A cold pack marketed for sports injuries consists of a sealed pouch that contains…
CHEMISTRY · Multi-part · AP · Chemistry · Chemical Reactions · Chemical Reactions
Problem
A cold pack marketed for sports injuries consists of a sealed pouch that contains solid NH4NO3 separated from liquid water by a thin barrier. When the barrier is broken and the pouch is shaken, the temperature of the pack drops from 25 °C to 4 °C in about 20 s. After use, the solution can be evaporated to dryness, and a white crystalline solid identical to the original NH4NO3 can be recovered.
Some reference sources list the dissolution of NH4NO3 in water as a chemical change, whereas others list it as a physical change.
Analyze the process using particulate-level reasoning to arrive at the most defensible classification.
(1) Describe the principal types of interactions that are disrupted and those that are formed as solid NH4NO3 dissolves in water.
(2) Use your description from part (a) to determine whether any intramolecular (ionic) bonds within NH4NO3 are broken during the dissolution. Explain your reasoning.
(3) Justify whether the overall process should be classified as physical or chemical. In your justification, address both (i) changes—or lack of changes—in the chemical identity of the solute and (ii) the energy evidence provided by the observed temperature drop.
(4) The recovered solid after evaporation is found to be chemically identical to the original NH4NO3. Discuss how this observation supports or contradicts your classification in part (c).