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Topic: When is ΔH soln expected to be negative?  (Read 12547 times)

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Offline JamesIon

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When is ΔH soln expected to be negative?
« on: June 19, 2012, 05:48:07 PM »
For which case would ΔH soln be expected to be negative?

A) if the solute-solute interactions are much greater then solvent-solvent and solute-solvent interaction.

B) if solvent-solvent interactions are much greater then solute-solvent and solute-solute interactions

C) if solute-solvent interactions are much greater then solvent-solvent and solute-solute interactions

D) if solute-solvent interactions are the same as solvent-solvent and solute-solute interactions

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I think its C, mainly because I know that if a solute and solvent are combined, the end result is an exothermic reaction and solution...

-ΔH soln = ΔH solute + ΔH solvent or -ΔHsoln = ΔH1 + ΔH2 + ΔH3


Am I right or am I missing it all together... it all seems like the answers are in tongue to me..  :P

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Also, this seems to follow the same path.  ???

Cold Packs contain solid NH4NO3 and a pouch of water. The temp. of the pack drops as NH4NO3 is dissolved in water. Therefore, for the dissolving of NH4NO3 in water...

A) ΔH soln  is negative and ΔS soln  may be negative or positive

B) ΔH soln is negative and ΔS soln is positive

C) ΔH soln is positive and ΔS soln may be negative or positive

D) ΔH soln is positive and ΔS soln  is positive

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I'm completely lost on this one, if the temp. drops does that means the solution is endothermic (positive)? Other then that guess I don't know what ΔS soln means, I'm assuming it C...
« Last Edit: June 19, 2012, 06:10:46 PM by JamesIon »

Offline Darren

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Re: When is ΔH soln expected to be negative?
« Reply #1 on: June 19, 2012, 09:32:51 PM »
C is correct because solute-solvent interactions account for energy released while solvent-solvent and solute-solute interactions account for energy absorbed to break them. So solvation to be exothermic, the energy released has to be greater than the energy absorbed.

For the second question, delta S refers to change in entropy, ehich means disorder of the system. Since you are dissolving an ionic compound in water, there is an increase in the number of ways you can arrange the water molecules around the ions of the compound, thus there is greater disorder and delta S is positive. So im thinking it is D. Have you learnt about delta G? Gibbs free energy, which tells you whether the reaction is spontaneous or not. If delta G is negative, the reaction spontaneously takes place. This further confirms the ans to be D as ΔG=ΔH-TΔS. Since the solvation is spontaneous, delta G is negative, and the negative delta H accounts for the negative sign of delta G, and if delta S is positive, this further provides a greater negative sign for delta G at all temperatures. We cant determine whether delta may or may not be negative because not enough information is given on the spontaneity of the reaction at all temperatures. Because at different temperatures, if delta S is negative, the solvation may or may not be spontaneous.

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