The synthesis and analysis of hexaammine nickel(ii) chloride

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Milo Freeman
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1 Candidate 1 The synthesis and analysis of hexaammine nickel(ii) chloride 1.Abstract Aim and Findings The aim is clear and the findings match the aim. The Abstract immediately follows the contents and is separate to the Introduction section. There is an error in the average % yield given in the conclusion the average % yield should be 56.35% as stated in the abstract and so this is not penalised here. There is some good chemistry included in this report and many of the main points have been covered. Coordination number, oxidation number, ligands and dative bonds are all mentioned. There is a good explanation of d splitting. Equations have been given. Some of the theory included could have been further expanded, for example a description of a dative covalent bond could have been included. 2.Introduction Underlying Chemistry 4 3 The orbital box notation does not match the spectroscopic notation, both claim to be for the ion (although it does not specify which ion so assume 2+), orbital box is correct for nickel atom, spectroscopic is not correct for neither the atom nor the ion. A more detailed explanation for the six water ligands for nickel(ii) chloride could have been given as well as an explanation as to why one salt cyrstallises and the other does not. A discussion about complementary colours should have been included. Back titration is not well explained and there is a charge has not been superscripted on the ammonium ion. The text suggests that it is the number of moles of ammonia in the hexaaqua complex that will be determined. There is an error in the colour change for phenolphthalein (page 6). The charge on EDTA is wrong in the equation should be superscripted 4-. 1

2 A diagram of the octahedral EDTA complex could have been included. There is no discussion about the indicator to be used in the EDTA titration nor why two indicators are used on the standardisation of HCl. Although there is no requirement for a complete or perfect underlying chemistry, the minor errors and omissions would suggest this is a reasonable rather than a good account. 3.Procedures 4.Results a.appropriate to aim(s) b.procedures clearly described c.appropriate level for Advanced Higher d. Duplication e. Accuracy of measurem ents f. Risk assessme nt Quantitative or qualitative The procedures used are appropriate to the aims. All the procedures have been described and the main procedures (synthesis, back titration and EDTA titration) could be repeated. There are some errors - not enough detail has been given for standard solution preparation. The wrong colour change was given for KHP standardisation with NaOH (no indication as to what KHP is either). There is no detail for quantitative transfer on. The mass given for KHP used in the results are very different from the procedures. The evaluation suggests that a standard solution of nickel salt was prepared for using in the back titration but this doesn t appear here. The procedures are at an appropriate level for Advanced Higher level (1). Two techniques have been carried out - synthesis and titrations (1). Mostly the procedures have been duplicated (standardisation was not duplicated but this would not be considered to be appropriate). From the description it is not possible to tell if the standard solutions were made accurately, for example, it is not clear if the solids were dissolved in the beaker then transferred with rinsings. This affects concentration and so affects the results. Major hazards (1) and control measures (1) given. Quantitative (1) The results are relevant to the aims. (1) Raw data is given and accurate. It is missing the weighing by difference masses for nickel salt used in EDTA titration this can be treated as minor since the rest is given. (1) Significant figures are acceptable. (0) There are 2 different Gram Formula Mass 2

3 5.Discussion a. Conclusion b. Evaluation of procedures c. Evaluation of results (GFM) quoted for hexaaqua complex correct one used in calculation. Calculation for % yield in run 2 should be 56.9% using these figures. The theoretical yield is possibly wrong. Again, % yield answer is based on correct numbers used incorrect ones are written in the sample calculation. One calculation is unclear (page 13) the procedure stated that a 1 moll -1 solution of sodium carbonate was used. This implies a solution of approximately moll -1 was prepared. There is no calculation for the exact concentration of this solution either, using the mass that was actually measured of solid sodium carbonate. It is unclear which value is accurate phenolphthalein or methyl orange for the standardisation. In the KHP calculations it is unclear which mass to use for concentration of KHP as the mass in the results is different to that used in procedures. Neither of the accurate HCl concentrations calculated from the standardisation has been used in the calculation (page 17). The wrong GFM (page 19) is used to calculate % by mass Ni. Titration tables have no units. No marks are awarded for presentation of raw and processed results. (1) Correct observation (document page 8, 9 and 10) (the wrong one on document page 9 is ignored) The yield quoted should be or 56.4% and not the 56.3% that has been stated. Although this could be considered to be a typo, it also amounts to a rounding error and means that the abstract was not consistent with the conclusion. In this instance, the decision was made to remove the mark here since the abstract has the correct value quoted. Some suggestions have been given for yield being less than 100. Uncertainties in equipment have been quoted. The candidate mentions making a standard solution of complex for use in the back titration but no mention of this in procedure so it is not clear if the g of salt used in the calculation (document page 17) was dissolved in 250cm 3 standard flask or not. The candidate compares the % nickel to the theoretical and so a generous 1 mark has 3

4 been awarded. 6.Presentation d. Quality of project a. Structure b.references This project does not merit the mark for quality as there are too many minor errors throughout the report. Although it seems a large project, there are many pages with few words on them and so is likely to be within the word limit. There is a title, contents page and page numbers. References have been cited and these have been referenced correctly at the back of the report. Total 30 marks 21 4

