A disposable "instant cold pack" contains solid ammonium nitrate sealed inside a thin plastic pouch within a larger bag of water. When the bag is squeezed, the pouch breaks and the solid quickly dissolves, causing the bag to feel noticeably colder to the touch. A student records that the temperature of the surrounding water drops from 22.0 °C to 12.8 °C during the first minute after activation.
(a) Clearly identify the system and the surroundings for this process.
(b) State the algebraic sign (positive or negative) of the enthalpy change ΔH for the dissolution of ammonium nitrate and justify your answer using the observed temperature change.
(c) Describe the direction of heat flow between the system and the surroundings during the first minute after activation.
(d) At the particulate level, discuss how changes in potential energy associated with breaking and forming intermolecular forces during dissolution lead to the observed change in the average kinetic energy of the water molecules.