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My chemistry book states that for example the bond S-O is preferred over the bond S-S because the first has a bond energy that is almost twice as the second one. Since the energy released by the formation of the bonds is equal to the energy needed to let the reaction occurr,why does stronger bond are preferred over lower ones? I mean, if a S atom needs let's say 170 kJ/mol to start a reaction with another S, shouldn't this be more likely to happen respect to a bond S-O that needs 340 kJ/mol?

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  • $\begingroup$ more energy released means more energy required to break the bond and if more energy is required to break the bond the bond is stable.I let you conclude from there. $\endgroup$ – I am Back Oct 11 '16 at 17:06
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When a mole of $\ce{S-O}$ bond is formed, $340$ kJ of heat is released.

This $340$ is not how much energy is needed to start a reaction. That energy (activation energy) cannot be derived from the bond dissociation energy.

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  • $\begingroup$ Oh thank you I didn't understand very well. I was very confused with the bonds energy. $\endgroup$ – Mirko Oct 12 '16 at 15:24
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The energy in forming a bond is heat released, and so must be absorbed when breaking a bond to form separate atoms. To see if one type of molecule will react with another we compare the heats of formation (approx. sum of bond strengths) to obtain a heat of reaction. If negative the reaction is exothermic then heat is released on reaction so products have lower energy and are more stable than reactants or if not then the reaction is endothermic which is the other way round.

The heat of reaction does not tell us whether the reaction is going to be fast or slow. This depends on the activation energy, which is a potential energy barrier between reactants and products, and has to be overcome if reaction is to occur. The more frequently this happens the faster the reaction. The activation barrier exists only very briefly, and is caused by rearranging the atoms in the reactive molecules, after they have collided with one another, and as new molecules are formed.

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