5 Candidate 2 Investigating the binding between ligands and transition metal complexes 1.Abstract Sub-category Aim and Findings 2.Introduction Underlying Chemistry 4 1 The abstract immediately follows the contents page and although it is not essential, the abstract is under a separate heading. The aim is clear and the findings are consistent with the conclusion. The first paragraph of the introduction describes transition metal complexes, ligands and dative bonds. The candidate could have shown diagrams here to demonstrate denticity and why ammonia has a lone pair of electrons. An explanation of what EDTA means could also have been given. The second paragraph discusses the origin of colour in complexes, d-d transitions and the spectrochemical series. This theory could easily have been improved with the usual diagrams showing octahedral complexes and associated splitting patterns. The candidate could also have given equations for the ligand exchange reactions that take place. The candidate shows a basic misunderstanding of this theory by claiming that ligands high in the spectrochemical series cause transitions of lower energy values. In addition there is a fundamental flaw in the theory presented by the candidate. Analysis of the colours absorbed by these complexes does not lead to a value for bond enthalpies per mole of solution. The candidate could have used the results from the experiments to confirm the spectrochemical series as the values calculated are related to the splitting energy not bond enthalpy. The candidate gives a brief account of colorimetry. This could have been improved by including a more detailed explanation of the role of specific filters, a diagram of how a colorimeter operates and examples of spectra showing lambda max for different coloured complexes. The candidate has also given the equation for converting wavelength to energy. However in this discussion the candidate demonstrates a lack of understanding that was is being measured is the difference in splitting caused by ligand exchange in the hexaaquacobalt(ii) ion and in the 5

6 hexaaquanickel(ii) ion. a. Appropriate to aim(s) b. Procedures clearly described 2 0 The procedure set out does not allow the stated aim to be achieved. The procedure included is the only procedure attempted in this project. The procedure does not detail the concentration of the EDTA and the results indicate some kind of spectrophotometer, not a colorimeter, was used. 3.Procedures c. Appropriate level for Advanced Higher 2 1 d. Duplication e. Accuracy of measureme nts 1 st mark awarded for complexity colorimetry. 2 nd mark not awarded as there are no controls of modifications in the light of experience. The water used to zero the colorimeter is not considered a control. There is evidence of duplication seen as there are two sets of results and two computer generated graphs. Due to the nature of the experiment a high level of accuracy is not required. A dropper used to dispense solutions into cuvettes will not effect lambda max. f. Risk assessment 2 1 Concentrated ammonia and concentrated hydrochloric acid are identified and the hazards are noted. No hazards given for transition metal salts 1 st mark is not awarded. 2 nd mark is awarded as precautions are given for NH 3 and HCl the identified hazards. Relevant (0/1) Since the procedure is not relevant to the stated aim then the results can t be relevant either. 4.Results Quantitative or qualitative 6 3 Raw data recorded (0/1) As lambda max cannot be determined accurately from the graphs, the raw data is not acceptable in this case. Raw and processed results are presented (2/3) 1 mark awarded for tables headings and units are acceptable. The table has kj instead of kj mol -1 - this is treated as a slip as the units are correct elsewhere. 1 mark awarded for significant figures. In the final results these range from 2 to 4 ideally 6

7 these should be quoted to the same number of significant figures however using the Marking Instructions for 2016, this mark was awarded as all the values fall within the accepted range for significant figures (one fewer and two greater). While the calculations are correctly carried out they cannot be verified as the values for the wavelengths cannot be determined from the graphs. Observations (1/1) Although many opportunities for more obvious observations have been missed, the candidate states the solutions were different colours and that specs were seen in some samples. 5.Discussion a. Conclusion b. Evaluation of procedures 3 1 The candidate has not determined the bond enthalpy stated in the aim. In addition the ligands involved are water, ammonia, EDTA and the chloride ion. HCl is not the ligand. The candidate has not interpreted the results correctly The candidate makes one or two valid points regarding the procedure particles floating in the cuvettes may have affected the colorimeter readings; there appears to be a suggestion that the solutions were diluted to give measurable absorbance values - (this confirms the need for the concentration of EDTA in the procedure). c. Evaluation of results 3 1 The candidate realises that without appropriate gridlines the graph cannot provide accurate values for lambda max and that this will cause errors in the final results. 6.Presentatio n d. Quality of project a. Structure b. References Total 30 marks 12 This project does not achieve the quality mark. The project is within the word count limit. The project contains a title, a contents page and page numbers throughout. There are only three references listed at the back of the report. Two of these are obtained from websites. As there are no dates given for when these websites were accessed the reference mark cannot be awarded. 7

Part of the practical procedure is given below.

Part of the practical procedure is given below. A peptide is hydrolysed to form a solution containing a mixture of amino acids. This mixture is then analysed by silica gel thin-layer chromatography (TLC) using a toxic solvent. The individual amino acids