Claim: The reaction occurring in instant cold packs is characterized by a negative change in enthalpy (ΔH < 0), a positive change in entropy (ΔS > 0), and results in a negative change in Gibbs free energy (ΔG < 0), indicating that the process is spontaneous and endothermic.
Evidence: Instant cold packs typically contain an ammonium nitrate (NH₄NO₃) salt which, when dissolved in water, absorbs heat from the surroundings, making the pack feel cold. The dissolution of NH₄NO₃ in water can be represented as an endothermic reaction, where the energy required to break the ionic bonds in the solid salt is greater than the energy released during the hydration of the ions.
This process shows that as the salt dissolves, the system absorbs heat, which leads to a decrease in temperature and the sensation of cold. Furthermore, the dissolution increases the disorder of the system; solid ammonium nitrate transforms into dispersed ions in solution. Therefore, there is an increase in entropy.
To assess the spontaneity of the reaction, we apply the Gibbs free energy equation:
\[ \Delta G = \Delta H - T\Delta S \]
Given that ΔH is negative (heat is absorbed) and ΔS is positive (disorder increases), we can infer that at a given temperature (T), the term \( -T\Delta S \) becomes significantly negative, thus making ΔG negative as well. This indicates that the reaction is thermodynamically favorable and occurs spontaneously.
Reasoning: The endothermic nature of the reaction (ΔH < 0) highlights that energy is absorbed from the environment, resulting in the cold sensation. Concurrently, the positive change in entropy (ΔS > 0) indicates an increase in disorder as solid salt transforms into dispersed ions. Together, these factors lead to a negative Gibbs free energy change (ΔG < 0), which confirms that the reaction is spontaneous under standard conditions. Therefore, the relationship between enthalpy, entropy, and Gibbs free energy in the context of instant cold packs illustrates a process that effectively absorbs heat and promotes increased disorder, reinforcing the overall effectiveness of the cold pack in treating injuries.