Endo & Exo- thermic Reactions

Review: What is a Chem. Rxn? Chemical Energy Chemical Energy O O Chemical Energy Energy Pow H H C Chemical Energy Pow Energy H H Chemical Energy O O Old Bonds Break New Bonds Form When reactions occur, the atoms change partners and new bonds form When this happens, something also causes changes with the energy Q: What energy is being used to hold the atoms together in molecules? A: Chemical Energy

H C What about the energy? “Chemical Energy” is what makes these atoms “stick” together. What we call “bonded” H C These two atoms are bonded together by the chemical energy Similar to magnets, there has to be something to hold the atoms to each other! Without it there is NO BONDING!

What is the energy doing? During a chemical reaction a change in energy occurs. Some reactions absorb energy, other reactions release energy. Remember that energy can change from one form/type into another! EX: Heat into Chemical or visa versa Endothermic Reactions: a reaction in which heat energy is “absorbed” from the surrounding air Exothermic Reactions: a reaction that “releases” energy in the form of heat & sometimes light into the air.

Overview Exothermic: A reaction that “feels” warm/hot A change in temperature without purposely adding or removing heat energy, “indicates” a chemical reaction! EX: Exothermic Rxn vs. Boiling Water Exothermic: A reaction that “feels” warm/hot Endothermic: A reaction that “feels” cool/cold Reaction The heat energy leaves and warms up everything around it (like the thermometer!) Heat leaves The new molecules didn’t need as much energy for their new bonds. So the extra Chemical energy is turned into heat that goes somewhere else. The heat energy is absorbed and the outside is now cooler (like the thermometer!) The new molecules need more energy to create The new bonds so they absorb energy from the space around the reaction = heat is turned into Chemical Energy (Bonds)!!! Reaction Heat comes in

Temperature of an Exothermic Reaction What would you expect the line to do with this reaction? ? Temperature Time

Temperature of an Endothermic Reaction What would you expect the line to do with this reaction? ? Temperature Time

Exothermic Reaction Reactants must be close enough together O O H H C Has to have enough energy to make it happen (Activation Energy) O O When bonds break and reform we call it a chemical reaction

BLAMO! Exothermic Reaction Release of Energy O O H H C H H O O Think: What indicators would you expect with this type of reaction? Exothermic Reactions give off the extra energy it didn’t need to form the new bonds.