Exothermic and Endothermic reactions stability

We Know that when a system becomes stable,it gives out energy.


In case of exo reaction,heat is liberated resulting in energy loss and thereby stabilizing the system.


In case of endo reaction,heat is taken in,thereby increasing the energy of the system thus making the system unstable.


So why would a endothermic reaction occur to make the reactants even more unstable..(we know that reaction occurs to make the reactants stable)




**TL;DR**


System spontaneously tries to reach most stable state. This depends on not only enthalpy, but entropy and hence, Gibbs free energy.
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"Why is water a solid below $\pu{0^\circ C}$, a vapour above $\pu{100^\circ C}$ and a liquid between these two states, all under atmospheric pressure conditions?"




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*A substance prefers to exist in the most stable form it can* - let us consider this generally true, and consider a relatable example. This is a qualitative explanation.


Consider a system which contains pure $\ce{H2O}$ - just one component and one phase. Suppose you are able to maintain it at any desired pressure and temperature. You set it at atmospheric pressure. You can vary the temperature.


**You know** that the stable state for $\ce{H2O}$ below $\pu{0^\circ C}$ is solid. Suppose you take not-solid $\ce{H2O}$ (i.e. water or steam) in your system and bring it to atmospheric pressure and a temperature below $\pu{0^\circ C}$. You will need to cool the system, meaning that you will need to take the evolved heat away. It is becoming more stable at atmospheric pressure and a temperature below zero by giving heat out.


**You know** that the stable state for $\ce{H2O}$ above $\pu{100^\circ C}$ is gas. Suppose you take not-gaseous $\ce{H2O}$ (i.e. ice or water) in your system and bring it to atmospheric pressure and a temperature above $\pu{100^\circ C}$. You will need to heat the system, meaning that you will need to provide heat to the system. It is becoming more stable at atmospheric pressure and a temperature above zero by absorbing heat.


You need to know about another thermodynamic property - **entropy**, represented using $S$. A system is considered stable when it does not generate any more entropy. The heat you mentioned in your question is called **enthalpy**, represented using $H$.


It is found that the **Gibbs free energy**, which is defined as $G=H-TS$, always decreases while approaching the stable state. When it eventually 'reaches' the stable state, i.e. the **equilibrium** condition, it stops decreasing. So, at a constant pressure and temperature, $\Delta G=\Delta H-T\cdot \Delta S$. If a system is approaching a stable state, its value of $G$ is decreasing. So, $\Delta G$ is negative. So, as a system goes to a stable state, it has to maintain $(\Delta H-T\cdot\Delta S)<0$. The heat you mentioned in the question covers only the $\Delta H$ term.


When you factor in the positive $\Delta S$ due to melting of ice and vaporisation of water, everything will fall in line.


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We Know that when a system becomes stable,it gives out energy. It depends on what you mean by *stable*, e.g. a green algal cell is 'stable', but it is absorbing light energy all the time.


In case of endo reaction,heat is taken in,thereby increasing the energy of the system thus making the system unstable. This is **not true** for spontaneous physical reactions.


Secondly I can not think of any **spontaneous** chemical reaction that is endothermic - you are not taking into account whether heat energy *input* is driving the reaction or the high potential energy of the reactants is driving the reaction


So why would a endothermic reaction occur to make the reactants even more unstable (we know that reaction occurs to make the reactants stable)?


Reactions occur to make the entropy of the whole system less - not the reactants. that is where you have to be careful.




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$$\ce{2H2(unstable) + O2(unstable) -> 2H2O(stable) + heat}$$
$$\ce{CaCO3(stable) + heat -> CaO(unstable) + CO2(stable)}$$


The above describes a hot hydrogen fuel flame that is causing the calcium carbonate to slake.


You have to consider the **whole system** not just one reaction. At the beginning you have 2x-unstable, at the end you have only 1x-unstable. The amount of instability is less.


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