Potassium Trihydrate

SYNTHESIS OF POTASSIUM TRIS(OXALATO) CHROMIUM(III) CHROMIUM(III) TRIHYDRATE : K3[CR(C2O4)3].3H2O

ABSTRACT

The aim of this experiment is to synthesize potassium tris(oxalato)chromium (III) via the addition of oxalic acid acid and potassium dichromate, K 2Cr2O7  to form a reagent. It will then being added with potassium oxalate monohydrate, K 2C2O4.H2O after a few minutes of cooling process from previous reactions. A dark green precipitate will be formed after several process and it will be dried in the oven and a total of 1.23g will be obtained. Later, the theoretical mass and the percent yield will be calculated. Precipitate obtained will be kept in the vial to be used in next experiment.

INTRODUCTION

Potassium tris(oxalato)chromium (III) is a metal complex of iron with three oxalate ligands 2-

(C2O4 ) bonded to every central metal atom. These ligands are bidentate, meaning that each of them binds to the metal atom at two different places. It has the chemical formula K3[Cr(C2O4)3]·3H2O, and the three-dimensional structure proposed in Figure 1. Potassium tris(oxalato)ferrate tris(oxalato)ferrate (III) is hygroscopic and light sensitive in nature.

Figure 1: 3-dimensional structure of potassium tris(oxalato)chromium (III)

In this experiment, the desired product was produced in the form of dark green precipitate. The oxalic acid utilized in the first step of this reaction scheme can be synthesized by 1

hydrolyzing cyanogen  or by oxidizing sucrose or glucose with nitric acid in the presence of a 2

small amount of vanadium pentoxide.  Another method of forming oxalic acid involves the oxidative carbonylation of alcohols followed by hydrolysis.

1

After successfully synthesizing the product, the percent yield is calculated using the mass of precipitate obtained and theoretical yield using the formula below.

METHODOLOGY

The first part of this experiment involved synthesizing the desired product, potassium tris(oxalato)chromium (III). This was done by mixing 5.00g of oxalic acid (H 2C2O4·2H2O) in 10 mL of water with potassium dichromate (K 2Cr2O7) in a 25 mL BOECO conical flask. A filter funnel was placed on the flask. A blue-black or dark blue mixture was formed and it was spontaneously warm up almost to boiling. Bubbles formation was observed during this process. After about 15 minutes,the reaction was subsided and 2.10g of potassium oxalate monohydrate, K2C2O4.H2O was dissolved in the mixture. It was then being heated up on a FAVORIT hot plate at level 4 temperature. After 13 minutes,the mixture was boiled and it was left to cool down to room temperature. Then, 10mL of 95% ethanol was added before the mixture in the flask was cooled further in a bath of ice for about 15 to 30 minutes. A dark green precipitate will be formed at the bottom of the conical flask. The second part of this experiment involved suction filtration process of the mixture. WHATMAN filter paper was used in this process. The remaining precipitate was collected an put on a glass plate. It was then being allowed to dry in a MEMMERT oven at the temperature of 73 Celcius degree for 20 minutes. After that,the solid was taken out from the oven and being weighed on a balance. The theoretical yield and percentage yield was obtained and the solid precipitate was kept in a vial to be used in next experiment.

RESULT AND OBSERVATION 1. THEORETICAL

YIELD

OF

K3[CR(C2O4)3].3H2O

K2Cr2O7 + 7 H2C2O4 + 2 K2C2O4 → 2 K3[Cr(C2O4)3] + 6 CO2 + 7 H2O From above equation, 1 mol of K2Cr2O7 → 2 mol of K3[Cr(C2O4)3] No. of mole K2Cr2O7 used = mass/RMM = 1.8g/2(39.1) + 2(52) + 7(16) = 0.0061 mol Thus, 0.0061 mol K2Cr2O7 → 0.0122 mol of K3[Cr(C2O4)3] Theoretical Mass of K 3[CR(C2O4)3].3H2O = 0.0122 mol X RMM K3[CR(C2O4)3].3H2O = 0.0122 mol X 451.3632g/mol = 5.51g

2. MASS OF K3[CR(C2O4)3].3H2O OBTAINED

Glass plate mass = 25.97 g Glass plate + K 3[CR(C2O4)3].3H2O mass = 27.20 g Mass of K3[CR(C2O4)3].3H2O = 27.20 g – 25.97 g = 1.23 g

3. PERCENT YIELD OF K3[CR(C2O4)3].3H2O

= 1.23 x 100 5.51 = 22.32 %

(MASS)

4. WHAT IS THE OXIDATION STATE OF CHROMIUM IN K 2Cr2O7 AND IN THE PRODUCT?

The oxidation state of chromium, K2Cr2O7, Let Cr be y, +1(2) + y(2) + (-2)(7) = 0 2 + 2y – 14

=0

2y – 12

=0

2y

= 12

Y

= +6

The oxidation state of Cr in K2Cr2O7 is +6 The oxidation state of Cr in K 3[CR(C2O4)3].3H2O, Let Cr be y, 3K + [y + 3(-2)] + 3[2(+1) + (-2)) = 0 3 + y – 6

=0

y – 3

=0

y

= +3

The oxidation state of Cr in K3[CR(C2O4)3].3H2O is +3

5. HALF-EQUATIONS FOR EACH REACTION INVOLVED IN THE PREPARATION OF THE COMPLEX AND BALANCED EQUATION FOR OVERALL REACTION

Reduction: +

2-

2-

2-

14 H  + Cr2O7 + 6C2O4  + 6e- → 2 Cr(C2O4)3  + 7H2O Oxidation: 3C2O4 → 6CO2 + 6eOverall Equation: K2Cr2O7 + 7 H2C2O4 + 2 K2C2O4 → 2 K3[Cr(C2O4)3] + 6 CO2 + 7 H2O

There are some errors happening throughout the experiment such as the systematic error of the balance. It has a an error value of  – 13.05 g. There are also some random errors done by students when reading the measured value for any liquid,determining the colour of mixture and solution and also while using the equipment. Althought this is an individual experiment,the ice bath was shared by more than three students which somehow affecting the stirring action for everyone. Time taken for cooling down process in the ice bath,waiting for reaction to cool down to room temperature and also time taken for the precipitate to dry are different for everyone. All of the above aspects are more or less affecting the end results. Some precautionary steps that could be taken to minimize errors are by reducing systematic errors and random errors. Besides that, an abundant amount of equipment should also be prepared to avoid oversharing between students.

CONCLUSION

The

experiment

has

successfully

synthesized

the

desired

product

potassium

tris(oxalato)chromium (III), K 3[CR(C2O4)3].3H2O with a yield of 22.32 %. Overall equation for the synthesis is K2Cr2O7 + 7 H2C2O4 + 2 K2C2O4 → 2 K3[Cr(C2O4)3] + 6 CO2 + 7 H2O