The following data compare the standard enthalpies and free energies of formation of some crystalline ionic substances and aqueous solutions of the substances:
| Substance |
ΔHfo(kJ/mol) |
ΔGfo(kJ/mol) |
| AgNO3(s) |
-124.4 |
-33.4 |
| AgNO3(aq) |
-101.7 |
-34.2 |
| MgSO4(s) |
-1283.7 |
-1169.6 |
| MgSO4(aq) |
-1374.8 |
-1198.4 |
(a) Write the formation reaction for AgNO3(s). Based on this reaction, do you expect the entropy of the system to increase or decrease upon the formation of AgNO3(s)?
(b) Use ?H°f and ?G°f of AgNO3(s) to determine the entropy change upon formation of the substance. Is your answer consistent with your reasoning in part (a)?
(c) Is dissolving AgNO3 in water an exothermic or endothermic process? What about dissolving MgSO4 in water?
(d) For both AgNO3 and MgSO4, use the data to calculate the entropy change when the solid is dissolved in water. (e) Discuss the results from part (d) with reference to material presented in this chapter and in the "A Closer Look" box on page 540.