OCR A Jun 2017 Paper 2 Q1 (with explained solutions) |

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OCR A Jun 2017 Paper 2 Q1

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Answer all the questions. 1 Barium combines with oxygen, chlorine and nitrogen to form ionic compounds. (a) Barium oxide, BaO, has a giant ionic lattice structure. (i) State what is meant by the term ionic bond.[1] (ii) Draw a dot-and-cross diagram to show the bonding in barium oxide. Show outer electrons only. (iii) Calculate the number of barium ions in 1.50 g of barium oxide. Give your answer in standard form and to three significant figures. [2] number of barium ions =[2] (b) Barium chloride, BaCl2, is soluble in water. Explain your answer in terms of the particles involved. (i) Compare the electrical conductivities of solid and aqueous barium chloride.[2]<br />
(ii) Describe the use of aqueous barium chloride in qualitative analysis.[2] (iii) Hydrated barium chloride can be crystallised from solution. Hydrated barium chloride has the formula BaCl2xH2O and a molar mass of 244.3 g mol 1. Determine the value of x in the formula of BaCl2xH2O. Show your working. (c) Barium nitride is formed when barium is heated with nitrogen. (i) Complete the electron configuration of a nitride ion. x =[2] 1s2[1] (ii) Solid barium nitride is reacted with water, forming an alkaline solution A and an alkaline gas B. Identify A and B. Write an equation, including state symbols, for the reaction. AB[4]<br />

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H032/02 Question Answer Mark Scheme June 2017 (a) (i) Electrostatic attraction between positive and negative ions(ii) Ba shown with either 0 or 8 electrons AND O shown with 8 electrons with 6 dots and 2 crosses (or vice versa) Correct charges on both ions(iii) FIRST CHECK THE ANSWER ON THE ANSWER LINE IF answer = 5.891021 award 2 marks for calculation Moles of barium oxide n(BaO) =1.50/153.3 OR 9.78103Number of barium ions (9.781036.02 x 1023) = 5.8910213 SF AND standard form required (b) (i) Barium chloride does not conduct electricity when solid AND Marks Guidance ALLOW oppositely charged ions ALLOW cations and anions ALLOW + for positive and - for negative IGNORE references to metal and non-metal IGNORE references to transfer of electrons For first mark, if eight electrons are shown around Ba, the extra electrons around O must match the symbol chosen for the electrons for Ba. IGNORE inner shells Circles not required Brackets not required ALLOW 0.00978 up to calculator value 0.009784735 ALLOW ECF from incorrect moles of BaO Common incorrect answers are shown below IF 137.3 is used for the molar mass ALLOW 1 mark total for 6.58 x 1021 (0.010924981 mol) OR 6.56 x 1021 (0.0109 mol) IF 153 is used for the molar mass ALLOW 1 mark total for 5.901021 IGNORE use of free instead of mobile ALLOW ions are not free to move<br />
H032/02 Question Answer because it has ions which are fixed (in position/in lattice)Marks Mark Scheme June 2017 Barium chloride conducts when in aqueous solution AND because it has mobile ions 2(ii) Test for sulfate/SO4 White precipitate forms (when barium chloride solution is mixed with a solution containing sulfate ions) (iii) FIRST CHECK THE ANSWER ON THE ANSWER LINE IF answer = 2 award 2 marks M(BaCl2) = ((137.3 + (35.52)) = 208.3 (g mol1)244.3208.3 = 36 AND 36/18 = 2 (1s2) 2s2 2p6 (c) (i) (ii) Products of reaction A = Barium hydroxide/Ba(OH)2 B = Ammonia/NH3 Formula for barium nitride Ba3N2 Balanced equation AND state symbolsBa3N2(s) + 6H2O(l) 3Ba(OH)2(aq) + 2NH3(g)State symbols are required Total 16 Guidance ALLOW ions are held (in position/in lattice) ALLOW ions are not mobile IGNORE charge carriers DO NOT ALLOW electrons moving ALLOW one mark for comparison that does not identify (s) and (aq). IGNORE hydrochloric acid ALLOW white solid IGNORE cloudy DO NOT ALLOW test result linked to incorrect anion ALLOW 208 (g mol1) ALLOW ECF for incorrectly calculated molar mass provided the final answer is rounded to nearest whole number IGNORE 1s ALLOW upper case letters AND subscripts ANNOTATE ANSWER WITH TICKS AND CROSSES ETC ALLOW one mark for correct products incorrectly labelled Formulae must be correct No ECF from any incorrect formula ALLOW multiples Correct equation with state symbols scores 4 marks seen twice<br />

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