Redox reactions involve the transfer of electrons from one reactant to another. The substance that loses electrons is oxidized and acts as the reducing agent, while the substance that gains electrons is reduced and acts as the oxidizing agent. This is demonstrated through an example of the combustion of ethane, where carbon is oxidized and loses electrons, acting as the reducing agent, while oxygen is reduced and gains electrons, acting as the oxidizing agent. A practice problem on the reaction between chlorine and potassium iodide is then worked through using the same steps to determine oxidation, reduction, and the reducing and oxidizing agents.

1. REDOX REACTIONS
Overview and Practice

2. Review
• Redox reactions involve the transfer of electrons
from one reactant to another
• When there is oxidation, there is also reduction
• The substance that loses electrons = oxidized
and is the reducing agent
• The substance that gains electrons = reduced
and is oxidizing agent

3. 2 Questions to Ask
• Does one gain electrons? If yes, then it has been reduced
• Does one lose electrons? If yes, then it has been oxidized

4. Example
Ethane burns in oxygen:
C2H6 + O2  CO2 + H2O
Steps:
1. Assign oxidation numbers (in red)
C2
-3 H6
+1 + O2
0  C +4O2
-2 + H +1
2O-2

5. Steps Continued
2. Write down which ones gain and which ones lose
electrons
Oxygen gained electrons (0 charge to -2 charge)
Carbon lost electrons (-3 charge to +4)
3. Determine which one is reduced and is the oxidizing
agent:
Oxygen is reduced and is the oxidizing agent
4. Determine which one is oxidized and is the reducing
agent:
Carbon is oxidized and is the reducing agent

6. Practice
Try out the 4 steps on the reaction below:
Cl2 + 2KI  I2 + 2KCl
Go to the next slide to see if you are correct!

7. 1. Assign oxidation numbers:
Cl2
0 + 2K+1 I-1  I2
0 + 2K+1Cl-1
2. Which ones gain and which ones lose electrons:
Cl gains electrons
I loses electrons
3. Determine which one is reduced and is oxidizing agent:
Cl is reduced and is oxidizing agent
4. Determine which one is oxidized and is reducing agent:
I is oxidized and is reducing agent