Edexcel Jan 2012 Paper 5 Q21 (with explained solutions) |

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Edexcel Jan 2012 Paper 5 Q21

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PMT SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 (a) Chromium is a typical transition metal, although its electronic configuration does not fit the general trend found in the first transition series. Complete the electronic configurations in s,p,d notation for vanadium and chromium. (1) Vanadium: [Ar]Chromium: [Ar](b) Some interconversions found in the chemistry of chromium are shown below. Use this information to answer the questions that follow. [Cr(H2O)6]2+ sky blue metal + H2SO4 step 1 [Cr(H2O)6]3+ violet step 2 2 CrO4 yellow H2NCH2CH2NH2 charcoal catalyst step 4 OH(aq) H3O+(aq) step 3 Cr2O7 2 orange [Cr(H2NCH2CH2NH2)3]3+ deep blue (i) State two typical properties of transition metals, other than the formation of coloured ions, which are shown in the diagram above. (2)10 *P35302A01028*<br />
PMT (ii) Use E (cid:57) values from your data booklet to suggest a metal that could be used for step 1. Justify your answer by calculating E (cid:57) for your cell. (2)(iii) Explain, using oxidation numbers, whether or not the conversion in step 3 is a redox reaction. (2)(iv) The organic compound H2NCH2CH2NH2 that is used in step 4 is 1,2-diaminoethane, often called ethylenediamine. It is a bidentate ligand. Explain the meaning of this term. (1)(v) Explain, in terms of its structure, how H2NCH2CH2NH2 can act as a bidentate ligand whereas H2NNH2 cannot. (2)*P35302A01128* 11 Turn over<br />
PMT (c) The half-equations relating the interconversion of the species Cr2+(aq), Cr3+(aq) and Cr2O7 2(aq) are given below. Half-equation I: Cr3+(aq) + e (cid:111) Cr2+(aq) Half-equation II: Cr2O7 2(aq) + 6e + 14H+(aq) (cid:111) 2Cr3+(aq) + 7H2O(l) (i) Use your data booklet to find E (cid:57) for each of the above half-equations. (1) Half-equation IVolts Half-equation IIVolts *(ii) Write the overall equation for the disproportionation of Cr3+ into Cr2+ and Cr2O7 2. Use the E (cid:57) values you have obtained in (c)(i) to show whether or not this disproportionation is feasible under standard conditions. (4)(Total for Question 21 = 15 marks) 12 *P35302A01228*<br />

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Section B Question Number 21(a) Acceptable Answers 3d34s2 OR 4s23d3 3d54s1 OR 4s13d5 both must be correct. ALLOW Electron numbers could be on the line or as subscripts IGNORE case of letters Acceptable Answers Question Number 21(b)(i) Variable/varying/different/several/ more than one oxidation state /number (1) Complex (ion formation) (1) Treat Physical properties (if correct) including catalytic activity as neutral Acceptable Answers Question Number 21(b)(ii) The following metals scores (2) Reject Mark Mark Reject Each metal has a different oxidation number Ligand exchange Reject Mark marks with correct E value: Mg 1.96, Ce 1.92, U 1.39, Al 1.25, Mn 0.78, V 0.77, Zn 0.35 NOTE: Positive sign/unit not needed, but penalise negative value The following metals score (1) mark with correct E value: Li 2.62, Rb 2.52, K 2.51, Ca 2.46, Na 2.30, Cr 0.33, Fe 0.03 NOTE: Positive sign/unit not needed, but penalise negative value All other metals 0/2 6CH0501 1201<br />
Question Number Acceptable Answers 21(b)(iii) Not a redox process Reject Mark Chromate and dichromate both the same/no change in oxidation number (1) contain Cr(VI) 6/6+ (1) Mark independently OR Not redox and both contain Cr(VI) 6/6+ (2) 6CH0501 1201<br />
Mark Reject to the molecule Mark Reject N=N, or triple bond in hydrazine max 1 or if implies only en has lone pairs max 1 Acceptable Answers Question Number 21(b)(iv) Forms two (dative/covalent) bonds/has two lone pairs (to the Transition Metal/ion) OR donates two pairs of electrons (to the Transition Metal/ion) Check answer to (v) if mark not awarded here Acceptable Answers Question Number 21(b)(v) Any two from Both have two nitrogen atoms with lone pairs or implied (1) or Far enough apart/longer chain in between in en (but not in hydrazine)/too close in hydrazine/hydrazine is too short/not as long (1) or Dative bonds/lone pairs too close/repel in hydrazine (1) OR for two marks Forms 5-membered ring (with en with no angle strain/stable) (2) or Bond angles too acute/too much ring strain in hydrazine (2) Mark for iv can be awarded here. 6CH0501 1201<br />
Question Number 21(c)(i) Acceptable Answers - 0.41 (V) +1.33 (V) Both answers needed, with number and sign, for 1 mark IGNORE additional words Reject Mark 6CH0501 1201<br />
Question Number *21(c)(ii) QWC Acceptable Answers Reject Mark 2- ALLOW 6Cr3+ + 2Cr3+ instead of 8Cr3+ Combines the equations to obtain 8Cr3+ + 7H2O 6Cr2+ + Cr2O7 +14H+ 1 max for the equation if electrons are IGNORE state symbols even if shown balanced wrong or unbalanced species (1), balance (1) =1.74V (1) Eo So not feasible on condition of negative value (1) OR 6Cr2+ + Cr2O7 7H2O If fully correct (1) Eo Disproportionation not feasible on condition of positive value but reject reaction is spontaneous (1) Other wrong equations IF Cr2O7 Then + 1.74 V (1) If Cr3+ alone on the left Then -1.74 V (1) and reaction not feasible (1) reaction = + 1.74V (1) 2- +14H+ 8Cr3+ + reaction 2- or Cr2+ on left 6CH0501 1201<br />

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