Igive 15 points +++ ! three types of chemical reactions are synthesis, decomposition, and replacement. in a synthesis reaction, two or more elements or compounds combine to form a more complex substance. a decomposition reaction occurs when compounds break down into simpler products. in a replacement reaction, one element replaces another element in a compound (single replacement) or two elements in different compounds trade places (double replacement). compare and contrast what is the difference between a synthesis and a decomposition reaction?