12.1 Chemical reactions that involve heat

THermochemistry Def: the study of changes in heat in chemical reactions (from Greek thermes, “heat”) Exothermic: reactions or processes that release heat (or other energy) Endothermic: reactions or process that absorb heat (or other energy)

Exothermic reactions C3H8 + 5 O2  3 CO2 + 4 H2O + 2043 kJ This means that when 1 mole of propane (C3H8) reacts with 5 moles of oxygen, 2043 kJ of heat is produced. Heat is produced because the energy released as new bonds form is greater than the energy required to break the bonds. Because heat and energy are related, the unit of joule (J) is used for heat.

Endothermic reactions C + H2O + 113 kJ  CO + H2 Heat (in the form of the 113 kJ) is a reactant. Here, one mole of carbon will react with one mole of water IF 113 kJ of heat are input into the system. Heat is needed because the energy released as new bonds form is less than the energy required to break the bonds. The energy absorbed is stored in the new chemical bonds.