EJC Nov 2019 H1 Chemistry 8873 Paper 2 Suggested Solutions

* 0011376599402 * I IIIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIII IIII 2 Section A Answer all the questions in this section in the spaces provided. 1 ■ Deduce the symbol of this species and include its charge, if any, and its atomic and mass numbers. · l..::_ucLES & MOE 2019 � 8873/02/O/N/19 _J � (a) Define the term isotopes. Isotopes are atoms of the same element with the same number of protons but different ············· .. number of neutrons······· ........................... ............................................................................................... [1] (b) Compared to the isotope �He, a species has • 8 times as many neutrons, • 9 times as many electrons, • 13 more protons. 31 P3− · 15 ·············································· ............................... Note: atomic number, mass number and charge must all be included in one single symbol·····································:·························· [3] Working: Number of protons = 2 + 13 = 15 Hence, the unknown species is P. Number of electrons = 9(2) = 18 Hence, charge of unknown species = 3− Number of nucleons = 8(4−2) + 15 = 31

* 0011376599403 * I IIIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIII IIII 3 (c) The relative atomic mass of an element depends on the percentage abundance of each isotope present. (i) A sample of sulfur contains four isotopes, as shown in Table 1.1. Table 1.1 isotope relative percentage isotopic mass abundance I% 328 31.972 95.02 33s 32.971 0.75 343 33.968 4.21 363 35.967 0.02 Calculate the relative atomic mass of sulfur in this sample, giving your answer to two decimal places. (ii) Suggest why the relative atomic mass in the Periodic Table for nickel is lower than that for cobalt. ....................................................................................................................................... [1] (d) Nickel reacts with sulfur to form nickel(II) sulfide, NiS. State the electronic configurations of the nickel ion, Ni2+, and the sulfide ion, s2-. Ni2+ 1s2 .................................................................................................................................. . s2- 1 s2 .......... .................................... .............. ................. ...... ....... .......... .... .......................... . [2] [Total: 9] l.!,ucLES & MOE 2019 � 8873/02/O/N/19 [Tumov� Note: answer must be in 2 d.p. (as stated in the question) or no credit will be given A sample of cobalt contains primarily of the isotope 59Co, while in a sample of nickel, 58Ni is the isotope with the highest percentage abundance. 2s2 2p6 3s2 3p6 3d8 2s2 2p6 3s2 3p6

Na is oxidised as the oxidation number of Na increases from 0 in Na to +1 in Na2O. KNO3 is reduced as the oxidation number of N decreases from +5 in KNO3 to 0 in N2. Na → Na+ + e− 2NO3− + 10e− → N2 + 6O2− Note: species such as H2O, H+ and O2 should not appear in the half equations as they are not present in reaction 2. Acid-base reaction Na produced from reaction 1 will react with the KNO3 present via reaction 2 to form additional N2 gas, leading to the pressure in the airbag being greater than 1.00 atm.

KNO3 is added to remove the Na produced in reaction 1 which is highly reactive and potentially explosive if it comes into contact with water. SiO2 is added to remove the Na 2O and K 2O produced in reaction 2 which will produce corrosive NaOH and KOH if it comes into contact with water. OR HN3 2NaN3 + 2HNO2 → 3N2 + 2NO + 2NaOH

The standard enthalpy change of combustion is the energy released when 1 mole of a substance is completely burnt in excess oxygen at 1 bar and at a specified temperature, usually 298 K.

Bond energy values involve bond breaking in the gaseous state. However, in the reaction involved in the enthalpy change of combustion of butane, water is in the liquid state. Note: not higher or lower It will be more exothermic than that calculated in (b) as energy is released when water changes from the gaseous to liquid state. Lattice energy of sodium chloride is less exothermic than that of magnesium oxide as and Na+ and Cl− have lower charges and larger ionic radii than Mg2+ and O2−. Note: not atomic radii Note: Comparisons of both the charges and ionic radii are required as both factors differ in the ions involved.

2Fe3+ + Zn → 2Fe2+ + Zn2+ Amount of Fe2+:Amount of Fe3+ = 0.0468:0.0932 = 1:2. For every Fe2+ ion, there will be two Fe3+ ions, resulting in a total charge of +8. Hence, four O2− ions are required to balance the charge of three iron ions, and the Fe:O mole ratio is 3:4. Error 1 will result in lower amounts of both Fe2+ and Fe3+ in solution X and hence less Fe2+ in solution Y. This leads to lower titre values in both titrations and hence lower calculated amounts of both Fe 2+ and Fe 3+. Error 2 will have no effect on the amount of Fe 2+ calculated in (a)(i) . However, it will result in less Fe 2+ in solution Y and hence lower titre value in the second titration, leading to a lower amount of Fe3+ calculated in (c).