Chemical Equations
& Reactions
Chemical Equations
4 Al(s) + 3 O2(g)
2 Al2O3(s)
4 g Al + 3 g O2 yield 2 g Al2O3
This equation means:
4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3
or
4 Al moles + 3 O2 moles yield 2 moles of Al2O3
4 mol Al@27g/mol
108 g
3 mol O2@32g/mol
+
96 g
2 mol Al2O3@102g/mol
=
204 g
Visualizing a Chemical Reaction
2 Na
10 mole Na
___
+
Cl2
5 mole Cl2
___
2 NaCl
10
? mole NaCl
___
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
Practice: Balance and Classify
1. Ca(OH)2 +
HCl 
CaCl2 +
H 2O
________________________
2. C2H4 + O2  CO2 + H2O
________________________
3. N2 + O2  N2O
________________________
4. SrCO3  SrO + CO2
________________________
5. NaI + Br2  NaBr + I2
________________________
6. C2H4O + O2  CO2 + H2O
________________________
7. MgBr2 + (NH4)2SO3 
________________________
MgSO3 + NH4Br
8. AgClO3 + (NH4)2Cr2O7  Ag2Cr2O7 + NH4ClO3______________________
9. Cs + H2O  CsOH + H2
________________________
10. Fe + O2 
________________________
Fe3O4
Symbols Used in Chemical Equations
“Yields”; indicates result of reaction
Used to indicate a reversible reaction (equilibrium)
(s)
A reactant or product in the solid state;
also used to indicate a precipitate
Alternative to (s), but used only to indicate a precipitate
(l)
A reactant or product in the liquid state
(aq)
A reactant or product in an aqueous solution
(dissolved in water)
(g)
A reactant or product in the gaseous state
Additional Symbols Used in Chemical Equations
Alternative to (g), but used only to indicate a gaseous product
D
2 atm
pressure
Reactants are heated
Pressure at which reaction is carried out, in this case 2 atm
Pressure at which reaction is carried out exceeds normal
atmospheric pressure
0 oC
Temperature at which reaction is carried out, in this case 0 oC
MnO2
Formula of catalyst, in this case manganese (IV) oxide,
used to alter the rate of the reaction
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 input or release difficult to reverse
gases or vapor
of energy
Combustion
C4H10 +
2
13/2 O2
4 CO2 +
13
8
General form: CxHx
carbon-hydrogen
compound
+
O2
oxygen
5 H2O
10
CO2
carbon
dioxide
+
H2O
water
Writing Equations Practice
Write a balanced equation for the reaction between chlorine
and solid sodium bromide to produce bromine and solid
sodium chloride.
1. Write a word equation for the reaction
chlorine + sodium bromide  bromine + sodium chloride
2. Write the correct formulas for all reactants and products,
(with correct phases of matter)
3. Balance the resulting equation
Cl2(g)
+
2 NaBr(s)

Br2(l)
+
2 NaCl(s)
Write the balanced equation for the aqueous reaction
between aluminum sulfate and calcium chloride to form
a white precipitate of calcium sulfate.
1) Write a word equation for the reaction
?
?
aluminum sulfate + calcium chloride  calcium sulfate + aluminum chloride
2) Write the correct formulas for all reactants and products
3) Balance the resulting equation
Al2(SO4)3(aq)
+ 3 CaCl2(aq)

3 CaSO4(s)
+
2 AlCl3 (aq)
Oxidation-Reduction Reactions
• “Redox” reactions involve the transfer of
electrons (e-)
• Reduction: gain e-
• Oxidation: lose e“LEO the lion says, ‘GER’”
“OIL RIG”
• Use oxidation states to keep track of the e-
My name is Leo.
Grr-rrrr…
Leo says Ger
“Lose electron  oxidation”
Zn  2e- + Zn2+
“Gain electron  reduction”
2e- + Cu2+  Cu
Assigning Oxidation States
Specific rules for assigning Ox #’s
•
•
•
•
•
Usually the same charge assigned by the PT
H is almost always +1
O is almost always -2
F is always -1 in compounds
For elements (H2, O2, F2, Ca, K, etc )
the oxidation state always = 0
Some exceptions do exist!
