Are all naturally occuring reactions exothermic?

If a reaction is endothermic, then the reaction must take energy from the environment to occur (as far as I know), and if this is the case, then to me the reaction seems energetically unfavorable, as there is only so much energy you can get from the environment. Does this means all naturally occurring reactions, (that don't just happen once in a blue moon but at a steady rate) are exothermic? This seems pretty silly, so if not, why not?




It's not necessary that all naturally occurring reactions are exothermic. The definition of Gibbs free energy is:


$$\Delta G = \Delta H - T\Delta S$$


A spontaneous reaction needs to have $\Delta G<0$, and therefore for spontaneous endothermic reactions to occur we only need that $\Delta H < T\Delta S$.


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> 
> Does this means all naturally occurring reactions, (that don't just
>  happen once in a blue moon but at a steady rate) are exothermic?
> 
> 
> 


**No**, not all naturally occurring reactions are exothermic ($\Delta H > 0$) or exergonic ($\Delta G > 0$). For example, the reaction fundamental to our existence, photosynthesis, where carbon dioxide is converted to oxygen and sugar is both [endothermic](http://en.wikipedia.org/wiki/Photosynthesis) and [endergonic](http://en.wikipedia.org/wiki/Endergonic_reaction) (see the figure).


$$\ce{n CO2 + 2n H2O + photons → (CH2O)\_n + n O2 + n H2O}$$


The energy required to surmount the barrier is provided by photons (radiation).


Numerous other biochemical reactions are also endothermic and endergonic. [This reference](http://en.wikipedia.org/wiki/Endergonic_reaction) mentions protein synthesis and the $\ce{Na+/K+}$ pump which drives nerve conduction and muscle contraction.


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An actual reaction is given here. Strongly basic barium hydroxide octahydrate reacts with a weakly acidic ammonium salt, given ammonia which becomes gaseous and so absorbs heat while increasing entropy. You can actually freeze water underneath the beaker with this reaction.


<https://youtu.be/bVBjc8eH1Co>


Thermichemical data indicate that the reactions of ammonium chloride with calcium and magnesium hydroxides are endothermic too. But the limited solubility of the lighter alkaline earth hydroxides means you don't get as good a visual effect.


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