While baking soda and powder are in the same category of chemical leaveners, their chemical make up is different. This is why it is not appropriate to replace baking soda with baking powder of the same amount or vice versa.
The process of leavening occurs when the gluten structure or air spaces is filled with a leavening agent, making the dough or batter to rise and expand during baking. Although carbon dioxide is the primary cause of leavening, other gases, such as ammonia gas, water in the form of steam, and integrated air (added during mixing), also contribute to the expansion of baked goods.