The thermal decomposition of solid ammonium nitrate is represented by A workbook…

CHEMISTRY · Multi-part · AP · Chemistry · Thermochemistry · Thermochemistry

Problem

The thermal decomposition of solid ammonium nitrate is represented by NH4NO3(s)→N2O(g)+2 H2O(g)NH_{4}NO_{3}(s) \rightarrow N_{2}O(g) + 2\,H_{2}O(g) A workbook solution uses Hess’s law with the three steps below but arrives at ΔH=−158 kJ\Delta H = -158\,\text{kJ}. Close inspection suggests multiple subtle errors, including one related to physical state. (1) NH4Cl(s)+NaNO3(s)→NaCl(s)+NH4NO3(s)NH_{4}Cl(s) + NaNO_{3}(s) \rightarrow NaCl(s) + NH_{4}NO_{3}(s)  ΔH=+3.2 kJ\Delta H = +3.2\,\text{kJ} (2) NH4Cl(s)→NH3(g)+HCl(g)NH_{4}Cl(s) \rightarrow NH_{3}(g) + HCl(g)  ΔH=+176 kJ\Delta H = +176\,\text{kJ} (3) NH3(g)+HCl(g)+NaCl(s)→N2O(g)+2 H2O(l)+NaCl(s)NH_{3}(g) + HCl(g) + NaCl(s) \rightarrow N_{2}O(g) + 2\,H_{2}O(l) + NaCl(s)  ΔH=−325 kJ\Delta H = -325\,\text{kJ} Assume the tabulated ΔH\Delta H values are accurate for the equations exactly as written. (a) Identify all species, including their physical states, that fail to cancel properly when the three reactions are added as presented. Summarize these inconsistencies in a well-organized table. (b) Propose a corrected sequence of reactions that produces only the species in the target decomposition equation. You may reverse or scale any of the given reactions but may not introduce new reactions. (c) Calculate the corrected ΔH\Delta H for the decomposition of ammonium nitrate using your sequence from part (b). Indicate clearly how you handled the phase discrepancy involving water. (d) Discuss how neglecting the phase change from H2O(l)H_{2}O(l) to H2O(g)H_{2}O(g) would numerically affect the magnitude and sign of ΔH\Delta H for the overall reaction.

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