Assigning Oxidation Numbers
• Overall charge = sum of the oxidation states of
all atoms in it
Neutral Compounds (e.g. H2O, CO2, CH4)
• H2O : H = +1 and O = -2
The overall charge is 2(1) + -2 = 0
• CO2: What is the oxidation state of C?
Since C + 2 (O) = 0…
C + 2(-2) = 0, thus… C = +4
• CH4: Is C still +4?
H is always +1
To remain neutral… 4(1) + C = 0
C must = - 4
Assigning Oxidation Numbers
• Charged compounds (e.g. NO3-, CO32-)
NO3- or (NO3)- : What is the oxidation # of N?
O is -2, and the overall charge is -1
So N + 3(O) = -1 or N + 3(-2) = -1
N=+5
(CO3)2-: What is the oxidation # of C?
O is -2, and the overall charge is -2
So C + 3(O) = -2 or C + 3(-2) = -2
C = +4
The oxidation # of ions = charge of ions
Mn3+ has an oxidation # of +3
S2- has an oxidation # of -2
Assigning Oxidation # Practice
Assign oxidation numbers to each atom
Cl2
Cl: 0 (element)
Fe2+
Fe: 2+ (ion)
ClO3-
O: 2-, 3(2-) + Cl = 1-…Cl: 5+
ClO4-
O: 2-, 4(2-) + Cl = 1-…Cl: 7+
IO2-
O: 2-, 2(2-) + I = 1-…I: 3+
CrO42-
O: 2-, 4(2-) + Cr = 2-…Cr: 6+
Fe3(PO4)2
Fe: 2+ (ion) PO4:3- (ion)….O:2-, 4(2-) + P = 3-, P: 5+
CoSO4
Co: 2+ (ion) SO4:2- (ion)….O:2-, 4(2-) + S = 2-, S: 6+
Assigning Oxidation Numbers Review
• Try these…MnO4-, Cr2O72-, C2O42• (MnO4)O = -2, so [4(-2) + Mn = -1]
Mn = +7
• (Cr2O7)2O = -2, so [7(-2) + 2Cr = -2]
2Cr = 12, therefore… Cr = +6
• (C2O4)2O = -2, so [2C + 4(-2) = -2]
2C = 6, therefore…C = +3
Oxidation-Reduction Reactions
• Two separate reactions occurring
simultaneously
• Oxidation: oxidation # of an atom increases
• e.g. Fe(s) → Fe3+(aq) (ox # goes from 0 → +3)
• Reduction: oxidation # of an atom is
“reduced”
• e.g. O2(g) → O2-(aq) (oxidation # goes from 0 → -2)
When occurring together…
• Fe(s) + O2(g) → Fe3+(aq) + O2-(aq)
• This is the redox reaction responsible for rust!
But, how do we balance this?
Balancing by Half-Reactions
*in acidic solution
1. Assign oxidation states for each element.
2. Write separate half-reactions for the reduction/oxidation
reactions.
3. Balance all the atoms EXCEPT O and H.
4. Balance the oxygen with water (H2O).
5. Balance the hydrogen with hydrogen ions (H+).
6. Balance the charge with electrons.
7. Multiply each half-reaction by an appropriate number to
make the electrons equal.
8. Combine both reactions into one and cancel the e -
Balancing by Half-Reactions
*in acidic solution
CH3OH (aq) + Cr2O72-(aq) → CH2O(aq) + Cr3+(aq)
1. Assign oxidation states.
C-2H4+O2- + (Cr26+O72-)2- → C0H2+O2- + Cr3+
2.
