Baking soda and baking powder are different products that are non-interchangeable. Even though both products are commonly used as a leavening agent, baking soda is made of sodium bicarbonate alone. Baking powder contains not only sodium bicarbonate, but also other chemicals that produce a "double acting" leavening process.
Normally, baking powder also includes monocalcium phosphate, which reacts with sodium bicarbonate the moment it becomes moist. It also includes sodium acid pyrophosphate or sodium aluminum sulfate, which reacts once it is heated and wet. Baking soda is normally used when the recipe already has an acidic ingredient, such as vinegar or buttermilk.