exothermic/endothermic

[wait4me]

I am wondering what are some key words to look for in passages to clue me into whether the reaction is exothermic or endothermic?

I know the easy stuff like:

Exothermic: "heat is given off/released" "delta H is negative"

Endothermic: "the reaction is being heated up" "delta H is positive"

Sometimes i get confused when they are talking about heat like whether they are putting it in or getting it out. for example, what about burning and combustion? I know it is exothermic so you are releasing heat...but dont u also put it in to get it out? Are all oxidation reactions exothermic?

What are some more discrete ways of saying this? Are there certain types of reactions that are ALWAYS exothermic (neutralization) or always endothermic? If something is being dissolved is that exothermic or endothermic?

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[inaccensa]

I am wondering what are some key words to look for in passages to clue me into whether the reaction is exothermic or endothermic?

I know the easy stuff like:

Exothermic: "heat is given off/released" "delta H is negative"

Endothermic: "the reaction is being heated up" "delta H is positive"

Sometimes i get confused when they are talking about heat like whether they are putting it in or getting it out. for example, what about burning and combustion? I know it is exothermic so you are releasing heat...but dont u also put it in to get it out? Are all oxidation reactions exothermic?

What are some more discrete ways of saying this? Are there certain types of reactions that are ALWAYS exothermic (neutralization) or always endothermic? If something is being dissolved is that exothermic or endothermic?

Combustion, crystillization, free radical halogenation are examples of reaction that are awlays exothermic. Combustion reaction is exothermic, although to initiate the reaction, heat is added. But the purpose of the reaction is really to gain heat as a product. the heat given off is much greater than added, so there is a net release of heat. I think the key here is to remember the phase diagram. Some reactions will be easy to remember with phase diagrams. , Melting, Evaporation, sublimation are usually endothermic reactions. Solid is more stable than liquid, thus heat must be added to break strong bonds and form weaker ones. Going the other way around, deposition, crystillization, condensation are usually exothermic.

 

[inaccensa]

dissolution can be either exo or endothermic. It really depends on whether the solute-solute or the solvent-solvent bonds that are being broken are more stable than the new solute-solvent bonds. If so, then the reaction will be endothermic or else it will be exothermic.

 

[ThirdEye]

Endothermic - You put energy in to break bonds (because molecules go from low energy to high). Exothermic - Energy is released when bonds are formed (because they go from high energy to low)

So think about what is happening in a reaction. For instance, a condensation reaction has gas condensing into a liquid.... bonds are forming so it's exothermic. Another way to look at is is water vapor is water with extra energy. When vapor condenses it is cooling so it must be releasing energy and is thus exothermic.

This is also why humidity sucks because as the vapor condenses on our skin, we absorb the energy that is released. On the other hand, when we sweat, water is evaporating off of our skin, using the heat from our bodies to do so (endothermic)