UNIVERSITI MALAYSIA SABAH
SEKOLAH SAINS DAN TECHNOLOGI
PANDUAN AMALI
SK 2161
KIMIA AMALI IV
NAMA:
NO. MATRIK:
KUMPULAN:
CONTENTS
EXPERIMENT 1
PREPARATION OF POTASSIUM TRISOXALATOALUMINATE (III) TRIHYDRATE {K3[Al(C2O4)3].3H2O}
EXPERIMENT-2
DETERMINATION OF THE OXALATE CONTENT OF THE COMPLEX {K3[Al(C2O4)3].3H2O}
EXPERIMENT 3
PREPARATION OF POTASSIUM BISOXALATODIAQUACHROMATE (III)
DIHYDRATE {K[Cr(C2O4)2(H2O)2].2H2O}
EXPERIMENT 4
PREPARATION OF A COMPLEX COMPOUND [Cu(NH3)4]SO4.H2O
EXPERIMENT 5
DETERMINATION OF ZINC ION WITH ETHYLENEDIAMINETETRA ACETIC ACID (EDTA): A COMPLEX-FORMING CHELATING AGENT
EXPERIMENT 1
PREPARATION OF POTASSIUM TRISOXALATOALUMINATE (III) TRIHYDRATE {K3[Al(C2O4)3].3H2O}
Introduction
Aluminium occurs widely in nature as aluminosilicate minerals and as bauxite, Al2O3.xH2O from which the metal can be produced by electrolysis after dissolving in molten cryolite, Na3AlF6. The metal is mainly used in aluminium alloys. The organoaluminium compounds (e.g., Et3Al) are used in the catalysts involved in the polymerisation of ethene.
Group 13 (IIIb) of the Periodic Table includes the elements boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). The electronic configuration is ns2np1 and the group valency is three. Aluminium (At. No. = 13) is present in the third period and contains empty 3d orbitals and favours the formation of complexes with six coordination number. It forms stable octahedral complexes with such ligands as 8-hydroxyquinoline also known as “oxine” (C9H7NO) and with the oxalate anion (C2O42-). In this experiment, you are to prepare an oxalato- aluminium (III) complex and study its reactions.
Preparation
(1) Dissolve 7 g (7/666.42 = 0.0105 mole) Al2(SO4)3.18H2O in 100 mL distilled water. Prepare a solution of 4.0 g (4.0/40.0 = 0.100 mole) of NaOH in 20 mL of distilled water and add to the above solution dropwise with stirring.
(2) Filter the freshly precipitated Al(OH)3 using a vacuum filtration technique (see the demonstrator) and wash the precipitate 6 times using 20 mL distilled water each time.
(3) Weigh 4 g (4/126.04 = 0.0317 mole) oxalic acid dihydrate and 6 g (6/184.2 = 0.0326 mole) potassium oxalate monohydrate and dissolve them in 100 mL distilled water in a 250 mL beaker. Add Al(OH)3 to the solution and heat the mixture on a steam bath with stirring using a glass rod to dissolve Al(OH)3 (sometimes a little precipitate may remain).
(4) Evaporate the solution to about 30 mL using a rotary evaporator (see the demonstrator) or a hot plate. Gravity filter the concentrated solution into a 250 mL beaker.
(5) Add 70 mL of ethanol to the solution dropwise over 10 minutes and cool in ice to precipitate completely colourless crystals of the product.
(6) Vacuum filter the product as in step (2) and wash four times with 10 mL ethanol, dry completely by vacuum suction.
(7) Keep the dry product for the next experiment.
(8) Weigh the product on a top loading balance. Transfer about 0.5 g into a small beaker to carry out the following tests for the presence of potassium, aluminium and oxalate in the product and the rest into a properly labelled vial and store over CaCl2 in a dry jar. Hand in the jar to the Demonstrator/Technician.
Qualitative Analysis
(1) Test for potassium
Your Demonstrator will show you the flame-test for potassium to be performed in a fume cupboard using a platinum wire. Note the colour of the flame.
(2) Test for aluminium
Dissolve about 0.2 g of the product in 5 mL water and add 2 mL of dilute sodium hydroxide solution. Note any precipitate in the solution. Now prepare a fresh solution of 0.2 g of the product in 10 mL dilute HCl, warm the solution and add NaOH solution dropwise until a precipitate is formed and redissolved in excess NaOH. Shake the solution after each addition.
