Opiates block sensations of pain because their chemical structure, is this true?

Yes: Opioids work, even though they may have very different chemical structures, because some part of their molecule fits into the receptors that evolved in animals in conjunction with the development of endorphins and enkephalins, our bodies' own opioids. We produce these when we are in pain and when we are stressed, but only in limited amounts. Natural and synthetic opioids are far more potent.