|I understand that to calculate the heat released or absorbed during a nuclear reaction you find the difference between the product mass and reactant mass and use the formula (E=mc2). But what about heat released or absorbed during a chemical reaction? The book I have says that mass is conserved during a chemical reaction, so where does the heat energy come from? — TC
While your book's claim is well intended, it's actually incorrect. The author is trying to point out that atoms aren't created or destroyed during the reaction and that all the reactant atoms are still present in the products. But equating the conservation of atoms with the conservation of mass overlooks any mass loss associated with changes in the chemical bonds between atoms. While bond masses are extremely small compared to the masses of atoms, they do change as the results of chemical reactions. However even the most energy-releasing or "exothermic" reactions only produce overall mass losses of about one part in a billion and no one has yet succeeded in weighing matter precisely enough to detect such tiny changes.