The Nature of Chemical Reactions Chapter 6.1

Chemical Reactions Occur when substances undergo chemical changes to form new substances Old bonds are broken and new bonds are formed

Clues… Signs of a chemical reaction: Production of gas Change in color Ex: Bread rising when it is baked (chemical change!) Change in color Bread changing color when it is baked (turning brown)

Chemical reactions rearrange atoms… Example: When gasoline is burned… C8H18 (with heat)  CO2 + H2O Reactants = substance that participates in a chemical reaction C8H18 Products = substance that forms in a chemical reaction CO2 + H2O

Energy must be added to break bonds Therefore energy is needed for chemical reactions to occur (to break bonds) This energy can be provided in the form of heat, a spark, electricity, sound, or light

Energy is released when new bonds form… When new bonds are formed, energy is released as heat, electricity, sound, light, etc.

Energy is conserved in chemical reactions Chemical energy—the energy released when a chemical compound reacts to produce new compounds The total energy before the reaction is equal to the total energy of the products and their surroundings

Two types of reactions… Exothermic reaction—a chemical reaction where heat is released to the surroundings (heat EXits) The surrounding environment becomes warmer! Endothermic reaction—a chemical reaction that requires heat (heat ENters the products and leaves the environment) The surrounding environment becomes colder!

Reaction Graphs Exothermic Reaction: Endothermic Reaction: Energy is released (so products have less energy than reactants) Endothermic Reaction: Energy is required to be input (so the products have more energy than reactants)

Exothermic or Endothermic?

Exothermic

Exothermic or Endothermic?

Exothermic