(3) Test for oxalate
Dissolve 0.5 g of the product in 20 mL distilled water. Divide the solution into two almost equal portions. Add 5 mL dilute H2SO4, in one of the solutions. Heat both the solutions and add in each solution 3-4 drops of dilute solution of KMnO4. Note the colour of the resulting solutions.
Report:
(Result sheet is provided. Anwer the questions in the result sheet according to the information given below)
(1) Write an ionic equation for the formation of Al(OH)3 using Al2(SO4)3.18H2O and NaOH. Indicate which is the limiting reagent in the reaction.
(2) The formation of the trisoxalatoaluminate (III) complex anion is carried out by reacting Al(OH)3 with the exact number of moles of the oxalate ion and oxalic acid. The number of moles of each reactant is given in step (3) of the preparation section. Using this information, balance the equation for the reaction:
Al(OH)3 + (C2O42-) + H2C2O4 => [Al(C2O4)3]3- + H2O
(3) Aluminium is present as a free cation when Al2(SO4)3.18H2O is dissolved in water. Which ions are present when K3[Al(C2O4)3].3H2O is dissolved in water? The function of acid used in carrying out test (ii) is to release Al3+ from the complex ion, [Al(C2O4)3]3-. The [Al3+(H2O)6] ion written in simple form as Al3+ is then identified as described in test (ii). Write equations for the reaction of the acid with the product and for the reaction which indicates the presence of aluminium (III) in the product.
(4) Aluminium (III) hydroxide is an amphoteric substance i.e., it reacts both with acids and alkalis to form the aluminium salt of the acid and AlO2- anion respectively. Write equations for the reactions of Al(OH)3 with HCl and NaOH to indicate the amphoteric nature of Al(OH)3.
(5) The oxalate ion is oxidised by the permanganate ion in the acid solution. Follow the following steps to derive the two half ionic equations for this reaction:
Oxidation of oxalate ion to carbon dioxide
(i) Write C2O42- on the left and CO2 on the right of =>.
(ii) Balance the carbon and oxygen atoms.
(iii) Balance the charge by writing electrons (e-) on the right hand-side.
Why is it called an oxidation reaction?
Reduction of MnO4- to Mn2+ in acid solution
(i) Write MnO4- on the left and Mn2+ on the right of =>.
(ii) Balance oxygen by writing H2O on the right hand-side.
(iii) Balance hydrogen by writing H+ on the left hand-side.
(iv) Balance the charge by writing electrons (e-) on the left hand-side.
Multiply with the appropriate factor to obtain the same number of electrons in each half-equation. Add the two half-equations to obtain the overall equation for the oxidation-reduction reaction. Why is it called reduction reaction?
What evidence did you see for this redox reaction in test (iii) to indicate the presence of oxalate in your product?
(6) The presence of water of crystallation in the complex can be tested by heat a small quantity of the complex in a test tube and holding a spetula containing anhydrous CuSO4 in the vapours. Note the colour of CuSO4. If it turns blue, water is confirmed. Write a balanced equation for this test after finding the information from the book about copper sulphate. Note the presence of water can also be tested from the I.R. spectra of K3[Al(C2O4)3].3H2O.
(7) Calculate the % yield of the product.
(8) Draw the structure of the oxalatoaluminate anion (read the Introduction Section), taking oxalate as a bidentate ligand.
EXPERIMENT-2
DETERMINATION OF THE OXALATE CONTENT OF THE COMPLEX {K3[Al(C2O4)3].3H2O}
Introduction
The complex, potassium trisoxalatoaluminate (III) trihydrate, contains potassium as the counter ion and the complex anion, [Al(C2O4)3]3- with three moles of water. The purity of the complex can be determined by analysing the quantity of one or more of its constituents (potassium, aluminium and oxalate) and comparing the experimental value with the calculated value. The relative percentages of all the constituents can be determined in order to confirm the above stoichiometry of the complex.
In this experiment, you are to determine the oxalate content of the complex and therefore find the purity of the complex which is decomposed by addition of excess sulfuric acid.
K3[Al(C2O4)3] + 3H2SO4 => 3H2C2O4 + 3K+ + Al3+ + 3SO42- (1)
The oxalic acid thus released into the solution can be determined by titration with a standard solution of KMnO4 which acts as an oxidant. The oxidation state of manganese in KMnO4 is (VII) i.e., Mn+7 which is reduced direct to manganese (II) i.e., Mn+2 in hot acid solution (or else it is converted to MnO2) by accepting five electrons from the oxalate ion which in turn acts as a reductant and is oxidised to CO2 as shown in the reaction equations:
MnO4- + 8H+ + 5e => Mn2+ + 4H2O (2)
C2O42- => 2CO2 + 2e (3)
Exprimental
Carry out the following steps for the determination of oxalate.
