1. Electrochemistry
Lesson 4
Using Oxidation
Numbers

2. Using Oxidation Numbers
If the oxidation number of the central atom increases going from left to right, its oxidation.
ClO2-  ClO4-
Increase is oxidation +3 +7
oxidation
reducing agent
4 electrons lost

3. Using Oxidation Numbers
If the oxidation number of the central atom decreases going from left to right, its reduction.
NO3-  HNO2 +5 +3
decrease is reduction
reduction
2 electrons gained
oxidizing agent

4. Label the oxidizing agent and the reducing agent.
ReO4- + IO-  IO3- + Re
Reduced Oxidized
Ox Agent Red Agent

5. Label the oxidizing agent and the reducing agent.
3As2O NO H2O + 4H+  6H3AsO NO
oxidized reduced
red agent ox agent

6. Predicting Spontaneous Reactions Using the Standard Reduction Chart
Does Au3+ react with Cl-?
If it does write the spontaneous reaction.

7. The top reaction is written forward- reduction
The bottom reaction is written in reverse- oxidation
(Au e  Au(s)) 2
(2Cl-  Cl2(g) + 2e-) 3
2Au Cl-  3Cl2(g) Au(s)

8. Does Ag+ react with Br-?

9. nonspontaneous

10. Some ions are on both sides of the table and are oxidizing or reducing agents. For example:
Fe  Fe2+  Fe3+
Sn  Sn2+  Sn4+   
Cu  Cu+  Cu2+

11. Does Sn2+ react with Cr?

12. spontaneous 3 (Sn2+ + 2e-  Sn(s)) 2 (Cr(s)  Cr3+ + 3e-)
Check both
spontaneous 3
(Sn e-  Sn(s)) 2
(Cr(s)  Cr e-)
2Cr (s) Sn2+  2Cr Sn(s)

13. Can you keep HCl in a Cu container? Explain!

14. nonspontaneous

15. Can you keep HCl in a Cu container? Explain!
Yes, nonspontaneous.
H+ is a weaker oxidizing agent than Cu2+ and Cu+.

16. Can you keep HCl in a Zn container? Explain!

17. Spontaneous- no
H+ is a stronger oxidizing agent than Zn2+

18. Write the spontaneous reaction.
2H e  H2(g)
Zn(s)  Zn e-
2H Zn(s)  H2(g) + Zn2+

19. Can you keep HNO3 in a Au container? Explain!

21. Can you keep HNO3 in a Au container? Explain!
Yes, nonspontaneous.
HNO3 is a weaker oxidizing agent than Au3+.

22. Can you keep HNO3 in a Cu container? Explain!

23. Take the strongest ox agent- higher
Take the strongest red agent- lower

24. Can you keep HNO3 in a Cu container?
No, spontaneous.
HNO3 is a stronger oxidizing agent than Cu2+.
Write the spontaneous reaction.
NO H e-  NO(g) H2O x 2
Cu(s)  Cu e- x 3
2NO H Cu(s)  2NO(g) H2O Cu2+