Chemistry Chapter 8 – Chemical Equations and Reactions
Section 8-1: Describing Chemical Reactions
Objectives:
List three observations that suggest that a chemical reaction has taken place
List three requirements for a correctly written chemical equation
Write a word equation and a formula equation for a given chemical reaction
Balance a formula equation by inspection
A. Introduction
1. chemical reaction – process by which one or more substances are changed into
one or more different substances
a. reactants – original substances; substance or molecule that participates
in a chemical reaction
b. products – resulting substances; substance that forms in a chemical
reaction
c. Law of Conservation of Mass – total mass of reactants must equal the
total mass of products for any given chemical reaction
2. chemical equation – representation of a chemical reaction that uses symbols to
show the relationship between the reactants and products
Example: Fig. 1, p. 261, bottom right
(NH4)2Cr2O7(s) N2 (g) + Cr2O3(s) + 4H2O(g)
B. Indications of a Chemical Reaction; to know for certain requires evidence that one
or more substances have undergone a change in identity
1. Evolution of energy as heat and light; Fig. 1, p. 261, bottom right
2. Production of a gas; Fig. 2a, p. 262, top left
3. Formation of a precipitate – solid that is produced as a result of a chemical
reaction in solution and that separates form the solution;
Fig. 2b, p. 262, top right
8.1b
C. Characteristics of Chemical Equations; requirements to be followed
1. The equation must represent known facts.
2. The equation must contain the correct formulas for the reactants and products.
a. diatomic elements must be written in the diatomic form;
Table 1, p. 263, top
b. other elements are represented by their atomic symbols without
subscripts except for sulfur, S8, and phosphorus, P4
3. The law of conservation of mass must be satisfied.
coefficient – small whole number that appears in front of a formula in a
chemical equation to specify the relative number of moles of the
substance; no coefficient is assumed to be 1
4. Word and Formula Equations
a. word equation – an equation in which the reactants and products in a
chemical reaction are represented by words; qualitative meaning
example: methane + oxygen  carbon dioxide + water
b. formula equation – an equation which represents the reactants and
products of a chemical reactions by their symbols or formulas
CH4(g) + O2(g)  CO2(g) + H2O(g) (not balanced)
c. balanced equation – an equation which represents the relative amounts
of reactants and products to satisfy the law of conservation of mass
(1) numbers and types of atoms must be the same on each side of
the equation; adjusted by inserting coefficients
(2) count the atoms of elements that are combined with atoms of
other elements and that appear only once on each side of
the equation
CH4(g) +
O2(g) 
CO2(g) +
H2O(g)
8.1c
balanced equation
CH4(g) + 2 O2(g) 
CO2(g) + 2 H2O(g)
(3) balance equation from left to right
(4) balance single elements last and H2O next to last
(5) balancing one element may unbalance another element
5. Additional Symbols Used in Chemical Equations
a. Table 2, p. 265, top; summarizes the symbols commonly used in
chemical equations
b. catalyst – substance that changes the rate of a chemical reaction and
that can be recovered unchanged; name or formula is written above
the reaction arrow
c. reversible reaction – chemical reaction in which the products re-form
the original reactants; indicated by using two arrows pointing in
opposite directions
d. other conditions for the reaction can be written above the reaction arrow
Sample Problem A: p. 267, bottom
Sample Problem B: p. 268, top
8.1d
D. Significance of a Chemical Equation
1. The coefficients of a chemical equation indicate relative, not absolute amounts
of reactants and products.
a. shows the smallest numbers of atoms, molecules, or ions that will
satisfy the law of conservation of mass
b.
H2(g) + Cl2(g)  2 HCl(g)
c. smallest ratio is: 1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl
d. also considered in terms of moles: 1 mol H2 : 1 mol Cl2 : 2 mol HCl
2. The relative masses of the reactants and products of a chemical equation can
be determined from the coefficients of the equation.
a. amount in moles is converted to grams using the molar mass
b. reactions occur amount to amount and not mass to mass; Fig. 4, p. 269
3. The reverse reaction for a chemical equation has the same relative amount of
substances as the forward reaction.
4. information not provided by a chemical equation
a. no indication of whether a reaction will actually occur
b. no information about the speed at which reaction will occur
c. no information about how bonding among atoms or ions changes during
the reaction
E. Balancing Chemical Equations
1. Identify the names of the reactants and the products, and write a word
equation.
water  hydrogen + oxygen
8.1e
2. Write a formula equation by substituting correct formulas for the names of the
reactants and the products.
H2O(l) 
H2(g) + O2(g)
(not balanced)
3. Balance the formula equation according to the law of conservation of mass.
a. Balance the different types of atoms one at a time.
b. First balance the atoms of elements that are combined and that appear
only once on each side of the equation.
c. Balance as single units polyatomic ions that appear on each side of the
equation.
d. Balance H atoms and O atoms after atoms of all other elements have
been balanced.
e. Balance single elements last and H2O next to last.
4. Count atoms to be sure that the equation is balanced.
If the coefficients do not represent the smallest possible whole-number
ratio of reactants and products, divide the coefficients by their greatest
common factor in order to obtain the smallest possible whole-number
coefficients.
5. Avoid the most common mistakes.
a. incorrect chemical formulas
b. changing subscripts; Subscripts may not be added, deleted, or changed.
Sample Problem C: p. 272, top
The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride
and hydrogen gas. Write a balanced chemical equation for the reaction.
8.1f
Sample Problem D: p. 273, top
Solid aluminum carbide, Al4C3, reacts with water to produce methane gas and solid
aluminum hydroxide. Write a balanced chemical equation for this reaction.
Sample Problem E: p. 273, bottom and p. 274, top
Aluminum sulfate and calcium hydroxide are used in a water-purification process. When
added to water, they dissolve and react to produce two insoluble products,
aluminum hydroxide and calcium sulfate. Write a balanced chemical equation for
the reaction.