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Today, in class, I learned about the two reactions. I know connecting bond releases energy, and breaking it requires and aborb/ consumes energy. But on my reference table I, 2C+H2 yielding C2H2 realeasing 227.4 kJ of energy. My point is why it releases energy? Also, where the activation energy comes from?

The paradox exists in that lower energy level means more stability, but nature also likes to form bonds thus to increase stability. However, sometimes forming bonds increases the energy level.

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  • $\begingroup$ You should remember that stability depends on lower energy and greater entropy. It is to be noted that both are independent factors. $\endgroup$ – Immortal Player Mar 19 '14 at 6:30
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    $\begingroup$ You got it all wrong. Acetylene formation is endothermic; it consumes energy rather than releases. $\endgroup$ – Ivan Neretin Jun 29 '16 at 7:49
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Your idea is in principle correct. The reaction you mentioned works well, you break one H-H bond and create 1 C-C bond (triple bond, in fact) and two C-H bonds. The energy is released. And in most crude approximation it also says where the activation energy barrier comes from - you have to break a H-H bond.

As for the paradox - that is the principle of chemistry, to make bonds we wish for. If it means increasing the energy level, thus being less favored by nature, you have to pay for it. By sacrificing other bonds. Those are the highly reactive chemicals humbly stated above reaction arrow, which quite often do all the magic.

As the most revealing example in this I consider the Nucleophilic Acyl Substitution reaction, where you have a series of derivatives of increased stability. You can always and simply make the more stable out of the less stable, having wide choice of reactions. And there are just a few to climb up, using very nasty chemicals.

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The energy released that you mention is the free energy between reactants and product. This is a thermodynamic quantity and in this case only tell us what the difference in free energy is between reactants and products. It does not allow us to predict the activation barrier or how fast a reaction will be.
As molecules do not react instantly on meeting one another this means that there is an energy barrier between reactants and products. The height of this (potential energy) barrier is the activation energy. The higher the barrier the slower the reaction. Its value cannot generally be predicted except for very simple reactions such as atom + diatom reactions, e.g. H+HCl. Reaction occurs because occasionally a reactant will randomly accumulate enough energy from the surrounding solvent (or gas) to surmount the barrier and so react. Most collisions between reactants do not lead to reaction as there is often not enough energy. Thus only occasionally when molecules collide does reaction occur.

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