1. Topic 9: REDOX
Topic 9.1: Redox (reduction-oxidation) reactions play a key role in many chemical and biochemical processes.

2. Important terms for this section
Oxidation
Half-equations
Reduction
Voltaic cell/battery
Electrolysis
Electrochemical cells
Redox reactions
Oxidation state
Oxidation number
Oxidizing agent
Reducing agent
Reductant
Oxidant
Displacement
Activity series
Titre
Dissolved oxygen
Biological oxygen demand
Winkler method

3. REDOX reactions
Used to be defined as the gain of Oxygen (oxidation) and gain of Hydrogen (reduction) but not anymore
NOW it is defined as the gain or loss of ELECTRONS
Loss of electrons = Oxidation (OIL)
Gain of electrons = Reduction (RIG)
When one atom is oxidized, something else is reduced
They occur together

4. Oxidation numbers
Sometimes it’s hard to tell how electrons are transferred
Such as in covalent bonds
Using oxidation states, we can determine if an atom is oxidized or reduced
Oxidation state: the apparent charge that an atom has in a molecule or ion
Written with charge first and then number: +3 or -2
Regular charges are written 2+ or 3-

5. Oxidation numbers Rules for assigning oxidation states:
1. atoms in the free element have an oxidation state of 0
Mg, O2, Li
2. in simple ions, the oxidation state is the same as the charge (written differently)
Mg2+ = +2, O2- = -2
3. oxidation states of all atoms in neutral state must add up to zero
H2SO4 = 0
4. oxidation states of all atoms in polyatomic ion must add up to the charge of ion
SO42- = -2
5. usual oxidation state for an element is same as the charge on most common ion
H = +1, O= -2
6. most main group non-metals, bottom of group 14, transition metals have varying oxidation states depending on compounds

6. Oxidation numbers

7. Interpreting Oxidation States
Sulfur as an example, can have several oxidation states
A lower oxidation state (more negative) is at it’s most reduced form
A higher oxidation state (more positive) is at it’s most oxidized form
Therefore:
An increase in oxidation number = oxidation
A decrease in oxidation number = reduction
Reduced  oxidized
H is oxidized 0 O is reduced 0-2

8. IUPAC names use oxidation states
Example in names, ferrous compounds Fe = +2 and ferric compounds Fe = +3
This is where the roman numerals came from in the names!
FeO = ferrous oxide or iron(II) oxide
Fe2O3 = ferric oxide or iron(III) oxide

9. REDOX equations: half-equations
Half-equations are used to separate out the oxidation from reduction reactions
Electrons are added to one side of each equation to balance out charges

10. REDOX equations: half-equations
We may not know the equation, but know the species involved
Use half-equations to solve by adding together (stack and destroy!)
Many use acidified solution, so use H2O and/or H+ to balance
Example:

11. REDOX equations: half-equations

12. REDOX equations: half-equations

13. REDOX equations: half-equations

14. REDOX equations: half-equations

15. Oxidizing and reducing agents
RXT that accepts the electrons = oxidizing agent or oxidant
In return gets reduced
RXT that donates electrons = reducing agent or reductant
In return gets oxidized
Oxidant Fe2O3 is reduced +2  0
Reductant C is oxidized 0  +2
Examples:

16. Reducing agents More reactive metals are stronger reducing agents
Tendency to form cations, they often lost electrons so they can be good reductants
More reactive metals lose e- more easily = stronger reductant
Example
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17. Reducing agents The copper ion displaces the zinc atoms
Zinc is the reducing agent and therefore gets oxidized
Thus, zinc is more reactive metal
Copper is the oxidizing agent and therefore gets reduced
The reverse reaction would not go (zinc sulfate and copper metal)
These levels of reactivity make up the activity series: Table 25 in booklet
DON’T MEMORIZE IT! You need to know how to interpret it

18. Oxidizing agents
More reactive non-metals are stronger oxidizing agents (oxidants)
Halogens’ reactivity reduces going down the group, so the oxidizing ability also decreases
Think of the displacement reactions from chapter 3

19. REDOX titrations Similar to acid-base reactions
Difference is equivalence point = where RXTs react stoichiometrically (transfer electrons)
Uses same kind of set up with burette, one standard solution, sometimes an indicator (some just change color at equivalence point anyway)
3 examples:
Analysis of Iron with manganate(VII)
Iodine-thiosulfate reaction
Winkler method to calculate dissolved O2

20. REDOX titrations: Analysis of Iron with manganate(VII)
Acidic KMnO4 solution = oxidant
Iron = reductant
Example problem:

21. REDOX titrations: Analysis of Iron with manganate(VII)

22. REDOX titrations: Analysis of Iron with manganate(VII)

23. REDOX titrations: Iodine-thiosulfate reaction
Iodine can be made by oxidizing it from iodine ions
Some oxidizing agents: KMnO4, KIO3, K2Cr2O7, NaOCl
Then iodine is reacted with sodium thiosulfate (Na2S2O3)
Starch is used as an indicator (iodine and starch turn purple)
As the iodine reacts, the purple color disappears

24. REDOX titrations: Iodine-thiosulfate reaction
REDOX equations:
Oxidation: 2S2O32-  S4O e-
Reduction: I2 + 2e-  2I-
Example problem:

25. REDOX titrations: Iodine-thiosulfate reaction
Solution:

26. REDOX titrations: Iodine-thiosulfate reaction

27. REDOX titrations: Winkler method to calculate dissolved O2
Used often for determining health/quality of water
BOD (biological oxygen demand) is a way of measuring the degree of pollution
Defined as the quantity of oxygen needed to oxidize organic matter in a sample of water over a 5-day period at a specified temperature
High BOD means more organic waste in the water = lower level of dissolved O2

28. REDOX titrations: Winkler method to calculate dissolved O2
Three steps to the Winkler method
1. Dissolved O2 is “fixed” by adding manganese(II) salt (i.e. MnSO4) in basic sol’n
The manganese will oxidize (i.e. Mn(IV))
2. Acidified iodide ions added to solution and oxidized by Mn(IV) to I2
3. Iodine produce is titrated with sodium thiosulfate

29. REDOX titrations: Winkler method to calculate dissolved O2
Example:

30. REDOX titrations: Winkler method to calculate dissolved O2
Solution:

31. REDOX titrations: Winkler method to calculate dissolved O2