Transcript JUST REMEMBER . . .

JUST REMEMBER . . .
“OIL RIG”
(oxidation is losing, reduction is
gaining)
REDOX
 In a REDOX reaction, when something is
being oxidized, it is called the reducing
agent, and when something is being
reduced it is called the oxidizing agent
Oxidation States
 Oxidation States are assigned to atoms to identify how
many electrons are either gained or lost by an atom
 For example metals in Group 2 (like Ca) have a +2
oxidation state. Therefore Metals in Group 2 lose two
electrons when they form compounds
 Changes in oxidation numbers indicate that a redox
reaction has occurred.
 It is important to learn the rules for assigning oxidation
states to atoms in order to determine whether
oxidation or reduction has occurred.
Oxidation State Rules
1) Free elements (not combined with
any other element) have an
oxidation number of zero.
Ex: Na, O2, H2
2) All metals in Group 1 have an
oxidation number of +1.
3) All metals in Group 2 have an
oxidation number of +2.
4) F (fluorine) always has an oxidation
of -1
5) The oxidation of simple ions is
equal to the charge on the ion. Ex:
Mg+2 has an oxidation number of
+2.
6) The sum of the oxidation numbers
must equal 0
Examples: NaCl & MgCl2
7) In polyatomic ions, the sum of the
oxidation numbers of all the atoms
must equal the charge of the ion.
Example: sulfate ion SO4-2 .
8) In general, oxygen has an oxidation
number of -2. Oxygen has an
oxidation number of -1 in peroxides
(O2-2) Example: H2O2. Oxygen has
an oxidation number of +2 in
compounds with fluorine Example:
OF2
9) Hydrogen has an oxidation number
of +1 in all compounds combined
with a non-metal. Hydrogen has
an oxidation number of -1 when it
is a metal hydrides (metal and
hydrogen. Example: LiH, and CaH2
Assigning Oxidation Numbers
Dr. McGuiness’ “bookend” technique to assigning oxidation
numbers to compounds with more than two elements.
1.
2.
3.
Identify the oxidation # of the last element (overall charge)
Identify the oxidation # of the first element (overall charge)
If there is no charge to the compound, then the overall
charge must be 0, therefore you can determine the oxidation
# of the element in the middle.
+1 x 1 = +1
Li(MnO4)
(+1) + (?) + (-8) = 0
-2 x 4 = -8
Half-Reactions
Example:
Cu + Ag(NO3) → Cu(NO3)2 + Ag
1. Assign oxidation numbers to everything
2. See which oxidation numbers change from reactant
side to product side.
3. Determine which half-reaction is oxidation (loses e), and which is reduction (gains e-)


OX: Cu0 → Cu2+ + 2eRED: Ag+ + 1e- → Ag0
Balancing REDOX reactions
Cu + 2Ag(NO3) → Cu(NO3)2 + 2Ag
 OX: Cu0 → Cu2+ + 2e RED: 2(Ag+ + 1e- → Ag)  2Ag+ + 2e-
2Ag0
 When copper is oxidized, it loses 2 electrons, which
are gained by the silver ion.
 Copper = reducing agent
 Silver = oxidizing agent
Activity Series – Table J
 Table J compares how active
each metal and nonmetal is.
 Metals higher up are more
active, and replace metals from
below them from compounds
(remember single
replacement???)
 In a single replacement, the
free element has to be more
reactive than the element in
compound in order for the
reaction to be spontaneous.
 If it isn’t – the reaction does
NOT GO!!
Electrochemical Cells
 There are two types of electrochemical cells, Voltaic &
Electrolytic
 These cells rely on REDOX reaction in different ways
to either generate energy or to separate elements in
compound that would normally not exist on their
own in nature.
Voltaic
Electrolytic
 Spontaneous
 Non-spontaneous
 Chemical  Electrical Energy
 Electrical  Chemical
 Anode – oxidation (-)
 Requires a battery as a
 Cathode – reduction (+)




source of energy
Example: BATTERY
 Anode – oxidation (+)
Electrons travel from anode to  Cathode – reduction (-)
cathode (wire)
 (+) charged ion moves
More reactive metal ALWAYS
toward the cathode
the site of oxidation
 (-) charged ion moves
Salt bridge is for the flow of
toward the anode
ions from one half-cell to
another
“RED CAT”
“AN OX”
Electroplating
 Another example of an
electrolytic cell.
 The process of electroplating
requires a layer of metal such
as silver or copper, coating or
covering any object to be
plated (spoon or fork)
 The item being plated is the
cathode - reduction/(-)
 The electrode must be the
same metal that you are
plating the object in.
Cathode (-)