Describing Reactions (Chp 7.1)

Chemical Reaction A chemical change in which the atoms of one or more substances are rearranged to form new substances Most reactions provide evidence Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com)

Evidence of a chemical reaction Temperature changes Color changes Odor The formation of a solid

Chemical equations Chemical equation - is a representation of a chemical reaction in which the reactants and products are expressed as formulas Shows what was present before and after the reaction Reactants are substances that undergo change during a chemical reaction Products are the new substances that are formed as a result of the reaction

Representing Chemical Reactions Use equations to show the reactants and products Shows the identities and relative amounts Shows the direction of the reaction Example: reactant1 + reactant2 product Carbon + Oxygen Carbon Dioxide C + O CO2

Word Equations Indicates the reactants and products using names Example: hydrogen + bromine --> hydrogen bromide

Skeleton Equations Uses chemical formulas instead of words Word equation: hydrogen + bromine --> hydrogen bromide Skeleton equation: H + Br --> HBr

The Law of Conservation of Mass States that mass is neither created nor destroyed in a chemical reaction The mass of the reactants must equal the mass of the products