Controlling Chemical Reactions Explain how activation energy is related to chemical reactions. Identify factors that affect the rate of a chemical reaction.

Energy and Reactions Activation Energy is the minimum amount of energy needed to start a chemical reaction Chemical reactions require activation energy to get started. The energy is used to break the chemical bonds of the reactants so the atoms can begin to form the new chemical bonds of the products. Example: Hydrogen and oxygen form water Reaction gives off a large amount of energy(exothermic) Electric spark (energy) gives enough activation energy for the molecules to react

Exothermic and Endothermic Reactions Every chemical reaction needs activation energy. Whether or not is needs more energy depends on if it is exothermic or endothermic. Exothermic: products have less energy than the reactants at the end (burning of fuel) Endothermic: needs energy to keep going besides activation energy; energy of the products is higher than that of the reactants How are the two graphs the same? (Both graphs plot energy against time for chemical reactions. In both graphs, the reaction begins when energy rises to a peak, the activation energy, which is about the same for both reactions.) How are the two graphs different? (The graph on the left is for an exothermic reactions, and the graph on the right is for an endothermic reaction. In the exothermic reaction, heat is released, and the products have les energy than the reactants. In the endothermic reactions, heat is absorbed, and the products have more energy than the reactants.) If the energy level for the exothermic reaction had started out as low as the energy level for the endothermic reaction, would the activation energy also be lower? Why? (No, because the activation energy is the minimum amount of energy required to sate the reaction.)

Factors Affecting Rates of Chemical Reactions 1. Surface Area When a chunk of solid substance reacts with a liquid or gas, only the particles on the surface of the solid come into contact with the other reactant. But if you break the solid into smaller pieces, more particles are exposed and the reactions happens faster. (Chewing your food breaks it into smaller pieces that your body can digest more easily and quickly.) 2. Temperature The higher the temperature the faster the reactions (faster-moving particles have more energy) Example: Bacteria in milk 3. Concentration The higher the concentration of the reactants the more particles react

Factors Affecting Rates of Chemical Reactions Continued 4.Catalysts A catalyst is a material that increases the rate of reaction by lowering the activation energy. They are not reactants because they aren’t permanently changed by the reaction. Enzymes in your body are specific examples of a catalysts. Each one is specific, affecting only one chemical reaction. 5. Inhibitors Inhibitors slow down the rate of chemical reactions. They prevent the reactants from coming together. Preservatives added to food products prevent them from becoming stale or spoiling.