QUESTION in Gen Chem

[SgtSadhu]

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I was looking through an SATII Chem book by Sparknotes and came across this problem:

Each of the two solutions are mixed separately, and both solutions are found to be the same temperature. The two solutions are mixed, and a thermometer shows that the mixture's temperature has decreased in temperature. Which of the following statements are true?

a) The chem rxn is exothermic
b) The chem rxn is absorbing energy
c) the energy released could be found by multiplying the temp's together
d) the energy absorbed by the solution is equal to the difference in temp. of the solutions

The answer is B, but i dont know why or how someone explain or tell me if the book has made a mistake

thanks all

 

[gdunk]

gonna need a chem person on this one...i thought if the temp dropped it would be releasing energy and therefore a exothermic reaction.

 

[Djapprentice]

Hey Don't quote me on this one, but bond formation is endothermic, and bond breaking is exothermic. This is how I would view it because by mixing the two solutions, you think think of bonds of some type, (ex: Hydrogen & Covalent, etc forming) so since bonds are forming, energy is being absorbed. If however, you try to isolate the two solutions, you have to break those bonds between them, & in that case it would be exothermic. A great way to remember that bond breaking is exothermic, is melting of ice. Water molecules and strong Hydrogen bonds contribute to its uniform crystal lattice and once its heated, those bonds gradually melt, due to the higher boiling point of water (100 degrees Celsius).

 

[Chga]

when the temp goes down, it means that the reaction is Endothermic
Endothermic= absorbs energy

 

[whiskeysour]

I was looking through an SATII Chem book by Sparknotes and came across this problem:

Each of the two solutions are mixed separately, and both solutions are found to be the same temperature. The two solutions are mixed, and a thermometer shows that the mixture's temperature has decreased in temperature. Which of the following statements are true?

a) The chem rxn is exothermic
b) The chem rxn is absorbing energy
c) the energy released could be found by multiplying the temp's together
d) the energy absorbed by the solution is equal to the difference in temp. of the solutions

The answer is B, but i dont know why or how someone explain or tell me if the book has made a mistake

thanks all

c and d can be eliminated right away because energy is heat, NOT temp!!

 

[RockstarDMD]

c and d can be eliminated right away because energy is heat, NOT temp!!

If the mixture's temperature is decreasing during the reaction it means the reaction is endothermic. Meaning the reaction is absorbing heat (energy) from the mixture and thus decreasing the mixture temp. 👍

 

[tom_servo_dds]

If the mixture's temperature is decreasing during the reaction it means the reaction is endothermic. Meaning the reaction is absorbing heat (energy) from the mixture and thus decreasing the mixture temp. 👍

This may help too. What the mixture in an endothermic reaction is doing is drawing heat from the environment (system) in. So relative to the system it will show a lower temperature. I'll quote Djapprentice here:

bond formation is endothermic, and bond breaking is exothermic

At some point when the reaction is complete the reaction vessel will equilibrate with the environment and both will register the same temperature. This would be like touching a cold radiator and having it draw heat from your hand until it doesn't feel cold anymore to you.

Alternatively, it may help to think of the opposite occuring. If the reaction were exothermic, heat would be given off and the temperature would have INCREASED since before they were mixed. This would be like touching a hot radiator. So really the temperature is relative to the system. If you can think of it in terms of what's happening with it inside the system then you should be able to think through these problem. (watch me get something like this wrong on the dat anyway 🙂 )

Hope that helps. Good luck!

 

[Yeyyy]

gonna need a chem person on this one...i thought if the temp dropped it would be releasing energy and therefore a exothermic reaction.

doesnt temp. drop mean absorbing the energy?

 

[tom_servo_dds]

doesnt temp. drop mean absorbing the energy?

Yep, it draws in heat for bond formation. (Edit to my previous post: More specifically, it draws in heat because the amount of energy given off in creating the bond is less than the activation energy supplies, so it draws in energy in the form of heat to continue the reaction.)

 

[SgtSadhu]

tHANKS ALOT GUYS THAT REALLY HELPED!

 

[keibee82]

Hey Don't quote me on this one, but bond formation is endothermic, and bond breaking is exothermic. This is how I would view it because by mixing the two solutions, you think think of bonds of some type, (ex: Hydrogen & Covalent, etc forming) so since bonds are forming, energy is being absorbed. If however, you try to isolate the two solutions, you have to break those bonds between them, & in that case it would be exothermic. A great way to remember that bond breaking is exothermic, is melting of ice. Water molecules and strong Hydrogen bonds contribute to its uniform crystal lattice and once its heated, those bonds gradually melt, due to the higher boiling point of water (100 degrees Celsius).

Its the other way around. Bond formation is exothermic and bond breaking is endothermic. Energy is needed to break the bonds, therefore absorbing. Nature tends to move from higher energy to lower energy. When bond is formed by a chemical reaction, that energy is released and products formed are lower in energy (more stable).

 

[tom_servo_dds]

Its the other way around. Bond formation is exothermic and bond breaking is endothermic. Energy is needed to break the bonds, therefore absorbing. Nature tends to move from higher energy to lower energy. When bond is formed by a chemical reaction, that energy is released and products formed are lower in energy (more stable).

Actually, energy IS released in forming a bond, but in an endothermic reaction, the amount of energy released is LESS than the activation energy needed to continue the reaction, so it draws in energy in the form of heat from the surroundings to continue the reaction. (Note: this is why exothermic reactions are typically spontaneous whereas endothermic reactions are not spontaneous.) This is also why the "thermometer shows that the mixture's temperature has decreased in temperature" and why the answer was B, which was the OP's question.

For more info:
http://en.wikipedia.org/wiki/Endothermic_reaction
http://en.wikipedia.org/wiki/Exothermic