Break a bond: pull objects apart that attract each other— increase P.E.Form a bond: objects that attract each other movecloser together— decrease P.E.Break weak bonds, form strong bonds: new molecules are more stable than old ones (new molecules have lower potential energy than old ones).Energy is released from the reaction, releasing heat (cf. combustion, a flame is hot).Frequently, when reactions occur, bonds are broken and other bonds are formed, so that the total P.E. after the reaction is different from that before the reaction occurs:Burn CH4CH4(g) + 2O2(g) →CO2(g) + 2H2O + heat Exothermic (exoergic) Plants convert CO2to O2and glucose by photosynthesisCO2(g) + 6H2O + solar energy C6H12O2+ 3O2(g) Endothermic(endoergic) lchlorophylstepsmany⎯⎯⎯→⎯End 6.4
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