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The first experiment aimed to calculate the accurate value of water of hydration(x) in hydrous copper(II) sulphate.
The second experiment aimed to observe the behavior of different ligands of copper(II).
The value of x hydrated crystals are common, copper(II) sulphate is one: CuSO4:xH2O (S). The value of x can be calculated. We need to know the mass of CuSO4:xH2O (m1) and the mass of CuSO4 (m2), hence xH2O can be solved: m1- m2.X can be calculated by this way :( m1- m2)/18.
Copper(II) sulphate is triclinic crystals, which has three axes and the length is not equal to each other, has blue colour and is transparent. Anhydrous copper(II) sulphate is gray white particles, which are easy to dissolve in water and be hygroscopic that means easy moist.(Lister and Renshaw, 2000). The equation is
CuSO4:xH2O (S) -->CuSO4(S) + xH2O(l)
Complex ions can identify other electronic pairs replace water molecules which surround the cation. Chemguide(2003) indicates compounds have colours because they absorb lights, such as gamma rays, X-rays, infra-red rays, radio waves, etc. It shows as figure 1 (Chemguide 2003).
Therefore, when the sunlight pass the copper (II) sulphate solution, red region of the spectrum is absorbed, hence, hcopper (II) sulphate solution will appear cyan, show as figure 2 (Chemguide 2003).
When excess of ammonia solution was added, a colour change from cyan to dark blue can be seen, figure 3 (Chemguide 2003). According to Chemguide, The H2O entirely wraps up a Cu ion using all 6 of the positions that we have seen before. When excess of ammonia solution was added, ammonia replaces 4 or 6 water molecules, the energy of the ions become larger, so that it can absorb yellow region, which has higher energy than red region. (Chemguide 2003)
Chemicals: blue copper(II) sulphate(aq), concentrated hydrochloric acid, ammonia solution. Equipment: crucible, spatula, paper clip, burner, tongs, electronic balance, stand, conical flask and pipettes (Lane, 2009).
The first experiment follow these steps. Firstly, the inside of a crucible was cleaned using tissue. It was weighted on the electronic balance; a paper clip was also included in the mass. The weight of crucible was made down. About 2g of copper sulphate was placed, inside and reweighted. Subsequently, the crucible was placed on the stand and for heating for about 5minutes. The hydrous crystals were stirred with the paper clip. The change of colour was observed carefully. After 5 minutes, the hot crucible was removed by tongs into dessicator for cooling. When cool enough to touch, the crucible was weighted. The process was repeated (Lane, 2009).
The second experiment follow these steps. Initially, hydrous copper(II) sulphate was put into three conical flasks and 100cm3 water was added. Then the three conical flasks were shaken, to dissolve the solid.
Secondly, concentrated hydrochloric acid was added to one, and observed.
Thirdly, some ammonia solution was dropped into a second flask until the colour changes with shaking (Lane, 2009).
As can be seen from table 1, the value of water of crystallization of copper(II) sulphate can be calculated:
CuSO4:xH2O (S) -->CuSO4(S) + xH2O
The mass of CuSO4:xH2O is 2.22g; the mass of CuSO4 is 1.4g, also the Mr of CuSO4:xH2O is 160+18x and the Mr of CuSO4 is 160. Lister states that Mr is the molecule mass, which compare with an atom of hydrogen (Lister and Renshaw, 2009). Thus, xH2O can be solved 2.22g/(160+18x)=1.4g/160, the x is 5.2, approximately CuSO4:5H2O (S) ?CuSO4(S) + 5H2O(l).
The error maybe comes from two sides: first, calculation; second, the copper(II) sulphate has not completely change to anhydrous salt, copper(II) sulphate may continue to absorb the water vapor in the air. Because of time, the copper(II) sulphate was just heated twice. Repeat heating will make sure the reaction has finished. In addition, the crucible must be cooled down before weighing. If overheating, the compound can turn black:
CuSO4 (S) -->CuO (S) +SO3 (g) (Chemguide 2003)
The CuO is black. Moverover, improve next time it should avoid hydrated copper(II) sulphate being in the air too long and must quickly put it into a dessicator. Repeat heating until the mass of anhydrous copper(II) sulphate remain unchanged.
Part B experiment, it was noted that the copper(II) sulphate solution and the concentrated hydrochloric acid solution reacted and the solution colour change to green. This can be explained as follows:
[Cu (H2O) 6]2+ (aq) + 4Cl+ (aq) --> [CuCl4] + (aq) + 6H2O (l) (Chemguide 2003).
That means six water molecules were replaced by four chloride ions, the colour change was mention in the mention the colour again.
Furthermore, the ammonia solution reaction has a cyan colour:
[Cu(H2O) 6]2+ (aq)+ 2NH3 (l)?[Cu(H2O) 4(OH) 2] (aq)+ 2NH 4+(aq) (Chemguide 2003).
After more ammonia is added the colour change to dark blue is result of [Cu (H2O) 6]2+ + 4NH3 ? [Cu (NH3) 4(H2O) 2] 2+ + 4H2O (Chemguide 2003).
To summarise, the hydrated formula of CuSO4:xH2O (S) was calculated to be CuSO4:5H2O. The accurate value of x in the equation for hydrate copper(II) sulphate is similar to theory. And observe the different colours of complex ions in solution, the colour change from blue to green of Cl- ligands and from blue to cyan then to dark blue of NH4- ions, demonstrate the process was clearly.
- Chemguide [online] (2003) Copper Available at: http://www.chemiguide.co.u/inorganic/transition/copper.html Adapted date 2009.12.27
- Lane, R. (2009) Chemistry Practical 2 Handout
- Lister, T. and J. Renshaw (2000). Chemistry for Advanced Level(3rd edition). Cheltenham: Nelson Thornes Ltd