2017 HCI H1 Chemistry Prelims P2 Answers

1 2017 H1 Chemistry Preliminary Examination Paper 2 Answers Section A 1 (a) (i) Since the forward reaction is exothermic, a low temperature will cause the position of equilibrium to shift to the right to produce more heat and thus producing more NH 3. However, the rate of reaction will be slow at low temperature, therefore, to increase the rate of reaction, a moderate temperature of 450oC is used. (ii) Iron catalyst provides an alternative reaction pathway with a lower activation energy. (iii) At a higher pressure, position of equilibrium shifts to the right so as to decrease the pressure by favouring the production of fewer number of moles of gases. The equilibrium is not affected as it is only affected by temperature. (iv) Kc = 3 22 2 3 ]][H[N ][NH (v) N2 (g) + 3H2 (g) 2NH3 (g) Let the number of moles of N2 reacted be x N2 3H2 2NH3 Initial no. of moles / mol 4 8 0 Change in no. of moles / mol - x - 3x + 2x Equilibrium no. of moles / mol 4 - x 8 - 3x 2x Given that the total number of moles of gases at equilibrium is 8, 4 - x + 8 – 3x + 2x = 8 Solving for x, x = 2 At equilibrium, there is 2 mol of N2, 2 mol of H2 and 4 mol of NH3. (vi) Kc =      3 2 22 24 22 = 1.00 mol-2 dm6

2 (b) (i) NH3(g) + 4 7 O2(g)  NO2(g) + 2 3 H2O(g) (ii) ΔH = ΔHf(NO2) + 2 3 ΔHf(H2O) - ΔHf(NH3) - 4 7 ΔHf(O2) ΔH = (-34) + ( 2 3 × -242) – (-46) – 0 = -351 kJ mol-1 (iii) ΔH = BE(bonds broken) - BE(bonds formed) Bonds broken = 3BE(N-H) + 4 7 BE(O=O) = 3 × 390 + 4 7 × 496 = 2038 kJ mol-1 Bonds formed = 3BE(O-H) + 2BE(N-O) = 3 × 460 + 2BE(N-O) = 1380 + 2BE(N-O) -351 = 2038 – 1380 – 2BE(N-O) BE(N-O) = +504 kJ mol-1 (iv) n(ammonia) = 17 44 = 2.59 mol = n(NO2) Theoretical mass of NO2 = 2.59 × 46 = 119.14 g Percentage efficiency = 119.14 89 × 100 = 74.7% (c) NH3 + CH3CH2Br  CH3CH2NH2 + HBr Ethanolic concentrated NH3, heat in a sealed tube The organic products belongs to the primary amine functional group. [Total: 20] 2 (a) (i) Al2O3 dissolves in both HCl and excess NaOH. P4O10 dissolves in NaOH. (ii) Al2O3(s) + 6HCl(aq)  2AlCl3(aq) + 3H2O(l) Al2O3(s) + 2NaOH(aq) + 3H2O(l)  2Na[Al(OH)4](aq) P4O10(s) + 12NaOH(aq)  4Na3PO4(aq) + 6H2O(l)

3 (b) (i) The aluminium atom in A lCl3 has only 6 electrons surrounding it hence it is electron-deficient. The lone pair of electrons from a chlorine atom in a neighbouring molecule would be donated via a dative bond, resulting in a dimer. (ii) Al2Cl6 has simple molecular structure. The dispersion forces between the A l2Cl6 molecules are weak and hence, only a small amount of energy is needed to overcome the weak interactions. [Total: 10] 3 (a) Cl2(g), AlCl3, (rt) (b) Step 2: oxidation Step 3: reduction (c) B dissociates to give H + and the conjugate base, C 6H5CO2ˉ (benzoate). The negative charge on the benzoate ion is delocalised equally over two highly electronegative oxygen atoms. The negative charge is dispersed and the carboxylate anion is greatly stabilised. (d) Reagents and conditions: Br2(l), UV light The substitution is uncontrollable and multi-substituted products may be obtained. (e) Step 1: Ethanolic KCN, heat Step 2: Dilute H2SO4, heat [Total: 10] Al Cl Cl Cl Al Cl Cl Cl CH2Br CH2COOHCH2CN

