1. 3.2. - Recognizing Redox Reactions

2. Example 1:
Oxidation numbers are a convenient way of identifying redox reactions and also indicating which element is oxidized and which is reduced. Here's an example - the reaction between sodium metal and chlorine gas:
2 Na + Cl2 → 2 NaCl
It is often useful to write the oxidation number for every element, in every compound, above the element in the equation. Thus for our reaction we have:
   -1
2Na + Cl2 → 2 NaCl
Be sure to note that the balancing coefficients in the equation (the "2" in front of Na and in front of NaCl) do not affect the value of the oxidation numbers. We'll return to these coefficients soon.

3. change in electrons (e-)
A chart is a useful way for us to summarize the changes in oxidation number for each element:
element
initial ox no
final ox no
change in electrons (e-)
oxidized or reduced Na → +1
lost 1 e-
oxidized Cl -1
gain 1 e-
reduced

4. We see several important things in our table:
Since oxidation numbers did change, this was a redox reaction
Na's oxidation number increased - from 0 to +1. An element becomes more positive by losing electrons.
Loss of Electrons is Oxidation (LEO)
Cl's oxidation number decreased, from 0 to -1, as chlorine gained electrons.
Gain of Electrons is Reduction (GER)

5. To summarize:
An increase in oxidation number indicates oxidation
A decrease in oxidation number indicates reduction

6. Example 2: Consider the reaction 2 Mg + O2 → 2 MgO
Determine oxidation numbers for all elements in every compound:
   -2
Summarize the changes, determine the number of electrons transferred per atom, and identify what is oxidized and what is reduced:

7. change in electrons (e-)
Example 2:
element
initial ox no
final ox no
change in electrons (e-)
oxidized or reduced Mg → +2
lost 2e-
oxidized O -2
gain 2e-
reduced

8. Oxidation & Reduction Agents
Reducing agent
the substance that is oxidized. It allows another element to be reduced.
Oxidizing agent
the substance that is reduced. It allows another element to be oxidized.
In our last example
2 Mg + O2 → 2 MgO
Magnesium was oxidized, therefore it was the reducing agent.
Oxygen was reduced, therefore it was the oxidizing agent.

9. Example 3: Consider the reaction N2 + 2H2 → 2 NH3
Determine oxidation numbers for all elements in every compound:

10. Example 3:
Summarize the changes, determine the number of electrons transferred per atom, and identify what is oxidized and what is reduced, and identify the oxidizing agent and reducing agent:
element
initial ox no
final ox no e-
oxidized or reduced
Agent N → -3
gain 3e-
reduced
oxidizing agent: N2 H +1
lose 1e-
oxidized
reducing agent: H2

11. Oxidation & Reduction Agents – Example 4
By convention we often refer to the oxidizing agent and reducing agents as the entire compound the element is in, not just individual element.
(N)   (Zn)   +4 (N)    
Zn + HNO3 → Zn(NO3)2 + NO2 + H2O

12. Summarize:
element
initial ox no
final ox no e-
oxidized or reduced
Agent Zn → +2
lost 2e-
oxidized
reducing agent: Zn N +5 +4
gain 1e-
reduced
oxidizing agent: HNO3
We see that HNO3 is referred to as the oxidizing agent, not just N.
One final thing to note in this example - notice that the nitrogen in Zn(NO3)2 did not undergo a change in oxidation number.