Honors
Chemistry
Unit 7:
Chemical Equations
Evidence of a chemical reaction:
odor
heat
sound
light
gas emitted
color change
A reaction has occurred if the chemical and physical
properties of the reactants and products differ.
For a reaction to occur,
particles of reactants
must collide, and with
sufficient energy
collision theory
activation energy: energy needed to start a reaction
Chemical reactions release or absorb energy.
exothermic
reactions
The reaction in an oxyacetylene torch is exothermic.
endothermic
reactions
Photosynthesis is an
endothermic reaction.
catalyst: speeds up reaction wo/being consumed
-- it lowers the activation energy (AE)
Energy
without catalyst
AE
with catalyst
AE
time
time
Examples:
-- enzymes catalyze biochemical reactions
-- catalytic converters convert CO into CO2
Factors that influence
the rate of a reaction
To make reaction
rate increase…
concentration of reactants
particle size
temperature
mechanical mixing
catalyst
use one
nature of reactants
N/A
Balancing Chemical Equations
In a reaction:
atoms are rearranged
mass
AND
are conserved
energy
charge
law of
conservation
of mass
=
same # of atoms
of each type on each
side of equation
coefficients
To balance, modify only __________.
subscript changes the substance.
Changing a ________
superscripts don’t enter into
Right now, ___________
our “balancing” picture.
The thermite reaction
(2 Al + Fe2O3  2 Fe + Al2O3)
releases enough heat that it
is used for underwater welding.
SiCl4 + 2 H2O 
SiClClClCl
HHO
SiO2 + 4 HCl
SiOO
HHO
HCl
HCl
HCl
HCl
Rule of Thumb A: Save the simplest substances
for last (i.e., choose a complex
substance on each side first).
Au2S3 + 3 H2  2 Au + 3 H2S
AuAuSSS
HH
Au
HHS
HHS
HH
HH
Au
HHS
V2O5 + 6 HCl  2 VOCl3 +
VVOOOOO
3 H2O
HCl
HCl
VOClClCl
HHO
HCl
HCl
VOClClCl
HHO
HCl
HCl
HHO
Rule of Thumb B: If polyatomic ions are the same
on both sides of the equation,
balance them first.
3 Hg(OH)2 + 2 H3PO4 
HgOHOH
HHH#
HgOHOH
HHH#
HgOHOH
Hg3(PO4)2 + 6 H2O
HgHgHg##
HHO HHO
HHO HHO
HHO HHO
CaC2(s) +
H2O(l)
CaO(s)
3 CaSi2 + 2 SbI3
6 Si + 2 Sb + 3 CaI2
2 Al + 3
6 CH3OH
2 Al(CH3O)3 + 3 H2
** 2 C2H2(g) + 5 O2(g)
**
**
C2H2(g) +
2 CO2(g) + 2 H2O(l)
4
C3H8 + 5 O2
3 CO2 + 4 H2O
C5H12 + 8 O2
5 CO2 + 6 H2O
** = complete combustion of a hydrocarbon
yields CO2 and H2O
Reaction Conditions and Terminology
Certain symbols give more info about a reaction.
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous
(dissolved in H2O)
NaCl(s)
NaCl(aq)
More on aqueous…
-- synonyms: “soluble” or “in solution”
-- acids and bases are aqueous solutions
Other symbols…
(i.e., clues about the reaction)
means...“yields” or “produces”
heat is added to the reaction
means ______
MgCO3(s)
MgO(s) + CO2(g)
Temp. at which we perform rxn. might be given.
C6H5Cl + NaOH
400oC
C6H5OH + NaCl
The catalyst used might be given.
C2H4(g) + H2(g)
Pt
C2H6(g)
precipitate: a solid product that forms in an
aqueous solution reaction
When ionic substances have “(aq)” written after
them, the individual ions have dissociated from
the ionic crystal and are floating around separately.
Na3PO4(aq) means… 3 Na+(aq) + PO43–(aq)
Sodium phosphate, Na3PO4,
(sometimes called “sodium phosphate, tribasic”)
is a cleaning agent and food preservative.
Na2CO3 (aq) + Ca(NO3)2 (aq)
Na+
Na+
CO32–
clear
Na2CO3
solution
Ca2+
NO3
CaCO3(s) + 2 NaNO3(aq)
–
NO3–
clear
Ca(NO3)2
solution
ppt
“chunks”
“sinkies”
“floaties”
cloudy solution
containing CaCO3(s)
and NaNO3(aq)
Word Equations
Solid iron reacts with oxygen gas to yield solid
iron(III) oxide.
word equation: iron + oxygen  iron(III) oxide
Fe3+
balanced equation:
O2–
4 Fe + 3 O2  2 Fe2O3
Write a balanced equation (w/rxn conditions)
from the following word equations.
