Ch. 9 Chemical Reactions and Equations Chemical Reaction (rxn) – a process that turns substances into new substances. (chemical changes) Chemical Equation – indicates the reactants and products in a rxn; it also tells you the relative amounts of reactants and products involved in the rxn. EX: 2 Mg + O2  2 MgO reactants products “magnesium reacts with oxygen to produce magnesium oxide”

Indicators of a chemical rxn: 1) Production of heat or light

2) Production of a gas

3) Formation of a precipitate 4) Color change

Requirements for a correct chemical equation: It must identify all the reactants and products. It must have the correct formulas for the reactants and products. It must satisfy the law of conservation of mass (it must have the same number of atoms of each element on both sides of the equation.)

Word Equations – use words (names) to identify reactants and products. EX: Write a word equation for the reaction in which hydrogen burns with oxygen in the air to produce water. hydrogen + oxygen  water

Coefficient – the number before each formula in an equation. Formula Equation – uses chemical formulas to represent the reactants and products. 2 H2 + O2  H2O *diatomic elements must written as diatomic when in the elemental state: H2, N2, O2, F2, Cl2, Br2, I2, At2 coefficient Coefficient – the number before each formula in an equation. (If there is no coefficient, the coefficient is understood to be “1”.) Coefficients are used to balance the number of atoms on each side of the equation.

Coefficients can stand for: 1) The relative number of atoms, molecules, or formula units 2) The relative number of moles 3) The relative volume of gases EX: 2 H2 + O2  2 H2O 2 molecules H2 + 1 molecule O2  2 molecules H2O 2 mol H2 + 1 mol O2  2 mol H2O 2 volumes H2 + 1 volume O2  2 volumes H2O

Coefficients give you the ratio of the substances involved in a rxn. EX: CH4 + 2 O2  CO2 + 2 H2O Additional symbols used in equations: (s) solid; (l) liquid; (g) gas; (aq) aqueous – dissolved in water

very very slow Chemical equations do not tell you if a rxn will necessarily occur, or how fast they will occur. EX: 4 Fe(s) + 3 O2(g)  2 Fe2O3(s) Cu(s) + ZnCl2(aq)  Zn(s) + CuCl2(aq) does not occur X

Counting Atoms