ACJC H2 CHEM P3 Questions Prelim

2 © ACJC 2014 9647/03/Prelim/14 [Turn over Answer any four questions. 1 (a) Dinitrogen tetroxide N 2O4 is one of the most important rocket propellants developed. N2O4 forms an equilibrium mixture with nitrogen dioxide NO2. NO2 is favoured at higher temperatures, while at lower temperatures, N 2O4 predominates. (i) Draw dot-and-cross diagrams to show the bonding in the molecules of NO2 and N2O4. (ii) Suggest a value for the bond angle in each of the above two molecules. [3] (b) The chemistry of nitrogen oxides is very versatile. (i) Given the following reactions and their standard enthalpy changes Reaction 1 NO(g) + NO2(g) N 2O3(g) Ho r = -39.8 kJ mol-1 Reaction 2 NO(g) + NO2(g) + O2(g) N 2O5(g) Ho r = -112.5 kJ mol-1 Reaction 3 2NO2(g) N2O4(g) Ho r = -57.2 kJ mol-1 Reaction 4 2NO(g) + O2(g) 2NO 2(g) Ho r = -114.2 kJ mol-1 Reaction 5 N2O5(s) N 2O5(g) Ho r = +54.1 kJ mol-1 Calculate the Ho r for Reaction 6 N 2O3(g) + N2O5(s) 2N 2O4(g) (ii) By considering the entropy and enthalpy change during reaction 5 and reaction 6, suggest how the standard Gibbs free energy change of the two reactions will compare in sign and in magnitude. Hence predict which reaction will be more spontaneous. Explain your reasoning. [5] (c) Compounds of elements in the second and third period of the Periodic Table show similar trends of periodicity. The Period II oxides are given as Li2O BeO B 2O3 CO 2 N 2O3/ N2O5 (i) The melting points of Li2O and CO2 are 1440 oC and -79 oC respectively. Explain for the differences in melting points. (ii) BeO is an amphoteric oxide. Write equations for the reaction between BeO with an acid and with a base. (iii) B2O3 is weakly acidic. The oxide acidity of the Group III elements decreases down the group. Suggest a possible reason for this trend. [6]

3 © ACJC 2014 9647/03/Prelim/14 [Turn over (d) Halogens are also commonly found in many organic compounds, such as an aromatic compound X with the molecular formula of C8H6Cl2O. Given that one mole of X reacts with one mole of dimethylamine to form a neutral product Y and Y does not react with hot ethanolic ammonia, suggest the structures for compounds X and Y, explaining your reasoning. Hence, discuss the reactivities of the two chlorine atoms in compound X towards substitution. [6] [Total: 20]

4 © ACJC 2014 9647/03/Prelim/14 [Turn over 2 (a) Sodium chloride and silver chloride are two simple salts and their solubilities in water are being considered in this question. Salt Hsoln / kJ mol-1 Ssoln / J mol-1K-1 NaCl +3.6 +43.2 AgCl +65.7 +34.3 (i) Use the values given in the table to calculate Gsoln  for each of the salts and hence deduce its solubility in water. (ii) The solubility product, K sp, of AgC l is related to Gsoln (AgCl) by the following equation, Gsoln  = -2.303RT lg K sp where R is 8.31 J mol -1K-1 and T is the temperature in K. Use the equation given above to calculate the value of Ksp of AgCl at 298K. (iii) Explain how solubility of AgCl will change with increasing temperature? [5] (b) (i) Draw a fully labelled diagram of the electrochemical cell you would use to determine the standard electrode potential of the Ag +(aq) l Ag(s) electrode system and show the direction of electron flow . (ii) When aqueous sodium chloride is added to the Ag +(aq)lAg(s) electrode system in the above electrochemical cell in (b)(i), explain qualitatively how the Ecell will change as a result. (iii) At 298K, the equation below relates the concentration of silver ions in solution with the electrode potential under non-standard conditions. E = E  + 0.060 lg [Ag+(aq)] where E= electrode potential of silver under non-standard conditions E  = standard electrode potential of silver The addition of excess aqueous sodium chloride, NaCl(aq), to the Ag+(aq) l Ag(s) half-cell results in a chloride ion concentration of 2.1 mol dm-3. Using the value of the Ksp of AgCl calculated in (a)(ii), calculate the value of E, the electode potential of the Ag +(aq)lAg(s) electrode system, after the addition of excess aqueous sodium chloride to the Ag+(aq)lAg(s) half-cell. [You may use this value of K sp of AgC l = 2.00 x 10 -10 for this calculation if you did not get an answer for (a)(ii)] [7]

5 © ACJC 2014 9647/03/Prelim/14 [Turn over (c) Silver-oxide primary batteries account for over 20% of all primary battery sales in Japan. It is available in small sizes as button cells and are used in watches, cameras, heart pacemakers and hearing aids due to its very steady output. A silver-oxide battery uses silver oxide as the positive electrode and zinc as the negative electrode and an alkaline electrolyt e such as sodium hydroxide. The chemical reaction that takes place inside the battery is as follows: Zn + Ag 2O  ZnO + 2Ag Ecell = +1.6V (i) Write the two half-equations that occur at the anode and cathode respectively. (ii) Suggest a reason why this button battery is often used as stated in the question. [3] (d) Electrodes used in electrocardiography are disposable and many incorporate silver. The silver metal is deposited in a thin layer on a small plastic “button” and then some is converted to AgCl. Ag(s) + Cl-(aq) AgCl (s) + e- The volume of silver used in plating one electrode is 1.5 x 10 -2 cm3. The density of silver metal is 10.5 g cm-3. (i) What is the mass of Ag used for plating one electrode? (ii) If Ag is plated on the “button” from an Ag + solution with a current of 12.0 mA, how long does the plating take? [3] (e) Diammine silver (I) has the formula [Ag(NH 3)2]+ and is the active ion in Tollens’ reagent. It produces a characteristic silver mirror on the side of the test-tube and this is used as the basis of the test for aldehydes. Tollens’ reagent also gives a positive result with methanoic acid which is fully oxidised to water and carbon dioxide. Write two half-equations showing the reduction of Tollens’ reagent and the oxidation of methanoic acid, followed by the balanced equation for the reaction between Tollens’ reagent and methanoic acid. [2] [Total: 20 marks]

6 © ACJC 2014 9647/03/Prelim/14 [Turn over 3 This question is about halogens and halogenated organic compounds. (a) One way to prepare brominated products is described as below: Cyclohexene reacts with N-bromosuccinimide (NBS) in the presence of UV light to give 3-bromocyclohexene resulting from the substitution of hydrogen by bromine at the allylic position – the position next to the double bond. The overall equation is shown below. The allylic bromination with NBS works in a similar manner as free radical substitution. The steps in the mechanism are shown below. Step 1: Homolytic cleavage of the N-Br bond of NBS generates a Br radical. Step 2: The Br radical abstracts an allylic hy drogen from the cyclohexene to form an allylic radical. Step 3: The HBr formed reacts with NBS to produce a Br 2 molecule. Step 4: This Br2 molecule reacts with the allylic radical formed in step 2, regenerating a new bromine radical that can begin the cycle again. (i) Describe the mechanism, using curly arrows to represent the flow of electrons, of the reaction between cyclohexene and NBS. (You need not show arrows for step 3) (ii) Suggest why NBS is used as the bromine source instead of adding Br 2 directly? (iii) The enthalpy cha