Unfavorable reactions
If the process we're discussing is an equilibrium, and if the forward reaction is "unfavorable" (I take that to mean that the products are of higher energy then the reactants), then there are still ways to shift the equilibrium in whichever direction you want.  For example

A &lt;-> B + C

using LeChatelier's principle, if we increase the pressure in our vessel, we will push the equilibrium back towards A, the opposite will occur if we decrease the pressure.  Temperature, catalysts, reagent concentrations, etc. can all affect the position of an equilibrium.