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Chemical Reactions: Chp 8 1 Describing Chemical Change: • To understand chemical reactions we need to describe them in writing. 2 Describing a Chemical Change: • Chemical reaction = one or more substances change into one or more new substances 3 Describing Chemical Change: • Reactants vs. Products • Reactants = those substances that are reacting or changing • Products = those substances that are being formed from the reactants 4 Describing Chemical Change: • In writing a chemical reaction, an arrow is used to separate the reactants from the products • The arrow ( ) = yields, gives or reacts to produce 5 Describing Chemical Change • Remember: All chemical reactions must follow the law of conservation of matter • What is this law? 6 Describing Chemical Change: • Chemical equations: using chemical formulas to describe in writing a chemical reaction • The arrow separates the formulas of the reactants from the formulas of the products 7 Describing Chemical Change: • Skeleton equation: –Does not indicate relative amounts –Shows just the formulas of the reactants and products • Example: Fe + O2 Fe2O3 8 Describing Chemical Change: • You can indicate the state of each substance by using symbols to represent solid (s), liquid (l), gas (g) and aqueous (aq) • Example: Fe(s) + O2(g) Fe2O3(s) 9 Describing Chemical Change: • Catalyst ? • A substance that speeds up the rate of a reaction but is not used up in the reaction 10 Balancing Chemical Equations: • If there is no indication of the quantity of reactants and products in a equation, it is unbalanced 11 Balancing Chemical Equations: • Balanced equations usually contain coefficients: • Coefficients = numbers placed in front of the symbols for the respective parts 12 Balancing Chemical Equations: • Balanced chemical equation = has the same number of atoms of each elements on each side of the equation • 4 Fe(s) + 3 O2(g) 2 Fe2O3(s) 13 Balancing Chemical Equations: • Example: –C(s) + O2(g) CO2(g) • Each side of the equation has the same number of carbon and oxygen atoms • The equation is balanced 14 Balancing Chemical Equations: • Rules: page 208 15 Balancing Chemical Equations: • Additional rules: • 1. Solve the more complicated compounds first • 2. Any polyatomic ions present on each side of the equation count as one unit • 3. Solve the single elements last 16