CHE5301B-INORGANIC CHEMISTRY.pdf PDF

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UNIVERSITY OF EAST ANGLIA School of Chemistry Main Series UG Examination 2016-17

INORGANIC CHEMISTRY CHE-5301B

Time allowed: 2 hours

Answer THREE questions. You are advised to spend an equal amount of time on each question. All questions carry an equal number of marks. Answer EACH question in a SEPARATE answer book. The breakdown of marks within each question is indicated by the percentage figures in brackets on the right.

The following are provided: Periodic Table.

Notes are not permitted in this examination. Do not turn over until you are told to do so by the Invigilator.

(CHE-5301B) Copyright of the University of East Anglia

Module co-ordinator: John Fielden (CHE) Version 1

2 1.

Answer BOTH parts.

(a)

Consider compounds A to D and their associated magnetic moments, eff: A

VCl4

B

K2[Ni(H2O)6](SO4)2  eff = 3.3 B

C

K3[Fe(CN)6]

 eff = 2.4 B

D

Cr(acac)3

 eff = 3.8 B

eff = 1.6 B

Using a simple ligand field approach give for A, B, C, and D: (i) The oxidation states for the transition metals and the numbers of delectrons. (ii) The d-orbital splitting diagrams for the transition metals showing the electronic configurations, stating the number of unpaired electrons in each case. (iii) The total ligand field stabilisation energy. (iv) Are the given values of  eff consistent with the spin-only formula? Explain any deviations. [80%] (b) Account for the pattern of the lattice enthalpies of the metal difluorides MF2, shown below. Include the metals Ca, Mn and Zn in your explanation. 3100

lattH kJ/mol

3000 2900 2800 2700 2600 2500 Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

[20%] (CHE-5301B) 5/22/2017 4:11 PM

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3 2.

Answer BOTH parts.

(a) Three reactions of main group hydrides and fluorides, and a description of the NMR spectra of the products are shown below.

1

BH3

H-

BH411

2

3

PF5

PF5

F-

B-NMR: 1:4:6:4:1 Quintet

PF631

H2O

P-NMR: 1:6:15:20:15:6:1 Sextet

PF3O + 2 HF 31

P-NMR: 1:3:3:1 Quartet

(i) Use your chemical and spectroscopic knowledge to deduce and draw the structures of the products, indicating their stereochemistry. (ii)

Explain the observed 11B and 31P NMR spectra.

(iii) Predict the 1H-NMR spectrum of the product in 1, and 19F-NMR spectra of the products in 2 and 3. (iv) The reaction of PF5 with aqueous HF may produce a range of fluorophosphorus species, for example PF3O, PF6-, PF2O2-. Is 31P or 19F-NMR the better spectroscopic tool to investigate the products? Explain why. [60%]

question 2 continues…/

(CHE-5301B) 5/22/2017 4:11 PM

TURN OVER Version 1

4 …/question 2 continued (b) The starting materials in the reactions above, BH3 and PF5, are both Lewis acids. (i) Define the term Lewis acid and describe the differing origins of Lewis acidity in BH3 and PF5. (ii) What type of Lewis acid reaction is occurring in 1 and 2 above, and in the first step of reaction 3? What is the structure of the intermediate formed in reaction 3? (iii) What would occur if H2O were replaced by NEt3 in reaction 3? Predict the structure of the final product and its 19F{31P}-NMR spectrum. [40%] 1

H, 100%, I = ½ B, 80.1% I = 32 19 F, 100%, I = ½ 31 P, 100%, I = ½ 11

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5 3.

Answer ALL parts (a) to (e).

(a) Assign the electron counts of compounds E to J, demonstrating how you arrived at these counts. [24%] (b) Assign the formal oxidation states of the metals in compounds E to J, showing your reasoning. [18%] (c) Explain which of the compounds H, I or J has the highest CO stretching frequency and give the reasons. [18%] (d)

(i) Is anion H converted to I using NaCl or Cl2? Explain your reasoning, making reference to the oxidation state of Fe. (ii)

(e)

Give the balanced chemical equation for this reaction.

[20%]

(i) What is the role of Ag+ in conversion of I to J? What other reagent do you need? (ii)

Give the balanced equation for this reaction.

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[20%]

TURN OVER Version 1

6 4.

Answer TWO parts of (a) to (c)

(a) Account for the four absorptions labelled A to D in the UV/Visible spectrum of a 0.1 M solution of Cr(H2O)63+. In your answer explain whether the electronic transition giving rise to each absorption feature is an allowed one.

