How does the chemical reaction work in bread rising?

Basically, yeast (usually Saccharomyces cerevisiae) eats the sugars in the dough and produces carbon dioxide and alcohol as "waste." That CO2 gets trapped in the gluten network and makes the dough rise, creating those air pockets you see in finished bread. The longer you let it ferment, the more gas accumulates and the more the dough grows, which is why slow fermentation gives better results than fast fermentation.