Title:
Preparation And Studies of Potassium Diaquabis(Oxalato) Chromate (III) Dihydrate {K[Cr
(C2O4)2 (H2O)2].2H2O}
Objective:
To prepare potassium diaquabis(oxalate) chromate (III) dihydrate complexes and study its
reactions.
Introduction:
Chromium (Cr) is found in group 6 (VIB) of the Periodic Table along with
molybdenum (Mo) and tungsten (W). It is located at Period 4 and has atomic number of 24.
These metals, therefore, have 6 valence electrons. In the chromium atom, the 3d-orbital is
half-filled with 5 electrons and 6th electron is present in 3s orbital. It is in solid state at room
temperature and density of 7194 kgm -3. Electron configuration of chromium is [Ar] 3d54s1.
Boiling point and melting point of chromium are 2671 oC and 1907 oC respectively. Besides,
it has atomic mass of 51.9961 amu. This transition metal is a lustrous steel-gray color which
takes a high polish. It is hard and resistant to corrosion. Chromium has a high melting point,
stable crystalline structure, and moderate thermal expansion. All chromium compounds are
colored. Chromium compounds are toxic. Chromium is used to harden steel. It is a
component of stainless steel and many other alloys. The metal is commonly used for plating
to produce a shiny, hard surface that is resistant to corrosion. Chromium is used as a catalyst.
It is added to glass to produce an emerald green color. Chromium compounds are important
as pigments, mordants, and oxidizing agents.The principal ore of chromium is chromite
(FeCr2O4). The metal may be produced by reducing its oxide with aluminum. It can be used
directly for the manufacture of steel and other chromium compounds. An alloy of chromium
and nickel, nichrome is used for electrical heating elements. Chromium (III) compounds are
sparingly soluble in water.
Chromium (III), the most stable oxidation state of chromium, forms a number of
complexes, in which six monodentate ligands surround the cation octahedrally. The colors of
these complexes are determined by the ligands attached to the Cr centre. In the above
compound, two oxalate anions (bidentate ligands) and two water molecules coordinate the
chromium cation to provide oxygen atoms at the corners of the octahedron. Two geometrical
-
2-
isomers exist for the complex ion, [Cr(C2O4)2(H2O)2] , two (C2O4)
ions [or two H2O
molecules] can be on the same side of octahedron structure of the complex ion giving the cis
isomer or on the opposite sides giving trans isomer.
Preparation
(1)
A pestle and mortar were used to powder 0.0555 mole oxalic acid dihydrate and
separately 0.0068 mole K2Cr2O7. The powders were mixed thoroughly, regrinding
them gently.
(2)
The mixture was transferred into a 100 ml beaker. The beaker was covered with a
small watch glass and heated gently the beaker on the hot-plate. A vigorous reaction
commenced with the evolution of steam and carbon dioxide and with the formation
of a dark semi-solid compound.
(3)
The beaker was removed from the hot-plate and immediately added 15 ml absolute
ethanol. The mixture was stirred with a glass rod and the mixture was transferred into
a small mortar (or stir with a beaker and glass rod). Stirring was continued for further
5 minutes breaking up the product and decant the ethanol. A fresh 15 ml absolute
ethanol was added. The product was grinded using a beaker and glass rod until a
crystalline violet powder is obtained.
(4)
The crystals were filtered by vacuum suction using a sintered glass crucible. The
crystals were washed two times with 5 ml ethanol and dried in the air by vacuum
suction. The product was weighed on a top loading balance and transferred into a
properly labelled vial.
(5)
The following tests were carried out on the product. Qualitative Analysis
(i)
Test for potassium
The flame-test for potassium was performed in a fume cupboard using a platinum
wire. The colour of the flame was noted.
(ii)
Test for cis and trans isomers
The following test was carried out to determine the nature of the isomer prepared in
this experiment:
A few crystals of the product was moisten and placed on a small filter paper resting
on a watch glass, first with a few drops of dilute ammonia solution and then with
dilute hydrochloric acid. Observations were recorded. With the cis- form, a deepgreen solution rapidly forms and spreads over the filter paper, no solid remains. With
the trans- form there is no green colour, but light-brown solid forms and remains
undissolved. In each case, reaction with dilute HCl restores the original colour of the
isomeric form.