[50%] (b) The shapes of borane clusters are described by Wade’s rules, but these rules also apply to many other clusters of main group elements. Apply Wade’s rules to first classify the following clusters and then to predict and draw their structures: (i) (ii) (iii) (iv) (v) (c)

B4H62B5H72CB5H6B10H106Pb2Bi2

[50%]

(i) Give the balanced chemical equation for the preparation of ferrocene (Cp2Fe) from FeBr2 and a suitable reagent (Cp = 5-C5H5) and show the structure of the product. (ii) Give the balanced chemical equation for the reaction of Cp2Fe with iodine to give an ionic product K. Show the structure of K. (iii) Give the balanced chemical equation for converting the product K back to Cp2Fe and suggest suitable reagents. [50%] END OF PAPER

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CHE-5301B exam feedback 2016/17 Question 1 68 Answers, Mean = 68.91% SD = 18.87% Part (a) was very well answered and the common mistakes made were those that students had been reminded about a number of times. (i) Giving an octahedral splitting diagram and ligand field stabilisation energies for the tetrahedral case. (ii) Forgetting to label the d-orbital splitting diagram. (iii) Incorrectly determining the oxidation state and number of d-electrons. (iv) Confusing the two spin-only formulae for the magnetic moment. (v) Rather than using the magnetic moments given to confirm the electron configuration working out a wrong configuration and then attempting to explain a deviation in the magnetic moments. In contrast part (b) was poorly answered, only one or two recognised the double hump feature as being an example of ligand field stabilisation energies associated with partial filling of the d-orbitals in an octahedral ligand field. However, almost all could explain the general trend of increasing lattice energy in terms of Zeff and smaller ions though few correctly stated this in terms of the ionic model. Question 2 27 Answers, Mean = 64.72% SD = 23.74% Regrettably, the fear that NMR spectroscopy seems to inspire in many students stopped them from spotting what was an eminently doable question. If the 27 students who did it are representative of the cohort more generally, then the rest would have been rewarded with a higher mark had they taken it on instead of 3 or 4. The broad take home message should be that working at things that initially seem tough and require problem solving skills is rewarded – avoiding them is not. So, overall, the question was answered quite well. Nearly all students got the structures in Part A right (whether by recall, VSEPR or deduction from NMR was unimportant), the vast majority also succeeded in explaining the described spectra. Students also correctly understood what a Lewis acid is in part b, understood the origin of Lewis acidity and the type of reaction occurring in (a) 1 and 2. The most common mistakes were as follows: 

  



Finding axial and equatorial chemical environments in PF6- when in fact there is only one environment –all of the F atoms are identical by symmetry. This should be clear from the description of the 31P NMR spectrum. Not realising that coupling to and I = 3/2 nucleus will give a 1:1:1:1 quartet by 2I+1 (not a doublet) and not understanding the consequence of partial abundance properly. Forgetting that 31P has nuclear spin when predicting the 19F spectra. Not realising (from the predictions made) that 31P-NMR gives more information about the structure of fluorophosphate products (number of fluorines/fluorine chemical environments) than 19F-NMR does. Nonetheless, a number of students were awarded substantial credit for coherent arguments in favour of using 19F. Not understanding that N-C bonds will not break after Lewis adduct formation between NEt3 and PF5 (unlike OH) because they are so much less polar.

Question 3 62 Answers, Mean 52.85% SD 20.61% This question saw an improvement in mark from ca. 46% last year, in line with an improved overall average of nearly 60%, and is probably a very fair reflection of the standard of the class overall. Qu. 3 followed the same pattern of previous years and is always a popular question since many answers are predictable. This year, as well as last year, some main group compounds were introduced in the electron counting question. This should have been very easy and a large proportion of students got this right. An equally large proportion went spectacularly wrong. This can only be explained by the low attendance at relevant lectures. All this material was thoroughly presented, and included in revisions sessions.

Question 4 57 Answers, Mean 56.48% SD 16.65% (a) A mix of answers from those with no idea to those who clearly knew the features of the UV-vis spectrum. The poor answers given in a number of cases was especially disappointing given the topic formed the basis of one of the practical experiments and had been expressly described in lectures. But was also straightforward enough to be worked out by students who were aware of the selection rules in electronic spectroscopy. (b) The vast majority answered the Wade’s rules question well. Some marks were lost for failing to show clear working on Wade, or errors in calculations and drawings. Most students remembered to count 3 electrons for CH and Bi and two for Pb. Generally there were few serious problems and most students gained a large percentage of their marks from this part of the question. Unfortunately, though, it seems to have sucked students into answering a mixed question where they performed badly on the other parts. (c) The performance on this question was very disappointing as the reactions involved – synthesis of ferrocene and its oxidation and reduction – are simple. Once again, it highlights the phobia of many of our students in dealing with simple chemical equations. The material in this part-question has been the subject of revision lectures and follows closely from what was presented and asked in previous years....