JPJC Prelim 2025 Paper 3 Answers_H2 Chem

© Jurong Pioneer Junior College [Turn Over Answer Scheme Paper 3 Section A 1 Nitrogen, a Period 2 element, forms a wide variety of compounds with diverse structures, oxidation states, and properties. These include ammonia, ammonium salts, amines, and nitrates, all of which play important roles in industry, agriculture, and the environment. (a) The enthalpy changes involved in the formation of ionic and covalent compounds vary across the period. (i) Explain why the first ionisation energy of nitrogen is more endothermic than that of carbon and oxygen. [2] C: 1s2 2s2 2p2 N: 1s2 2s2 2p3 O: 1s2 2s2 2p4 Comparing C and N: N has a higher nuclear charge, while the shielding effect caused by inner shell electrons remains approximately constant. Therefore, it has a smaller atomic radius, and valence electrons experience a stronger nuclear attraction. Consequently, N has a higher first ionisation energy compared to C. Comparing N and O: Less energy is required to remove one of the paired 3p electrons in oxygen due to the larger interelectronic repulsion. Thus, O has a lower first ionisation energy than N. (ii) Some Period 3 elements can react with chlorine to produce chlorides, for example, NaCl and SiCl4. Describe and explain what happens when NaC l and SiCl4 are separately added to excess water. State the pH of the resultant mixtures and give relevant equations. [3] NaCl is an ionic chloride and will dissolve/hydrate in water to form a solution of pH 7. NaCl → Na+(aq) + Cl– SiCl4 is a covalent chloride and has energetically accessible vacant 3d orbitals that allow the water molecule to form dative bonds. Thus SiCl4 hydrolyse in water to form an acidic solution of pH 1-2. SiCl4 + 4H2O → Si(OH)4 + 4HCl

2 © Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAM/2023 1 (b) Ammonia (NH3), ammonium nitrate (NH4NO3), and ethanamide (CH3CONH2) are important compounds that can be used as fertilisers due to their high nitrogen content. Arrange these three compounds in order of increasing pH value of their aqueous solution. Explain your answer. [3] NH4NO3 < NH2CONH2 < NH3 NH4NO3 will dissociate into NH4+ and NO3- ions in solution. NH4+ will undergo hydrolysis to produce H + ions, forming an acidic solution. Thus, the pH value of NH4NO3 solution is less than 7. NH3 will be basic due to the availability of lone pair electrons on N for protonation. Thus, the pH value of the solutions will be higher than 7. CH3CONH2 will be neutral due to the delocalisation of the lone pair of electrons on N into the neighbouring  electron cloud of the electron - withdrawing C=O group, the lone pair of electrons in urea is not available for protonation than NH 3. Thus, it is a weaker base compared to NH 3, hence, lower pH value. (c) Aqueous iron(III) nitrate, Fe(NO3)3(aq), is yellow in colour. (i) Explain why the solution of iron(III) nitrate is yellow in colour. [3] The presence of ligan