Results:
Mass of oxalic acid dehydrate
6.9879g
Mass of K2Cr2O7
2.0058g
Mass of watch glass
19.8201g
Mass of watch glass and product
23.9701g
Mass of product
23.9701g-19.8201g
= 4.15g
Observations:
i)
Test for potassium
The colour of the flame is pinkish purple.
ii)
Test for cis and trans isomer
A deep green solution rapidly forms and no solid remains.
Discussion:
The purpose of doing this experiment is to prepare potassium diaquabis (oxalato)
chromate (III) dihydrate complexes and study its reactions. Chromium forms octahedral
complex ion like many transition metals do. In this experiment, potassium diaquabis (oxalato)
chromate (III) dihydrate was formed. Oxalic acid dihydrate and potassium dichromate were
as the starting material in this experiment to prepare the complex. Potassium dichromate
(K2Cr2O7) is a crystalline ionic solid in red orange colour and it has molecular weight of
294.15 g/mol. It is also an inorganic chemical reagent where mostly use as an oxidizing agent
in the reaction. Its molecular geometry is tetrahedral. Oxalic acid dihydrate (H2C2O4.2H2O) is
a colourless crystalline solid. It is soluble in water and its solubility increase with the
temperature. It is a strong organic acid and simplest dicarboxylic acid. The oxalate ion from
oxalic acid dehydrate, C2O42- is the conjugate base of oxalic acid, H2C2O4, acts as a
chelating agent for potassium ion because the oxalic acid is the reducing agent for this
oxidation-reduction reaction.
As the starting procedures in this experiment, both solution were mixed and heated on
a hot plate until the formation of a dark semi-solid compound with evolution of carbon
dioxide and steam was released. The formation of this compound indicates that complex was
started to form after the potassium dichromate react with oxalic acid dihydrate. Next, the
product was grinded until a violet powder is obtained. At the end, the product was being
filtered and thus the final product formed in this reaction is potassium diaquabis(oxalate)
chromate (III) dihydrate {K[Cr (C2O4)2 (H2O)2].2H2O}. This product formed is in solid state
which is in black colour under sunlight but has a deep purple colour under artificial light. The
product is in octahedral shape which has 6 coordination number. Half ionic equation for each
reaction involved in the preparation of the complex using oxalic acid dihydrate is H2C2O4 
2CO2 + 2H+ + 2e-. Next, the half ionic equation for potassium dichromate for the preparation
complex is Cr2O72−(aq) + 14H+ + 6e− → 2Cr3+(aq) + 7H2O. The balanced equation for overall
reaction is K2[Cr207] + 7H2C204*2H20 2{K[Cr(H20)2(C204)2].2H20} + 6CO2 + 13H2O.
In this experiment, distilled water was not allowed to rinse the product because it
may cause the product to dissolve and form another product which is hexaaquachromium
(III), [Cr(H2O)6] 3+ which is soluble in water. Therefore, absolute ethanol was used to wash
the crystals during the filtration process. Absolute ethanol is ethanol with low water content
(5%) and high percentage of ethanol (99%). By using absolute ethanol, the product formed
was less soluble in ethanol than in water so it will not dissolved much of the product in water.
In other words, absolute ethanol helped to reduce the amount of crystals from dissolved
further. Furthermore, by drying the product in the air by vacuum suction as in the experiment,
the product formed was far easier to dry with absolute ethanol in it than with water as ethanol
is volatile. That is why absolute ethanol was used to wash the product for few times.
After had collected the product, following test were carried out on the product. Flame
test was used to test for the presence of potassium using a platinum wire. Colour of the
product formed was determined under the flame of Bunsen burner. A flame test is to detect
the presence of certain element, basically metal ions which based on each element’s
characteristic emission spectrum. From the difference in colour for each of the element, we
can differentiate between one element to another element. In this experiment, the colour of
flame changes from orange to pinkish purple. This observation that obtained had indicates
that potassium ion was present in this compound. Next, test for cis and trans isomer by
adding ammonia solution and dilute hydrochloric acid. Both isomers are in equilibrium when
solid complex is dissolved in water. Cis form is more soluble and exists in high proportion
solution but it is slowly converted into trans from when the solution is evaporated slowly. In
solid state only one isomer can exits depending on the method of preparation of the complex.
