A laboratory inquiry into the thermochemical relationships in the reaction between aqueous solutions of NaHCO[subscript 3] and NaOH is described. The enthalpy change for this reaction, delta[subscript r]H, and that for neutralization of strong acid and NaOH(aq), delta[subscript n]H, 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 delta[subscript d]H. Then, they are required to show experimental evidence of the positive delta[subscript d]H. Examination of the temperature dependence of the acid dissociation constant K[subscript a] is performed by students through pH measurements of the solution at the half-neutralization point of the reaction between aqueous solutions of NaHCO[subscript 3] and NaOH at different temperatures. This provides a second means of predicting the sign of delta[subscript d]H; then, through introduction of the van't Hoff equation, a numerical value for delta[subscript d]H can be calculated. The goal of the inquiry activity is to verify Hess's law using the evaluated experimental values of delta[subscript r]H, delta[subscript n]H, and delta[subscript d]H. 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 delta[subscript d]G and entropy change delta[subscript d]S of acid dissociation of the hydrogen carbonate ion from the student data for the temperature dependence of K[subscript a] can be applied in an advanced lab activity. (Contains 5 figures.)