<p> Chapter 8: Chemical Reactions</p><p>I. Describing a Chemical Reaction A. Physical vs. Chemical Change 1. Physical: A change that affects physical properties (Solid, Liquid, Gas) 2. Chemical: A change that results in different substances with different properties. B. Chemical Equation 1. Description of the substances before the reaction (Reactants) and the substances after the reaction (Products). 2. May be in words:</p><p>Acetic Acid + Sodium Hydrogen Carbonate  Sodium Acetate + Water + Carbon Dioxide</p><p>3. Or Formulas:</p><p>HC2H3O2 + NaHCO3  NaC2H3O2 + H2O + CO2</p><p> a) A formula tells you which substances are involved in the reaction, and the quantities of each atom needed. C. Law of Conservation of Mass-The mass of the reactants of a reaction will always be the same as the mass of the products. 1. Balancing Equations-Not all equations are balanced in their simplest form. Sometimes more than one molecule of a product or reactant must be written to represent a balance on each side of the equation. The number that may be written in front of a molecule is called the Coefficient.</p><p>H2 + O2  H2O (Not Balanced) 2H2 + O2  2H2O (Balanced)</p><p>D. Energy in reactions 1. Endothermic-A reaction in which energy is absorbed. 2. Exothermic-A reaction in which energy is released.</p><p>II. Reaction Rate: the speed at which a reaction progresses A. Activation energy 1. Reactions happen when particles bump into each other 2. The particles must bump into each other with a minimum amount of energy to react 3. Even exothermic and endothermic reactions have a minimum energy level for activation B. What affects the rate? 1. Temperature-the faster the particles move, they more the bump together, and with greater energy 2. Concentration-more particles, more chance to bump into the right reactant 3. Size matters-with large particles, only the outside layer is available to react. Smaller particles mean more particles available to react. 4. Inhibitor-Chemicals that slow a reaction: Food preservatives 5. Catalyst a) a substance that speeds up a reaction without being involved in the reaction b) Aid in reducing the activation energy 6. Enzymes a) Organic catalysts. Speed up biological processes b) Usually end in –ase: Proteasebreaks down proteins for recycling.</p>