Question

How do we know if a chemical reaction is possible?
My lecturer mentioned something about Gibbs energy (it has to be minimized or something like that).

Can someone please explain?

Answer 1

Really, it has to do with equilibrium, temp/pressure and electron affinity... but there are a lot of details. This link explains things rather well.

http://en.wikibooks.org/wiki/Chemical_Principles/Will_It_React%3F_An_Introduction_to_Chemical_Equilibrium

Answer 2

For a spontaneous reaction, Gibbs energy is always negative. That is the best way top put it I guess.

Answer 3

Yes, for a spontaneous reaction, the energy of the reactants has to be greater than the energy of the products, for it to be favorable for the latter to form. The difference in energy between the reactants and products, as can be seen from an example of an energy profile diagram above, is called the Gibbs free energy, or deltaG. So,

Energy of Products - Energy of Reactants = deltaG = negative



In the above case, the reaction would not be possible as the products are at a higher energy than the reactants (i.e. they are essentially more unstable). So, it would not make sense for the reaction to proceed here, so we would say that it is not possible (i.e. deltaG would be positive).


In a lot of cases, a reaction may require a certain energy input to reach the products from the reactants. The energy difference between the reactants and the point of highest energy, where we would find some form of transition state for the reaction, is called the ACTIVATION ENERGY. Beyond this point in the reaction pathway, the reaction has sufficient energy to form the required products, which will still have a lower energy than the reactants. So, even though we had to overcome this energy barrier, once we did we would say that it was favorable for the products to form, as deltaG (energy of products - energy of reactants) would still be negative. So, this reaction would not be spontaneous, but given sufficient energy would still be possible...the key thing being this energy difference between what we started with and what we are left with in the end!

Sorry for the badly drawn graphs but hopefully that helps you out! :)