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?
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asked Mar 19, 2015 at 6:58
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1$\begingroup$ What is a "natural" reaction, which reactions are "unnatural"? $\endgroup$– GeorgMar 19, 2015 at 20:38
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3 Answers
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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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2$\begingroup$ Some examples should be good. Suggestion: enthalpy of dissolution of ammonium nitrate, decomposition of carbonates (e.g. in such natural settings as volcanic heat), and the endothermic reactions in equilibriums such as $\ce{NO2}$ (a very good example since the temperature has a great effect on the color of the gas) $\endgroup$ Mar 19, 2015 at 9:28
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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 and endergonic (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 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.
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.
answered May 10, 2017 at 10:32