Section 7-2: Chemical Equations
A. Describing Reactions
1. chemical equation – representation of a chemical reaction that uses symbols to show the
relationship between reactants and products
a. many ways to describe a chemical reaction: photograph, videotape, and word
equation:
methane + oxygen  carbon dioxide + water
b. chemical rxns are rearrangements of atoms; equations show products and reactants
3. balanced chemical equation – an equation that shows that the number of atoms of each
element in the reactants matches those same elements in the products; accounts for the
conservation of mass
a. unbalanced chemical equation shows simply the symbols for elements and cmpds.
b. chemical equations are written the same way worldwide
4. balancing chemical equations
a. subscripts must not be changed because the chemical formula would be changed
b. coefficient – numeral used to the left of a symbol or formula in a chemical equation
to indicate relative amounts of reactants or products; these are changed to
balance the equation
c. procedure for balancing equations
(1) write the chemical equation for the reaction
(2) count the number of atoms of each element on each side of the arrow
(3) place a coefficient to the left of a symbol or formula so that the number of
atoms of that element is the same on each side of the arrow
Balance single elements last and H2O next to last
d. additional tips for balancing equations
(1) balance elements from left to right.
(2) balance as single units, polyatomic ions that occur the same on each side
(3) balancing one element may unbalance others
7-2b
B. Determining Mole Ratios
1. balanced equations show the conservation of mass
2. Law of Definite Proportions – A compound always contains the same elements in the same
proportions by mass
3. mole ratio – relative number of moles of the substances required to produce a given amount
of product in a chemical reaction: 2H2O → 2H2 + O2
a. determined by the coefficients of the balanced chemical equation: 2: 2: 1
b. relative mass can be calculated from the mole ratios
c. multiply the molar mass of each substance by the mole ratio from the balanced
equation:
2(18 g/mol) 2(2 g/mol) + 1(32 g/mol)