Experiments of Copper (II) Sulphate

Aim

This experiment is to calculate the x which is water of crystallization and the chemical equation is CuSO4 (aq) · xH2O (s) and to observed colour changed when complex formation of copper (II) sulphate are formed.

Introduction

Molar mass is summation of all the atomic masses in a chemical equation.

Transition metals are the elements which are found in the d-block in The Periodic Table. The element which is in the first transition series is strong oxidizers and they can form bivalent metal ions which have reducibility and its charge is 2+. Because of empty of d-orbital, transition metals easily form coordination compounds. Copper can form coordination compound; however, its d10 structure is stable so that its highest charge is +3.

Hydrochloric acid, its chemical formula is HCl. It is a strong acid and is easily vaporize so that after opening the container which contains HCl, the acid vapour will be seeing very clearly. Because HCl can volatilize, it will combine with the water and form HCl drops (Baidu.Baike, 2009).

Copper (II) sulphate often exists as a crystallization as a pentahydrate which means a molecule contains five crystallization of water. It can be sale as blue vitriol which is a chemical material using to produce pigments, battery and so on (Chmicalland21, nd).

Ammonia solution is a colourless liquid and has pungent smell. If it is exposing to air it loses NH3 quickly (Down.Food126, 2008).

The displacement reaction is a simple substance reacts with a compound, and forms other kinds of simple substance and compound.

The Crystal Theory is a theory that search for the chemical bond of the transition metals. The transition metals ions are located on the crystal field and surrounding by anions and dipole molecules. The anions are called central ions, and the dipole molecules are called ligands. These ions can affect the energy which belongs to d-orbital, because of a splitting of energy levels. However, this theory can used to explain the colour changes of the complex (Science.Jrank, 2010).

Lister and Renshaw (2000, p469) has pointed out that the molecules or ions use lone pairs to form dative bonds with a d-block metal which is surrounding by a transition metal to form, it’s a complex. In addition, these kinds of molecules or ions are called ligands.

Method

Firstly, several apparatus were used in this experiment, for instance, crucible, spatulas, burner, tongs, electronic balance, stand, dessicator, paper clip, conical flasks and pipettes. Secondly, some chemical were used, for example, hydrated copper sulphate, water, concentrate hydrochloric acid, ammonia solution. The last but one, students should wear the coat and safety glasses in the experiment.

Part A

First, the inside of crucible was cleaned. Second, a paper clip was placed into the crucible and weighed by electronic balance. The weight was recorded down to 0.01g.

Third, 3g of copper sulphate was placed into the crucible using the spatula, and was weighed by the electronic balance. Fourth, the burner was lit, and was put under the stand. The paper clip and copper (II) sulphate was placed on the stand and was heated for about 5 minutes. The crystal was stirred with the paper clip. The colour was observed. Fifth, the crucible was placed inside the dessicator for 5 minutes to cool down using tongs (The paper clip was left in the crucible). Sixth, the crucible was weighted when it was cool enough. At last, some water was added into the crucible and indicates the blue colour which is the colour of crystallization of water.

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Part B

Firstly, some copper sulphate and water was put into 3 conical flasks and was shaken to dissolve. Secondly, concentrated hydrochloric acid was dropped into one conical flask using a pipette and observed. Lastly, some ammonia acid was dropped into a second conical flask using another pipette until the colour changed. The colour of this solution should observe carefully after different volume ammonia acid was added.

Discussion

Part A

The chemical formula in part A is

CuSO4 (aq) · xH2O (l) –> CuSO4 (aq) + xH2O (l)

Mr 160 + 18x 160

Mass 2.22g 1.4g

1.4 (160 + 18x) = 160 * 2.22

224 + 25.2 = 355.2

25.2 x = 131.2

x = 5.20634

Therefore, x is equal to 5.

Here is an explanation of the calculation. The Mr of CuSO4 is 160, and the Mr of water is 18x which the x is still a unknown value. The mass of CuSO4 and water is 2.22g and 1.4g. The number cross multiplies with each other. As a result, the x was come out. Thus, the value of x is slight greater than predict. According to the theory, the right result should be lower than 5 value, maybe the copper sulphate was oxidised, and the water of crystallization was not evaporated enough so that influence the final result.

The copper (II) sulphate became black because of the oxidation of the copper. The copper (II) sulphate in the experiment was heated twice in order to make the water of crystallization evaporate completely. If it heated too strongly, the colour of copper sulphate will turn to black.

The chemical equation of this reaction is:

CuSO4 (aq) –> CuO (s) + SO3 (g)

Part B

One of the reasons why the colour changed is causes by spectrum. The copper (II) sulphate indicated blue in the experiment. Because when the ordinary sunlight passes through the solution, the solution absorbed the wavelengths of the light. The red area in the spectrum was absorbed by the copper (II) ions. However, the sunlight which through the solution and on the other side of the conical flasks made the solution indicates the colour except red. The blue colour is the mixture of wavelengths (Chemguide, 2009).

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The colour changed in ammonia solution added into copper (II) sulphate is causes by the complex. The light blue colour substance was indicated, and this is basic copper sulphate precipitate. After more ammonia solution was poured into the solution, the precipitate will dissolve and produce [Cu (NH3)4] 2+ which is an indigo colour substance and replaced the basic copper sulphate, and water molecule was replaced by ammonia (Chemguide, 2009).

The chemical equation is:

Cu2 (l) + 4NH3 (l) –> [Cu (NH3)4] 2+ (aq)

The transition metals have their own colour causes by d orbital. When the HCl was poured into copper (II) sulphate solution, the [Cu (H2O) 6]2+ was produced, and the colour of this ion is blue. After that, the [CuCl4]2+ was produced, and its colour is yellow. However, the solution was turned to green. Lister and Renshaw (2000, p476) pointed out that the energy gap between d orbital is suitable for the frequency of electromagnetic radiation in the ordinary light, as a result, most of the transition metal has its own colour.

Conclusion

To summarize, part A shows that how to calculate the water of crystallization, and colour changed between hydrated copper (II) sulphate and anhydrous copper (II) sulphate. In part B shows the definition of ligands, complexes and colour change cause by the spectrum.

Reference

• Baike.Baidu [online] (2009) Available at: http://baike.baidu.com/view/1729.htm?fr=ala0_1_1 Accessed date: December 28, 2009
• Chemguide [online] (2009) Available at: http://www.chemguide.co.uk/inorganic/complexions/colour.html Accessed date: January 6, 2010
• Chmicalland21 [online] (nd) Available at: http://chemicalland21.com/industrialchem/inorganic/COPPER%20SULPHATE%20PENTAHYDRATE.htm Accessed date: January 10, 2010
• Down.food126 [online] (2008) Available at: http://down.food126.com/download/view-software-3346.html Accessed date: December 28, 2009
• Lister, R. and J., Renshaw (2000). Chemistry for Advanced Level (3rd edition). Cheltenham: Nelson Thornes Ltd, Accessed date: December 28, 2009
• Lane, R (2009) Chemistry Practical 2 Handout Accessed date: December 28, 2009
• Lookchem [online] (2008) Available at: http://www.lookchem.com/COPPER-II-SULFATE-PENTAHYDRATE-1-1-5-/ Accessed date: December 28, 2009
• Science.Jrank [online] (2010) Available at: http://science.jrank.org/pages/13786/crystal-field-theory.html Accessed date: January 6, 2010