5.1.4—Energy transfers occur during chemical

Breaking a chemical bond separates bonded atoms and requires energy, so it is an energy input. In a reaction, identify the bonds present in the reactants that must be broken before the atoms can be rearranged.

Forming a chemical bond releases energy as the new bonded arrangement is established. Identify the new bonds in the products and treat their formation as an energy output.

The reaction enthalpy is the net result of these transfers: energy required to break bonds is balanced against energy released when bonds form. More energy released on forming products gives an exothermic result; more energy required to break reactant bonds gives an endothermic result.

Use a balanced equation to track which bonds change, but do not claim that the sign of ΔH alone identifies a reaction mechanism or rate. The bond-energy calculation route belongs to the next objective; this card establishes the causal energy account.