Experiment 2: A Study of Copper (II) Complexes with Different Ligands Objective 1. To observe a various colour change in a complex ion due to ligand substitution
Materials Reagent
Apparatus
Copper Sulphate( 0.025 M CuSO4 )
Conical Flask (250 ml )
Concentrated ammonia solution (NH3)
Pipette
Concentrated hydrochloric acid (HCl)
Dropper
Ethylenediamine(en)
Boiling tube
Ethyldiaminetetraacetate (EDTA) Potassium cyanate KCN ,1M
Introduction Transition metals differ from main group metals in several key properties. One of the more interesting aspects of transition metals is their ability to form coordination compounds. Coordination compounds are formed between a metal ion and a molecule with one or more unshared electron pairs, called a ligand. Ligands may be classified according to the number of donor atoms they contain. A monodentate ligand donates a single electron pair to the metal or metal ion. Common examples of monodentate ligands include NH3, H2O, NO2-, and CN-. A bidentate ligand, as the name suggests, donates two electron pairs to the metal or metal ion. A good example is ethylenediamine,NH2CH2CH2NH2. Anions as well as neutral molecules may act as ligands. If one or more neutral molecules coordinate to metal ion, the resulting species retains the charge of the transition metal ion and is called a complex ion. For example, most transition metal ions form complex ions with water molecules when in aqueous solution. Examples include [Co(H2O)]6]3+ and [Ni(H2O)6]2+. If one or more anions coordinate to a metal ion, a complex ion with an overall negative charge may ma y result.
In writing formulas for complex ions and coordination compounds, the molecules inside the brackets represent ligands physically coordinated to the metal ion. Anything outside of the brackets is present for charge balance. Therefore the compounds [Co(H2O)6]Cl3 and [Ni(H2O)6]Cl3 contain positively charged complex ions in which the chloride ions are present for charge balance. The compounds K3[Co(NO2)6] and K4[Fe(CN)6] contain negatively charged complex ions with potassium ions present for charge balance
Methodology 5ml of concentrated hydrochloric acid were placed inside a test tube. since hydrochloric acid is corrosive, it was handled very carefully to not allow it make contact with skin or clothing. After that, 5ml of each another ligand which were ammonia (NH3), ethylenediamine (en), ethyldiaminetetraacetate (EDTA), potassium cyanate (KCN) were placed into another test tubes. Then, 5ml of 0.025M CuSO4 was placed into a test tube and was marked as X. After that 5ml of 0.025M CuSO4 was added to each test tube. The colour changes in each of the test tube was compared to the colour of solution in test tube x was observed and recorded.
Result
Ligand Hydrochloric Acid (HCl) Ammonia (NH3) Oxalic Acid
Ethyldiaminetetraacetate (EDTA) Potassium Cyanate (KCN) Cooper Sulphate CuSO4
Colour Before
After
Colourless
Greenish Yellow
Colourless
Dark Blue
Colourless
Cloudy Blue
Colourless
Blue
Yellow
Pale Yellow
Pale Blue
Discussion Cooper Sulphate (CuSO4) when dissolved in water, it produces the pale-blue [Cu(H2O)6]2+ ion. This colour of solution acted as the control for the experiment. For the first test tube, CuSO4 was reacted with HCl. HCl which is colourless when reacted with CuSO4 resulted with greenish yellow solution. The reaction that occurs was: -
2-
2-
CuSO4•5H2O + 4Cl → [CuCl4] + SO4 2-
[CuCl4] is an ion with square planar geometry. For the second test tube, CuSO4 was reacted with ammonia. Ammonia which is colourless when reacted with CuSO4 resulted with dark blue solution. The reaction that occurs was: 2+
CuSO4.5H2O + 4NH3 → [Cu(H2O)2(NH3)4] + SO4
2-
This reaction is usually used as a qualitative test for the copper (II) ion.For the third test tube, CuSO4 was reacted with oxalic acid. Oxalic acid which is colourless when reacted with CuSO4 yielding cloudy blue solution. The reaction that occurs was:
For the fourth test tube, CuSO4 was reacted with Ethyldiaminetetraacetate (EDTA). EDTA turns from colourless when reacted with CuSO4 to blue solution. The reaction that occurs was: Cu
2+
(aq) +
2 – (aq)
(EDTA)
2 – (aq) +
→ Cu(EDTA)
+ (aq)
2H
2 –
EDTA forms a more stable complex and frees the indicator, which then displays its original color. The appearance of the free indicator means that all metal ions have been 2 –
complexed by EDTA , which signals the end point. For the fifth test tube, CuSO4 was reacted with Potassium Cyanate (KCN). KCN turn into pale yellow from its original colour which is yellow when reacted with CuSO4. The reaction that occurs was: 2 KCN + CuSO4 → Cu(CN)2 + K 2SO
Conclusion There are various changes of colour occur towards CuSO4 when reacting with different complex ions. Greenish yellow solution formed when HCl is added, dark blue solution formed when NH3 is added, Cloudy blue formed when oxalic acid is added, clear blue solution formed when EDTA is added and lastly pale yellow solution resulted when KCN is added. Reference House, J. E. (2013). Inorganic Chemistry 2nd Edition. Oxford: Elsevier. McClure, M. (2009). Retrieved December 2, 2014, from University of North Carolina at Pembroke web site: http://www2.uncp.edu/home/mcclurem/courses/chm226/introduction_Coordination_Chemistr y.pdf Silberberg, M. S. (2013). Chemistry: The Molecular Nature of Matter and Change 6th edition. New York: McGraw-Hill.
