Understanding About the Homogeneous Precipitation

Understanding About the Homogeneous Precipitation
Synthesis and thermal analysis of the group 2(iia) metal oxalate hydrates Objective : 1. To run the synthesis of calcium oxalate via the precipitate from solution containing calcium ion and oxalate ion. 2. To do a thermo gravimetric analysis on calcium oxalate. 3. Understand and practice the method of homogeneous precipitation through this experiment. Introduction Thermo gravimetric analysis (tga) is one of the common analytical thermal analysis techniques that widely use to determine the thermal profile and stability of compound. This is rather important in the industry. When the thermal profile of certain substance was know, it can be produce in large quantity by an accurate reaction with the lowest cost. With the tga techniques, a sample of material is being heated, while the sample mass is recorded as a function of temperature. By doing so, the composition of the material is analyzed, both qualitatively (which components are present in the material) and qualitatively (how much of these components is present).
In this experiment, synthesis and thermo gravimetric analysis of calcium oxalate has been carried out. Synthesis of calcium oxalate was done by the reaction between calcium ions and oxalate ion: Ca2+(aq) + C2O42-(aq) arrow CaC2O4(s) In which prepared via precipitation from solution at about pH 5 called homogeneous precipitation. Since calcium oxalate is an oxalic acid, it dissolve in acidic solution, to make the calcium oxalate form precipitate, the solution was adjust to more basic by addition of ammonia, but this is not done by adding the ammonia solution directly into the solution. Conversely, make the ammonia forms slowly within the solution through the reaction of hydrolysis of urea: (H2N)2C=O + H2O arrow 2NH3 + CO2 Urea is a suitable compound in this reaction because it can decomposed easily to form ammonia.During the experiment, the solution was heated after urea has been added in, this is to increase the degree of hydrolysis of urea and therefore increase the formation of ammonia. The formation and hydrolysis of ammonia slowly increase the pH of solution to about 5, that is sufficient to generate free oxalate and precipitate calcium oxalate. In this condition, supersaturation is minimized and local built up of concentration of ammonia are avoid compare to add directly of ammonia from initial state. Summary of experiment procedure: - 25 mg of calcium carbonate was weigh in to a 25ml beaker follow by 2.0ml of deionized water and magnetic stirring bar. 6M HCl was added drop wise to the solid with stirring. The beaker was cover by a small watch glass and the solution was gently warm on a magnetic stirring hot plate until all the solid dissolve. - The solution was dilute to 10.0ml by deionized water and one drop of 1% methyl red indicator was added into the solution. After that, 1.5ml of saturated ammonium oxalate solution and 1.5 gram of solid urea was added into the solution. - With stirring, the solution was gently boiled until the colour changes from red to yellow. Some water was added during the heating process to compensate for loss of water due to evaporation. 6M ammonia should add into solution to neutralized any excess acid if there are no precipitate from. The solution was cool to room temperature and the product crystal was collected by using Hirsch funnel and water tarp by suction filtration. The product was then wash by cold water until free from chloride ion (test by AgNO3, where the stage of few drops of filtrate do not show any turbidity with a drop of 1% AgNO3 solution). The product was dry on a clay tile and the yield percentage was calculated. Result: (theoretical value) Weight of empty beaker = 36.0130mg Weight of beaker + sample = 36.0383mg Weight of sample = 0.0253 g Molecular weight of CaCO3 = [40.0780+12.0107+(3×15.9994)] = 100.0869 No. of mole of CaCO3 used = 0.0253g/100.0869g mol-1 = 2.5278 × 10-4 mole According to the equation: Ca2+(aq) + C2O42-(aq) arrow CaC2O4(s) 1 mole of calcium ion will produce 1 mole of calcium oxalates, therefore, 2.5278 × 10-4 mole of calcium oxalate will be produced. Molecular weight of CaC2O4 .H2O= [40.0780+(2×12.0107)(4×15.9994) (2×1.0079)+15.9994 ] = 146.1123 The mass of CaC2O4 .H2O produced = 2.5278 × 10-4 × 146.1123 = 0.03693g. The actual mass of CaC2O4 .H2O produce in this experiment cannot be calculated because the mass of the filter paper was not measured at the beginning, therefore the mass of filter paper cannot be subtracted from the total weigh of product and filter paper. It is recommended that each time running the experiment which involve the filtration, the mass of filter paper need to be measured. Discussion : 1.Generally, the reaction between the calcium carbonate and hydrochloric acid will produce the calcium chloride, water, and the carbon dioxide. CaCO3 + HCl arrow CaCl2+ H2O + CO2 When calcium carbonate dissolved in acid, it gave up the Ca2+ ion into the solution which reacted with oxalate ions to form calcium oxalate in this experiment. 