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