A disposable instant cold pack contains solid ammonium nitrate in a sealed inner pouch surrounded by water. When the user squeezes the outer bag, the inner pouch ruptures and the salt dissolves in the water. A student places the activated cold pack on a wooden lab bench and notices that the outer surface of the pack becomes noticeably colder than the bench within 10 s.
Answer the following questions about the energetic nature of the process and the flow of energy.
(1) Identify the system and the surroundings for the process described above. Justify your choices.
(2) Describe the direction of heat flow between the system and the surroundings during the first 10 s after activation.
(3) Classify the dissolution process of ammonium nitrate in water as endothermic or exothermic. Explain your reasoning based solely on the student’s macroscopic observation.
(4) Explain, at the particulate level, how the relative strengths of intermolecular or interionic interactions before and after dissolution account for the observed temperature change.