(1) Weigh out accurately using an analytical balance about 0.2 g of the complex in a weighing bottle. Note the mass of the complex up to the fourth decimal place. Carefully transfer the solid complex into a 250 mL conical flask. Add about 5 mL of 2M H2SO4 to the weighing bottle using a small measuring cylinder. Use a glass rod to dissolve any remaining complex in the weighing bottle and add the solution into the flask. Use a further two times about 5 mL of 2M H2SO4 for washing the weighing bottle and add the solutions to the flask. Now add an additional 10 mL of 2M H2SO4 to the flask.
(2) Heat the solution on a hotplate, at a low dial setting, so as not to let the solution boil but the flask be just “hot” enough to touch. While the solution is being heated on the hotplate, wash a 50 mL burette with distilled water and then twice with 5 mL of the standard (0.02M) KMnO4 solution provided and finally fill the burette to the mark. Titrate the hot oxalate solution adding slowly KMnO4 solution from the burette to a permanent pink colour. Note the volume of the KMnO4 solution. Repeat the above steps (1) and (2) to obtain two concordant results i.e., two values should be within 0.1 mL for the 50 mL burette.
(3) Calculate the volume of the KMnO4 solution required in each titration for 1 g of the complex by dividing each volume with the mass of the complex used for each titration. Calculate the average volume and find the number of moles of KMnO4 in that volume for 1 g of the complex.
(4) Write the balanced overall reaction for the oxidation of oxalate by permanganate from the above half equations (2) and (3) and calculate the equivalent number of moles of oxalate ion reacted with the above number of moles of KMnO4. Now multiply the number of moles of oxalate with its molar mass [mol. mass = 88] to obtain the mass of oxalate in 1 g of the complex and then convert it to a percentage value.
(5) Calculate the theoretical value of the percentage of oxalate in the complex, K3[Al(C2O4)3].3H2O [mol. mass = 462.39]. Find the percentage purity of the complex from the relationship:
% Purity = experimental value/ theoretical value x 100
Report
(Result sheet is provided. Anwer the questions in the result sheet according to the information given below)
(1) Show all the weighings and calculations carried out to find the purity of the complex.
(2) Comment on your result regarding the purity of the complex.
(3) Comment as to why no indicator is used in this titration and state the difference in reading the burette when using a coloured solution such as KMnO4 and a colourless solution such as NaOH.
(4) Read the Introduction Section carefully and state the reason as to why is it necessary to carry out the KMnO4 titration in hot acid conditions?
EXPERIMENT 3
PREPARATION OF POTASSIUM BISOXALATODIAQUACHROMATE (III)
DIHYDRATE {K[Cr(C2O4)2(H2O)2].2H2O}
Introduction
Chromium (Cr) is found in group 6 (VIA) of the Periodic Table along with molybdenum (Mo) and tungsten (W). 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. The most common ore of chromium is chromite [chrome ironstone/iron (II) chromite FeCr2O4 which 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), the most stable oxidation state of chromium, forms a number of complexes, in which six monodentate ligands surround the cation octahedrally. 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 isomers exist for the complex ion, [Cr(C2O4)2(H2O)2]-, two (C2O4)2- 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) Using a pestle and mortar, powder 7 g (7/126.04 = 0.0555 mole) oxalic acid dihydrate and separately 2 g (2/294.2 = 0.0068 mole) K2Cr2O7. Mix the powders thoroughly, regrinding them gently.
(2) Transfer the mixture into a 100 mL beaker. Cover with a small watch glass and gently heat the beaker on the hot-plate. A vigorous reaction commences with the evolution of steam and carbon dioxide and with the formation of a dark semi-solid compound.
(3) Remove the beaker from the hot-plate and immediately add 15 mL absolute ethanol. Stir with a spatula and transfer the mixture into a small mortar (or stir with a beaker and glass rod). Continue stirring for further 5 minutes breaking up the product and decant the ethanol. Add a fresh 15 mL absolute ethanol. Grind the product using a beaker and glass rod until a crystalline violet powder is obtained.