CHEMISTRY · Multi-part · AP · Chemistry · Chemical Reactions · Chemical Reactions
Problem
Ammonium chloride, NH₄Cl(s), when heated in a sealed test tube, disappears from the bottom of the tube and reappears as a white crystalline coating near the cooler mouth of the tube. The overall process may be summarized by the reversible equation
NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
(a) Describe the macroscopic observations a student would record while heating NH₄Cl(s) as outlined above and upon subsequent cooling of the tube.
(b) For the forward (heating) and reverse (cooling) directions separately, classify the change as physical, chemical, or both. Provide detailed particle-level justification for each classification.
(c) Design an experimental test—other than simple visual inspection—that would allow you to confirm whether new chemical species are present in the gas phase during heating. Outline the procedure and the expected positive result.
(d) Suppose a mixture of NH₃(g) and HCl(g) is allowed to escape into open air and immediately forms a dense white smoke of NH₄Cl(s). Explain, from the standpoint of intermolecular and ionic interactions, why the solid precipitates rapidly upon contact of the two gases.