Complex Ions of Copper (II)
The aim of this practical is to study complex ions of copper. There are two parts, firstly to determine the empirical formula of hydrated copper (II) sulphate, and secondly to make other copper (II) complex ions in solution and observe the different colors.
Many ionic compounds contain water molecules in their solid crystals, and called hydrates or hydrated salts or hydrous complexes. According to CP Chemical Inc, copper (II) sulphate is a large dark-blue granular crystal or blue crystalline powder, and it is toxic, odorless with metallic astringency. Copper (II) sulphate can dissolve into water, and the aqueous solution will be slightly acidic. It can not dissolve into ethanol; the density of copper (II) sulphate is 2.2844g/cm3, (CP Chemistry Inc, 1994). When the temperature is in excess of 150℃, copper (II) sulphate will lose water and crystalline white powder remains, called anhydrous copper (II) sulphate.
Anhydrous copper (II) sulphate is a white solid; its solution is acidic with irritating odor. It can dissolve into water and aqueous ethanol, but it can not dissolve into absolute ethanol, (CP Chemical Inc, 1994). It's easy to deliquescence for anhydrous copper (II) sulphate in moist air. It can also turn to black copper oxide at a high temperature. Anhydrous copper (II) sulphate is an important material to manufacture copper salts in the chemical industry. Hydrous copper (II) sulphate changes into anhydrous copper (II) sulphate by the chemical equation:
CuSO4·xH2O(s) → CuSO4(s) + xH2O(l).
Moreover, hydrochloric acid is a liquid which is colourless and easy to volatilize with irritating odor. The melting point of concentrated hydrochloric acid is -25℃, and the boiling point is 109℃, (Chemistry Safety Data, 2003). It can dissolve into water and alkaline agent.
MSDS (2008) stated that ammonia solution is a transparent liquid with irritating odor and corrosion behaviour. It can be decomposed when it met heat and light, and the boiling point is -77℃, (MSDS, 2008).
Equipment and material: crucible, spatula, burner, tongs, electronic balance, stand, paper clip, desiccators, pipettes, conical flasks, hydrochloric acid, ammonia solution.
Part A (Lane, 2009)
Firstly, a cloth was used to clean the inside of the crucible. A paper clip was placed in the crucible and both were weighed on the electronic balance. The weight was recorded to 0.01g. Then the crucible and paper clip were put on the small electric balance. Using the spatula, 3g of copper sulphate was placed in the crucible and accurately weighed. The burner was lit and put under the stand. In addition, the crucible and sample, was placed on the stand and heated for 5 minutes. Then the crystals were stirred with the paper clip and the change in colour was noted. The crucible was placed inside the desiccators for 5 minutes to cool down by the tongs. Moreover, the crucible was weighed when it was cool enough. Some water was added to the crucible. Finally, the weight was repeated six times to check and stayed the same after heating.
Part B (Lane, 2009)
Firstly, some copper (II) sulphate and water were put into 3 conical flasks and shaken to dissolve. Furthermore, the concentrated hydrochloric acid was added into one flask using a pipette. Finally, some ammonia solution was added into a second flask using another pipette and observed. The third flask of (CuSO4·xH2O)(aq) was used as a colour control.
The mass data of copper sulphate is in Table 1.
Mass of crucible (+ paper clip)
Mass of crucible (+ paper clip) + copper (II) sulphate
After first heating
After second heating
After third heating
Hydrated copper (II) sulphate
23.11 - 20.95 = 2.16
Anhydrous copper (II) sulphate
22.33 - 20.95 = 1.38
Amount of water
2.16 - 1.38 = 0.78
Table 1, measurements
The observation of complex ions in solution is in Table 2.
Heating hydrous copper (II)
During heating, the color of copper (II) sulphate
change from blue crystalline solid to a white
powder, and the condition is change from
crystal to powder.
Concentrated hydrochloric acid
After a few drops, the color of the liquid's surface changed from blue to slightly yellow; then it changes back to blue after shaking. It disappeared easily.
As more is added, the color changes from blue to bright green. The colour grows stronger and more permanent.
The color of the liquid's surface changes
from bright blue to dark blue, then
it changes back again to bright blue. After
about 50 drops, the color changes from
bright blue to dark blue, and some solid appears at the bottom of the flask. The colour intensifies and becomes more permanent.
Table 2 observations
However, there is also having some experimental phenomena to discuss. In part A, repeating the heating is to make sure the water of the hydrous copper (II) sulphate has been driven off completely. If it is heated too strongly, copper (II) sulphate can change into copper (II) oxide, and a color change is seen from blue to black solid. The number of moles of anhydrous copper sulphate is 1.38÷64+32+16×4 = 0.008625g; the mass of water is 23.11-22.33 = 0.78g; the number of moles of water is 0.78÷16+(1×2) = 0.043g. The mole ratio between the anhydrous copper (II) sulphate with water is 1:4.99, approximately 1:5. As a result, the formula of an-hydrous copper (II) sulphate is CuSO4·5H2O(s).
In part B, the reactions both have the productions of complex metal ions. During the addition of chloride ions, the reaction produced tetrachloride copper ions. Adding the ammonia solution, produces [Cu(NH3)4(H20)2]2+ which is indigo, (Clark.J, 2003).
All of two experiments are have their own result. In part A, the value for the number of water molecule in a copper (II) sulphate crystal was 5 which agree with the given true value. In addition, in part B, the reactions were used to make complex ions in solution.
Chemistry Safety Data, (2003) [online] Chemistry Safety Data: Hydrochloric Acid http://cartwright.chem.ox.ac.uk/hsci/chemicals/hydrochloric_acid.html [Accessed date 28/12/09]
CP Chemical Inc, (1994) [online] Copper Sulphate
http://pmep.cce.cornell.edu/profiles/extoxnet/carbaryl-dicrotophos/copper-sulfate-ext.html#top [Accessed date 28/12/09]
MSDS, (2008) [online] Ammonia Solution, Strong
http://www.jtbaker.com/msds/englishhtml/a5472.htm [Accessed date 28/12/09]
Lane, R (2009) IFY Lab Handout: Chemistry Practical 2:
Complex Ions of Copper (II)
Clark, J (2003) [online] The shapes of complex metal ions http://www.chemguide.co.uk/inorganic/complexions/shapes.html#top [Accessed date 05/01/10]