Chemistry
Unit 7:
Chemical Equations
In a reaction:
atoms are rearranged
mass
AND
are conserved
energy
charge
Balancing Chemical Equations
law of
conservation
of mass
=
same # of atoms
of each type on each
side of equation
CH4 + 2 O2  CO2 + 2 H2O
Reactants
1 C atom
4 H atoms
4 O atoms
Products
1 C atom
4 H atoms
4 O atoms
EX. solid iron reacts with oxygen gas to yield
solid iron (III) oxide
Fe3+ O2–
___Fe(s) + ___O2(g)
___Fe2O3(s)
If all coefficients are 1…
+ ___O
1 2(g)
1
___Fe(s)
1
___Fe
2O3(s)
+
If we change subscripts… But we can’t!!!
1
1 3(g)
1
___Fe
___Fe
2(s) + ___O
2O3(s)
+
subscript changes the substance.
Changing a ___________
coefficients
To balance, modify only _____________.
superscripts
Right now, _______________
don’t enter into
our “balancing” picture.
4
3 2(g)
+ ___O
___Fe(s)
2
___Fe
2O3(s)
+
Hint: Start with most complicated substances first
and leave simplest substances for last.
solid sodium reacts w/oxygen
to form solid sodium oxide
4
1 2(g)
___Na(s)
+ ___O
Na+
O2–
2
___Na
2O(s)
+
Aqueous aluminum sulfate reacts w/aqueous calcium
chloride to form a white precipitate of calcium sulfate.
The other compound remains in solution.
Al3+ SO42–
Ca2+ Cl–
1_ Al2(SO4)3 (aq) + 3_ CaCl2 (aq)
3
_ AlCl3 (aq)
_ CaSO4(s) + 2
Methane gas (CH4) reacts with
oxygen to form carbon dioxide
gas and water vapor.
Furnaces burn
primarily methane.
1
2 O2(g)
_ CH4(g) + _
1
2 H2O(g)
_ CO2(g) + _
1 CaC2(s) + _
1 H2O(l)
_
1
_ C2H2(g) + 1
_ CaO(s)
_
3 CaSi2 + _
2 SbI3
_ Si + 2
_ Sb + 3
_ CaI2
6
_
2 Al + _3
6 CH3OH
_1
2 Al(CH3O)3 + _3 H2
1 C2H2(g) + _
2
5 O2(g)
** _
1 C3H8 + _
5 O2
** _
1 C5H12 + _
8 O2
** _
2 CO2(g) + 1
4
2
_
_ H2O(l)
Odd # Oxy
3 CO2 + _
4 H2O
_
5 CO2 + _
6 H2O
_
** = complete combustion (+ O2) of a hydrocarbon
(CxHy) yields CO2 and H2O
Write equations for the combustion of C7H16 and C8H18
1 C7H16 + 11
_
_ O2
7 CO2 + _
8 H2O
_
1
2 C8H18 + 25
_
_ O2
8 CO2 +18
9 H2O
16
_
_
Odd # Oxy
Signs of Chemical Reactions
There are five main signs that indicate a chemical
reaction has taken place:
release
input
change in color
change in odor
production of new
gases or vapor
input or release
of energy
difficult to reverse
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.
