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Neutralization and Acid Dissociation of Hydrogen Carbonate Ion: A Thermochemical Approach

2013; American Chemical Society; Volume: 90; Issue: 5 Linguagem: Inglês

10.1021/ed300090g

1938-1328

Nobuyoshi Koga, Kana Shigedomi, Tomoyasu Kimura, Tomoyuki Tatsuoka, Saki Mishima,

Advanced Chemical Sensor Technologies

A laboratory inquiry into the thermochemical relationships in the reaction between aqueous solutions of NaHCO3 and NaOH is described. The enthalpy change for this reaction, ΔrH, and that for neutralization of strong acid and NaOH(aq), ΔnH, are determined calorimetrically; the explanation for the difference is the basis for the student inquiry. The contribution of acid dissociation of the hydrogen carbonate ion to the overall reaction is considered by students as a possible explanation for the difference. Using Hess's law, students propose a positive value for the acid dissociation enthalpy change ΔdH. Then, they are required to show experimental evidence of the positive ΔdH. Examination of the temperature dependence of the acid dissociation constant Ka is performed by students through pH measurements of the solution at the half-neutralization point of the reaction between aqueous solutions of NaHCO3 and NaOH at different temperatures. This provides a second means of predicting the sign of ΔdH; then, through introduction of the van't Hoff equation, a numerical value for ΔdH can be calculated. The goal of the inquiry activity is to verify Hess's law using the evaluated experimental values of ΔrH, ΔnH, and ΔdH. This lab activity is appropriate for advanced chemistry courses at high schools or general chemistry courses at colleges. Further, calculations of the Gibbs energy change ΔdG and entropy change ΔdS of acid dissociation of the hydrogen carbonate ion from the student data for the temperature dependence of Ka can be applied in an advanced lab activity.