Energy changesEnergy ChangesAny bond between two atoms has
energy Chemical reactions involve the breaking and formation of
chemical bondsThe general rule is:Energy is absorbed in the
breaking of chemical bonds, and energy is released when bonds are
formedExothermic and EndothermicEXOTHERMIC: reactions that release
heatThe energy required to break the bonds in the reactants is
exceeded by the energy release when chemical bonds are formed in
the productExothermic reactions are recognized by the temperature
of the surroundings increasing Exothermic and
EndothermicENDOTHERMIC: reactions that absorb heatMore energy is
required to break the bonds in the reactant than is released when
the products form Endothermic reactions are recognized by the
temperature of the surroundings decreasingEnthalpy ChangeThe symbol
H is used to represent the energy (called enthalpy) or stored heat
content of a chemical.Almost all chemical processes either absorb
energy from, or release energy to, the surroundings, generally in
the form of heatHeat changes measured at constant pressure are
called enthalpy changes (H)Enthalpy ChangeAlthough the enthalpy of
a chemical cannot be measured be itself, the change in enthalpy
during a reaction can be measured in units Joules (J) or kilojoules
(kJ)rH = H2 H1Change in enthalpy = enthalpy of products enthalpy of
reactantsEnergy Diagrams
