EK 1001 Chemistry # 494

[Oorham]

The questions states: When salt X is dissolved in water to form 1 molar unsaturated solution, the temperature of the solution is seen to drop. From this information, we can conclude that for dissolving this salt in water:

Answer: delta H of formation is positive; Delta G of formation is negative.

I'm wondering why delta H is positive? The answer explanation says " Since the temperature of the solution drops, new know that the reaction is absorbing heat from the environment, and so we know the reaction is endothermic."

This doesn't make sense to me. If we take the solution as our system; when its temperature drops, heat must be released to the environment; i.e. being exothermic.

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[chemphysicsinstructor]

The questions states: When salt X is dissolved in water to form 1 molar unsaturated solution, the temperature of the solution is seen to drop. From this information, we can conclude that for dissolving this salt in water:

Answer: delta H of formation is positive; Delta G of formation is negative.

I'm wondering why delta H is positive? The answer explanation says " Since the temperature of the solution drops, new know that the reaction is absorbing heat from the environment, and so we know the reaction is endothermic."

This doesn't make sense to me. If we take the solution as our system; when its temperature drops, heat must be released to the environment; i.e. being exothermic.

Delta H is positive since the reaction is endothermic. The system (i.e., the salt) absorbs heat from the water (the surroundings) so the temperature of the water drops.

Please let me know if you have any more questions.

 

[Oorham]

Delta H is positive since the reaction is endothermic. The system (i.e., the salt) absorbs heat from the water (the surroundings) so the temperature of the water drops.

Please let me know if you have any more questions.

Thanks for answering my question. Your explanation makes sense but question states the temperature of the solution ( water+salt), not the solvent, drops. That's why I considered water and salt as the system.

 

[chemphysicsinstructor]

Thanks for answering my question. Your explanation makes sense but question states the temperature of the solution ( water+salt), not the solvent, drops. That's why I considered water and salt as the system.

You can consider the system as either the pure crystalline salt, or the water+salt. Either way will work.

 

[Oorham]

You can consider the system as either the pure crystalline salt, or the water+salt. Either way will work.

But if I consider water+salt as my system, then how their temperature would drop in an endothermic reaction? If they're absorbing heat from the surrounding, then the temperature of the system (solution) shouldn't drop!

 

[chemphysicsinstructor]

But if I consider water+salt as my system, then how their temperature would drop in an endothermic reaction? If they're absorbing heat from the surrounding, then the temperature of the system (solution) shouldn't drop!

Here is a basic picture of what is happening: The salt absorbs heat from the water. This heat is used to break the ionic interactions of the salt. The ions now have ion/dipole interactions. The dissolution is endothermic if the salt absorbs more heat than the heat released when the ion/dipole interactions occur. The dissolution is exothermic if the salt absorbs less heat than the heat that is released when the ion/dipole interactions occur.

The most accurate analysis involves hydration from gas phase ions. It is probably a bit overkill to use hydration enthalpies and entropies.

Please let me know if you have any questions.