1. Redox Reactions, Acid, Base, Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Chapter Balancing redox equations
Redox reactions: reduction – oxidaton reactions.
Reduction: lowering of positive charge (increase of negative charge)
Oxidation: increase of positive charge (lowering of negative charge)
If an electron is transferred, then
the negative charge must decrease somewhere (where the electron comes from),
and it must increase somewhere else (where the electron goes),
hence reduction and oxidation must occur together.
There is a balance in such changes,
and reduction – oxidaton reactions provide good examples
for balancing chemical equations.
Redox Reactions, Acid, Base, Paul G. Mezey

2. Chapter 18: Electrochemistry
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Chapter 18: Electrochemistry
Chapter 18: Electrochemistry
18.2 Balancing Oxidation-Reduction Equations [19.1]
(Note: This section is usually covered with a very brief review of oxidation numbers and oxidizing/reducing agents, as covered in Chemistry 1010, which is found in section 4.9 of the Tro text).

3. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Oxidation numbers
Redox reactions are easier to balance if we understand where the electrons are coming from and where they are ending up.
Oxidation numbers help us figure this out.
Redox Reactions, Acid, Base , Paul G. Mezey

4. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
In a very simplistic way, the basis of oxidation numbers is the following idea:
Each (single) chemical bond between any two atoms X and Y is formed by two electrons.
There are three possibilities:
X attracts electrons more than Y
2. Y attracts electrons more than X
X and Y attract electrons exactly the same way (important: this happens only if X = Y)
Case 1. Even if the attraction is only slightly stronger by X than by Y, we pretend that the entire electron pair belongs to X.
Case 2. Similarly to case 1, we pretend that the entire electron pair belongs to Y.
Case 3. We assign one electron to X, and the other electron to Y.
If we carry out this formal assignment of electrons for each bond of the
molecule or ion, then the “charge” obtained on each atom is the
oxidation number of the given atom.
Redox Reactions, Acid, Base , Paul G. Mezey

5. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
A numerical measure of how strongly atoms attract electrons is electronegativity
We may phrase the concept of oxidation number in terms of electronegativity:
In calculating oxidation numbers the two electrons in a bond are completely assigned to the more electronegative element,
UNLESS
the bond is between two atoms of the same element,
in this case the electrons are shared equally so that
one electron is assigned to each of the atoms.
Redox Reactions, Acid, Base , Paul G. Mezey

6. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Based on this simple idea, we can follow a set of rules to assign oxidation numbers .
We go through the set of rules until we find the FIRST rule that applies to our specific atom in the compound or ion of interest.
Oxidation number rules (Rule 1)
Atoms of pure elemental compounds (e.g. metals, solid carbon, O2 gas, Br2 liquid, I2 solid, etc.) have an oxidation number of ZERO
Oxidation number rules (Rule 2)
Monatomic ions (like Mg2+, Li+, F-, S2-, etc.) have an oxidation number equal to the charge
Oxidation number rules (Rule 3)
Fluorine, as the most electronegative element, will ALWAYS have an
oxidation number of -1 EXCEPT in F2 where it has an oxidation number of ZERO (Rule 1)
Redox Reactions, Acid, Base , Paul G. Mezey

7. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Oxidation number rules (Rule 4)
Oxygen, as the second most electronegative element, will usually have an
oxidation number of -2 UNLESS it is bonded to another oxygen or fluorine
Oxidation number rules (Rule 5)
Hydrogen, will have an oxidation
number of +1 unless it is bonded to a metal atom, where it will have an
oxidation number of -1, or if it is bonded to another H atom, where it will have an oxidation number of 0 (Rule 1).
Oxidation number rules (Rule 6)
Halogens (Cl, Br, I, and At), generally have an oxidation number of -1
EXCEPT when bonded to F, O, or halogens of the same type or above it on the periodic table.
Oxidation number rules (Rule 7)
The sum of the oxidation numbers for ALL the atoms in a compound or
ion MUST ADD UP to match the total charge on the compound (zero) or
ion (ion charge).
Redox Reactions, Acid, Base , Paul G. Mezey

8. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Apply the rules in the order given.
Any atoms not specifically covered in the rules can usually be assigned oxidation numbers by applying Rule 7 and some logic.
If in any doubt, please remember that the main idea is to artificially assign the entire bonding electron pair to the more electronegative atom in each bond. If all bonding electron pairs are distributed this way, then the charge obtained on each atom becomes the oxidation number.
(Of course, the charge on each atom includes the protons in the nucleus as well as all the electrons of the atom).
Redox Reactions, Acid, Base , Paul G. Mezey

9. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Problems:
Assign oxidation numbers to every atom
in the following compounds and ions:
S8 LiH
TiO2 H2O
H2O2 HSO4-
Cr2O72- CaCO3
Redox Reactions, Acid, Base , Paul G. Mezey

10. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Using oxidation numbers, the balancing of redox chemical equations becomes simpler.
The main steps in balancing redox equations:
Using oxidation numbers, identify
what is oxidized (what loses electrons) and
what is reduced (what gains electrons).
What are the products after the oxidation and reduction take place?
Is the redox reaction done under acidic or basic conditions?
The information obtained from these 3 steps results only in an unbalanced
skeleton equation where we know generally what reactants and products
are specifically involved in the electron transfer (redox) process.
Redox Reactions, Acid, Base , Paul G. Mezey

11. Examples of skeleton equations with oxidation numbers shown:
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Examples
Examples of skeleton equations with oxidation numbers shown:
Redox Reactions, Acid, Base , Paul G. Mezey

12. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
How to proceed further?
Brake up the problem to two formal half-reactions :
We break the skeleton reaction into two unbalanced half-reactions where the
oxidation half-reaction has an atom where the oxidation number becomes more positive
and the
reduction half reaction has an atom where the oxidation number becomes more negative.
Redox Reactions, Acid, Base , Paul G. Mezey

13. Example: Half-reactions from skeleton reaction
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Example: Half-reactions from skeleton reaction
Oxidation half-reaction:
Reduction half-reaction:
Redox Reactions, Acid, Base , Paul G. Mezey

14. Steps for balancing half-reactions in ACIDIC solution:
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for balancing half-reactions
in ACIDIC solution:
1. Balance all atoms EXCEPT H and O in each half reaction:
2. Balance O atoms by adding water to the side missing O atoms:
Redox Reactions, Acid, Base , Paul G. Mezey

15. Steps for ACIDIC solution
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for ACIDIC solution
3. Balance H atoms by adding H+ to the side missing H atoms:
Oxidation half-reaction
Reduction half-reaction
Redox Reactions, Acid, Base , Paul G. Mezey

16. Steps for ACIDIC solution
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for ACIDIC solution
4. Balance charge by adding electrons to the side with more total positive charge:
Oxidation half-reaction
Reduction half-reaction
Redox Reactions, Acid, Base , Paul G. Mezey

17. Steps for ACIDIC solution
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for ACIDIC solution
5. Make the number of electrons the same in both half-reactions by multiplication, while avoiding a fractional number of electrons:
Oxidation half-reaction
Reduction half-reaction
Redox Reactions, Acid, Base , Paul G. Mezey

18. Steps for ACIDIC solution
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for ACIDIC solution
6. Add the half reactions together and then simplify by cancelling out species that show up on both sides:
Added together
Simplified (should have NO extra electrons!)
Redox Reactions, Acid, Base , Paul G. Mezey

19. Steps for ACIDIC solution
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for ACIDIC solution
7. Confirm that the reaction is balanced in number of atoms and total charge on both sides of the arrow.
If the reaction stoichiometry can be simplified by division without giving fractional coefficients, you can simplify further:
Redox Reactions, Acid, Base , Paul G. Mezey

20. Steps for balancing half-reactions in BASIC solution:
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for balancing half-reactions in BASIC solution:
First follow steps one to seven as seen in the case of acidic solution.
8. Add the same number of OH- groups as there are H+ present to BOTH sides of the equation:
Redox Reactions, Acid, Base , Paul G. Mezey

21. Steps for BASIC solution
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for BASIC solution
9. One side of the reaction has BOTH OH- and H+ present in equal amounts. Combine these together to make an equal amount of water:
Redox Reactions, Acid, Base , Paul G. Mezey

22. Steps for BASIC solution
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Steps for BASIC solution
10. Simplify by cancelling out an equal number of water from each side until one side has no water and confirm that the reaction is balanced in number of atoms and total charge on both sides of the arrow:
Redox Reactions, Acid, Base , Paul G. Mezey

23. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
Problem
Balance the following unbalanced redox skeleton equation in BASIC solution
Redox Reactions, Acid, Base , Paul G. Mezey

24. Redox Reactions, Acid, Base , Paul G. Mezey
Chapter 18.2 Balancing Oxidation-Reduction Equations [19.1]
End of section
Redox Reactions, Acid, Base , Paul G. Mezey