Unit 9 – Chemical Reactions
Section 9.1
Reactions and Equations
Section 9.2
Classifying Chemical Reactions
Section 9.3
Reactions in Aqueous Solutions
Review Vocabulary
 chemical change
 metal
New Vocabulary
 chemical reaction
 combustion reaction
 reactant
 decomposition reaction
 product
 single-replacement reaction
 chemical equation
 double-replacement reaction
 coefficient
 precipitate
 synthesis reaction
 aqueous solution
 solution





solute
solvent
complete ionic equation
spectator ion
net ionic equation
Section 9.1 Main Idea: Chemical reactions are represented by balanced chemical equations.
The process by which one or more substances are rearranged to form different substances is called a chemical reaction.
Evidence that a chemical reaction may have occurred:
Change in temperature
Change in color
Odor
Gas bubbles
Appearance of a solid
Light
Chemists use statements called equations to represent chemical reactions.
Reactants are the starting substances.
Products are the substances formed in the reaction.
Symbol
Purpose
+
Separates two or
more
reactants/products
→
Separates
reactants and
products
⇄
Separates
reactants and
products,
indicates
reaction is
reversible
(s)
Chemical is a
solid
(l)
Chemical is a
liquid
(g)
Chemical is a
gas
(aq)
Chemical is
dissolved in
water
In word equations, aluminum(s) + bromine(l) → aluminum bromide(s) reads as “aluminum and bromine react to
produce aluminum bromide”.
Skeleton equations use symbols and formulas to represent the reactants and products.
Al(s) + Br2(l) → AlBr3(s)
Both word and skeleton equations lack information about how many atoms are involved in the reaction.
A chemical equation is a statement that uses chemical formulas to show the identities and relative amounts of the
substances involved in a chemical reaction.
A coefficient in a chemical equation is the number written in front of a reactant or product, describing the lowest wholenumber ratio of the amounts of all the reactants and products.
Use the scoreboard method to balance equations
Write down the number of each atom on each side of the equation
Adjust coefficients until the number of atoms is equal on both sides
The most fundamental law in chemistry is the law of conservation of mass.
Balanced equations show this law.
Section 9.2 Main Idea: There are four types of chemical reactions: synthesis, combustion, decomposition, and
replacement reactions.
Chemists classify reactions in order to organize the many types.
A synthesis reaction is a reaction in which two or more substances react to produce a single product.
A + B → AB
When two elements react, the reaction is always a synthesis reaction.
In a combustion reaction, oxygen combines with a substance and releases energy in the
form of heat and light.
Heated hydrogen reacts with oxygen to produce heat and water in a
combustion reaction. This is also a synthesis reaction.
A decomposition reaction is one in which a single compound breaks down
into two or more elements or new compounds.
Decomposition reactions often require an energy source, such as heat, light,
or electricity, to occur.
AC → A2 + C
Replacement Reactions
A reaction in which the atoms of one element replace the atoms of another
element in a compound is called a single replacement reaction.
A + BX → AX + B
A metal will not always replace a metal in a compound dissolved in water
because of differing reactivities.
An activity series can be used to predict if reactions will occur.
Halogens frequently replace other halogens in replacement reactions.
Halogens also have different reactivities and do not always replace each
other.
Double replacement reactions occur when ions exchange between two
compounds.
AX + BY → AY + BX
The solid product produced during a chemical reaction in a solution is called
a precipitate.
All double replacement reactions produce either water, a precipitate, or a
gas.
Type of Reaction
Synthesis
Combustion
Decomposition
Reactants
Two or more elements or
compounds
A metal and oxygen
A nonmetal and oxygen
A compound and oxygen
One compound
Single Replacement
A metal and a compound
A nonmetal and a
compound
Double Replacement
Two compounds
Probable Products
One compound
Generic Equation
A + B → AB
The oxide of the metal
The oxide of the nonmetal
Two or more oxides
Two or more elements or
compounds
A new compound and the
replaced metal
A new compound and the
replaced nonmetal
Two new compounds, one
of which will be a solid,
water, or a gas.
D + O2 → AO
BC → B + C
A + BX → AX + B
AX + BY → AY + BX
Section 9.3 Main Idea: Double-replacement reactions occur between substances in aqueous solutions and produce
precipitates, water, or gases.
An aqueous solution contains one or more dissolved substances (called solutes) in water.
The solvent is the most plentiful substance in a solution.
Water is always the solvent in an aqueous solution.
There are many possible solutes—sugar and alcohol are molecular compounds that exist as molecules in aqueous
solutions.
Compounds that produce hydrogen ions in aqueous solutions are acids.
Ionic compounds can also be solutes in aqueous solutions.
When ionic compounds dissolve in water, their ions separate in a process called dissociation.
When two solutions that contain ions as solutes are combined, the ions might react.
If they react, it is always a double replacement reaction.
Three products can form: precipitates, water, or gases.
Aqueous solutions of sodium hydroxide and copper(II) chloride react to form the precipitate copper(II) hydroxide.
2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s)
Ionic equations that show all of the particles in a solution as they actually exist are called complete ionic equations.
2Na+(aq) + 2OH–(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+(aq) + 2Cl–(aq) + Cu(OH)2(s)
Ions that do not participate in a reaction are called spectator ions and are not usually written in ionic equations.
2Na+(aq) + 2Cl–(aq) → 2Na+(aq) + 2Cl–(aq)
Formulas that include only the particles that participate in reactions are called net ionic equations.
2OH–(aq) + Cu2+(aq) → Cu(OH)2(s)
Some reactions produce more water molecules.
No evidence of a chemical reaction is observable.
HBr(aq) + NaOH(aq) → H2O(l) + NaBr(aq)
Without spectator ions
H+(aq) + OH–(aq) → H2O(l).
Gases that are commonly produced are carbon dioxide, hydrogen cyanide, and hydrogen sulfide.
2HI(aq) + Li2S(aq) → H2S(g) + 2LiI(aq)
Another example is mixing vinegar and baking soda, which produces carbon dioxide gas.
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
H2CO3(aq) decomposes immediately.
H2CO3(aq) → H2O(l) + CO2(g)
Two reactions can be combined and represented by a single chemical reaction.
Reaction 1
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
Reaction 2
H2CO3(aq) → H2O(l) + CO2(g)
Combined equation
HCl(aq) + NaHCO3(aq) + H2CO3(aq) → H2CO3(aq) + NaCl(aq) + H2O(l) + CO2(g)
Overall equation
HCl(aq) + NaHCO3(aq) → H2O(l) + CO2(g) + NaCl(aq)