No, they’re not interchangeable. Baking soda is a single chemical, an alkaline substance which when combined with something acid like lemon juice or buttermilk gives off carbon dioxide bubbles. This is what causes the rise in baked goods which use these ingredients. Baking powder contains baking soda, but it’s mixed with a dehydrated acid, so by itself it raises quick breads, pancakes, waffles, and the like; there’s no need for acidic ingredients in the batter or dough.
Most baking powders these days are double-acting. They contain not just one, but two, dehydrated acids. One is activated by liquid, the other by heat. This makes it possible to prepare something like a muffin batter, and not bake it off right away. The acid which is activated by heat causes the second reaction, and the muffin will still rise in the oven.
There are recipes which call for both. This isn’t just for extra insurance that the product will rise. Because baking soda neutralizes acid, it affects the taste of baked goods. For example, baking soda will modify the taste of acidic ingredients like buttermilk, yogurt, and molasses. It also works on sour (not spoiled) milk. The effect doesn’t last long, so you have to use it up right away; but I’ve made pancakes with slightly sour milk with no one the wiser because I added a pinch of baking soda to it.
Baking soda also tenderizes by weakening proteins in flour, and promotes browning. If you omit the baking soda in a recipe like this, it will rise properly, but may be pale or less tender.
Baking is a science, and science requires accuracy for good, consistent results. It’s especially important when working with baking soda. Too little won’t leaven your batter. If you go overboard and use too much, all the carbon dioxide bubbles created will run into one another and burst, with a similar lack of leavening. And too much baking soda will create a nasty metallic taste in your baked goods.
Follow the recipe, then, and don’t try to swap one for the other.