Firstly, few drops of dilute ammonia solution and then dilute hydrochloric acid were added.
Based on observations obtained, a deep green solution was rapidly formed and spread over
the filter paper but there were no solid remains. This had showed for the cis-form on the
product. Furthermore, as the solution evaporated slowly, light brown solid was formed and
remains undissolved and there is no longer green colour solution.
In this experiment, percentage yield was calculated. By taking the theoretical mass
and actual mass of the product, percentage yield can be obtained. Thus, the formula is
Percentage yield (%) =
𝐴𝑐𝑡𝑢𝑎𝑙 𝑚𝑎𝑠𝑠
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑎𝑠𝑠
× 100%. From the equation, K2[Cr2O7] +
7H2C2O4.2H2O  2K[Cr(H2O)2(C2O4)2].2H2O + 6CO2
+ 13H2O, 1 mole of K2Cr2O7
produce 2 moles of K[Cr(H2O)2(C2O4)2].2H2O. From this equation, we can find for the
theoretical mass of the product. On the other hand, actual mass will be obtained through
conducting this experiment. The theoretical mass of this product is 4.63g and actual mass that
obtained is 4.15g. Therefore, percentage yield that get from this experiment is 89.63g.
There were some precautionary steps taken in this experiment. Firstly, before started
the experiment, all the apparatus must washed with acetone then put into the oven to remove
acetone. We have to use acetone instead of using water because the product formed later will
dissolved in water and this will affect the percentage yield of the product. Besides, before the
crystalline violet powder was being filtered by vacuum suction, the powder must be spread in
order to add in absolute ethanol. This is to ensure that the product can dry up faster and the
unreacted impurities are able to wash away. Moreover, absolute ethanol was chosen to be
used in this experiment because it has a high proportion of ethanol than water and thus,
product formed would not easily dissolved. Furthermore, gloves and safety googles must be
worn throughout the experiment to prevent any injuries and to avoid any direct contact with
chemicals.
Questions and Answers:
(1)
What is the oxidation state of chromium in K2Cr2O7 and in the product? Write two
half-equations for each reaction involved in the preparation of the complex using
oxalic acid dihydrate (H2C2O4.2H2O) and potassium dichromate (K2Cr2O7) and a
balanced equation for the overall reaction.
Oxidation state of chromium in K2Cr2O7 : +1(2) + 2x -2(7) =0
2x =12
X= +6
Oxidation state of chromium in product: x + 0(2) + 0(2) = -1
x= -1
Two half equations:
Oxalic acid dehydrate: H2C2O4  2CO2 + 2H+ + 2e- ( Oxidation reaction)
Potassium dichromate: Cr2O72−(aq) + 14H+ + 6e− → 2Cr3+(aq) + 7H2O
(Reduction reaction)
Balanced
equation
for
overall
equation:
2{K[Cr(H20)2(C204)2].2H20} + 6CO2 + 13H2O
K2[Cr207]
+
7H2C204*2H20
(2)
Calculate the % yield of the product.
K2[Cr2O7] + 7H2C2O4.2H2O 2K[Cr(H2O)2(C2O4)2].2H2O + 6CO2 + 13H2O
From the equation: 1 mole of K2Cr2O7 produce 2 moles of K[Cr(H2O)2(C2O4)2].2H2O
Molar mass of K2Cr2O7 = 294.185 g/mol
Molar mass of K[Cr(H2O)2(C2O4)2].2H2O = 339.1943 g/mol
Number of moles of K2[Cr2O7] =
=
𝑀𝑎𝑠𝑠 (𝑔)
𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠(
𝑔
)
𝑚𝑜𝑙
2.0058𝑔
294.185 (
𝑔
)
𝑚𝑜𝑙
= 6.82 × 10-3 mol
Number of moles of K[Cr(H2O)2(C2O4)2].2H2O = 6.82 × 10-3 mol × 2
= 0.01364 mol
Theoretical mass of K[Cr(H2O)2(C2O4)2].2H2O = no. of moles× molar mass
= 0.01364 × 339.1943
= 4.63 g
Actual mass of K[Cr(H2O)2(C2O4)2].2H2O = 4.15 g
𝐴𝑐𝑡𝑢𝑎𝑙 𝑚𝑎𝑠𝑠
Percentage yield (%) = 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑎𝑠𝑠 × 100%
4.15𝑔
= 4.63𝑔 × 100%
= 89.63 %
(3)
Write the chemical equations for the reactions involved in the tests for the detection of
chromium (III) and oxalate ions. Indicate in each case whether it is an acid-base or
oxidation-reduction reaction.
