qualitatively: Why are thioesters more reactive than regular esters? In ochem students are taught that shorter bonds (like alkynes) have more energy than longer bonds such as alkanes. However in biochem we are taught that thioesters are very energetic bonds and release alot of energy upon breaking. 

Is there a specific mechanism that explains the relationships between bond energy and length?

background: a university student with a chem minor
http://guweb2.gonzaga.edu/faculty/cronk/CHEM440pub/thioesters.html

"thioester cannot make as much of a contribution because the required overlap between carbon's 2p orbital and the sulfur 3p orbital is ineffective. The large difference in energy between these two orbitals means they cannot "mix" as well as the 2p orbitals of oxygen and carbon."