A model is prepared for the hydrogenation reactions of ammonia and methanol synthesis that are carried out in solid electrolyte cell-reactors. Hydrogen is supplied in the form of H+ through the proton conducting wall and the thermodynamic activity of hydrogen on the catalyst surface is controlled electrochemically. Under certain conditions, the yield of the cell-reactor is orders of magnitude higher than that of the corresponding conventional catalytic reactor. The key parameters that affect the product yield of either reaction are identified and their role is discussed.