Endothermic Reaction
Energy + Reactants  Products
Energy
Activation
Energy
Products
Reactants
Reaction progress
Exothermic Reaction
Reactants  Products + Energy
10 energy
=
8 energy
+ 2 energy
Energy of reactants
Energy
Energy of products
Reactants
Products
Reaction Progress
Decomposition of Nitrogen Triiodide
N2
NI3
2 NI3(s)
I2
N2(g) + 3 I2(g)
catalyst: speeds up reaction w/o being consumed
… it lowers the activation energy (Ea)
Energy
without catalyst
AE
with catalyst
AE
time
time
Examples:
enzymes catalyze biochemical reactions
catalytic converters convert CO into CO2
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…
“soluble” or “in solution” indicates that a
substance is dissolved in water (usually)
all acids 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)
Factors that influence
the rate of a reaction
To make reaction
rate increase…
concentration of reactants
particle size
temperature
mechanical mixing
pressure
catalyst
nature of reactants
use one
N/A
Types of Chemical Reactions
Synthesis (combination) reaction
A + B  AB
AB  A + B
Decomposition reaction
ASingle-replacement
reaction
BDouble-replacement
reaction
Combustion reaction
(of a hydrocarbon)
Ause
A + BC  AC + B
element
compound
compound element
AB + CD  AD + CB
All compounds…
CxHy + O2  CO2 + H2O
activity series to predict products/reactivity
Buse solubility chart to predict products/reactivity
Balance and classify the following reactions:
1
_ F2 + 1_ MgI2
_ I2
_1 MgF2 + 1
Single replacement
4
_ C3H5N3O9
1 O2
_ H2O + _
6
_ CO2 + 10
_ N2 + 12
Decomposition
1 C5H12 + _
8 O2
_
5 CO2 + _
6 H2O
_
Double Replacement?
Combustion
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)
Single and Double Replacement
Reactions
Single-replacement reaction
Mg
+
CuSO4
General form:
A
+ BC

MgSO4

AC
+
+
Cu
B
Double-replacement reaction
CaCO3
+
General form:
AB
+
2 HCl

CaCl2
+
H2CO3
CD

AD
+
CB
single-replacement:
one element
replaces another
AB + C
A + CB
sodium
chlorine +
bromide
Na+ Br–
1
_ Cl2 + 2
_ NaBr
copper (II)
aluminum +
sulfate
Cu2+ SO42–
2
_ Al + 3
_ CuSO4
AB + C
B + AC
sodium
+ bromine
chloride
Na+ Cl–
2
_ Br2
_ NaCl + 1
aluminum
?
+ copper
sulfate
Al3+ SO42–
1_ Al2(SO4)3 + 3
_ Cu
Activity Series
Ca
Foiled again:
Aluminum is
knocked out by
Calcium
Printable
Version
of
Activity
Series
Element Reactivity
Li
Rb
K
Ba
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Ni
Sn
Pb
H2
Cu
Hg
Ag
Pt
Au
Halogen Reactivity
F2
Cl2
Br2
I2
Predict if these reactions will occur
3 Mg
+ 2 AlCl3
2 Al
+ 3 MgCl2
Can magnesium replace aluminum?
• Activity Series
Al
+
YES, magnesium is more reactive than aluminum.
MgCl2
NR (No Reaction)
Can aluminum replace magnesium? Therefore, no reaction will occur.
NO, magnesium is less reactive than aluminum.
MgCl2
+
Al
No reaction
Order of reactants
DOES NOT
determine how
they react.
We must determine if the lone element is more reactive than the bonded
one… metals replace metals or non-metals replace nonmetals
How do we know if a reaction will occur?
For single-replacement reactions, use Activity Series.
In general, elements above replace elements below.
1
1 FeSO4
_ Ba + _
3
_ Mg + 2
_ Cr(ClO3)3
1
_ Fe + 1
_ BaSO4
2_ Cr + _
3 Mg(ClO3)2
_ Pb + _ Al2O3
NR
_ NaBr + 1
_ Cl2
2
_
_ Br2
2 NaCl + 1
_ FeCl3 + _ I2
1_ CoBr2 + 1
_ F2
NR
1 CoF2 + 1
_
_ Br2
double-replacement:
AB + CD
iron (III)
+
chloride
Fe3+ Cl–
1
_ FeCl3 +
AD + CB
potassium
hydroxide
K+ OH–
3_ KOH
iron (III) potassium
?
