WJEC Chemistry for AS: Study and Rev Guide

Rhiannon’s answer David’s answer (a) A big jump in ionisation energy means that an electron has been removed from a new shell. 3 There is a big jump after removing 6 electrons, so there are 6 electrons in the outer shell and 2 electrons in the inner shell. 3 ✗  (b) O – (g) + e —> O 2– (g) X (c) (i) Oxygen has more protons so nucleus pulls electrons tighter. 3 ✗  (ii) Repulsion between paired electrons in oxygen makes it easier to remove one of the electrons. 3 ✗  (iii) Oxygen has one shell less so outer electron is closer and there is less shielding. 33 (a) There is a large jump after removing 6 electrons, so there are 6 electrons in the outer shell and 2 electrons in the inner shell. 33 There is not much difference between electron 5 and 6 so these two electrons are in the same orbital in the outer shell. 3 (b) O + (g) + e —> O 2+ (g) 3 (c) (i) Oxygen has a greater nuclear charge and there is no extra shielding. 33 (ii) Nitrogen only has unpaired electrons while oxygen has a pair of electrons in the 2p orbital. 3 The repulsion between paired electrons makes it easier to remove one of the electrons. 3 (iii) Because its electron is closer to the nucleus and there is less shielding. 3 ✗  Examiner commentary  To gain the third mark, Rhiannon needs to mention the different orbitals in the outer shell.  Rhiannon needs to state that the outer electrons are in the same shell or that there is little extra shielding.  Rhiannon has not made it clear that there are only unpaired electrons in nitrogen’s outer orbital. Rhiannon achieves 6 out of 10 marks. Examiner commentary  David needs to specify that the outer electron is closer to the nucleus to gain both marks. David achieves 9 out of 10 marks. Ionisation energies Q & A AS Chemistry: Study and Revision Guide 116 2 The graph below shows the logarithm of the successive ionisation energies of oxygen plotted against the number of the electron removed. 3 1 2 3 4 5 6 7 8 3.5 4 4.5 5 No. of electron removed log I.E. (a) Explain the shape of the graph in terms of the electronic structure of oxygen. [3] (b) Write an equation to represent the second ionisation energy of oxygen. [1] (c) Explain why oxygen’s first ionisation energy is: (i) higher than that for carbon [2] (ii) lower than that for nitrogen [2] (iii) higher than that for sulfur. [2]