1. Redox Reactions.
Reduction
Oxidation

2. What is Redox? Loss Electrons = Oxidation Gain Electrons = Reduction
30/09/99
REDOX stands for REDuction/OXidation
Oxidation is often thought of as a combination of a substance with oxygen (rusting, burning)
Just like with acid/base definitions the definition of oxidation is expanded
Oxidation refers to a loss of electrons
Reduction refers to a gain of electrons
Oxidation and reduction always occur simultaneously
As a mnemonic remember LEO says GER
Loss Electrons = Oxidation
Gain Electrons = Reduction

3. What’s the point ? REDOX reactions are important in …
30/09/99
What’s the point ?
REDOX reactions are important in …
C3H8O + CrO3 + H2SO4  Cr2(SO4)3 + C3H6O + H2O
Purifying metals (e.g. Al, Na, Li)
Producing gases (e.g. Cl2, O2, H2)
Electroplating metals
Electrical production (batteries, fuel cells)
Protecting metals from corrosion
Balancing complex chemical equations
Sensors and machines (e.g. pH meter)

4. RedOx Oxidation: Reduction: Gain of oxygen Loss of electrons
Loss of oxygen
Gain of electrons
Increase in
oxidation
number
Decrease in
oxidation
number

5. Balanced symbol equation
4 Experiments:
Burning magnesium
Copper in silver nitrate solution
Chlorine solution and potassium iodide solution
Exploding hydrogen
Word equation
Balanced symbol equation

6. Oxidised – gains oxygen
2Mg(s) + O2(g)  2MgO(s)
Must be a redox!
Mg  Mg2+
Oxidised – loss of e-
+2e-
Put the
e- in.
O  O2-
Reduced – gain of e-
+2e-

7. Cu(s) + 2AgNO3(aq)  Cu(NO3 )2(aq) + 2Ag(s)
Complete the half-equations
Oxidised?
Reduced?
Cu  Cu2+
Oxidised – loss of e-
+2e-
Ag  Ag
Reduced – gain of e-
+e-

8. H2(g) + ½ O2(g)  H2O(g)
Covalent!
Need a new
definition.
No H+ or OH-

9. RedOx Oxidation: Reduction: Gain of oxygen Loss of electrons
Loss of oxygen
Gain of electrons
Increase in
oxidation
number
Decrease in
oxidation
number

10. Oxidation Numbers
The oxidation number of an atom in an element is zero. E.g. Mg in Mg, O in O2.

11. Oxidation Numbers F -1 O -2 H +1 Cl Oxidation state of C in CO2?
The oxidation numbers of atoms in a compound add up to zero. F -1 O -2 H +1 Cl
Oxidation state of C in CO2?
? – 4 = 0
? = +4
Put the +!

12. Oxidation Numbers F -1 O -2 H +1 Cl +2 Oxidation state of Mg in MgCl2?
The oxidation numbers of atoms in a compound add up to zero. F -1 O -2 H +1 Cl
Oxidation state of Mg in MgCl2? +2

13. Oxidation Numbers F -1 O -2 H +1 Cl -3 Oxidation state of N in NH3?
The oxidation numbers of atoms in a compound add up to zero. F -1 O -2 H +1 Cl
Oxidation state of N in NH3? -3

14. Oxidation Numbers F -1 O -2 H +1 Cl Oxidation state of S in SO42-?
The oxidation numbers of atoms in an ion add up to the charge on the ion. F -1 O -2 H +1 Cl
Oxidation state of S in SO42-?
? – 8 = -2
? = +6

15. Oxidation Numbers F -1 O -2 H +1 Cl -2 Oxidation state of S in S2-?
The oxidation numbers of atoms in an ion add up to the charge on the ion. F -1 O -2 H +1 Cl
Oxidation state of S in S2-? -2

16. Oxidation Numbers F -1 O -2 H +1 Cl -3 Oxidation state of N in NH4+?
The oxidation numbers of atoms in an ion add up to the charge on the ion. F -1 O -2 H +1 Cl
Oxidation state of N in NH4+? -3

17. H2(g) + ½ O2(g)  H2O(g)
Covalent!
Need a new
definition.
No H+ or OH-

18. RedOx Oxidation: Reduction: Gain of oxygen Loss of electrons
Loss of oxygen
Gain of electrons
Increase in
oxidation
number
Decrease in
oxidation
number

19. H2(g) + ½ O2(g)  H2O(g) H +1 O -2 Covalent! Need a new definition.
Covalent! +1 O -2
Need a new
definition.
No H+ or OH-

20. H2(g) + ½ O2(g)  H2O(g) H +1 O -2 Oxidised? Reduced? O – decrease+1 O -2
Oxidised?
Reduced?
O – decrease
in oxidation
number
H – increase
in oxidation
number

21. Oxidation Numbers and names
To avoid any confusion when an element can have several oxidation numbers, the oxidation number is usually mentioned in the compound’s name. In names like “elementate(X)”, the number refers to “element” and not the associated oxygens.
So if we look at some examples , we get the following names:-
KMnO4 potassium manganate(VII)
NaClO3 sodium chlorate(V)
POCl2F phosphorus(V) oxydichlorofluoride
NaH2PO3 sodium dihydrogenphosphate(III)
K2Cr2O7 potassium dichromate(VI)
Check the
numbers.

22. Mg is the reducing agent
LEO says GER :
- Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent.
- Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent.
Mg(s) S(s) → MgS(s)
Mg is oxidized: loses e-, becomes a Mg2+ ion
Mg is the reducing agent
S is the oxidizing agent
S is reduced: gains e- = S2- ion

23. OIL RIG

24. Not All Reactions are Redox Reactions
- Reactions in which there has been no change in oxidation number are NOT redox reactions.
Examples:

25. Trends in Oxidation and Reduction
Active metals:
Lose electrons easily
Are easily oxidized
Are strong reducing agents
Active nonmetals:
Gain electrons easily
Are easily reduced
Are strong oxidizing agents

26. Well done!