Combustion Rxn is exothermic??

[userman]

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Heat is added to the system in order to run the rxn, I was wondering how is this exothermic??


Also, can you think of any other rxns that seem endothermic/exothermic, but are the opposite?

 

[UMICHPremed]

Yes, combustion requires an initial input of energy (a little flame or a spark); however, the output energy is much greater than the initial input. So overall, the reaction is exothermic.

Think about it in terms of burning gasoline. Initially, you require a little electric spark to get the reaction going; however, once it gets going, the flame/output is much greater than the initial energy investment

 

[Phantastic]

Many reactions require passing through a "transition state": who's energy resides at the top of that activation energy hill you might have seen in a reaction coordinate diagram. The key is that the products are at a lower energy than the reactants, which is where our heat comes from. But yea, gotta have that first Ea.

 

[navyrick]

i get a lot of questions similar to this...and the answer is always the same...

think about it completely removed from your study situation--if you were running your own little combustion reaction..i.e...burning something...you can physically experience the heat evolved from the reaction.

yes, you do have to input heat, but more is evolved, thus exothermic.

 

[userman]

Thanks for the answer, but the answer I was most interested in was...


Are there any other kinds of reactions like this that I must be aware of (that will not be covered in passages)?

 

[Phantastic]

Also, can you think of any other rxns that seem endothermic/exothermic, but are the opposite?

Well, how about diamond's spontaneous conversion to graphite? It takes a really, really long time, but the products are at a lower energy than the reactants, so that would make it exothermic. It just takes a long time for it to acquire the activation energy for even a small of amount to convert.

You should take a look at those thermo rxn graphs.

 

[userman]

Thank you~~

It is just that for these reactions, for someone like me who hasn't had much experience in chemistry, it is quite downright IMPOSSIBLE to know which reactions are exothermic/endothermic if they seem counter-intuitive.

I perfectly understand the concept that a bit of activation energy is required to allow the reaction to proceed and release energy as it reaches a more stable state.


But I mean, coMe On~~~
If you don't know anything about a reaction, and heat is added for it to progress, it is just natural to assume it has an endothermic nature.

So pretty much ALL of the reactions that I have been exposed to that require heat input, I assume they are endothermic...

 

[loveoforganic]

ALL reactions require energy input. The only theoretical reactions that wouldn't require energy input would be when the "reagent" is actually the transition state, but that's impossible.