Inorganic more soluble in hot solvents than

Inorganic Experiment 1: Chromium Complexes

Name: Gemma Deeny, Student No: 16322189

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Date: 16/1/18

Lab partner’s name: Jessica Tighe

Demonstrator: Joseph

 

Aims:

The aim of this experiment is to prepare samples of
chromium(III) complexes and to examine them in terms of their solubility and
reactivity with various solvents.

Introduction:

This experiment involves the formation of some chromium(III)
complexes via reduction reactions involving a chromium(VI) compound, potassium
dichromate. Chromium is a transition metal which exists mainly in its +3 and +6
oxidation states as in these states, its valence shell is filled and half-filled,
respectively, making these more stable than its other oxidation states. The
product of part (i), which we did not complete during the lab, and that of part
(ii) are a cis/trans isomeric pair. They are an example of stereoisomers ie.
they differ only in their three-dimensional spatial arrangement of atoms. In
the cis-product, the two H2O groups are adjacent to each other while
in the trans-product, they are in opposite positions. 1

 

 

 

 

 

                                Cis                                                                                          Trans

The two oxalate ligands in these complexes are bidentate
ligands, meaning that they attach to two coordination sites of the central
metal atom and form ring structures.

The overall reaction which occurs in part (ii) of the experiment
is:

The oxalic acid dihydrate acts as a reducing
agent causing the reduction of the chromium in the potassium dichromate from an
oxidation number of +6 to +3 in the cis-diaquabis(oxalate)chromate(III)
dihydrate product. Oxalic acid dissociates according to the following half
equation:

It therefore supplies the electrons needed for the reduction
of chromium.

The overall reaction that occurs during part (iii) of the
experiment is:

 

 

 

 

 

 

                Tris product

The potassium cis-diaquabis(oxalato)chromate(III) dihydrate
and potassium tris(oxalato)chromate(III) trihydrate both have optical isomers
and are examples of chiral molecules. This means that they have no plane of
symmetry and have non-superimposable mirror images of each other. They also rotate
the plane of polarised light in opposite directions.

Recrystallisation is used at the end of each part to obtain
the final product. Recrystallization is a method of obtaining a pure solid from
an impure compound dissolved in solution. It follows the concept that solids
are more soluble in hot solvents than cold ones. A solvent is used that will
not dissolve the solid when cold and when crystals form after cooling the
solution, any impurities should remain dissolved in the solvent. 2

Vacuum filtration is used throughout this experiment to
collect and dry the products. They are collected in the Buchner funnel while
any impurities are sucked through into the conical flask in the solvent. The
suction of air through the solid product also helps to remove remaining moisture.

Pre-Practical
Questions:

1.      
Bidentate ligands have two pairs of donor
electrons that bind to a metal atom ie. they bind to it at two places.

A chelating ligand is one with more than
one donor site (polydentate) that binds to a metal atom, forming a ring.

 

2.      
The most common oxidation states of chromium are
+6 and +3. Chromium is in its +6 oxidation state in dichromate, Cr2O72-.
It is in its +3 oxidation state in chromium(III) oxide, Cr2O3.

 

3.      
Cr2O72- + 14H+
+ 6e- ? 2Cr3+ +
7H2O

In dichromate, the chromium is in its +6
oxidation state and in Cr3+, it’s in its +3 oxidation state.

 

 

 

Experimental
Procedure:

12g (0.0982 moles) oxalic acid dihydrate and 4g (0.0136
moles) potassium dichromate were ground separately to fine powders, in the fume
hood. The white oxalic acid and orange potassium dichromate powders were then
mixed together and transferred to a dry 150cm3 beaker. The beaker
was placed on a warm hot-plate and covered with a clock glass. A reaction was
observed that produced bubbles of carbon dioxide and a black syrup. 20cm3
alcohol was added to the beaker and the beaker was warmed (below 100oC).
The syrup was stirred until a black crystalline product formed in the alcohol
solvent. The product was filtered through a Buchner funnel using vacuum
filtration, washed with less than 20mL alcohol and then dried in the air. The
yield and appearance of the resulting dark grey/black product was recorded.

