Why are spontaneous reactions usually exothermic?

[confusedbyphysics]

I am having trouble understanding this..

If reactions are spontaneous when they create more entropy (disorder), why is it that most spontaneous reactions are exothermic? Such as an exothermic reaction like A + B ---> AB + Energy, AB then has lower energy than the reactants, which also means it is more stable in that form. If it is more stable, doesn't that mean it should have more order (less entropy)? Also wouldn't the molecule AB have less microstates than A + B, and thus less entropy? A + B atoms would have a lot more different positions than the AB molecule by itself.

Even though I know it is wrong, it seems like endothermic reactions should be spontaneous since AB + Energy---> A + B would make the atoms less stable and give them more microstates.

What am I missing?

 

[James R]

The product AB, by itself, probably has less entropy than A and B separately, but the entropy of the system of AB + energy released is higher than the system prior to the reaction.

 

[GCT]

What am I missing?

You notion of entropy and spontaneity is foggy, you'll need to review the definitions.

Spontaneity refers to the total entropy change which is the sum of the entropy change of the system and surroundings for a process. For constant pressure and temperature systems the entropy change of the surroundings is actually associated with enthalpy (to which such terms as exothermicity applies). From such assumptions, the fundamental form of Gibbs energy is assigned and from which we ascertain the spontaneity of particular processes. Thus spontaneity is not exclusive to entropy nor enthalpy. A more meaningful explanation involves the relation of the Gibbs or hemholtz form to maximum work, however this may be beyond your scope.

 

[Gokul43201]

This is essentially what GCT covered above. You determine the spontaneity from the free energy change, df = dH - TdS.

For instance, if changes in H and S are positive for some reaction, then it is spontaneous at high temperatures, where the TdS term becomes dominant. Such a reaction is said to be "entropically driven at high temperatures". If dH and dS are negative, the free energy can be negative only at low temperatures, where the reaction is said to be enthalpically driven.

The other two cases lead to reactions that are spontaneous or non-spontaneous at all temperatures. Specifically, if dH is negative and dS is positive, the reaction is spontaneous at all temperatures.

To summarize, if you want df to be negative, it helps to have dH be negative (exothermic). That should answer the question in your title.

 

[NoNufin]

well simply if you take ice from the fridge and put it at room tempreture then it will absorbs the heat which makes it a endothermic reaction but the solid would turn into liquid, which gives it a higher entropy. So it would be the opposite if you were freezing water...and i don't know about microstates because i havnt learned that yet :)

 

[Kushal]

i don't know if this is correct but when a reaction is spontaneous, it means that it has the necessary activation energy. so, it does not require heat from outside, it is not endothermic.