The Nature of Chemical Rxns Chapter 5.1

Signs of a Reaction 1.Heat production (sometimes) 2.Color change (sometimes) 3.Fire/Smoke 4.Property change 5.Solid precipitate 6.Gas production 7.Energy released (or absorbed) Light, heat, sound, electricity

Writing the Reaction H 2 O  H 2 + O 2 Na + Cl 2  NaCl reactants  products

Energy and Reactions Where does the energy come from? In the atom? –No Outside the atom? –Yes –In the bonds between atoms!!

Bonds and Energy It takes energy to break bonds It takes energy to make bonds What ’ s the importance of the Law of Conservation of Energy? –Energy is conserved in a reaction

Energy – Is it coming or going? Sometimes there is more energy in the reactants than in the products This results in extra energy after the reaction happens Reactions that release energy are exothermic Sometimes there is more energy in the products than in the reactants This results in extra energy needed for the reaction to happen. Reactions that absorb energy are endothermic

Exothermic Reactions Examples –Fires –Explosions –Some hot packs –Bioluminescence Endothermic Reactions Examples –Baking –Some cold packs –Electrolysis –Photosynthesis Energy – Is it coming or going?