OCR A Jun 2012 Paper 1 Q3 (with explained solutions) |

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OCR A Jun 2012 Paper 1 Q3

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3 Magnesium and strontium are typical Group 2 metals. (a) Complete the electron configuration of a magnesium atom. 1s2[1] (b) The second ionisation energy of magnesium is 1064 kJ mol1. (i) Write an equation to represent the second ionisation energy of magnesium. Include state symbols.[2] (ii) Magnesium has a greater second ionisation energy than strontium. Explain why. In your answer, you should use appropriate technical terms spelled correctly.[3]OCR 2012<br />
(c) A student watches a demonstration in which strontium is added to water. The equation for the reaction is shown below. Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g) (i) Write the formulae of the aqueous ions formed when this reaction takes place.[2] (ii) In terms of electron transfer, explain why strontium has been oxidised.[1] (iii) Suggest two compounds which could be reacted together to produce Sr(OH)2.[1] (d) Sr(OH)2 can react with an acid to form SrSO3. (i) The systematic name for SrSO3 is strontium sulfate(IV). Why does the number (IV) have to be used here rather than simply naming this compound as strontium sulfate?[1] (ii) Suggest the formula of the acid used to form SrSO3.[1] [Total: 12]OCR 2012 Turn over<br />

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F321 Mark Scheme June 2012 Answer Marks Question 3 (a) (1s2) 2s2 2p6 3s2 (cid:57) (b) (i) Mg+(g) (cid:198) Mg2+(g) + e Equation correct (cid:57) State symbols correct (cid:57) (ii) Ionic radius mark Mg(+) has smaller (ionic) radius OR has less shells (cid:57) Shielding mark (outermost electron) of Mg(+) experience less shielding (cid:57) Nuclear attraction mark More nuclear attraction on (outermost electrons) OR Outer electrons are attracted more strongly (to the nucleus) (cid:57) ORA throughout Guidance IGNORE 1s2 seen twice ALLOW subscripts ALLOW Mg+(g)e (cid:198) Mg2+(g) for 2 marks The second mark is dependent upon the first mark except for the following close attempts for the first mark: ALLOW the following for one mark as states are correct Mg(g) (cid:198) Mg2+(g) + 2e Mg(g) + e- (cid:198) Mg2+(g) + 2e ALLOW e for electron IGNORE states on electron Use annotations with ticks, crosses, ECF etc for this part ALLOW Mg(+) has less energy levels ALLOW Mg(+) has electrons in lower energy level ALLOW Mg(+) has electrons closer to nucleus IGNORE Mg(+) has less orbitals OR less sub-shells IGNORE atomic for ionic IGNORE different shell ALLOW screening for shielding ALLOW Mg(+) has less electron repulsion from inner shells Quality of Written Communication: nuclear OR nucleus OR electron(s) spelled correctly once and used in context for the third marking point ALLOW Mg(+) has more nuclear pull IGNORE Mg(+) has more effective nuclear charge DO NOT ALLOW more nuclear charge for more nuclear attraction for the third mark<br />
June 2012 Guidance ALLOW 2OH ALLOW 2 marks for Sr(OH)2 (cid:198) Sr2+ + 2OH ALLOW 1 mark for Sr2+ + 2OH (cid:198) Sr(OH)2 IGNORE H+ ALLOW Sr (cid:198) Sr2+ + 2e IGNORE references to oxidation numbers ALLOW acceptable alternatives from Sr salts and alkalis eg SrCl2 + NaOH DO NOT ALLOW the charge on sulfur DO NOT ALLOW shows the oxidation number of the sulfate ALLOW Otherwise it could be SrSO4 ALLOW Sulfur has different oxidation numbers AW F321 Question (c) (i) Sr2+ (cid:57) OH (cid:57) Answer Mark Scheme Marks (ii) Sr has lost (two) electrons (cid:57) (iii) SrO AND H2O (cid:57) (d) (i) It shows the oxidation number of the sulfur OR the name without the IV is ambiguous (cid:57) (ii) H2SO3 (cid:57) 12 Total 10<br />

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