The test for chromium (III) is an acid- base reaction. Product was dissolved in water
and form hexaaquachromium(III) ions [Cr(H2O)6]3+. When a base was added into
the solution, neutral complex and precipitate were formed. For example, reactions
with ammonia solution. The ammonia acts as base and a ligand. By adding small
amount of ammonia solution, hydroxide ions (OH-) are pulled off the hexaaqua ionto
give the neutral complex.
Equation: [Cr(H2O)6]3+ + 3NH3  [Cr(H2O)3(OH)3] + 3NH4+
By adding excess ammonia solution, ammonia will replace water as a ligand to give
hexaamminechromium(III) ions.
Equation: [Cr(H2O)6]3+ +6NH3  [Cr(NH3)6]3+ +6H2O
Next, the test for oxalate ion is oxidation-reduction reaction.
Purple colour of
potassium permanganate will decolourise immediately if react with acid like dilute
sulphuric acid while purple colour of potassium permanganate decolourise slowly
without dilute H2SO4. In this case, acid acts as a catalyst in this redox reaction. The
oxalate ion is oxidised by the permanganate ion in the acid solution. Potassium
permanganate (KMnO4) is acts as oxidising agent. In this oxidation-reduction
reaction, the C2O42- is oxidised to CO2.
Equation: C2O42-  2CO2 + 2eThe permanganate ion (MnO4-) will be reduced to Mn2+.
Equation: MnO4- + 5e-  Mn2+
(4)
Draw the structures of the two possible geometrical isomers of the complex. What is
your conclusion regarding the nature of the isomer in the test (iv)? When dilute
ammonia is added to the solid complex, one molecule of the ligand H2O is replaced
by OH- ion. Write equations for both the isomers to represent this reaction as well as
for the reaction of the hydroxo product and HCl to restore the original complex
The geometrical isomer that exists in {K[Cr(H20)2(C204)2].2H20} is cis isomer
because it formed a deep green solution and spreads over the filter paper and there
was no solid remains. The ammonia acts as both a base and a ligand. When dilute
ammonia is added to the solid complex, one molecule of the ligand H2O is replaced
by OH- ion.
Equation for both isomers:
K[Cr(C204)2 (H20)2].2H20 + NH3  K[ Cr(C204)2 (H20)(OH)].2H2O + NH4+
K[Cr(C204)2 (H20)(OH)].2H20 + HCl  K[Cr(C204)2 (H20)2].2H20 +ClConclusion:
The product formed in this experiment is potassium diaquabis (oxalato) chromate (III)
dihydrate complexes, {K[Cr (C2O4)2 (H2O)2].2H2O} which is in octahedral shape. This
complex has two oxalate anions bidendate ligand and two water molecules coordinate the
chromium cation to provide oxygen atoms. This product was prepared form potassium
dichromate and oxalic acid dihydrate. The percentage yield of the product is 89.63%. Pinkish
purple flame can be seen when the product was tested for flame test showing the presence of
potassium ion. Next, a deep green solution was formed and no solid remains showing the
product with cis isomer.
Reference:
1.) Chromium. Available from: http://chemistry.about.com/od/elementfacts/a/chromium.html.
[Access on 1 March 2014]
2.) Reactions of chromium (III) ions in solution. Available from:
http://www.chemguide.co.uk/inorganic/transition/chromium.html. [Access on 1 March 2014]
3.) Inorganic Synthesis. Available from: http://files.rushim.ru/books/polytom/inorganicsynthesis/inorganic-synthesis17.pdf. [Access on 1 March 2014]