Chemical Reactions PS-4.6: distinguish between chemical changes (including the formation of gas or reactivity with acids) and physical changes (including changes in size shape, color, and/or phase) PS-4.7: summarize characteristics of balanced chemical equations (including conservation of mass and changes in energy in the form of heat- that is, exothermic or endothermic reactions) PS-4.8: summarize evidence (including the evolution of gas; the formation of a precipitate; and/or changes in temperature, color, and/or odor) that a chemical reaction has occurred

Chemical Reactions Understanding Chemical Changes Change in arrangement of atoms New substance is formed Reaction with an acid Acid reacts with metal and forms salt Acids react with bases to form water and salt

Chemical Reactions Signs a Chemical Reactions has Taken Place Bubbles form…gas has been released Ex: adding Zinc to HCl- hydrogen gas is given off Precipitate forms Ex: Silver Nitrate added to sodium chloride- white precipitate forms

Chemical Reactions Signs (cont…) Energy change Ex: paper burns- heat and light given off Color change Ex: when silver tarnishes it changes color Odor given off Ex: ammonium carbonate is heated the odor of ammonia gas can be detected

Chemical Reactions Understanding physical change New substance is not produced Change size…broken into smaller pieces Change shape…bent or stretched Expand or contract due to temperature Color change Phase change

Chemical Reactions What is a chemical reaction? Change there one or more substances are made into a new substance Reactants= the original substances Products= the new substance Reactants  Products

Chemical Reactions Conservation of Mass Matter can neither be created nor destroyed…it can only be changed Ex: Mercury (II) oxide = oxygen + mercury 10.0g0.7g9.3g

Chemical Reactions Exergonic Reactions Reactions that release energy Less energy is needed to break the bonds…excess energy is given off as light or heat Reactions is referred to as exothermic if energy is given off as heat

Chemical Reactions Endergonic Reactions Reactions that requires more energy to break bonds Energy can be in form of light, thermal energy, or electricity When energy absorbed is in form a heat reactions is referred to as endothermic

Chemical Reactions Changing the speed of a reaction Catalyst Substance added that speeds up the chemical reaction without permanently altering the product Inhibitors Used to slow down a chemical reaction