Generation and absorption of CO2 gas CO2 is generated by dissolving carbonates in in hydrochloric acid according to the following equation: CaCO3(s) + 2 HCl(l) = CaCl2(aq) + CO2(g) + H2O(l) One convenient way for carbon dioxide generation is the Kipp generator (see: http://en.wikipedia.org/wiki/kipp%27s_apparatus). You will also see an example of this apparatus in the lab, however, during the exercise, another apparatus, shown in Figure 3, is used: The CO2 gas is produced from the reaction of marble pieces (CaCO 3 ) and 1:3 hydrochloric acid in the reaction flask (Schlenk flask). The generated gas passes through a buffer vessel (gas wash bottle hooked up reversed), and a bubbler functioning also as an absorber for acid droplets. The gas is bubbled through a saturated Ca(OH)2 solution to produce CaCO3. Since the CO 2 dissolution is an equilibrium process (CO 3 /HCO 3 equilibrium), in case we use CO 2 in excess, after a certain point the precipitated CaCO 3 starts to dissolve in the form CaHCO 3. Ca(OH)2(aq) + CO2(g) = CaCO3(s) + H2O(l) CaCO3(s) + CO2(g) + H2O(l) Ca(HCO3)2(aq)
Figure 2: Effect of physically dissolved CO 2 on the solubility of CaCO 3 Figure 3: Apparatus for gas formation reaction (Note, this is not exactly the equipment we use!!) Such gas forming reactions, which are exothermic or require heating, are done in a heatresistant glass reaction flask, fire resistant porcelain or in a quartz tube (depending on the temperature). If the reaction happens between a solid and liquid component or between two liquids at a moderately high temperature (max until 200 C), a glass reaction flask (see Schlenk flask) based setup is the most practical solution. The solid component is placed at the bottom of the flask, while the liquid component is carefully dosed through the dropping tunnel by controlling the dosage with the tunnel s stopcock. The produced gas leaves the reaction flask through its sidearm, which is attached to a bubbler and to a gas wash bottle with plastic tubes. If all the components are liquid, it is important to keep in mind their total volume and choose a reaction flask which is larger than this. Which component is in the reaction flask and
which one is dosed through the dropping funnel can vary and always depends on the two liquids properties. For example, if one of the components is concentrated sulfuric acid, it evidently has to be the component dosed through the funnel (mind the dilution of concentrated acids!) Otherwise, we might blow up the equipment due to the sudden pressure increase caused by heat release and/or gas formation. Laboratory project aims: CO 2 gas generation from CaCO 3 (marble pieces) and hydrochloric acid CaCO 3 formation with the produced gas Study the CO 3 /HCO 3 equilibrium and the properties of the CO 2 gas Study the thermal stability of carbonates Analytical detection of CO 3, Cl, NO, NO 3 and SO 4 anions Exercise 1: Place the pieces of marble into the reaction flask. Use a paper funnel to avoid contaminating the grease on the ground glass joint of the flask. Add 100 cm 3 of saturated Ca(OH)2 solution into a ~150 cm 3 Erlenmeyer flask. (Use the solubility product constant of the solution to calculate the Ca 2+ ion concentration, L Ca(OH)2 = 4.86 10 6.) Immerse the glass tube connected to the outlet of the gas generator into the Erlenmeyer flask and start the gas generation by letting a small amount of acid into the reaction flask. The amount of acid dropping onto the marble should be regulated to maintain a slow but steady gas flow (~12 bubbles/s) through the Ca(OH)2. (Use 1:3 HCl, and not a more concentrated hydrochloric acid, to avoid the formation of solid CaCl 2 which would form a crust on the CaCO 3 pieces within the reaction flask and therefore stop the CO 2 generation.) After a few minutes the production of CaCO3 ceases, the excess CO2 begins to dissolve the CaCO3 (see graph in Figure 2). Stop the gas flow (take the inlet tube out of the Erlenmeyer flask and close the valve of the dropping funnel. The point of CaCO 3 dissolution can also be detected by the ph of the solution. While a carbonate ion solution has a ph around 12, HCO 3 ions start to appear due to the excess gas dissolution at ph 10 (see Figure 1). Measure the ph of the solution in the Erlenmeyer flask with universal ph indicator and stop the reaction when the ph goes below 10. Place the Erlenmeyer flask on the hot water bath for ~1h to drive the excess CO2 from the solution. Weigh a glass filter (dried in the oven beforehand) on an analytical balance. Filter the precipitated carbonate on the glass filter with the help of the water jet pump. Dry the precipitate in the oven set to ~140 ºC until constant weight and weigh it again on an analytical balance. Calculate the net mass and the reaction yield. Exercise 2: Place a sample for each following carbonate salts in separate test tubes: Na 2 CO 3, ZnCO 3, CuCO 3. Attach the CO 2 detecting tube filled with saturated Ba(OH) 2 (see Figure 4) to the test tube with a rubber plug. Heat the test tube using the Teclu burner and observe the changes within the test tube. (What happens to the solids within the test tube, is there a detectable gas formation or not?)
Exercise 3: Identification of inorganic anions Identification of carbonate ions Put solid carbonate or a solution containing carbonate ions in a test tube. Add 2M HCl solution in excess. Attach the CO 2 detecting tube filled with saturated Ba(OH) 2 (see Figure 4) to the test tube with a rubber plug. The CO 2 gas generated from the carbonates forms a white precipitate (excess CO 2 gas forms bicarbonate ions causing the precipitate to dissolve again). CO 3 (aq) + 2 H + (aq) = CO 2 (g) + H 2 O(f) CO 2 (g) + Ba(OH) 2 (aq) = BaCO 3 (sz)+ H 2 O(f) Figure 4. : CO 2 detection tube Identification of chloride ions Chloride ions give a white precipitate with silver ions. The reaction is done in acidic media to avoid the precipitation of other ions. Cl (aq) + AgNO 3 (aq) = AgCl (s) + NO 3 (aq) The AgCl precipitate dissolves in ammoniumhydroxide readily while a complex ion forms. AgCl(s) + 2NH 4 OH (aq) = [Ag(NH 3 ) 2 ]Cl(aq) + 2H 2 O(l)
Identification of sulfate ions Sulfate ions form a fine, white, highly insoluble precipitate with barium ions. The precipitate does not even dissolve in 2M HCl. Ba 2+ (aq) + SO 4 (aq) = BaSO 4 (sz) Figures and calculations needed in the laboratory report: 1. Drawing of the laboratory apparatus used during the laboratory practice 2. The chemical reaction of the gas generation 3. Chemical reactions of the precipitated CaCO 3 formation, observations, discussion of the observations, calculation of the reaction yield: m product 100 m stochiometric : calculated mass based on the chemical reaction m stochiometric m product : measured net mass of the precipitated product 4. Drawing of the equipment used for the thermal stability study of carbonates, chemical reactions, observations 5. Analytical detection of CO 3, Cl, NO, NO 3 and SO 4 anions, chemical reactions and observations