| Date | November 2014 | Marks available | 5 | Reference code | 14N.3.hl.TZ0.2 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Describe and State | Question number | 2 | Adapted from | N/A |
Question
Waste water from an abandoned copper mine is suspected of polluting a community’s drinking water supply, causing concern that the concentration of dissolved copper compounds might exceed the legal limit of 1.3 ppm.
The concentration of transition metal ions can also be determined by measuring the colour intensity of solutions of their complex ions. In the case of copper(II) ions, excess aqueous ammonia is sometimes added before measuring the absorption.
(i) Describe why adding excess ammonia to aqueous copper(II) ions causes the shade of the blue colour to change.
(ii) State one other factor, apart from a change of ligand, which could affect the colour of a transition metal complex.
Markscheme
(i) ligands cause splitting of d-orbitals;
colour depends on movement of electrons between d-orbitals;
(four of the) water ligands replaced by ammonia;
splitting of orbitals depends on the ligand present;
ammonia ligands increase the splitting of the d-orbitals / cause a greater splitting than water ligands / ammonia higher than water in spectrochemical series;
Accept “Ammonia affects splitting of d-orbitals”.
spectrum moves to the blue end / absorbs a higher frequency/shorter wavelength;
(ii) oxidation state/number of the metal/charge on metal ion/charge density;
Accept “metal ion involved / geometry of the complex ion / coordination number of ligands of metal ion”.
Examiners report
Atomic Absorption was usually correctly identified and good answers were given in (b) when the calibration curve was mentioned. The answers to (c) (i) were rather patchy with few recognizing that there is a replacement of ligands and few mentioning that colour is caused by movement of electrons between d-orbitals. This was not quite the “usual” why are TM complexes coloured? type of question. Most gave a good answer to (c) (ii).
