Reactions
CH.3
Balancing Reactions
Reaction Types
4-1
A Chemical Reaction
Reactants
Products
Types of Chemical reactions:
There are many types of reactions but
most can be classified into a few simple
reactions types.
How do you know a chemical reaction has
occurred?
4-2
Chemical equations
Chemist’s shorthand to describe a reaction.
It shows:
• All reactants and products
• The state of all substances
• Any conditions used in the reaction

CaCO (s)
CaO (s) + CO
2 (g)
3
Reactant
Products
A balanced equation shows the relationship
between the quantities of all reactants and products.
4-3
Balancing chemical equations
Each side of a chemical equation must have
the same number of each type of atom.
CaCO3 (s)
Reactants
1 Ca
1C
3O
CaO (s) + CO2 (g)
Products
1 Ca
1C
3O
4-4
Balancing chemical equations
Step 1
Count the number of atoms of each
element on each side of the
equation.
Step 2
Determine which atom numbers are
not balanced.
Step 3
Balance one atom at a time by
using coefficients in front of one or
more substances.
Step 4
Repeat steps 1-3 until everything is
balanced.
4-5
Chemical Changes
Temperature
Changes
Light
Gas formation
Precipitation
Color changes
4-6
Types of Chemical Reactions
Reactions involving electron transfer
– synthesis or combination
– decomposition
– Electrochemical or single replacement
– Combustion of hydrocarbons.
Reactions that involve rearrangement but
not necessarily involve electron transfer
– Metathesis or Double Substitution
– Precipitation reactions and
– Acid Base Reactions
Polymerization reaction
4-7
Properties of
aqueous solutions
There are two general classes of solutes.
Electrolytic
• ionic compounds in polar solvents
• dissociate in solution to make ions
• conduct electricity
• may be strong (100% dissociation) or
weak (less than 100%)
Nonelectrolytic
• do not conduct electricity
• solute is dispersed but does not
dissociate
4-8
Dissolving ionic compounds
When an ionic solid dissolves in water, the
solvent removes ions from the crystal.
NaCl + H2O  Na+(aq) + Cl- (aq)
4-9
Dissolving covalent
compounds
Covalent compounds do not dissociate.
C6H12O6  C6H12O6 (aq)
4 - 10
Ionic equations
When ionic substances dissolve in water, they
dissociate into ions.
AgNO3
KCl
H2O
H2O
Ag+ + NO3K+ + Cl-
When a reaction occurs, only some of the
ions are actually involved in the reaction.
Ag+ + NO3- + K+ + Cl-
AgCl(s) + K+ + NO34 - 11
Ionic equations
To help make the reaction easier to see, we commonly
list only the species actually involved in the reaction.
Molecular equation
KCl + AgNO3
AgCl(s) + KNO3
Full ionic equation
Ag+ + NO3- + K+ + ClNet ionic equation
Ag+ + Cl-
AgCl(s) + K+ + NO3-
AgCl(s)
NO3- and K+ are referred to as spectator ions.
4 - 12
Some simple solubility rules
• All acids are soluble.
• All Na+, K+ and NH4+ salts are soluble.
• All nitrate and acetate salts are soluble.
• All chlorides except AgCl and Hg2Cl2 are soluble.
PbCl2 is slightly soluble.
• All sulfates are soluble except PbSO4, Hg2SO4, SrSO4
and BaSO4. Ag2SO4 and CaSO4 are slightly soluble.
• All sulfides are insoluble except those of the Group IA
(1), IIA (2) and ammonium sulfide.
• All hydroxides are insoluble except those of the group
IA(1) and Ba(OH)2. Sr(OH)2 and Ca(OH)2 are slightly
soluble.
4 - 13
Metathesis Reactions
Precipitation reactions
the formation of a solid upon mixing two
solutions.
Gas formation
The formation of a gas when two mixtures
are reacted
Acid Base titration
The neutralization of acids and bases to
form a salt in water.
4 - 14
Precipitation
NaCl + AgNO3 = AgCl (ppt) + NaNO3
Pb(NO3)2 + K2CrO4 = PbCrO4 (ppt) + KNO3
4 - 15
Gas Formation
Vinegar + Baking soda = Carbon dioxide
CH3COOH + NaHCO3 = NaCH3COO + H2O + CO2
4 - 16
Acid Base Neutralization
Vinegar + Ammonia = Salt + water
CH3COOH + NH4OH = NH4CH3COO + H2O
4 - 17
Common REDOX Reactions
Combustion
Corrosion
Photosynthesis
Kreb’s Cycle
Synthesis and Decomposition
Single Replacement
4 - 18
Oxidation numbers and
the periodic table
Some observed trends in compounds.
• Metals have positive oxidation numbers.
• Transition metals typically have more than
one oxidation number.
• Nonmetals and semimetals have both
positive and negative oxidation numbers.
• No element exists in a compound with an
oxidation number greater than +8.
• The most negative oxidation numbers
equals 8 - the group number
4 - 19
Oxidation Numbers
1. An atom in its elemental state has an
oxidation number of 0.
2. An atom in a monatomic ion has an
oxidation number identical to its charge.
3. An atom in a polyatomic ion or in a
molecular compound usually has the same
oxidation number it would have it were a
monatomic ion.
4. The sum of the oxidation numbers is 0 for a
neutral compound and equal to the net
charge for a polyatomic ion.
4 - 20
Identifying oxidation-reduction
reactions.
Oxidation-Reduction - REDOX
A chemical reaction where there is a net
change in the oxidation number of one or
more species.
Both an oxidation and a reduction must
occur during the reaction.
Mg (s) + Cl2 (g)
MgCl2 (s)
Here the oxidation number of Mg has changed from
zero to +2. Cl has changed from zero to -1.
4 - 21
REDOX reactions
Oxidation
An increase in oxidation number.
Reduction
A decrease in oxidation number.
If the oxidation number of any element
changes in the course of a reaction, the
reaction is oxidation-reduction.
Example.
2 Fe(NO3)3 (aq) + Zn(s)
2 Fe(NO3)2 (aq) + Zn(NO3)2 (aq)
4 - 22
Half Reactions
The reactions that show the loss or gain of
electrons.
2 Mg + O2  2 MgO
What are the oxidation states of each of the
atoms in the above reaction?
Mg
O2


Mg+2
2 O2-
4 - 23
Oxidation
2 Mg + O2 
Mg
12 p, 12 e
Mg


2 MgO
Mg+2
12 p, 10 e
Mg+2 + 2 e
The loss of electrons
4 - 24
Single replacement reaction
Where one element displaces another in a
chemical compound.
H2 + CuO
Cu + H2O
• In this example, hydrogen replaces copper.
• This type of reaction always involves
oxidation and reduction (REDOX).
• Since one species is replacing another,
there are no spectator ions.
4 - 25
Single Replacement
4 - 26
Synthesis Reaction
The formation of a new compound from
several substances
2 H 2 + O2 = 2 H 2O
2 Na + Cl2 = 2 NaCl
CaO (lime water) + CO2 = CaCO3
(s)
4 - 27
Synthesis
4 - 28
Decomposition Reactions
HgO = Hg + O2
H2O2= H2O + O2
with MnO2 catalyst
4 - 29
1774: Antoine Lavoisier solves the question
of the burning process and chemical
processes
4 - 30
Combustion
The burning of a substance in
the presence of oxygen.
Gasoline burns when oxygen
is taken into your engine, the
products of the reaction are
carbon dioxide and water
C8H18 + O2 = CO2 + H2O
Fe + O2 = Fe2O3 (rust)
4 - 31
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