic bookwork that should have been done. Candidates are reminded to avoid giving unnecessary details for definition questions as this may result in the imposition of conditions which is not necessarily the case e.g. complex does not necessarily need to include transition metal atoms/ions. Non-transition metal atoms/ions can be part of a complex too. • For (b)(ii), it was encouraging to see that candidates generally knew how to describe the process of d orbital splitting, although some incorrectly identified the d orbitals present at each energy level. Candidates are reminded to revise the naming of d orbitals e.g. there are no such orbitals as dx, dy, dx or dx2y2 • For (b)(iii), a sizeable number of candidates were careless and labelled the axes incorrectly, especially when drawing the dz2 orbital for (b)(iii). Hence, no credit was given despite the correct shape drawn. • For (b)(iv), candidates were generally able to attribute the difference in colour of the complexes to the difference in energy gap.

4 2023 H2 Chem Prelim P3 (Ans) Tampines Meridian Junior College 2023 JC2 Preliminary Examination H2 Chemistry (iv) The Nernst equation can be used to determine the cell potential under non-standard conditions and is given as follows: Ecell = Ecell ⦵ – 0.0592 n log10 Q where the value of Q is calculated using this expression [Co2+][Ag(NH3)2 +] [Co3+][NH3]2 and n is the number of moles of electrons transferred per mole reaction. When the electrochemical cell described in (c) discharges, the concentration of Co3+ in the Co3+(aq)/Co2+(aq) half -cell decreases from 1.00 mol dm –3 to 0.75 mol dm–3. Show that the value of Q is 8.33 when [Co3+] is 0.75 mol dm–3. Hence, calculate the