2. Methyl red (2-(4-Dimethylaminophenylazo) benzoic acid) is in form of dark red crystalline powder. It is stable compound and incompatible with strong oxidizing agents, thus it has been widely use as indicator in many types of titration. Methyl red has a transition range from pH 4.4 to pH 6.2. In more acidic condition it shows red colour, while in less acidic condition it will change colour to yellow. The Molecular Weight of methyl red is 269.30 g/mol, its molecular formula is (CH3)2NC6H4N=NC6H4CO2H and its structure are shows below: [image] 3. The boiling point of water is on 100 oC. Therefore the water of hydration will lost when the compound was heated to 100 oC. Hydration enthalpy increases as the size of an ion decreases and as the charge on an ion increases. This is because both of these factors create a high charge density and cause the ion to attract more water molecules making hydration more exothermic. As going down group 2, the increase in cationic size decreases the hydration enthalpy. 4. When heat was applied on the carbonate of group 2(II2) metal, it will decomposed to form the respective metal oxide and gave up the carbon dioxide gas. MCO3 arrow MO + CO2 ,Where M = Be, Mg, Ca, Sr, Ba The table below shows the temperature where the carbonate of these elements decomposed: Carbonate Temperature / oC BeCO3 97 MgCO3 197 CaCO3 897 SrCO3 1277 BaCO3 1357 The above table clearly indicated that: When going down a group, the metal carbonate is increasing in temperature in which the compound decomposed. It is more and more difficult to get the compound to be decomposed as the as the period number of the element increases. This means that the thermal stability of the carbonate is increases down a group. 5. Since the solution was heat at a temperature of over 70 oC, the hydrated salt form is in monohydrate form.The water molecule was bond to the group2 (iia) oxalate in square planar form.â?Œ [image]6. The person who first determined the copper sulfate is a Swiss chemist Alfred Werner (1866-1919). Alfred Werner earned his Ph.D. from the University of Zürich (1890) for work with Arthur Hantzsch on the oximes, a class of organic nitrogen compounds. He received the Nobel Prize in chemistry in 1913 for his coordination theory of transition metal-amine complexes. Copper sulfate usually crystallizes as a pentahydrate compound containing five molecules of water (CuSO4.5H2O) and is known in commerce as blue vitriol. It is prepared by the treatment of copper oxides with sulfuric acid. Cu (s) + 2H2SO4 (l) + 3H2O (l) rightarrow CuSO4.5H2O (s) + SO2 (g) The beautiful blue color arises from water molecules attached directly to the copper (II) ion. The water/copper ion complex absorbs photons of yellow or red light which promotes an electron from the water to the copper (II) ion. Since only yellow or red light is absorbed, blue light is transmitted, and the crystals appear in blue colour. The structure of copper sulfate hydrate is shown as below: [image] [image] In this experiment, deionized water was use instead of distill water to avoid any unexpected reaction of the impurities ion that contain in the water. In order to get more accurate result and higher yield percentage of product, the experiment should be carry out in a larger scale. That is, use more reactant when perform this experiment, since the amount of reactant used is much, the weigh lost of yield product due to the error is negligible compare to total amount of the product, then the yield percentage can be calculate accurately. Besides that, a centrifuges can be used to helps to minimized the lost of calcium oxalates when collecting the precipitate. Conclusion: The theoretical mass of calcium oxalate should be produce in this experiment is 0.03693g and the yield percentage of product cannot be calculate since the weigh of product cannot be found. The very little of calcium carbonate used in this experiment makes the results be expected highly inaccurate. The calcium oxalate produce in this experiment is calcium oxalate monohydrate. Generally heating the solution below 70 oC will give the dehydrated salt while heating above 70 oC will give monohydrate salt.

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