HW 4.45


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Hannah Lee 2F
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Joined: Thu Jul 11, 2019 12:15 am

HW 4.45

Postby Hannah Lee 2F » Tue Feb 11, 2020 1:45 pm

4.45 Potassium nitrate dissolves readily in water, and its enthalpy of solution is 34.9 kJ/mol.
(a) Does the enthalpy of solution favor the dissolving process?
b) Is the entropy change of the system likely to be positive or negative when the salt dissolves? (c) Is the entropy change of the system primarily a result of changes in positional dis- order or thermal disorder? (d) Is the entropy change of the surround- ings primarily a result of changes in positional disorder or thermal disorder? (e) What is the driving force for the dissolution of KNO3?

Can someone help explain this? The solutions manual states that since the change in entropy of the surroundings is negative, and spontaneous reactions are usually accompanied by an increase in entropy, enthalpy of solution does not favor dissolution. However, I thought that spontaneity was determined by the total enthalpy (ΔStot = ΔSsys + ΔSsurr), not just ΔS of surroundings?

Ariel Davydov 1C
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Re: HW 4.45

Postby Ariel Davydov 1C » Fri Feb 14, 2020 8:02 am

Delta S is entropy, not enthalpy. Since the reaction is endothermic (positive enthalpy), it requires heat input in order to occur, which indicates that this reaction is not typically spontaneous unless at a high temperature and with a high entropy.

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