Write separate half-reactions for the reduction
and oxidation reactions. (only keep charges that
are changing…)
Ox: C-2H4O → C0H2O (C is going from -2 to 0)
Red: (Cr26+O7)2- → Cr3+
(Cr is being reduced from +6 to +3)
Balancing the half reactions…
3 ( C2-H4O → C0H2O + 2H+ + 2e- )
Ox:
Red: 6e- +14H+ +(Cr26+O7)2- → 2 Cr3+ + 7H2O
3CH4O → 3CH2O + 6H+ + 6e3 CH4O + 8 H++ Cr2O72- → 3 CH2O + 2 Cr3+ + 7 H2O
3.
4.
5.
6.
7.
8.
For each half reaction, balance all atoms EXCEPT O and H.
Balance the oxygen by adding water (H2O).
Balance the hydrogen by adding hydrogen ions (H+)
Balance the charge by adding electrons.
…use the oxidation state as a guide
Multiply each half-reaction by an appropriate number to make
the electrons equal.
Add the reactions together and cancel e-/simplify.
Practice Balancing Redox Reactions
Unbalanced reaction (in acid):
MnO4 + Fe2+  Mn2+ + Fe3+
Balanced Reduction half-reaction:
8H+ + MnO4 + 5e  Mn2+ + 4H2O
Balanced Oxidation half-reaction:
5(Fe2+  Fe3+ + e)
Balanced overall reaction:
8H+ + MnO4 + 5Fe2+  Mn2+ + 5Fe3+ + 4H2O
Balancing by Half-Reactions
*in basic solution
1. Assign oxidation states.
2. Write separate half-reactions for the reduction/oxidation
reactions.
3. Balance all the atoms EXCEPT O and H.
4. Balance the oxygen by adding water (H2O).
5. Balance the hydrogen by adding H+.
6. Balance the charge by adding electrons.
7. Multiply each half-reaction by an appropriate number to
make the electrons equal.
8. Combine both reactions into one and cancel.
9. Add OH- to both sides to cancel out H+ and create H2O.
Simplify further, if necessary.
Balancing by Half-Reactions
(in basic solution)
Let’s balance a previous example in basic solution
Remember, it is all the same steps up to this point
3CH4O + 8H+ + Cr2O72- → 3CH2O + 2Cr3+ + 7H2O
+ 8OH+ 8OH3CH4O + 8H2O + Cr2O72- → 3CH2O + 2Cr3+ + 7H2O + 8OH-
3CH4O + H2O + Cr2O72- → 3CH2O + 2Cr3+ + 8OH-
Practice Balancing Basic Redox Rxns
Unbalanced reaction:
ClO + Zn  Cl- + Zn2+
Balanced Reduction half-reaction:
2e- + 2H+ + ClO-  Cl- + H2O
Balanced Oxidation half-reaction:
Zn  Zn2+ + 2eBalanced overall reaction (acidic):
2H+ + ClO + Zn  Zn2+ + Cl- + H2O
Balanced overall reaction (basic):
H2O + ClO + Zn  Zn2+ + Cl- + 2OH-
Activity Series
Ca
Foiled again:
Aluminum is
knocked out by
Calcium
Element Reactivity
Li
Rb
K
Ba
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Ni
Sn
Pb
H2
Cu
Hg
Ag
Pt
Au
Printable
Version
of
Activity
Series
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, aluminum is less reactive than magnesium.
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
More SR Reactions…
Activity Series
“Magic blue-earth”
Fe + CuCl2
FeCl2
+
Cu
Can Fe replace Cu? Yes
Zinc in nitric acid
Zn
+ 2 HNO3
Can Zn replace H?
Zn(NO3)2
+ H2
Yes
NO REACTION
MgCl2
+
Can Br replace Cl?
