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Properties of complex ions

Aim

The aim of this experiment is to investigate some properties of ionic complex compounds which contain water molecules, namely experiments of blue copper (II) sulphate hydrated. The number of coordinated water will be determined.

Introduction

The calculation of the value of x (the number of combined water) is based on the 'relative molecule mass (Mr)' and 'the mole'. Lister and Renshaw (2000) stated that, Mr is the mass of a molecule compared to the mass of 1 H atom, and is the sum of the relative atomic mass. The number of moles equals (mass in gram) / Mr. The relative atomic mass of Cu, H, O and S are 63.5, 1, 16 and 32 respectively and CuSO4 is 160. (Lister and Renshaw, 2000).

Copper (Cu) is a d-block element in The Periodic Table, and is a member of transition elements. Lister and Renshaw (2000) pointed out that transition elements including copper has several special properties of which are usually coloured compared to s-block metals and have ability of forming complex with dative bonds. In copper's complex, other molecules such as NH3 which has lone pair can form dative bonds with copper, and these molecules are called ligands. Such H2O, NH3 and Cl- are ligands (Lister and Renshaw, 2000).

According to Clark (2000), dative covalent bond which is also called coordinate bond, is a covalent bond (share a pair of electrons) where both electrons are supplied by the same atom. Usually, a lone pair (unshared pair of electrons) is accepted cation such as copper (II) in order to help to obtain a full outer shell (Lister and Renshaw, 2000). A typical example is NH3 in which has a lone pair to form dative bond.

Hydrated salts are compounds containing water molecules (Farlex Inc., 2009). When a crystal of the substance forms, some waters are combined. They will be driven off when the crystal is heated, and becomes an anhydrous salt. A typical example is copper (II) sulphate. According to Bennett (1998), hydrated copper (II) sulphate has four waters straight bound to the copper (by dative bonds) and forms a Cu(H2O)42+ ion firstly. In that ion, the copper is lying at the center of a square surrounded by the oxygens of the water. One sulphate ion and one water are bonded each other by hydrogen bond and connect with Cu(H2O)42+ as a line. Hydrated copper (II) sulphate becomes anhydrous with the formula below:

CuSO4•xH2O(s) --> CuSO4(s) + xH2O(l)

A significant nature of transition elements (T.E.) including copper is that they are coloured. Lister and Renshaw (2000) stated that this feature is caused by the energy gap between two energy levels in d orbital, which T.E. all have, and the energy gap need to absorb light energy to be filled. A equation that E=hv where E refers to energy gap, h is a constant and v is the frequency, shows that if the v of the substances are in the region of visible in the spectrum, the rest lights (not be absorbed) will appear as the substances' colour. It is the ligands that make the difference of energy level. According to Clark. J (2000), when ligands such as H2O, NH3 and Cl- approach the ions of T.E., there is a repulsion between the electrons form ligands and d orbital of T.E., as a result, the ligands split the energy into two groups, of which a group have promote to a higher energy level that make a gap.

Method

The source of this method is Lane, R (2009)

These chemical were provided:

Copper Sulphate (s), concentrated hydrochloric acid (l) and ammonia solution (l)

These apparatus were provided:

Spatula, tongs, paper clip, electric balance, dessicator, crucible, burner, stand, pipeclay triangle and conical flask

Part A:

First in order to record data, a crucible was cleaned with a tissue. After a paper clip was placed in the crucible and both weighed using an electric balance and recorded to 0.01g. Then 2-3g copper sulphate was added the crucible with paper clip and weighed.

After these were done, hydrated copper (II) sulphate was heated. Over an alcohol burner placed under a stand. The crucible was placed on the stand and heated for 5 minutes. The crystal was stirred and broken up using the paper clip and observed. Next, using sing tongs, the crucible was placed inside a drying dessicator for 5 minutes to cool down (the paper clip remained in the dish). Until it was cool enough to touch and reweighed. The process was repeated twice until a constant was achieved.

