Destroyer 2010 Genchem #179

[flin5845]

Which of the following are false.

A.If a rxn is exothermic, the system evolves heat to surroundings
B. During an exothermic rxn, a chemical substance increases potential Energy
C. During an endothermic rxn, the temp of the surrounding is lowered.
D. B and C
E. A and B

Solution: Only B is false, If a reaction is exothermic, heat is given off, and heat is evolved to the surroundings. We see and increase in kinetic energy, but a decrease in potential energy.

I do not know how to make a relationship to kinetic energy. All I know is that in gasses, the Kinetic energy is directly proportional to temperature. I do not know if this holds true for solutions.

Initially I thought that since KE=1/2MV^2, if the system is loosing heat, things slow down so then V would decrease, and if V decreased then KE would decrease and PE would increase.

Any advice??

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[Bis-GMA111]

if you look at a reaction coordinate diagram (the one with the hill), you will see that for an exothermic reaction you start off with more potential energy, and you end up with much less (since it is expelled into the environment)

 

[Bis-GMA111]

http://www.engineering-resource.com/fig03.gif

 

[ChrisM07]

I try to think of it like this. I hope it makes sense (it does in my head at least ):

When you have a molecule of something, there is energy locked up in those bonds. Some things have more energy than others. Take a hydrocarbon for instance such as octane (or gasoline). As a liquid, there is a lot of energy that is stored in all of those bonds. They have the POTENTIAL to explode. However, when you put a match to the octane and cause the gasoline to explode (an exothermic reaction), the bonds break and the heat is released. Those same bonds don't have the potential to explode anymore. So, there is a great reduction in potential energy because you're sliding down the "hill" and an increase in kinetic energy.

If you were to take the products of the combustion and put the octane together again, you would have to put IN a lot of energy to walk back up that hill. This is an exothermic reaction and when you finally reach the top, your bonds would be back to having the potential to explode again. Hope it makes sense.