Oxidation/Reduction Reactions

REDOX REACTONS!

All chemical reactions fall into two categories those that are redox and those that are not redox!

Redox reactions involve the transfer of electrons.

2Zn(s) + O2(g)  2ZnO(s) is an example of a redox reaction, watch the following video to see what happens to the Zn metal
and the oxygen gas.

http://www.youtube.com/watch?v=e6Xxz-VBE6s

Remember that ZnO(s) is an ionic compound composed of Zn2+ and O2- ions.
What is oxidation and reduction?


Oxidation

Early chemists thought of oxidation as the combining of an element with
oxygen, as in combustion.

Now we refer to oxidation as the loss of electrons.

In the previous example Zn loses two electrons, Zn(s)  Zn2+ + 2e-
Reduction
 Early chemists thought of reduction as the loss of oxygen from a
compound.
 Now we refer to reduction as the gain of electrons.
 In the previous example O must gain the electrons that Zn loses, O + 2e O2-
Redox reactions without oxygen:

Mg(s) + S(s)  MgS(s)

This reaction can be broken down into two steps:

Mg  Mg2+ + 2e-
(loss of electrons: oxidation)

S + 2e-  S2-
(gain of electrons: reduction)

H2(g) + Cl2(g)  2HCl(g)

Hydrogen and chlorine undergo the following changes to form a hydrogen
chloride molecule:
 H  H+ + e(loss of electrons: oxidation)
 Cl + e-  Cl(gain of electrons: reduction)
“LEO the lion says GER”
Elements can be oxidized or reduced depending on the reaction. Observe Iron
in the following reactions: (remember that Iron III oxide is an ionic compound)


Oxidation

4Fe(s) + 3O2(g)  2Fe2O3(s) - combustion or rusting

Fe  Fe3+ + 3e-
- Loss of Electrons - Oxidation
Reduction

2Fe2O3(s) + 3C(s)  4Fe(s) + 3CO2(g)-loss of oxygen

Fe3+ + 3e-
 Fe
-Gain of Electrons - Reduction
Oxidation and Reduction
must occur simultaneously:

If one element is oxidized then another must be reduced!

Example:
2AgNO3(aq) + Cu(s)  2Ag(s) + Cu(NO3)2(aq)

Net Ionic:

Half Reactions:
2Ag+ + Cu  2Ag + Cu2+
• Nitrates ions are spectator ions!

2Ag+ + 2e-
 2Ag
Reduction

Cu
 Cu2+ + 2e-
Oxidation

These reactions occur simultaneously and the electrons lost must equal
the electrons gained.

Remember that silver and copper II nitrate are soluble and hence are dissociated ions AgNO3(aq) is Ag+(aq) and NO3-(aq)

When silver tarnishes it forms a black silver sulfide compound according to
the following reaction:

2Ag(s) + S(s)  Ag 2 S(s)

Observe the following video and write the oxidation and reduction half
reactions:
http://www.youtube.com/watch?v=a6RR4kPsnlE&feature=related

Ag  Ag+ + 1e-
(oxidation)

S + 2e-  S2-
(reduction)
Chapter 20.1- Read pages
631  638

Complete question #1, #2 page 634

Complete question #3  #8 page 638

Complete Worksheet – Redox - 1