Synthesis of Cr-acetate, Lab Reports of Inorganic Chemistry

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Mohammed Faijul Mannan

Abstract: In laboratory, there are several ways of synthesizing Cr (II)-acetate. The lab was conducted for 2 weeks. On beginning of the first week, sky blue solution of Cr (II)- acetate was prepared using potassium dichromate, zinc, sodium acetate trihydrate, hydrochloric acid. Then side arm test tube and U-shaped glass tube were used to remove hydrogen from Cr (II)-acetate solution and brick red Chromium was the outcome in this process. After that, 0.63 g of red crystals of Cr (II)- acetate were synthesized through vacuum filtration with washing by deoxygenated water, ethanol, and diethyl ether. A grind portion of this crystals was used for X-ray diffraction. On second week, photometric quantification as absorbance of Chromium was deduced from 5 different concentrated solutions of Cr (II)-Acetate stock solution against a blank of potassium hydroxide and potassium chromate. Next, Cr (II)-acetate solution was neutralized using nitric acid, potassium hydroxide and hydrogen peroxide which later used for measuring absorbance against the prepared blank solution.

Introduction: Cr (II)-acetate is also referred as Chromium acetate hydrate. Chemical formula for this compound is Cr 2 (CH 3 CO 2 ) 4 (H 2 O) 2. It is a brick-red coloured octahedral compound with a short M-M bond and provide D4h symmetry.^1 First introduced as dimeric by Eugene-Melchior Peligot in 1844. Later, in 1951 this structure was discovered.

Calculation of required K 2 CrO 4 : Molar mass of 1 mol K 2 CrO 4 = 194.1896 g/mol 0.001 mol of K 2 CrO 4 = 0.1942g Second week: At first, I prepared a blank using 4 mL 2M KOH and filled that upto 100 mL in volumetric flask. Then, I prepared a stock solution using 0.1942g of K 2 CrO 4 , 4 mL 2M KOH and filled that up to 100 mL in volumetric flask. Then I prepared 5 different concentrated solution using my stock solution. For instance, I added 2 mL stock solution into 4 mL 2M KOH, and filled that flask with water up to 100 mL and concentration of this solution was 0.0002M. Thus I made 5 others solutions of 0.00001M, 0.00002M, 0.00005M, 0.0001M, 0.0002 M with required volume of solution and water.

Calculation of required sample of Cr(II)-acetate: Given, I= 1cm A= 0. ε= 4820 l/mol*cm λ= 374 nm

A= εbC or, 0.5=4820 l/molcm * 1cm * C or, C= 1.03710^-4 M Number of mole = 1.03710^-4 M * 250 mL * = 2.59210^-5 mol required Cr (II)-acetate = 2.592*10^-5 mol * 376. 198 g/mol = 0.195 g

For measuring Cr (II)-acetate 0.195 g of sample product was taken and HNO 3 was added dropwise in it till product was dissolved. Then, I filled the flask up to 100 mL. Then, 5 mL of this solution was neutralized using 2M KOH till the solution became transparent in 250 mL volumetric flask. Then, Additional 10 mL of KOH added to it which changed the color to green. After that, 10 drops of H 2 O 2 were added to the solution, heated it gently till the mixture turned to deep green. After that, filled the solution up to 250 mL and it turned to light green. Measured the absorbance of solution against blank which had a wavelength of 374 nm.

Results and Discussion: Absorbance is tabulated below for 5 solutions along with their physical appearances:

Concentration(M) Volume added(mL) Color Change Absorbance(A) 0.00001 0.1 Clear with a slight yellowish

0.00002 0.2 Lighter than the next shade Light yellow

0.00005 0.5 Light yellow 0. 0.0001 1 Light yellow green 0. 0.0002 2 Yellow-green 1.

Formation of Cr(II)-acetate: (^2) 2 Cr Cr2+ 3+ (^) + 4 OAc^ + Zn → 2− (^) + HCl + 2 H Cr2+^ + Zn2+ 2 O → Cr2(OAc)4(H2O) 2

K 2 Cr 2 O 7 was used as a initial reactant which later oxidized by gaining electrons and become Cr(II) from Cr(III). Zn is the oxidizing agent. Next, HCl was added which is a reducing agent. NaOOCCH 3 .3H 2 O later added in it which supplies the ligands of Cr 2 (OAc) 4 (H 2 O) 2. It has a octahedral shape 2 Cr in the centre surrounded by ligand 4(OAc) and 2 H 2 O. Structure is given below:

Figure: Showing molecular structure of Cr (II)-acetate. Point group of Cr (II)-acetate is S 4.

Since the product has paired electrons in valence electron shell it shows diamagnetic behaviour.

Nature of Cr-Cr bond and bond length of Cr-Cr bond in Cr(II)-acetate: From above, Cr-acetate structure is consist of 4d orbitals overlapping of 2 Cr. Sigma bond take places by overlapping dz^2 where other bonds d (^) xz , dyz orbitals construct pi bonds and rest one d xy provides delta bonds. Bond length of Cr-Cr in Cr(II)-acetate is 236.2+/-0.1 pm. 4

Absorbance of solution after neutralization with KOH: 1.672 A against 374 nm blank solution. The change in color of product after one week, shows that there is contamination lies in the product. For which the absorption also increased over the week so as concentration.

TGA shows that how much product was gained or lost during the process.

Summary:

Over 2 weeks of period, 0.63 g of red brick Cr(II)-acetate was synthesized by vacuum filtration after preparing solution in sidearm test-tube. Loss of the product is around 22.26% after gaining product from 1.25g of starting K 2 Cr 2 O 7. Percent yield is around 77.74% which gain some additional chemical during the process for which it later turns to purple from red brick after a week. Absorption increases along with the increasing concentration. But, 1.672 A^0 deviates from the related absorbance after neutralization with KOH which is the measuring absorbance of final product on second week, which initial reactant turns violet from previous week’s red brick color.