Concept preview

What you will be able to do

1. 01Explain energy changes in terms of breaking and forming bonds
2. 02Distinguish exothermic and endothermic reactions
3. 03Interpret an energy term in an equation and the sign of ΔH

Molecules are held together by chemical bonds, and energy is needed to break a bond — just as it takes energy to break a piece of string. Conversely, when atoms come together to form a bond they must give off their excess energy in order to 'stick'. So a reaction both absorbs energy (breaking the reactant bonds) and releases energy (forming the product bonds); whether the reaction overall gives off or takes in energy depends on which is larger.