Solid sodium reacts w/oxygen
to form solid sodium oxide.
4 Na(s) +
O2(g)
Na+
O2–
2 Na2O(s)
+
Aqueous aluminum sulfate reacts w/aqueous calcium
chloride to form a white precipitate of calcium sulfate.
The other compound remains in solution.
Ca2+ Cl–
Al3+ SO42–
Al2(SO4)3 (aq) + 3 CaCl2 (aq)
3 CaSO4(s) + 2 AlCl3 (aq)
Methane gas (CH4)
burns in oxygen.
Much of the methane that is liberated during
the extraction of petroleum is simply burned
because the cost of transporting it is too high.
CH4(g) + 2 O2(g)
CO2(g) + 2 H2O(g)
Write an equation for the combustion of C8H18.
2 C8H18 + 25 O2
8 CO2 + 18
9 H2 O
16
Classifying Reactions
four types
synthesis: simpler substances combine to form
more complex substances
A+B
AB
AB + C
sodium + chlorine gas
2 Na +
Cl2
ABC
A+B+C
sodium chloride
2 NaCl
ABC
decomposition: complex substances are broken
down into simpler ones
AB
A+B
ABC
lithium chlorate
Li+ ClO3–
2 LiClO3
AB + C
ABC
A+B+C
lithium chloride + oxygen
Li+
Cl–
2 LiCl + 3 O2
single-replacement:
one element
replaces another
AB + C
A + CB
copper(II)
aluminum +
sulfate
Cu2+ SO42–
2 Al + 3 CuSO4
AB + C
B + AC
aluminum
?
+ copper
sulfate
Al3+ SO42–
Al2(SO4)3 + 3 Cu
double-replacement:
AB + CD
AD + CB
lead(IV)
calcium
+
nitrate
oxide
Pb4+ NO3– Ca2+ O2–
Pb(NO3)4 + 2 CaO
lead(IV)
calcium
+
?
oxide
nitrate
PbO2 + 2 Ca(NO3)2
How do we know if a reaction will occur?
For single-replacement reactions, use Activity Series.
In general, elements above replace elements below.
Ba +
FeSO4
3 Mg + 2 Cr(ClO3)3
Pb +
Al2O3
2 NaBr + Cl2
FeCl3 +
I2
CoBr2 +
F2
Fe +
BaSO4
2 Cr + 3 Mg(ClO3)2
NR
2 NaCl +
Br2
NR
CoF2 +
Br2
For double-replacement reactions,
reaction will occur
water a gas
if any product is:
a precipitate
driving forces
Check new combinations to decide.
Pb(NO3)2(aq) + 2 KI(aq)
Pb2+ NO3–
K+ I–
2 KOH(aq) +
H2SO4(aq)
K+ OH–
FeCl3(aq) +
Fe3+ Cl–
H+ SO42–
Cu(NO3)2(aq)
Cu2+ NO3–
PbI2(s) + 2 KNO3(aq)
Pb2+ I– (?)
(ppt)
K+ NO3– (?)
(aq)
K2SO4(aq) + 2 H2O(l)
K+ SO42– (?) H+ OH– (?)
(aq)
(water)
NR
(aq)
(aq)
Quantitative Relationships in Chemical Equations
4 Na(s) +
O2(g)
2 Na2O(s)
Particles
4 atoms
1 m’cule
2 m’cules
Moles
4 mol
1 mol
2 mol
Grams
4g
1g
2g
** Coefficients of a balanced
equation represent # of
particles OR # of moles,
but NOT # of grams.
SUBSTANCE “A”
Mass
Vol.
Part.
SUBSTANCE “B”
Use coeff. from bal. eq. to go
from “moles of A”
to “moles of B”
MOL
(known)
MOL
(unknown)
Mass
Vol.
Part.
4 Na(s) + O2(g)
2 Na2O(s)
Na
Na
2O
Na
Na
O22O
How many moles oxygen will react with 16.8 moles sodium?
1 mol O2
16.8 mol Na
= 4.20 mol O2
4 mol Na
How many moles sodium oxide are produced from
87.2 moles sodium?
2 mol Na2O
87.2 mol Na
= 43.6 mol Na2O
4 mol Na
(
(
)
)
How many moles sodium are required
to produce 0.736 moles sodium oxide?
0.736 mol Na2O
(
)
4 mol Na
2 mol Na2O
= 1.47 mol Na
0
0