Electrochem Homework
3, 15, 17, 23, 27, 29, 35, 37, 41,
44, 49, 51, 53, 59, 63, 89, 91,
97a, 99, 117
Oxidation Numbers
• # of e- gained or lost when forming a compound
• If ion, actual charge
• Use the Periodic Table!
General Rules
• Free uncombined state of atoms -> 0
Ex.
Na, K, S8 , Cl2
• Sum of oxidation #’s in neutral compound is zero
• Polyatomic ions, oxidation #’s add up to the charge
on the ion
• F has a -1 ox. state
General Rules Continued
• H usually has an ox. state of +1
Exception: NaH where H is -1
• O usually has an ox. State of -2
Exceptions: O-1 and O-1/2
• Use the Periodic Table to find the location of the
element
Examples
• HNO3
• NO3-1
New Method
# of valence electrons
- # of unshared electrons
Midway number
- # of bonding electrons assigned to it*
oxidation number
* Higher electronegative atom gets the e-
Examples
• NH3
• CH3OH
Why do we need a new method?
• SO4-2 vs. S2O3-2
Electrochemistry
• Study of the relationship between chemical change
and electrical work
• Application of thermodynamics in everyday
applications
Ex. Radios, iPods, calculators
• Battery houses a spontaneous chemical reaction
that releases free energy to produce electricity
Electrochemistry cont.
• 2nd type of reaction:
Nonspontaneous and absorbs free energy
from an external source of electricity
Ex. Electroplating, recovery of metals from
ores
Electrochemistry cont.
• Amount of energy (electrical) consumed or
produced in chemical reactions can be
accurately measured
• All electrochemical reactions involve the
transfer of electrons
Redox reactions!
• The electrochemical cell contains the
reacting system
Electrochemistry cont.
• Sites of oxidation and reduction separated
physically so that reduction occurs at one
electrode and oxidation occurs at the other
electrode
Red Cat and An Ox
Examples of Redox
reactions
4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)
Balancing redox reactions
1.
2.
3.
4.
5.
Write the unbalanced equation
Assign oxidation numbers
Choose coefficients that make the transfer of
electrons the same
Balance the charge
Balance remaining elements if necessary
Balancing In Acidic Media
• Add H+ and H2O to balance
charge
Ex. Cr2O7-2 + ICr+3 + IO3-
Balancing In Basic Media
• Balance charge by adding OHand H2O
Ex. Cr2O7-2 + ICr+3 + IO3-
Balance in Acid
• MnO4- + Br-
MnO2 + BrO2-1
Balance in Base (Drano)
• Al
+
H2O
[Al(OH)4]- + H2
Balance in Acid
Cu + NO3-
Cu+2 + NO
Half Rxn Method for balancing
redox equations
• Overall reaction split into two half reactions
• Separates oxidation and reduction half reactions
• Easy to balance redox reactions occurring in acidic
or basic solutions
How?
1. Divide equation into two half reactions
2. Balance the atoms and charges in each half
reaction
- e- added to left is reduction
- e- added to right is oxidation
3. Multiply by integer so e- lost = e- gained
4. Add balanced equations
Ex. Balance in acidic media
Ex. Cr2O7-2 + I-
Cr+3 + I2
Ex. Balance in basic media
MnO4-1 + C2O4-2
MnO2 + CO3-2
Breathalyzer reaction
Cr2O7-2 + C2H5OH + H+
Cr+3 + C2H4O + H2O
Equivalents
• Acids and Bases  Just deal with the number of H+
or OH- being donated
Ex. HCl -> 1 equivalent
H3PO4 -> 3 equivalents
NaOH -> 1 equivalent
Ca(OH)2 -> 2 equivalents
Acid Reaction
H2SO4
1 mol
98.0 g/mol
+ H2 O
2 H+ + SO4-2
2 mol
1 mol
2.0 g/mol 96.0 g/mol
2 equivalents
1 equivalent HNO3 – 63.01 g
1 equiv. CH3COOH – 60.03 g
1 equiv. H2SO4 - 49.04 g
1 equivalent of Ca(OH)2 - 37.05 g
Normality
• Molarity = moles of solute
Liters of solution
• Normality = equivalents of solute
Liters of solution
• N = M x #equivalents
Equivalents and Redox
• Not only do equivalents pertain to # H+ or #
OH- but also to electrons
• Equivalent – mass of oxidizing or reducing
substance that gains or loses 1 mole of
electrons
• 1 mole of electrons equals 1 equivalent of
electrons
Equivalents
• One equiv. oxidized reacts with one equiv.
reduced
1 eq. ox. = 1 eq. red.
• If redox equation is balanced – mole ratios
can be used in stoichiometry
• If NOT balanced – equivalents can be used
END!