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This practical focuses on the solid crystals of ionic copper (II) complexes and their characters. To calculate by experiment the number of water each molecule has.
Copper (II) sulphate exists naturally as one of only a few water-soluble sulphate minerals. It is often found in dry areas (Department of Chemistry, 2005). After heat, the bond between associated water and copper will break and the water becomes gas and evaporates away (Yahoo Answer, A, 2009). Lister and Renshaw (2002) state that, copper (II) sulphate dissolves in water and forms the tetraaquacopper (II) ion [Cu(H2O)4]2+. The Cu2+ cation is the cause for the blue colour of the solution. Adding hydrochloric acid into chloride ions, lead to the replacement. Lister and Renshaw suggest that the copper (II) sulphate will react with the chloride ion in 4 steps:
[Cu (H2O) 4]2+ (aq) + Cl-(aq) <==> [Cu (H2O) 3Cl] + (aq) + H2O (l)
[Cu (H2O) 3Cl]+ (aq) +Cl-(aq) <==> [Cu (H2O) 2Cl2] (aq) + H2O (l)
[Cu (H2O) 2Cl2](aq) +Cl-(aq) <==> [Cu (H2O) Cl3]-(aq) + H2O (l)
[Cu (H2O) Cl3]- (aq) +Cl-(aq) <==> [CuCl4]2-(aq) + H2O (l)
The blue colour is replaced by the yellow-green of [CuCl4] (aq)
When a little ammonia solution was added, it will form Cu (OH) 2(s), white precipitate is seen as a blue suspension appears. After more ammonia solution is added, [Cu(NH3)4]2+ forms and is seen as indigo coloured solution.
Requirements: crucible, spatula, burner, tongs, electronic balance, stand, conical flask, pipette, desiccators, copper (II) sulphate, hydrochloride acid, ammonia solution.
First of all, a crucible was put out and the inside of it was cleaned with a tissue. After a paper clip was placed in the crucible, both of them were weighed on an electronic balance. The weight was recorded to 0.01. Then, the crucible was put on the electric balance and 2-3g of copper (II) sulphate was placed in crucible, weighed accurately. Before the burner was placed under the stand, it was lit. Both crucible and its contents were placed on the stand and heated for about 5 minutes. After that, the crystals were stirred with the paper clip. All observations were noted. The last but one, the crucible was placed inside drying desiccators for about 5 minutes to cool down and stayed dry. In this step, tongs are used to protect against heat. The crucible was weighed when it had cooled. Moreover, the 4-6 steps were repeated to check weight after heating, to achieve a constant weight. Finally, some water was added into the crucible.
- Some copper (II) sulphate was put into 3 conical flasks with water and shaken to dissolve completely.
- Concentrated hydrochloric acid was added slowly into on flask using pipette.
- A little ammonia solution was dropped into a second flask and any changes were recorded. Finally excess ammonia solution was added into the same flask. The changes were noted.
The data in table 1 shown the number of water in a molecule of hydrated copper (II) sulphate is 5. The [CuCl4]2-(aq) is made by adding hydrochloric, so the colour changes to yellow-green (Lister and Renshaw, 2002). The colour of anhydrate copper (II) sulphate changes to blue after adding water to it. It shows the reversible between dehydration and hydration, at the same time, it is easy to do this transfer. In addition, the reason of hydrate CuSO4 shows, or Cu2+ cation, shows colour blue, is the absorbing of light. Accurately, the incompletely filled of d-orbital make the absorption. While the water interacts with copper (II) sulphat to form hydrate CuSO4, eletrons absorb energy for moving to 3d orbital to make Cu2+ cation without absorbing blue colour. (Yahoo Answer, C, 2009)The experiment of adding ammonia solution has selections. It shows how the observation will change depends on how much was added.
According to these experiments, the character of hydrate crystal is shown. Such as hydrous CuSO4 can be dried and rehydrated. This transformation seems so easy to them. Furthermore, copper can form other kinds of hydrate in the solution. Different volume of reactant makes the phenomenon different. All these do help for next experiments to make judgment for the reactions.
- Department of Chemistry, 2005 (Online), copper (II) sulphate in natural, (Available at: http://www.-teach.ch.cam.ac.uk/links/crystals/webCuSO4/.html) (Accessed: 27 December 2009)
- Lister, T and J Renshaw, Chemistry for Advanced Level, (2002: 474), third edition, Stanley Thornes, Licensing Agency Limited of 90 Totten ham Court Road, London on WIT 4 LP.
- Yahoo Answer: A, 2009(Online), What happens when you add heat to Copper sulphate pentahydrate? (Available at: http://answer.yahoo.com/question/ind ex?qid=20081106214336AArHP5c) (Accessed: 26 December 2009)
- Yahoo Answer: B, 2009(Online), Science question!! Copper sulphate +ammonia solution? (Available at: http://answer.yahoo.com/question/index?qid=2009051306451 5AAA823w) (Accessed: 26 December 2009)
- Yahoo Answer: C, 2009(Online), Why is copper (II) sulpahte blue, (Available at: http://answers.yahoo.com/question/ind ex?qid=20080908034443AAi4LFS) (Accessed: 26 December 2009)