Getting Exothermic and Endothermic straight...

[capostat]

I thought i had a handle on this, but it just keeps biting me.

If a reaction is EXOTHERMIC, it releases heat. Releases. So when i think "release", i often think, "it gives it away to someone else", so it that gets colder. But it doesnt get colder, because we know that exothermic reactions are hot. So then it's kind of releasing that heat to itself (I dont know if that's actually correct). Anyway, so then we say that the system gets hotter....and then the surroundings get hot too?? Doesnt somebody have to lose here?

Don't we say that the system lost heat to the surroundings? <--that's part of the exothermic process, no?

And then ENDOTHERMIC reactions. We say that the system absorbs heat from the surroundings. But the system doesnt get hot. So is it getting colder? Because the surroundings are supposed to get colder.

please help me put this issue to rest...

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[0Member0]

Try thinking exothermic and endothermic with the loss and gain of energy in the form of heat.

EXOTHERMIC
A+B --> C + heat

You are correct. Someone has to gain and someone has to lose. Assuming the reaction above goes to completion, the system loses heat to its surroundings. In really simple terms - it gives its heat to the products. Once that happens, you can consider the reactants as COLD, and the product as HOT.

ENDOTHERMIC
A+ heat --> B + C


The system absorbs heat to make the reaction go to completion. Again, the system gains heat from its surroundings. In really simple terms - it take heat to the reactants. Once that happens, you can consider the products as HOT and the reactant as COLD.


I hope this helps you. Please let me know if you more clarification is needed.

 

[vandyam]

The above is a very good explanation, and will help you in Le Chatlier type situations when dealing w/ temperature increase/decrease.

I also think it is important to note, that on the MCAT, when it says the temperature increases during the course of the reaction, then the rxn is exothermic. I got this right originally, but when I went to go review it I found myself thinking, 'if the temperature increases, it must have absorbed heat... and therefore endothermic'. This is not the case.

 

[Schemp]

This is definitely a confusing topic, and mostly because I think the meaning is interpreted differently based on the circumstances. I had exactly this question a few days ago.

For example, if they talk about mixing two concentrated substances together and they get colder (ie, release heat), I would say the reaction is exothermic.

However, in the case of calorimeter reactions, or heat of solution measurements, you're adding reactants to a relatively large amount of solvent, and the change in solution temperature is really a change in solvent temperature, ultimately. Therefore, in that case, if the solution gets colder the reactants are actually absorbing heat from the solvent (ie, the surroundings) and the reaction is endothermic.

Hopefully that makes it a little bit more clear.