A-LevelChemistryChemical periodicity9701/21 · Oct/Nov 2017 · Q214 Marks

The elements in the third period, and their compounds, show trends in their physical and chemical properties.

(a)A sketch graph of the first ionisation energies of five successive elements in the third period is shown. [Figure X.X]
(a)(i)Explain why there is a general increase in the first ionisation energy across the third period.[2]
(a)(ii)Sketch, on the graph, the position of the ionisation energies of the two elements that come before Mg in this sequence.[2]
(a)(iii)Explain, with reference to electron arrangements, the decreases in first ionisation energy between Mg and Al and between P and S. Mg and Al ................................................................................................................................................................................ ................................................................................................................................................................................................ P and S ................................................................................................................................................................................... ................................................................................................................................................................................................[4]
(b)The chlorides of the elements in the third period behave in different ways when added to water, depending on their structure and bonding. L and M are each a chloride of an element in Period 3. A student investigated L and M and their results are given. L is a white crystalline solid with a melting point of 987K. L dissolves in water to form an approximately neutral solution. Addition of NaOH(aq) to an aqueous solution of L produces a white precipitate. M is a liquid with a boiling point of 331K. M is hydrolysed rapidly by cold water to form a strongly acidic solution, a white solid and white fumes. Identify L and M. Explain any properties and observations described. Give equations where appropriate.
(b)(i)L is[3]
(b)(ii)M is[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] due to increasing nuclear attraction (for electrons)
[1] due to increasing nuclear charge / atomic / proton number AND similar shielding / same (outer/number of) shell / energy level
[1] Cross shown on first vertical line from the y-axis (Group 0 / Ne) is clearly higher than all shown
[1] Cross shown on second vertical line from the y-axis (Group 1 / Na) lower than all shown
[1] Al (the outer / valence) electron (which is lost) is in (3)p sub-shell (Mg is in (3)s subshell) OR Al (the outer / valence) electron (which is lost) is in higher energy sub-shell ora
[1] (electron to be removed) is more shielded / experiences greater screening effect ora
[1] S has a pair of electrons in (a) (3)p orbital / (a 3)p orbital is full ora
[1] electron pair repulsion
[1] (L=) MgCl2 / magnesium chloride
[2] Any two from: (giant) ionic (with strong attractions) ; Mg2+(aq) / Mg(H2O)62+(aq) is neutral / undergoes (partial) hydrolysis ; Mg(OH)2 is the white precipitate / solid / insoluble / partially soluble ; MgCl2 + 2NaOH → Mg(OH)2 + 2NaCl
[1] (M=) SiCl4 / silicon chloride
[2] Any two from: (simple) molecular / simple covalent ; hydrolysis possible due to available d orbitals ; forms HCl (aq) / hydrochloric acid / solution and / or HCl gas / fumes ; white solid is (hydrated) SiO2 ; SiCl4 + 2H2O → SiO2 + 4HCl