I H. Graden Kirksey Memphis State University Memphis. Tennessee 38152
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The Solray Synthesis of Sodium Hydrogen Carbonate An undergraduate laboratory experiment
The Solvay synthesis of sodium hydrogen carbonate has not been a popular experiment for introductory chemistry ~ t u . dents inrelent ye&. Only six of forty-eight randomly selected laboratory manuals written for first-year college chemistry courses contain a Solvay synthesis experiment. Only one of these six was published since 1970. I n all six manuals the students were told what reactants to use to ~ r e ~ aNaHCO?. re how much ofeach to mix, and the identity bfthe white product. Could it be that neelect for the Solvav svnthesis has come about becawse a sound &ratem has not he&devised using this process for teaching chemical synthesis? The strategy for the Solvay experiment described here was developed with theaim of teaching students to think and owanile their ideas inorder to plan a synthesis followed by lahoiatory work that will test their plan. When students have completed a study of acids, bases, and chemical equilibria they are probably asking, "What is the use of this stuff?" Now is the time to beein a svnthesis exneriment by asking students, "How would-you go about ireparing NaHC03?" T h e teacher may need to initiate discussion by suggesting that preparing an aqueous solution of sodium and hydrogen carbonate ions a t a concentration greater than the solubility of NaHC03 would solve the problem, but let the students examine soluhility data for NaHC03 and Cop and the equilibrium constant for the hydrolysis of COz to see if this suggestion is feasible. If calculations lead students to believe that it is not, how might the hydrolysis of CO2 be shifted to the right to produce more HC03- ions a t equilibrium? Students will more than likely answer that a base like NaOH would supply hoth OH- ions to enhance the hydrolysis of COz and a t the same time furnish Na+ ions to precipitate NaHC03 from solution. T h e decision concerning how much NaOH, Con, and water should be mixed in order to precipitate NaHC03 is left to the students. When thew decisions are made the students are ready to go into the laboratory and see if their plan will work.
-with 0.10M HCL These experimentssupportthe conclusion that the white d i d is a hydrate of NazC03 and not the expected prod-
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This attempted synthesis of NaHC03 has confronted the students with the reality of side reactionssince they negleded the hydrolysis of HCO~-ions. ~t is important that they now assume the job of thinking through their problem and devise a second plan for syntheaizine NsHCO,. Two options erisr for planning a successful synthesis. The first is to cuntrol ronditiuna no that when COz and NaOH are mired OHions are the limiting reactant. By making sure that the number of molw of C02in the reaction mixture w greater than twice the number of moles of NaOH theexcess Coywill react with C O F and water to form HCO3- after essentially allthe OH- ions have been consumed. Sodium hvdroeen carbonate will orecioitate from solution if thereaction miitwe-1s saturated in ~ a in $ brder to reduce the solubility of NaHC03 in the reaction mixture. A second solution to the problem is seen by students who realize that the reaction mixtuxe must have alow concentrationof OH- ions in order to prevent a high concentration of C032- ions from formine: yet a large numtwr 41f OH- ronx are needed u,ronven an eppreeiabie amount of ('09 to HCO4i. Thls rundrrron can be met by usmg a weak base like NH? In the reactton mlrture instead of NaOH. A second synthesisexp&entean then be performed by reacting Dry Ice with a solution saturated in both NH3and NaCI. Typical quantities of reactants used hy students for the first attempted synthesis ire 22 g of Dry Ice mixed with a solution of 20 g of NaOH in 75 ml of water, which yields NazC03H20 as the product. Those used in the two later syntheses are: (1) 33 g of Dry Ice mixed with s solution of 5 g of NaOH and 32 g of NaCl in 100ml of water; and (2) 66 g of Dry Ice mixed with 100rnl of a solution saturated in hoth NHBand NaCl(9MNH3 and 4M NaCI). Both of these mixtures yield NaHC03 when reacted in a flask capped with a balloon. No written laboratory directions were prepared for the students nor did they hear a lecture about the Solvay synthesis before or during the experiment.The success of this experimentas a teaching synthesis depends upon creating a learning situation in which students are free and willing to think and communicateamong themselves as they are challenged with the problems of a chemical synthesis. The Solvay synthesis itself serves only as a means to illustrate many of the uroblema common to chemical svnthesis: and amounta ,, ehoim ~~.~ . " ~~~~-, namelv. of reactants, limiting reactants, side reactions, aeid-base equilibria, solubilities in a multicompanent system, and the isolation, identification, and purification of a product. The original adaptation of this teaching strategy to the Solvay synthesis was done while the author was a member of the Physical Science Group at Newton College in Newton, Massachusetts.' ~~~~
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The first attempted synthesis can he performed by dropping a known mass of Dry Ice chips into a standard solution of sodium hydroxide and immediately capping the reaction flask with a large, sturdy balloon. If the sodium hydroxide solution used is sufficiently concentrated a white precipitate will appear after the reaction mixture has stood overnight. If a nrecioitste forms a student will usuallv consider his synthksis succksful~mtilthe suggestion is made that the synthesis experiment is successfulonly if the product can be shown to he NaHC03. The identity of the white product can be determined by loss of mass on heating and by the use of a p H meter to determine the number of endpoints during the titration of a sample of known
272 1 Journal of Chemical Education
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Presented at the Southeast Regional ACS Meeting, November I , 1973,Charleston, South Carolina. 'Physical Science Group, "Experimental Chemistry," Preliminary Edition, Boston University, Boston, 1973, p. 7-1.
