Practical chemistry on copper ions

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Aim

The aims of this chemistry practical are to find out how many water molecules compounds hydrated, associated with one copper (II) sulphate molecule. Secondly study two typical copper (II) complexes.

Introduction

As the study is about copper (II) ion and its complex compound and sulphate, this introduction will introduce the theories based on these substances.

As Cornell University reported 'copper sulphate is a fungicide used to control bacterial and fungal diseases of fruit, vegetables, nut and field crops'. It also reported that the solution makes the protective fungicide called Bordeaux mixture (Cornell University; 1994). The structure of hydrous crystalline copper (II) sulphate is shown in the figure below. (Look Chem; 2008).

The complex compounds are called Coordination Compounds. It means after react acid and base, can get a compound which combined by a neutral molecule or anions (these are called ligands) with a central metal atom or ion through dative bonds. (Chemscape Chime Plugin; 2009). Dative bond is a kind of covalent bonding which the coordinate electrons (lone pairs) is offered by the same atom. The ligands are the atoms that offer lone pairs in the complex compound (Clark; 2005 a).

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The central metal atoms such as copper, is the transition element. These are found in the d-Block of the Periodic Table, as the figure below shows (Mark; 2009).

The dative is also used in the hydrate metal ions, a kind of complex ions which the ligands are water molecule. The water molecule offer electrons to the empty hybrid orbital of central metal atoms, as figure 3 shows (Clark; 2005 b).

If the central molecule is copper, then it will have six dative bonding with water, as the structure below. (Adept from: Clark; 2005 c)

When the concentrated hydrochloric acid (HCl (aq)) is mixed with copper sulphate aqueous, the chloride anions are becoming the ligands of copper, the group of ion called tetrachlorocuprate(II) ion (Clark; 2009). The structure of tetrachlorocuprate (II) shows as below (MediaWiki; 2009). This complex ion is yellow when it in nonaaueous circumstance. (Hudong; 2009 c)

Method

Chemicals and Equipment

Copper (II) sulphate (CuSO4.xH2O)

Concentrated hydrochloric acid (HCl (aq))

Ammonia solution (NH3 (aq))

Dessicator

Crucible

Paper clip

Burner

Stand

Pipeclay triangle

Spatula

Crucible tongs

Electronic balance

Part one: To calculate the empirical formula of hydrated copper (II) sulphate

First the crucible was cleaned with a cloth. Then, a paper clip was placed in the cleaned crucible and weighed together using the electronic balance. After that, the result was written down and recorded to 0.01g. The crucible was weighed again after 2-3g of copper sulphate was placed. A burner was lit and placed under the stand, the crucible with solid was placed on the stand and was heated for about 5 minutes. Next, the solid was stirred using a paper clip and the colour changes were written down. The crucible was placed inside a dessicator, a utensil to dry the chemical substance to pledge the chemical are absolutely arid, for another 5 minutes to dry, using the tongs. After cool to touch, the crucible was weighed again and the result was recorded. The weight was checked by repeating the process. Lastly, some water was added into the crucible for testing is the hydrate copper sulphate cause the blue colour. (Lane; 2009 a)

Part two: Complex copper ions

Three conical flasks were prepared by adding approximately the same mass of copper (II) sulphate solid into each of them for first step. Then, water was added into each flask and gently shaken. Next, some of the concentrated hydrochloric acid was added into the first flask and observed. Some ammonia solution was dropped into the second flask using a pipette and observed. Last,the third flask acted as a colour control for comparison. All the changes of colour were recorded. (Lane; 2009 b)

Discussion

Table 1, from the experiment, that copper sulphate can lose waters of hydration by heating. The reaction is shown as below.

M of CuSO4 = 160 g/mol

M of H2O =18 g/mol

M of CuSO4.xH2O = (160 + x18) g/mol

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(Mr and M has the same numerical value but Mr do not have unit)

n = m/Mr

1.35 * (160+18x) = 2.13 * 160

x = 5.14 ? 5

X is the number of crystallization water, means 5 molecules water with one molecule CuSO4, thus the chemical formula of hydrate copper sulphate should be CuSO4.5H2O.

The reason of heating hydrate copper sulphate three times is to guarantee the crystallization water in the copper sulphate crystal evaporation. If the second value and third value are close, means the crystalwater is nearly evaporation.

There has a seriously error occurs in one process during experiment is that use tongs to mash the stick copper (II) sulphate, thus the powder was wasted. These will make the numerical value not accurately. The mistake should be careful next time.

A possibility error is that if the copper sulphate was over heated, black compound will occur. The reaction of this is show as below.

The CuO (copper (II) oxide) is black solid and the SO3 (sulphur trioxide) is a colourless gas, the black compound is CuO solid. However, it may not happen as the temperature of alcohol burner can only reach approximately 400-500? (Hudong; 2009 a), while the required temperature of forming the CuO from CuSO4 is about 650?. (Hudong; 2009 b)

The introduction shows that [CuCl4]2- should be a yellow in nonaqueous situation. Actually it soak in the aqueous, the [Cu (H2O) 4]2+ appears blue, thus the final solution emerges a green colour. As the reaction is a reversible reaction, the first observation is the green-blue colour occurs but disappears immediately. Clear green colour solution is due the reaction attains equilibrium concentration.

Conclusion

By calculating the statistics that obtained in experiments, the empirical formula of copper sulphate crystal is CuSO4.5H2O. The copper ion can combine with other anions to from complex such as [CuCl4]2- ions or combine molecules like [Cu (H2O) 4]2+ and [Cu (NH3) 4]2+ ions to form complex ions. This experiment is can calculate the number of crystallized water molecules in the complex and some of the complex compound. The misoperation which was done during the experiments must be improving next time.

References

  • Clark (2005 c) [online] co-ordinate (dative covalent) bonding http://www.chemguide.co.uk/atoms/bonding/dative.html (Access date: 10/01/2010)
  • Chemscape Chime Plugin (2009) [online] What is a coordination compound? Available at: http://www.chem.purdue.edu/gchelp/cchem/ (Access date: 28/12/2009)
  • Cornell University(1994)[online] Copper sulfate http://pmep.cce.cornell.edu/profiles/extoxnet/carbaryl-dicrotophos/copper-sulfate-ext.html (Access date: 22/12/2009)
  • Hudong (2009 a) [online] Copper sulphate http://www.hudong.com/wiki/%E7%A1%AB%E9%85%B8%E9%93%9C
  • Hudong (2009 b) [online] Alcohol burner http://www.hudong.com/wiki/%E9%85%92%E7%B2%BE%E7%81%AF
  • Hudong (2009 c) [online] Copper ions http://www.hudong.com/wiki/%E9%93%9C%E7%A6%BB%E5%AD%90
  • Look Chem (2008) [online] Copper (II) sulphate pentahydrate (1:1:5) http://www.lookchem.com/COPPER-II-SULFATE-PENTAHYDRATE-1-1-5-/ (Access date: 22/12/2009)
  • Lane. R (2009 a) Chemistry practical 2: complex ions of copper (II) Practical handout
  • Lane. R (2009 b) Chemistry practical 2: complex ions of copper (II) Practical handout
  • Mark. R (2009) [online] http://www.meta-synthesis.com/webbook/35_pt/pt_database.php?PT_id=14 (Access date: 27/12/2009)
  • MediaWiki (2009) [online] Tetrachlorocuprate (ii) http://commons.wikimedia.org/wiki/File:Tetrachlorocuprate_(ii).png (Access date: 10/01/2010)