3 Salty cold packs (7 points) Consider the dissolution of solid salt, NaCl, into

Question

3 Salty cold packs (7 points) Consider the dissolution of solid salt, NaCl, into its constituent ions in water (i.e., an aqueous solution): NaCl (s) -Na (a) Cl (aq) Keep in mind the thermodynamic properties of these compounds as given in pp. 404-405 of the textbook. The various quantities are measured at standard temperature, pressure, and concentration (298 K, 1 bar, and 1 mol/kg, respectively). (a) What is the Gibb's free energy change AG for this reaction? Does this reaction occur sponta- neously? (2 points) (b) What is the change in enthalpy AH? What can you conclude about the flow of heat? (2 points) (c) Suppose we dissolved 200 g of NaCl into 1 L of water at 298 K. What's the new water temper- ature? Do you think such a mechanism would make a good cold pack? It actually turns out that such a mechanism (with a different salt called ammonium nitrate) is actually what is used in instant cold packs. (3 points)

Explanation / Answer

Part a

Gibbs free energy of reaction = sum of Gibbs free energy of formation of products - sum of Gibbs free energy of formation of reactants

G = Gf (Na+) + Gf(Cl-) - Gf(NaCl)

= - 261.91 - 131.23 + 384.14

G = - 9 kJ

Since G is negative, the reaction is spontaneous

Part b

Change in enthalpy of reaction = sum of Enthalpy of formation of products - sum of Enthalpy of formation of reactants

H = Hf (Na+) + Hf(Cl-) - Hf(NaCl)

= - 240.12 - 167.16 + 411.15

= 3.87 kJ

The reaction is endothermic. Heat is absorbed by the reactants

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