Chapter 8
Chemical Equations and Reactions

A. Describing Chemical Reactions
1. Introduction

chemical reaction - definition
reactant - definition
product - definition
Law of Conservation of Mass - statement
chemical equation - definition
(NH4)2Cr2O7 -----> N2 + Cr2O3 + 4H2O

figure 8-1 page 241

2. Indications of a Chemical Reaction

evidence that a chemical reaction has occurred
analysis of the product is definitive proof
Various indications

-evolution of heat and light -- can also indicate physical change

-production of a gas

-formation of a precipitate

precipitate - definition

3. Characteristics of a Chemical Equation

reaction summarizes a chemical change

things to keep in mind about equations

1. the equation must represent the known facts
2. the equation must contain the correct formulas for the reactants and products
a) diatomic molecules
b) table 8-1 page 243
c) sulfur -- S8
d) phosphorus -- P4
3. the law of conservation of mass must be satisfied
a) coefficients must be used to balance the equation
b) coefficient - definition
c) if no coefficient - one is understood

Word and Formula Equations
1. need to represent the facts
2. word equation - definition
3. word equation is qualitative
e.g. methane + oxygen ----> carbon dioxide + water
4. figure 8-3a page 245
a) meaning of ---->
5. replace names of reactants and products with correct symbols and formulas
6. formula equation - definition
7. physical states in parentheses
8. gas, solid, liquid, aqueous solution
9. balance the equation by adding appropriate coefficients
H2O v 2H2O
figure 8-3b page 245

4. Additional Symbols Used in Chemical Equations

upward arrow

downward arrow

heat as the Greek letter delta

pressure value above the arrow

temperature value above the arrow

a chemical symbol/formula above the arrow - catalyst - definition

two opposing arrows -- sometimes written as an equal sign - reversible reaction - definition

table 8-2 page 246

Sample Problem 8-1 page 247

solid sodium oxide + water yields sodium hydroxide solution

-given the word equation
-write the symbol and charge for each element/ion
-balance the charges so compounds are neutral
-add subscripts to indicate physical state of each substance, if applicable
-balance the chemical equation

Homework: 8.1

5. Significance of a Chemical Equation

-arrow is like equal sign i.e. equal amounts of reactants and products
-coefficients indicate relative, not absolute amounts of reactants and products i.e. coefficients are usually the smallest whole number ratio
-to get larger amounts we multiply EACH coefficient by some factor such as 2 or 3 or 4
-relative masses can be determined from the coefficients, by converting moles into grams
-figure 8-4 page 249
-the reverse reaction has the same relative amount of substances as the forward reaction
-just because we can write an equation does not mean the reaction will happen
-the chemical equation does not tell us anything about the speed at which it will occur.

Homework: 8.2

6. Balancing Chemical Equations
sequence to follow is on page 251
balance the following
H2 + O2 ---> H2O
PbCl2 + Na2CrO4 ---> PbCrO4 + NaCl

Homework: 8.3

7. Types of Chemical Reactions

1. Introduction
a) Need to predict what the products of a chemical reaction will be
b) either memorize all reactions or classify they into groups with predictable products
c) five type of reactions: synthesis, decomposition, single replacement, double replacement, and combustion

Synthesis Reactions

- synthesis reaction - definition
- aka composition reaction
- generalized equation: A + X ---> AX
A and X can be elements or compounds
- Reactions of Elements with Oxygen and Sulfur
element + oxygen ---> oxide
e.g. word equation is magnesium + oxygen ---> magnesium oxide; chemical equation is Mg + O2 ---> MgO
2Mg + O2 ---> 2MgO

e.g. lithium + oxygen ---> lithium oxide
Li + O2 ---> Li2O

4 Li + O2 ---> 2Li2O

both group 1 and 2 metals also react with sulfur to produce sulfides
Rb + S8 ---> Rb2S

16Rb + S8 ---> 8Rb2S

Ba + S8 ---> BaS

8Ba + S8 ---> 8BaS

e.g. iron + oxygen ---> either iron (II) oxide or iron (III) oxide depending on the conditions under which the reaction occurs
2Fe + O2 ---> 2FeO
4Fe + 3O2 ---> 2Fe2O3

nonmetals also react with oxygen to form oxides
S8 + 8O2 ---> 8SO2

C + O2 ---> CO2 or
2C + O2 ---> 2CO
2H2 + O2 ---> 2H2O

figure 8-7 page 257

figure 8-8 page 257

Reactions of Metals with Halogens
Group 1 and Group 2 metals react with halogens to form ionic compounds of the general form MX or MX2
e.g. 2Na + Cl2 ---> 2NaCl
e.g. Mg + F2 ---> MgF2

Synthesis Reactions with Oxides
oxides of active metals react with water to form hydroxides
e.g. CaO + H2O ---> Ca(OH)2
calcium oxide is lime, calcium hydroxide is slaked lime, necessary in the setting of cement.
figure 8-9 page 258

oxides of elements in the upper right of the periodic table form oxyacids
e.g. SO2 + H2O ---> H2SO3
2H2SO3 + O2 ---> 2H2SO4
oxygen from air to produce sulfuric acid which produces acid rain
some metal oxides react with nonmetal oxides to form salts
e.g. CaO + SO2 ---> CaSO3

