In a student experiment to confirm Hess’s law, the reaction

was carried out in two different ways. First, 8.00 mL of concentrated NH₃(aq) was added to 100.0 mL of 1.00 M HCl in a calorimeter. (The NH₃(aq) was slightly in excess.) The reactants were initially at 23.8 °C, and the final temperature after neutralization was 35.8 °C. In the second experiment, air was bubbled through 100.0 mL of concentrated  NH₃(aq), sweeping out NH₃(g) (see sketch). The NH₃(g) was neutralized in 100.0 mL of 1.00 M HCl. The temperature of the concentrated NH₃(aq) fell from 19.3 to 13.2 °C. At the same time, the temperature of the 1.00 M HCl rose from 23.8 to 42.9 °C as it was neutralized by NH₃(g). Assume that all solutions have densities of 1.00 g/mL and specific heat capacities of 4.18 J g^-1 °C^-1.

(a) Write the two equations and ¢rH values for the processes occurring in the second experiment. Show that the sum of these two equations is the same as the equation for the reaction in the first experiment.

(b) Show that, within the limits of experimental error, Δ_rH for the overall reaction is the same in the two experiments, thereby confirming Hess’s law.

Transcribed Image Text:

NH3(concd aq) + HCl(aq) →→→ NH₂Cl(aq)