Explain the sign of the Delta H degree upon the fact that the dissolving of pota

Question

Explain the sign of the Delta H degree upon the fact that the dissolving of potassium nitrate involves the dissolution of an ionically bound solid, held together by strong ionic bonds, into a solution where the resulting ions are stabilized by relatively weak water-ion interactions. Was the potassium nitrate more or less soluble as the temperature increased? Explain in terms of LeChatelier's Principle and the sign of the Delta H degree. Explain the sign of the Delta S degree based upon the fact that the dissolving of potassium nitrate involves the dissolution of an ionically bound solid into a solution of separated ions. Explain the importance (i.e., which parameter dominates) of Delta H degree and Delta S degree in determining the sign of Delta G degree for the dissolving of potassium nitrate.

Explanation / Answer

1. Potassium nitrate KNO3 is an ionic compound that requires energy to separate its ions as the attarction between the ions in a crystal is very strong. It takes energy and gets separated into K+ and NO3- ions in solution, thus cooling down the solution temperature. The energy released upon solvation of these ions is much smaller and thus thr overall enthalpy change for the process is +ve, reaction being endothermic in nature.

2. For an endothermic reaction (dH +ve) as more heat in the form of increasing temperature is supplied, the reaction would take added excess energy as proceed in the forward direction. So the dissolution of KNO3 would be greater at higher temperature. Thus is according to LeChatellier's principle. The reaction would re-equilibriate upon addition of heat (increase temperature) so that heat is balanced in the reaction by forming products.

3. Entropy is degree of randomness. As we go from left to right in the dissolution reaction of KNo3, we form two ions starting with a KNO3 molecule. Thus, more ions means more randomness and entropy for the process increases (+ve).

4. Here, entropy dS value is the major contributor in deciding the sign of dG to make the process spontaneous. dG is -ve and thus the process for dissolution of KNO3 is spontaneous.

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