+
hydroxide
chloride
Fe3+ OH–
K+ Cl–
1
_ Fe(OH)3 + 3
_ KCl
lead (IV)
calcium
+
nitrate
oxide
lead (IV)
calcium
+
?
oxide
nitrate
Pb4+ NO3– Ca2+ O2–
1
_ Pb(NO3)4 + 2
_ CaO
Pb4+ O2–
Ca2+ NO3–
1_ PbO2 + 2_ Ca(NO3)2
Double Replacement Reactions
Formation of a solid: AgCl
AgNO3(aq) + KCl(aq)  KNO3 (aq) + AgCl(s)
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.
precipitate: a solid product that forms in an
aqueous solution reaction
Na2CO3 (aq) + Ca(NO3)2 (aq)
Na+
Na+
CO32–
clear
Na2CO3
solution
Ca2+
CaCO3(s) + 2 NaNO3(aq)
NO3–
NO3–
clear
Ca(NO3)2
solution
ppt
“chunks”
“sinkies”
“floaties”
cloudy solution
containing CaCO3(s)
and NaNO3(aq)
Predict if a reaction will occur when you combine aqueous
solutions of iron (II) chloride and sodium carbonate…
If the reaction does occur, write a Balanced
balanced chemical equation
showing it (be sure to include phase notation).
iron (II) chloride + sodium carbonate
Fe2+ Cl -
Na+ CO32-
FeCl2
Na2CO3
sodium chloride + iron (II) carbonate
Na+ CO32-
Fe2+ Cl-
FeCO3 (s)
NaCl (aq)
Using the SOLUBILITY TABLE:
sodium chloride is soluble
iron (II) carbonate is insoluble
FeCl2 (aq)
+
Na2CO3 (aq)
2 NaCl (aq)
+
FeCO3 (s)
For double-replacement reactions,
reaction will occur
water a gas
if any product is:
a precipitate
driving forces
Check new combinations to decide.
1 Pb(NO3)2(aq) + _
2 KI(aq)
1 PbI2(s) + 2
_
_
_ KNO3(aq)
Pb2+ NO3–
K+ I–
_ KOH(aq) + _
2
1 H2SO4(aq)
K+ OH–
H+ SO42–
_ FeCl3(aq) + _ Cu(NO3)2(aq)
Fe3+ Cl–
Cu2+ NO3–
Pb2+ I– (?)
(ppt)
K+ NO3– (?)
(aq)
_ K2SO4(aq) + 2
_ H2O(l)
1
K+ SO42– (?) H+ OH– (?)
(aq)
(water)
NR
(aq)
(aq)
Ions in Aqueous Solution
Pb(NO3)2(aq)
Pb(NO3)2(s)
Pb2+(aq) + 2 NO3–(aq)
Pb2+
NO3
–
NO3–
add
water
NO3–
Pb2+
NO3–
dissociation: “splitting into ions”
NaI(aq)
NaI(s)
Na+(aq) + I–(aq)
Na+
I–
add
water
Na+
I–
ChemThink
Mix them and get the overall ionic equation…
Pb2+
NO3–
reactants
I–
Na+
NO3–
Na+
–
+(aq) + __I
1 2+(aq) + __NO
2
2
2 –(aq)
__Pb
3 (aq) + __Na
yields
–(aq) + __Na
+(aq)
1
2
2
+
__NO
__PbI
(s)
3
2
Na+
Pb2+
I–
NO3–
I–
Na+
products
NO3–
Cancel spectator ions to get net ionic equation…
2+(aq) + __I
1
2 –(aq)
__Pb
1
__PbI
2(s)
I–
Pb2+
I–
Pb2+
I–
I–
I–
Polymers and Monomers
polymer: a large molecule (often a chain) made of
many smaller molecules called monomers
Polymers can be made more rigid if the chains are
linked together by way of a cross-linking agent.
Monomer
Polymer
H H O amino acids………… proteins
H–N–C–C–O–H
w/Nnucleotides
(
R
bases A,G,C,T/U)….. nucleic acids
styrene……………… polystyrene
PVA…………………. “slime”
polyvinyl
alcohol