This document discusses oxidation-reduction (redox) reactions through examples of writing complete and net ionic equations, identifying oxidizing and reducing agents, writing half-reactions, and balancing redox reactions. Key points covered include: 1. Redox reactions involve the transfer of electrons between atoms. 2. Net ionic equations show the ionic form of the reactants and products. 3. The atom that loses electrons is oxidized and acts as the reducing agent. The atom that gains electrons is reduced and acts as the oxidizing agent. 4. Half-reactions allow identifying how many electrons are lost or gained in the oxidation and reduction steps. 5. Balancing redox reactions

1. +
Oxidation
Reduction
Reactions
November 23rd, 2011

2. +
What are redox reactions?
 We have already talked about oxidation-
reduction reactions:
 Anytime electrons are transferred from
one atom to another the reaction is called
an oxidation-reduction reaction or redox
reaction.

3. +
Net ionic equation
 A net ionic equation is a reaction written in terms of its ions.
 Examples:
 First let’s look at the complete chemical equation (the complete
reaction)
 2Ca + O2  2CaO
 Now let’s look at the net ionic equation:
 Ca + O2  2Ca2+ + 2O2-
 Notice how in the net ionic equation the product shows the
individual oxidation numbers of the compound calcium oxide

4. +
Now you try one
Synthesis reaction of sodium chloride
First
write the complete chemical
equation:
Then write the net ionic equation

5. +
Answer
 Complete chemical equation:
 2Na + Cl2  2NaCl
 Net ionic equation:
 2Na + Cl2  2Na+ + 2Cl-
 What happened to the oxidation numbers?
Did they change?

6. +
Now try it with single displacement
reactions
 Single displacement reaction between Calcium and
sodium chloride
 Complete chemical equation:
 Ca + 2NaCl  2Na + CaCl2
 Net ionic equation:
 Ca + Na+ + Cl-  Na + Ca2+ +2Cl-
 What happened to the oxidation numbers? Did they
change?

7. +
NOTE
 Notice how in each of the previous examples the
OXIDATION NUMBERS of the elements changed
from the reactants to the products.
 If you hadn’t notice go back and check
 This is due to the oxidation-reduction reaction

8. +
What happens
 When an atom loses an electron it is said to be OXIDIZED.
 When an atom gains an electron it is said to be REDUCED.
 An easy way to remember is:
 LEO the lion says GER
 Loss of Electrons is Oxidation, Gain of Electrons is
Reduction.

9. +
Continued…
 The substance that causes another substance to oxidize by
ACCEPTING its electrons is called an OXIDIZING AGENT.
 The substance that reduces another substances by
DONATING its electrons is called a REDUCING AGENT.
 In other words:
 The substance that is reduced (gains electrons) is the
OXIDIZING AGENT.
 The substance that is oxidized (loses electrons) is the
REDUCING AGENT

10. +
Let’s go back
 Let’s review our previous equations to see what is reduced and
what is oxidized.
 Ca + O2  2Ca2+ + 2O2-
 Calcium: oxidized (lost electrons). It is also the reducing agent.
 Oxygen: reduced (gained electrons) It is also the oxidizing
agent.

11. +
Now you try the other ones
 2Na + Cl2  2Na+ + 2Cl-
 Sodium:
 Chlorine:
 Ca + Na+ + Cl-  Na + Ca2+ +2Cl-
 Calcium
 Sodium
 Chlorine

12. +
Answer
 2Na + Cl2  2Na+ + 2Cl-
 Sodium: oxidized
 Chlorine: reduced
 Ca + Na+ + Cl-  Na + Ca2+ +2Cl-
 Calcium: oxidized
 Sodium: reduced
 Chlorine: neither… oxidation number stayed the same

13. +
Cont.…
 Oxidation and Reduction always occur together!!!
 In order to know which atom is reduced and which is oxidized you
ALWAYS have to write the
NET IONIC EQUATION

14. +
Half Reactions
 Inorder to know how many electrons were gained
or lost by the atoms, half reactions can be written.
 A half-reaction is one of the two parts of a redox
reaction.
 Either the oxidation half or the reduction half.

15. +
Examples
 Let’s do half-reactions with the reactions we previously used.
 First let’s write the net ionic equation of the synthesis of
calcium oxide:
 Ca + O2  2Ca2+ + 2O2-
 Now let’s do the half reactions:
 Oxidation: Ca  Ca2+ + 2e-
 Reduction: O2 + 2e-  2O2-
 Notice that electrons are added in the half reactions in order for
the reaction to be balanced.

16. +
Now you try it!
 2Na + Cl2  2Na+ + 2Cl-
 Oxidation:
 Reduction:
 Ca + Na+ + Cl-  Na + Ca2+ +2Cl-
 Oxidation:
 Reduction:

17. +
Answers:
 2Na + Cl2  2Na+ + 2Cl-
 Oxidation: Na  Na+ + e-
 Reduction: Cl2 + 2e-  2Cl-
 Ca + Na+ + Cl-  Na + Ca2+ +2Cl-
 Oxidation: Ca  Ca2+ + 2e-
 Reduction: Na+ + e-  Na

18. +
Balancing a redox reaction
 There are various ways in which redox can be used to balance
equations, but we will focus on one of them.
 The steps to balancing a redox reaction are the following:
 1. Write the net ionic equation for the reaction, and omit
spectator ions.
 2. Write the oxidation reduction half reactions for the net ionic
equation.
 3. Balance the charges in each half reaction.
 4. Adjust the coefficients so that the number of electrons lost in
oxidation equals the number of electrons gained in reduction.
 5. Add the balanced half-reactions and return spectator ions.

19. +
Example
 Let’s do an example with the equations we have used.
 Let’s do the single replacement reaction of calcium and sodium
chloride. We have already done steps 1, 2 and 3 (In this the
spectator ion was chlorine because its oxidation state did not
change). Here are the half-reactions.
 Oxidation: Ca  Ca2+ + 2e-
 Reduction: Na+ + e-  Na
 Now we need to adjust the number of electrons so that the
number of electrons gained equals the number lost. In order to
do this we multiply the reduction reaction by 2.
 Oxidation: Ca  Ca2+ + 2e-
 Reduction: (Na+ + e-  Na)2

20. +
Cont.
 Now the half reactions look like this:
 Oxidation: Ca  Ca2+ + 2e-
 Reduction: 2Na+ + 2e-  2Na
 All that is left to do is step 5
 Ca + 2NaCl  Na + CaCl2

21. +
This is it!
We will do more examples in class!!!