Br2
MgBr2
+
No
General Form
A
+
BC
AC + B
Cl2
Li
Rb
K
Ba
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Ni
Sn
Pb
H2
Cu
Hg
Ag
Pt
Au
F2
Cl2
Br2
I2
Double Replacement Reactions
K2CO3 (aq)
Potassium carbonate
+
BaCl2 (aq)
Barium chloride
2 KCl (aq)
Potassium chloride
+
BaCO3 (s)
Barium carbonate
Formation of a solid precipitate:
AgNO3(aq) + KCl(aq)  KNO3 (aq) + AgCl(s)
acetate
bromide
carbonate
chloride
chromate
hydroxide
iodide
nitrate
phosphate
sulfate
sulfide
TABLE OF SOLUBILITIES IN WATER
aluminum
ss
s
n
s
n
i
s
s
i
s
d
ammonium
s
s
s
s
s
s
s
s
s
s
s
barium
s
s
i
s
i
s
s
s
i
i
d
calcium
s
s
i
s
s
ss
s
s
i
ss
d
copper (II)
s
s
i
s
i
i
n
s
i
s
i
iron (II)
s
s
i
s
n
i
s
s
i
s
i
iron (III)
s
s
n
s
i
i
n
s
i
ss
d
lead
s
ss
i
ss
i
i
ss
s
i
i
i
magnesium
s
s
i
s
s
i
s
s
i
s
d
mercury (I)
ss
i
i
i
ss
n
i
s
i
ss
i
mercury (II)
s
ss
i
s
ss
i
i
s
i
d
i
potassium
s
s
s
s
s
s
s
s
s
s
s
silver
ss
i
i
i
ss
n
i
s
i
ss
i
sodium
s
s
s
s
s
s
s
s
s
s
s
zinc
s
s
i
s
s
i
s
s
i
s
i
Legend
SOLID
i = insoluble
SOLIDsoluble
ss = slightly
AQUEOUS
s = soluble
d = decomposes
n = not isolated
Solubility Rules
1.
Most nitrates are soluble.
2. Most salts containing Group I ion and ammonium ion, NH4+,
are soluble.
3. Most chloride, bromide, and iodide salts are soluble, except
Ag+, Pb2+ and Hg22+.
4.
Most sulfate salts are soluble, except BaSO4, PbSO4, Hg2SO4,
and CaSO4.
5. Most hydroxides except Group 1 and Ba(OH)2, Sr(OH)2, and
Ca(OH)2 are only slightly soluble.
6. Most sulfides, carbonates, chromates, and phosphates are
only slightly soluble.
Ohn-Sabatello, Morlan, Knoespel, Fast Track to a 5 Preparing for the AP Chemistry Examination 2006, page 91
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)
Predict if a reaction will occur when you combine aqueous
solutions of potassium chloride and sodium nitrate…
If the reaction does occur, write a balanced chemical equation
showing it (be sure to include phase notation).
potassium chloride + sodium nitrate
K+
Cl KCl(aq)
Na+ NO3NaNO3 (aq)
sodium chloride + potassium nitrate
K+
Cl-
NaCl(aq)
Na+
NO3-
KNO3(aq)
Using the SOLUBILITY TABLE:
sodium chloride is soluble
potassium nitrate is soluble
Notice that nothing has really changed because the
ions are still dissolved in water!
Ions in Aqueous Solution Expt.
Pb(NO3)2(s) + H2O(l)
Pb(NO3)2(aq)
Pb2+(aq) + 2 NO3–(aq)
Pb2+
NO3–
add
water
NO3–
NO3–
Pb2+
NO3–
in solution,
aqueous,
soluble,
dissolved
Dissociation reactions: solids mixed with water dissociate into ions
NaCI(aq)
NaCI(s) + H2O(l)
Na+(aq) + CI–(aq)
Na+
CI–
Na+
CI–
• Mix them and get…
• Balance to get complete ionic equation…
• Cancel spectator ions to get net ionic equation…
Chem
Think
Mix them and get…
Pb(NO3)2(aq) + 2 NaCI(aq)
Solubility
Chart
PbCI2(s) + 2 NaNO3(aq)
NO3–
Pb2+
Pb2+
NO3
Na+
Na+
CI–
–
CI–
CI–
Na+
Na+
CI–
Solid
(precipitate)
NO3–
NO3–
in solution
(aqueous)
Balance to get complete ionic equation…separate anything (aq)
Pb2+(aq) + 2 NO31–(aq) + 2 Na1+(aq) + 2 CI–(aq)
PbCI2(s) + 2 NO31–(aq)+ 2 Na1+(aq)
Cancel spectator ions to get net ionic equation…
Pb2+(aq) + 2 CI–(aq)
PbCI2(s)
Pre-lab:
1. What ions are present in the following solutions?
Na+(aq)
Cl-(aq)
NaCl(aq)  ____________________
AgNO3(aq)

+
-
Ag (aq)
NO3 (aq)
____________________
2. When these solutions are mixed together, a precipitate is seen. What are the new
combinations of ions that could have formed the precipitate?