Part B:

Copper (II) sulphate here was made in solution and reacted with other solutions. Copper (II) sulphate and water were put into 3 conical flasks and shaken to dissolve. Then, by using a pipette, concentrated hydrochloric acid was dropped into one flask and observed. The process was repeated in flask 2 with ammonia solution replacing hydrochloric acid. Two aspects were performed, first to add a little ammonia and second to add excess solution.

Discussion

The calculation of the value of x:

Number of mole of anhydrous copper (II) sulphate: m1(g)/M1 (g/mol) = 1.35g/(63.5+32+16•4)g . mol-1 ? 8.46 x 10-3 mol

The mole of combined water: m2/M2= 0.78g/18g•mol-1 ? 0.043 mol

Hence, the value of x = 0.043mol/8.46•10-3 mol ? 5.12 ? 5 moles

The empirical formula of hydrated copper sulphate is CuSO4•5H2O

From the calculation in the result, there are approximately five water molecules surrounding each copper (II) sulphate. As the coordinated water were lost gradually, it is supposed that the bonds between water molecules and copper ion broke one bye one. The hydrogen bond may break firstly as its bond energy is low (Lister and Renshaw, 2000), and successively the dative bonds break. The fewer bonds combined to copper may result in higher energy required to break them. The further formula can be written:

CuSO4•5H2O(s) --> CuSO4•H2O (s) -->CuSO4(s)

so CuSO4•5H2O(s) --> CuSO4(s) + 5H2O(l)

The way that copper (II) exists in the water is [Cu(H2O)4]2+ and is formed by dative bonding (Lister and Renshaw, 2000). Here water molecules whose oxygen has lone pairs are attracted to copper (II) to fill its empty orbital. When the hydrochloric acid was added to copper (II) sulphate, in this case the Cl- is ligand which forms stronger bonds than water molecules, in other word, water slightly split the energy level and Cl- can produce a large energy gap. As a result the anion replaces H2O (Lister and Renshaw, 2000). The reversible equation can be written below:

[Cu(H2O)4]2+ (aq) +4Cl-(aq) --> [CuCl4]2-(aq) + 4H2O(l)

Here [CuCl4]2- is yellow for several reasons. According to Clark. J (2000), Cl- had splited the copper's energy level more slightly related to water and procured smaller energy gap which determines the wavelength of light being to be absorbed. Thus the wavelength of the substance is higher and the lower energy (dark colour) light was absorbed. Consequently, in spectrum, the lighter colour namely yellow appears (from magenta to red as the wavelength increases). As the reaction happened gradually and is reversible, the colour of solution changed slowly from blue to green (the mixed colour of blue and yellow).

When ammonia solution was added to copper sulphate solution, it is again replacement, copper (II) firstly reacts with OH- from ammonia water to form Cu(OH)2 so that a small amount of blue suspension was produced. Due to the few difference between OH- and water in splitting, the colour change little. Then the Cu(OH)2 reacted with ammonia solution. Similar to the reaction with hydrochloric acid, ammonia molecules replaced water and the complex produced is indigo and soluble in water. This process proceed darker colour because ammonia make large energy gap, as a consequence, lower wavelength of light was absorbed and remained darker colour namely indigo. The two reaction equations are:

Cu2+(aq) + 2NH4 +OH-(aq)--> Cu(OH)2(s) + 2NH4+(aq)

Then a reversible equation

Cu(OH)2(s) + 2NH4 +OH-(aq) --> [Cu(NH3)4]2+(aq) + 4H2O(l)

There some errors found during this experiment. While desiccating the copper (II) sulphate the third time, the desiccator was not covered, resulting in the moisture mixing with anhydrous copper (II) sulphate, so that the final record was greater than expectation. To improve, the whole experiment should be conducted in extremely dry condition in order to get rid of water.

Conclusions

The value of x is five, which means five water molecules are combined with one copper (II) sulphate. Copper (II) sulphate can react with hydrochloric acid, producing a green complex with dative bonds. Similarly, the reaction between copper (II) sulphate and ammonia solution is relevant to coordinate bonds and has two steps, forms indigo complex.

References

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