Examples of how the presence or absence 2'-hydroxyl groups influence physicochemical properties of DNA and RNA
As user137 said, the general base abstracts a proton from the 2'OH and subsequently the 2'O- renders a nucleophilic attack on the δ+ Phosphorous, leading to the hydrolysis of the phosphodiester bond. There can be slight variations in the mechanism and the intermediates; for details see this review.
             
After hydrolysis both 2'- and 3'- phosphates can form.
RNA is not thermodynamically unstable; apart from being unstable in the presence of a strong base RNA is biochemically unstable too. This is because there are several RNAses and most of them degrade RNA by a mechanism similar to alkaline hydrolysis (they cannot act on DNA). Also see this post.