1. Nature of Chemical Reactions Objectives: 1. Describe the Characteristics of a chemical reaction. 2. Distinguish between reactants and products. Key Terms: Chemical reaction, Reactants, Products, subscripts, coefficients, yield

2. Chemical Reactions A chemical reaction occurs when substances (reactants) are converted into a new substances (products) with new physical and chemical (intensive & extensive) properties. Consider the chemical reaction of photo synthesis: H 2 O + CO 2  C 6 H 12 O 6 + O 2 In this reaction water and carbon dioxide are changed into sugar and oxygen. H 2 O and CO 2 are the reactants of the reaction and C 6 H 12 O 6 and O 2 are the products. The arrow means yields (becomes). The change is permanent. That is your test for a chemical reaction.  If the reaction cannot be reversed physically and get the original reactants, a chemical change has taken place. A chemical reaction must follow the Laws of Conservation of Matter, Mass, and Energy.  Matter reactants = Matter products, Mass reactants = Mass products, Energy reactants = Energy products

3. Evidence of Chemical Reactions change in physical appearance (color, texture, luster) transfer on energy (exothermic vs endothermic) production of gas (effervescence) formation of a solid in solution (precipitate) pH change

4. Chemical Equations A chemical equation lists all the elements in the reactants and in the products they form  Must obey Conservation of Matter, Mass, & Energy 6H 2 O + 6CO 2  C 6 H 12 O 6 + 6O 2 The 6 listed before water, carbon dioxide, and oxygen is a coefficient  Multiplies the entire compound The subscripts tell how many atoms there are of the element directly preceding it The  is the yield. It is the action word in a word problem (yields, forms, becomes, produces, …) If there is a D above the , the reaction requires an input of energy.