Homework: 8.4

Decomposition Reactions

decomposition reaction - definition

AX ---> A + X where AX is a compound and A and X can be elements or compounds
-usually need some form of energy for the reaction to take place e.g. heat or electricity
-reverse of synthesis reactions

Decomposition of Binary Compounds
simplest type
2H2O -----> 2H2 + O2 with the help of electricity -- direct current
electrolysis - definition

oxides of metals located in the lower center of the periodic table decompose into elements when heated
e.g. 2HgO + heat ---> 2Hg + O2

Decomposition of Metal Carbonates
metal carbonate + heat ---> metal oxide and carbon dioxide gas
e.g. CaCO3 + heat ---> CaO + CO2

Decomposition of Metal Hydroxides
metal hydroxide + heat ---> metal oxide + water
e.g. Ca(OH)2 + heat ---> CaO + H2O
exception is group 1 metal hydroxides

Decomposition of Metal Chlorates
metal chlorate + heat ---> metal chloride + oxygen
e.g. 2KClO3 + heat ---> 2KCl + 3O2, uses MnO2 as a catalyst

Decomposition of Acids
some acid ---> nonmetal oxides and water
e.g. H2CO3 ---> CO2 + H2O

Homework: 8.5

Single Replacement Reaction

also known as a displacement reaction

single replacement reaction - definition
A + BX ---> AX + B or
Y + BX ---> BY + X where A, B, X and Y are elements and AX and BY are compounds

Replacement of a Metal in a Compound by Another Metal
e.g. 2Al + 3Pb(NO3)2 ---> 3Pb + 2Al(NO3)3

Replacement of Hydrogen in water by a Metal
for most active metals e.g. group 1 the products are hydroxide and hydrogen gas
e.g. 2Na + 2H2O ---> 2NaOH + H2

for less active metals the products are an oxide and hydrogen gas
e.g. 3Fe + 4H2O ---> Fe3O4 + 4H2

Replacement of Hydrogen in an Acid by a Metal
more active metals react with certain acid solutions -- hydrochloric acid and sulfuric acid
products are usually hydrogen gas and a salt
salt is the metal + the anion of the acid

Replacement of Halogens
one halogen replaces another
can only replace a halogen below it not above it
e.g. chlorine can replace bromine but not fluorine

Homework: 8.6

Double Replacement Reactions

double replacement reaction - definition

one of the compounds formed may be
a) a precipitate
b) an insoluble gas that bubbles out of the solution;
c) a molecular compound that bring the ions together

general equation is
AX + BY ---> AY + BX
where AX and BY are solutions of ionic compounds

1. Formation of a Precipitate

occurs when the cation of one reactant combines with the anion of another reactant to form an insoluble or slightly soluble compound e.g.

2KI(aq) + Pb(NO3)2(aq) ---> PbI2(s) + 2KNO3(aq)

the potassium and nitrate ions remain in solution just as they were as reactants -- they are not active participants in this reaction -- spectator ions

2. Formation of a Gas

one product is an insoluble gas which bubbles out of the solution
e.g. FeS(s) + 2HCl(aq) ---> H2S(g) + FeCl2(aq)

3. Formation of Water

water is a stable molecular compound formed when H+ and OH- ions come together
HCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(l)

Homework: 8.7

Combustion Reactions

combustion reaction - definition

figure 8-13

reaction is 2H2(g) + O2(g) ----> 2H2O(g)

C3H8(g) + 5O2(g) ---> 3CO2(g) + 4H2O(g)

Homework: 8.8

Activity Series of the Elements

activity series - definition
order determined by single replacement reactions
most active elements on top
an element can replace all elements below it but not above it
table 8-3 page 266
used to predict if a chemical reaction will occur
2Al(s) + 3ZnCl2(aq) ---> 3Zn(s) + 2AlCl3(aq)

Co(s) + 2NaCl(aq) ---> no reaction

order of activity series is based on experiments
notice reaction with acids and with water, both cold and hot

Homework: 8.9

end of notes

A chemical reaction is the process by which one or more substances are changed into one or more different substances.

Reactants are the original substances on the left of the chemical reaction.

Products are the resulting substances on the right of the chemical reaction.

The Law of Conservation of Mass states the total mass of the reactants must equal the total mass of the products for any given chemical reaction.

A chemical equation represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction.

A precipitate is a solid that is produced as a result of a chemical reaction in solution and that separates from the solution.

A coefficient is a small whole number that appears in front of a formula in a chemical equation.

A word equation is an equation in which the reactants and products in a chemical reaction are represented by words.

A formula equation represents the reactants and products of a chemical reaction by their symbols or formulas.

A catalyst is a substance that changes the rate of a chemical reaction but can be recovered unchanged.

A reversible reaction is a chemical reaction in which the products reform the original reactants.

A synthesis reaction (composition reaction) is a chemical reaction in which two or more substances combine to form a new compound.

A decomposition reaction is a chemical reaction in which a single compound undergoes a reaction that produces two or more simpler substances.

Electrolysis is the decomposition of a substance by a direct current. (DC)

A single replacement (displacement reaction) reaction is a chemical reaction in which one element replaces a similar element in a compound.

A double replacement reaction is a chemical reaction in which the ions of two compounds exchange places in an aqueous solution to form two new compounds.

A combustion reaction is a chemical reaction in which a substance combines with oxygen, releasing a large amount of energy in the form of light and heat.

The activity series is a list of elements organized according to the ease with which the elements undergo certain chemical reactions.