Destroyer GChem #52, 2010 edition

[Cofo]

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"Which statement is correct?

Choice C says this: When solid NH4NO3 is dissolved in water at 25 degrees, the solution temperature decreases. This reaction is endothermic."

I think this statement is false because if the solution temperature DECREASES...the reaction RELEASED heat...and is exothermic.

But the destroyer book says that the statement is true.

Am I right, or is destroyer? I feel like I'm taking crazy pills!

 

[rpatel8]

Hmm. When I first read the question, I was thought that the reaction is endothermic because energy is used to break bonds. So if you were at X temperature, and you used energy in the solution to break the bonds so it dissolves, you would be taking away from X so T decreases. Anyone else have thoughts?

 

[PreDental88]

I respectfully disagree because if the solution temperature decreases, then heat must be lost from the system. If heat is lost, it must have been absorbed by something- the reaction.

 

[Teeth4Life]

I respectfully disagree because if the solution temperature decreases, then heat must be lost from the system. If heat is lost, it must have been absorbed by something- the reaction.

Or given off to the atmosphere. But either way, all cases are correct. 🙂

 

[ZFT]

Choice C says this: When solid NH4NO3 is dissolved in water at 25 degrees, the solution temperature decreases. This reaction is endothermic."

I think this statement is false because if the solution temperature DECREASES...the reaction RELEASED heat...and is exothermic.

Definitely endothermic. Maybe if you thought about the situation piecewise it might help.

The reaction: NH4NO3 --> NH4+ + NO3-

The cause: heat from the water (that's why temp decreased)

Conclusion: The reaction (splitting of NH4NO3) used heat energy, so it's endothermic.

If you dropped an ice cube (NH4NO3) in a glass of water, water temp decreases because heat from the water was transferred to the cube, or the cube sucked in heat energy. So the melting of the cube (NH4NO3 --> NH4+ + NO3-) is endothermic. I hope this analogy makes sense...

The point here is to focus on the reaction (NH4NO3 --> NH4+ + NO3-), then ask if this used or released energy. The lowering of water temp is not the reaction, any energy that might have transferred into or out of the system (NH4NO3 + water) is also not the reaction. They are either the cause or consequence of the reaction, but the reaction itself is the splitting of NH4NO3.

 

[Cofo]

Definitely endothermic. Maybe if you thought about the situation piecewise it might help.

The reaction: NH4NO3 --> NH4+ + NO3-

The cause: heat from the water (that's why temp decreased)

Conclusion: The reaction (splitting of NH4NO3) used heat energy, so it's endothermic.

If you dropped an ice cube (NH4NO3) in a glass of water, water temp decreases because heat from the water was transferred to the cube, or the cube sucked in heat energy. So the melting of the cube (NH4NO3 --> NH4+ + NO3-) is endothermic. I hope this analogy makes sense...

The point here is to focus on the reaction (NH4NO3 --> NH4+ + NO3-), then ask if this used or released energy. The lowering of water temp is not the reaction, any energy that might have transferred into or out of the system (NH4NO3 + water) is also not the reaction. They are either the cause or consequence of the reaction, but the reaction itself is the splitting of NH4NO3.

Thank you so much. I understand now. 👍 you rock.