4 Section B 4 (a) (i) Cooking oil is primarily made up of long hydrocarbon chains, and experiences dispersion forces between molecules, whereas water experiences hydrogen bonding between water molecules. Because of the long hydrocarbon chains, the dispersion forces experienced by these molecules is extensive and stronger than the hydrogen bonding experi enced by water molecules. Thus, more energy is required to overcome the dispersion forces in cooking oil compared to water, and the boiling point of cooking oil is thus higher. (ii) The cooking oil and water is immiscible (no need to comment on relative density). This is because the molecules in cooking oil are largely non -polar, and do not form favourable interactions with the polar water molecules. (iii) When the noodles undergoes deep frying, the high temperature of the heated oil causes the water in the noodles to vaporize. As water molecules do not form favourable interactions with the cooking oil, the wat er molecules escape as steam, thus drying out the noodles. (b) (i) (ii) Sodium chloride has a giant ionic lattice structure, and experiences strong electrostatic forces of attraction between oppositely charged cations and anions. The attraction between oppositely charged ions is very strong and requires a lot of energy to overcome. Thus, a high temperature must be achieved to provide enough energy for melting to occur. In the giant ionic lattice, the cations and anions are arranged in an alternating fashion. When a force is applied, the lat tice structure is disrupted, causing similarly charged ions to be aligned with one another. The resulting repulsion between like charges causes the lattice to shatter, thus accounting for sodiu m chloride’s brittle nature. (iii) Lattice energy is the heat evolved when one mole of pure ionic solid is formed from its constituent gaseous ions. 𝐿𝐸 ∝ 𝑞+𝑞− 𝑟++𝑟− The chloride anion has a smaller charge (–1) compared to the oxide anion (–2). The chloride anion has a larger ionic radii compared to the oxide anion (period 3 vs period 2). Since lattice energy is directly proportional to charge and inversely proportional to ionic radii, the magnitude of the lattice energy of sodium chloride is smaller than that of sodium oxide.

5 (iv) Sodium chloride dissolves in water to form a colourless solution of neutral pH. NaCl(s) + aq  Na+(aq) + Cl−(aq) In an excess of water, AlCl3 dissolves to form a colourless solution of acidic pH. Al3+ forms a complex ion with 6 water molecules, as shown below. AlCl3(s) + 6 H2O(l)  [Al(H2O)6]3+(aq) + 3 Cl−(aq) [Al(H2O)6]3+(aq) + H2O(l) ⇌ [Al(H2O)5OH]2+(aq) + H3O+(aq) A violent reaction with water producing fumes of HCl gas. Complete hydrolysis occurs in this case. SiCl4(l) + 4 H2O(l)  SiO2.2H2O(s) + 4 HCl(aq) (v) Sodium chloride is an ionic compound while silicon chloride is a covalent compound. Therefore, as the degree of covalency increases across the period, so does the extent of hydrolysis with water. 5 (a) (i) First ionisation energy is the energy required to form 1 mol of unipositively charged cations from 1 mol of gaseous atoms. (ii) Potassium has one more quantum shell than sodium, thus the outermost electron is further away from the nucleus in potassium compared to sodium. Therefore, the attraction between the out ermost electron and the nucleus of potassium is weaker, and requires less energy to remove compared to that of sodium. The 3 p subshell of aluminium is further away from the nucleus than the 3 s subshell. There is weaker attraction between the nucleus and the outermost electron. Hence less energy is required to remove the 3 p electron from aluminium, resulting in a lower ionisation energy for aluminium. Sulfur has a set of paired electrons in the p subshell, whereas the p orbitals in phosphorus are singly filled. The inter -electronic repulsion between the paired electrons causes the outermost electron of sulfur to be easier to remove, thus less energy is required to ionise sulfur. Argon has more protons than chlorine, thus the nuclear charge of argon is higher. As electrons are added to the same quantum shell, shielding effect is relatively constant for chlorine and argon. Thus the effective nuclear charge increases from chlorine to argon, implying that the attraction between the