2023 TMJC H2 Chem Prelim P3 (QP)

2023 H2 Chem Prelim P3 (QP) 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] ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………...

2 2023 H2 Chem Prelim P3 (QP) 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] ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………...

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]

4 2023 H2 Chem Prelim P3 (QP) 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 cell potential at this Co3+ concentration. [3] ……………………………………………………………………………………………... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………... ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ………………………………………………………………………………………………

5 Tampines Meridian Junior College 2023 JC2 Preliminary Examination H2 Chemistry [Turn over ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… [Total: 18]

6 2023 H2 Chem Prelim P3 (QP) Tampines Meridian Junior College 2023 JC2 Preliminary Examination H2 Chemistry 2 Inorganic compounds have various uses in the food and medical industry. For example, sodium hydroxide is used in the chemical peeling of fruits and vegetables, whereas magnesium compounds are used as food additives. (a) Sodium hydroxide is formed in an explosive reaction between sodium metal and water, as shown in reaction 1 in Table 2.1. The table also shows some other equations and their related standard enthalpy changes. Table 2.1 (i) Deduce the sign of ∆S⦵ for reaction 1. [1] (ii) Hence, predict how an increase in temperature affects the spontaneity of reaction 1. Explain your answer. [1] (iii) Write an equation to represent the first ionisation energy of sodium. [1] (iv) Use the data in Table 2.1 and relevant data from the Data Booklet to draw an energy cycle to calculate the standard enthalpy change of reaction 1, ∆H1 ⦵ . [3] ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………