Analytical Chem istry - DePauw University

210 Analytical Chemistry 2.0Napoleon’s expedition to Egypt was thefirst to include a significant scientific presence.The Commission of Sciences andArts, which included Claude Berthollet,began with 151 members, and operatedin Egypt for three years. In addition toBerthollet’s work, other results includeda publication on mirages, and detailedcatalogs of plant and animal life, mineralogy,and archeology. For a review of theCommission’s contributions, see Gillispie,C. G. “Scientific Aspects of the FrenchEgyptian Expedition, 1798-1801,” Proc.Am. Phil. Soc. 1989, 133, 447–474.6AReversible Reactions and Chemical EquilibriaIn 1798, the chemist Claude Berthollet accompanied Napoleon’s militaryexpedition to Egypt. While visiting the Natron Lakes, a series of salt waterlakes carved from limestone, Berthollet made an observation that led himto an important discovery. When exploring the lake’s shore Berthollet founddeposits of Na 2 CO 3 , a result he found surprising. Why did Berthollet findthis result surprising and how did it contribute to an important discovery?Answering these questions provides an example of chemical reasoning andintroduces us to the topic of this chapter.At the end of the 18th century, chemical reactivity was explained interms of elective affinities. 1 If, for example, substance A reacts with substanceBC to form ABA+ BC → AB+Cthen A and B were said to have an elective affinity for each other. With electiveaffinity as the driving force for chemical reactivity, reactions were understoodto proceed to completion and to proceed in one direction. Onceformed, the compound AB could not revert to A and BC.AB + C → A+BCFrom his experience in the laboratory, Berthollet knew that addingsolid Na 2 CO 3 to a solution of CaCl 2 produces a precipitate of CaCO 3 .Na CO () s + CaCl ( aq) → 2NaCl( aq) + CaCO () s2 3 2 3Natron is another name for the mineralsodium carbonate, Na 2 CO 3 •10H 2 O. Innature, it usually contains impurities ofNaHCO 3 , and NaCl. In ancient Egypt,natron was mined and used for a varietyof purposes, including as a cleaning agentand in mummification.Understanding this, Berthollet was surprised to find solid Na 2 CO 3 formingon the edges of the lake, particularly since the deposits formed only whenthe lake’s salt water was in contact with limestone, CaCO 3 . Where the lakewas in contact with clay soils, there was little or no Na 2 CO 3 .Berthollet’s important insight was recognizing that the chemistry leadingto the formation of Na 2 CO 3 is the reverse of that seen in the laboratory.2NaCl( aq) + CaCO () s → NaCO () s + CaCl ( aq)3 2 3 2Using this insight Berthollet reasoned that the reaction is reversible, andthat the relative amounts of NaCl, CaCO 3 , Na 2 CO 3 , and CaCl 2 determinethe direction in which the reaction occurs and the final composition of thereaction mixture. We recognize a reaction’s ability to move in both directionsby using a double arrow when writing the reaction.Na CO () s + CaCl ( aq) 2NaCl( aq) + CaCO () s2 3 2 3Berthollet’s reasoning that reactions are reversible was an importantstep in understanding chemical reactivity. When we mix together solutionsof Na 2 CO 3 and CaCl 2 they react to produce NaCl and CaCO 3 . If during1 Quilez, J. Chem. Educ. Res. Pract. 2004, 5, 69–87 (http://www.uoi.gr/cerp).