+
+
-
-
Ag (aq)
Na (aq) NO3 (aq) and ____________________
Cl (aq)
____________________
3. Using the solubility table, which new combination will form a precipitate?
Ag+(aq)
Cl-(aq)
____________________
 AgCl(s)
4. Which new combination will remain in solution?
+
-
Na (aq) NO3 (aq)
____________________
5. Write the complete ionic equation for this reaction. Be sure to indicate the correct phase
(reaction condition) for each reactant and each product.
Na+(aq) + Cl-(aq)
+ Ag+(aq)
+ NO3-(aq)
AgCl(s) + Na+(aq) + NO3-(aq)
6. Write the net ionic equation for this reaction by canceling out spectators. Again, include
the phases (reaction conditions).
+
-
Ag (aq) + Cl (aq)
AgCl(s)
7. Explain why you would expect no reaction between solutions of KOH(aq) and NaOH(aq).
When the cations switch places they end with a hydroxide
(no new combination is formed)
Mix together Zn(NO3)2(aq) and Ba(OH)2(aq):
Solubility
Chart
Mix them and get…
Zn(NO3)2(aq) + Ba(OH)2(aq)
Zn(OH)2(s) + Ba(NO3)2(aq)
Zn(NO3)2(aq)
Ba(OH)2(aq)
Zn2+(aq) + 2 NO3–(aq)
Ba2+(aq) + 2 OH–(aq)
OH–
NO3–
Ba2+
Zn2+
NO3–
OH–
Balance to get complete ionic equation…
Zn2+(aq) + 2 NO3–(aq)
+ Ba2+(aq) + 2OH–(aq)
Zn(OH)2(s) + 2 NO3–(aq) + Ba2+(aq)
Cancel spectator ions to get net ionic equation…
Zn2+(aq) + 2 OH–(aq)
Zn(OH)2(s)
Separation of Cations
You have a solution containing Fe2+, Cu2+, Ba2+, Ag+
and K+ ions. By what means could you separate
these ions from each other?
• In Chem I, we discussed various ways to
separate things…
•
•
•
•
Distillation
Filtration
Centrifugation
Reactivity
Will any of these work to
separate aqueous ions?
Separation of Cations
Fe2+, Cu2+, Ba2+, Ag+, K+ (aq)
+ Cl- (aq)
AgCl(s)
Fe2+, Cu2+, Ba2+, K+ (aq)
+ SO42- (aq)
BaSO4(s)
Fe2+, Cu2+, K+ (aq)
+ CrO42- (aq)
CuCrO4(s)
Fe2+, K+ (aq)
+ S2-, PO43- OH- or
CO32- (aq)
FeS, Fe3(PO4)2 , Fe(OH)2 , or FeCO3(s)
K+(aq)
Separation of Cations
Try this example on your own…
Pb2+, Ca2+, Zn2+, NH4+ (aq)
+ CrO42-(aq)
PbCrO4 (s)
Ca2+, Zn2+, NH4+ (aq)
+ SO42- (aq)
CaSO4(s)
Zn2+, NH4+ (aq)
+ CO32-, PO43- OH- or
S2- (aq)
ZnCO3, Zn3(PO4)2 , Zn(OH)2, or ZnS(s)
NH4+ (aq)
Summary of Classes of Reactions
Chemical reactions
Double Replacement
reactions
Oxidation-Reduction
Reactions
Acid-Base
Reactions
Combustion
Reactions
Synthesis and
Decomposition
reactions
Single Replacement
reactions