9g (0.0737 moles) oxalic acid was ground in a mortar and
dissolved in 20cm3 water and warmed to 70oC on a hot
plate. 3g (0.0102 moles) potassium dichromate was gradually added in small
portions. When bubbles of CO2 were no longer observed, the solution
was boiled and 3.5g (0.019 moles) potassium oxalate was added. No visible
reaction was observed. When the potassium oxalate had dissolved, the solution was
cooled to room temperature, 4cm3 alcohol was added and the mixture
was cooled in ice. The crystallised product was filtered through a Buchner
funnel and washed with small volumes of 50% ethanol/water mixture and then with
alcohol. The product was dried in the air and the yield of dark green/black
product was recorded.

A series of test tube reactions were carried out on the two
products obtained. A sample of both products were placed in separate test tubes
and water was added to both tubes to test the solubility of both products. The
solubility and colour of the solutions were recorded when both products were
tested with dilute NH3, dilute HCl and BaCl2 solutions
respectively.

 

Results:

CHROMIUM(III) COMPLEXES

ii)
Preparation of cis-KCr(C2O4)2(H2O)2·2H2O

Weight of K2Cr2O7
taken: 3.98g

Moles of K2Cr2O7
taken: 0.01353 moles

Weight of cis-KCr(C2O4)2(H2O)2·2H2O obtained: 7.32g

Moles of cis-KCr(C2O4)2(H2O)2·2H2O
obtained: 0.0216
moles

% yield of
product: 79.75%

Physical
appearance of product: dark grey solid

iii)
Preparation of K3Cr(C2O4)3·3H2O

Weight of K2Cr2O7 taken: 2.99g

Moles of K2Cr2O7 taken: 0.01016 moles

Weight of K3Cr(C2O4)3·3H2O obtained: 3.38g

Moles of K3Cr(C2O4)3·3H2O obtained: 0.0069
moles

% yield of product: 33.99%

Physical appearance of product: dark green/black solid

Test

Compound
(ii)

Compound
(iii)

Water
solubility

Soluble

Soluble

Colour
in daylight

Violet

Violet

Dilute
NH3

Soluble – Green

Soluble – Violet

Dilute
HCl

Soluble – Violet

Soluble – Violet

BaCl2
solution

Soluble – Violet

Insoluble

 

The bubbles
of gas visible during the two reactions are due to the production of carbon
dioxide as shown in the reactions given in the introduction.

No visible
reaction was observed when potassium oxalate was added in part (iii). This is
because the chromium(VI) has already been reduced to chromium(III) and cannot
be reduced any further.

The cis
product was shown to give a green solution when tested with dilute NH3
while the tris product gave a violet solution. This is due to the reversible
ligand change in the cis product when a water molecule is deprotonated to give
an OH- ligand. It was proven that the colour change was not due to
the bonding of ammonia to the complex by testing the cis product with another
base, NaOH, which gave the same green colour. The tris product however gave
violet solutions in both low and high pH solutions.

 

 

 

 

 

 

 

Discussion:

A
percentage yield of 79.75% was obtained of the potassium cis-diaquabis(oxalato)chromate(III)
dihydrate. This indicates that although some product may have been lost due to human
error during transferral between containers, a substantial amount of product
was successfully produced.

However, a
yield of only 33.99% of potassium tris(oxalato)chromate(III) trihydrate was
obtained. This percentage yield is low, indicating that a significant amount of
product may have been lost due to human error or if the reactants did not fully
react.

 

Conclusion:

Samples of potassium
cis-diaquabis(oxalato)chromate(III) dihydrate and potassium tris(oxalato)chromate(III)
trihydrate were successfully produced during this experiment with percentage yields
of 79.75% and 33.99% respectively.

 

Post-Practical Qs:

1.     

 

 

 

 

 

 

 

 

 

                        Cis                                                                   Cis

 

 

 

 

 

 

 

                       

                                    Trans

 

 

 

 

 

           

Tris                                                      Tris

The cis-
and trans- products are stereoisomers as they are only different in terms of
their spatial arrangement of atoms.

The cis product
also has an enantiomer/optical isomer making it an example of stereoisomerism.

The tris
product again has an optical isomer.

 

 

2.     
Dichroism
is observed in the potassium tris(oxalato)chromate(III) trihydrate product. The
dry product is a dark green/black colour however, when dissolved in water, a
violet solution is observed. Dichroism is defined as the property of absorbing more
light in one incident plane than another, resulting in the exhibition of
different colours.

3.     
The
monodentate oxalate ligand, Cr(C2O4)69-
isn’t formed because the negative charges on the oxalate groups repel each
other. The chelate effect also means that the coordination of the second O-
atom in the bidentate oxalate ligand is favoured once the other O-
has bonded to the chromium ion. 

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