NJC Prelim P2_Solutions

1 NJC SH2 Preliminary Examination 9729 / 02 / 18 [Turn Over NJC SH2 H2 Chemistry P2 Solutions 1 The properties of elements and their compounds show similarities, differences and trends depending on the positions of the elements. (a) The elements in the third period, and their compounds, show trends in their physical and chemical properties. A sketch graph of the first ionisation energies of five successive elements in the third period is shown. (i) Sketch on the graph, the position of the ionisation energy of the two elements that come before Mg in this sequence. Cross shown on first vertical line from the y-axis (group 0/Ne) is clearly higher than all shown. Cross shown on second vertical line from the y-axis (group 1/Na) is clearly lower than all shown. [1] (ii) Explain, with reference to electronic arrangements, the decreases in first ionisation energy between Mg and Al and between P and S. Mg and Al: The most loosely held electron in A l is in the higher energy 3p subshell while that of Mg is in the lower energy 3s subshell. This outweighs the effect of the increase in nuclear charge from Mg to Al. Hence nuclear attraction for the most loosely held electron in Al is weaker, i.e. Al has a lower 1st IE. [1] P and S: The most loosely held electron in S is one of the paired electrons in 3p orbital while that of P is in the singly filled 3p orbital. Inter-electronic repulsion between the paired electrons in the same p orbital outweighs the effect of an increase in nuclear charge. Hence, nuclear attraction for the most loosely held electrons is weaker in S, i.e. S has a lower first IE. [1] [2] (b) The chlorides of the elements in the third period behave in different ways when added to water, depending on their structure and bonding. x x

2 NJC SH2 Preliminary Examination 9729 / 02 / 18 [Turn Over L is a chloride of an element in Period 3. A student investigated L and the results are as given below. • L is a white crystalline solid with a melting point of 987 K. • L dissolves in water to form a weakly acidic solution. • Addition of NaOH(aq) to an aqueous solution of L produces a white precipitate, M . (i) Identify L and M. L: MgCl2 M: Mg(OH)2 [1] (ii) Write an equation to illustrate the formation of the w