So we have seen so far when we looked at the Electron
Transfer Theory that Redox reaction involve the
transfer of electrons.
But we have also seen that when elements form
compounds or ions they do it so they can be more
stable…….So they don’t want to transfer electrons and
undergo redox reactions.
So then why do they do it? Any ideas?
Tug of War For Electrons
The two players in a redox reaction compete with each
Example:
other seeing who is strong enough to take the other
players electrons.
If one of the players is strong enough to win the
competition and take the others electrons a redox
reaction occurs….if not, nothing happens.
Example:
Zn
Oxidized
Cu
So which one is oxidized
and which one is reduced?
****Reduced: Gains electrons and its charge decreases (Less -) as
a result.
Oxidized: Loses electrons and its charge increases (more +) as a
result.****
Oxidizing and Reducing Agents (RA & OA)
Reducing Agent (RA): The reactant that causes the
other reactant to be reduced. (It actually gets oxidized)
Oxidizing Agent (OA): The reactant that causes the
other reactant to be oxidized. (It gets reduced in the
process)
RA & OA
Cu
Zn
Zinc gets oxidized
from Zn(s) to
Zn2+(aq)….it causes
Cu2+(aq) to be reduced
to Cu(s)……therefore
it’s the RA.
Copper gets reduced
from Cu2+(aq) to
Cu(s)….it causes Zn(s)
RA to
to be oxidized
Zn2+(aq)…therefore
it’s the OA.
OA
Recap of Oxidation/Reduction
So far we have learned that when a reaction occurs, one
reactant gets their valence electrons taken away and gets
oxidized.
AND
The other reactant takes the electrons and thus becomes
more negatively charge and gets reduced.
The reactant that gets oxidized and thus causes the other
reactant to become reduced is referred to as the Reducing
Agent (RA).
AND
The other reactant that gets reduced and thus causes the other
reactant to become oxidized is referred to as the Oxidizing
Agent (OA).
So….
You put two reactants….one solid and one not then one
will reduce and one will oxidized?
ERRRRR. Wrong
again…..Reactions
are NOT always
spontaneous.
Copper(II)
Nitrate….lots
of copper ions.
So there’s the question. How do we know if the
reaction will happen or not?
To Be or Not To Be……. Spontaneous
So to find out what combinations of solid reactants will
react spontaneous (right away as soon as their mixed) and
which will not…
We could just randomly mix different combinations of
metals and solutions and see which ones happen
spontaneously.
It would take forever….and why? Really, who has the time
for that?
And besides, somebody already did it for us…..so lets look
at the Redox table in your text book and learn how to use it.
That way we can predict without having to do all that work.
And Remember! Its not guessing…I prefer to call it
scientific guestimation…..sounds more scientific.
Redox Table and Ranking RA’s and OA’s
P. 569
Ranked According to Strength
So in this experiment scientists mixed a bunch of metals and metal ions
together to see which combinations would be spontaneous and which
would not.
They just tried every possible combination and listed them according to
which reactant work and caused a reaction the most times when combined
with another reactant. (The same way as you rank a team in a round
robin tournament in sports.
3
Metals/Ions
Ag(s)
Cu(s)
Pb(s)
Zn(s)
Ag+(aq)
Cu2+(aq)
Pb2+(aq)
Zn2+(aq)
Redox Table and Ranking RA’s and OA’s
P. 569
Ranked According to Strength
3
Metals/Ions
Ag+(aq)
Cu2+(aq)
Pb2+(aq)
Zn2+(aq)
Ag(s)
Cu(s)
Pb(s)
Zn(s)
Strongest
Oxidizing Agent
Weakest
Reducing Agent
Weakest
Oxidizing Agent
Strongest
Reducing Agent
Redox Table and Ranking RA’s and OA’s
Ranked According to Strength
Textbook Pg. 828
or
Data Booklet Pg. 7
“Table of Selected Standard Electrode Potentials*”
The table has all the possible redox reactions listed by relative
strength.
Along the right hand side all the Electrical Potentials E° (V) are
listed for each reaction as well…..
DON’T WORRY ABOUT THE E° (V), its used for something
else….they just list them their because its convenient.
You will also notice that on the bottom
“*For 1.0mol/L solutions at 298.15K (25.00 °C) and a pressure of
101.325 kPa”
Nice that they put it there…..BUT I HAVE NEVER USED IT FOR
ANYTHING.
It actually give ½ STP and ½ SATP….How useless.
Decreasing Strength
OA
If OA is above
RA on the table
SPONTANEOUS!
SPONTANEOUS
NOT SPONTANEOUS!
Decreasing Strength
RA
Predicting Redox Reactions
The first step to predicting a redox reaction is
to list all the species (Atoms/Ions/H+/OH-) that
are present (reactants only).
***REMEMBER to add H2O(l) as a species that’s
present because it’s a solution…ALWAYS!
***Keep an eye out to see if the example is
acidic or basic….if it is make sure to also
include H+ or OH-.
Predicting Redox Reactions
Example
HNO3(aq)
Au(s)
Did we get everything?
H+(aq)
Au(s)
H+(aq)
NO3-(aq)
NO3-(aq)
H2O(l)
H2O(l)
Predicting Redox Reactions
Example
OA
RA
RA
Next, you use the Redox table to assign all of
the entities as either an RA or an OA.
+ entities
- in the
***Look
the
table
Au(s) for H
NO3
H2O
(aq)
(aq)
(l) EXACTLY
as they are in your question…SAME STATE.
Predicting Redox Reactions
Example
Sixth,
rewrite
the
using
the ½
Fifth,you
balance
theSOA
number
of electrons
so both
OAtwo equations
balancing
add
the two equations
reactionsamounts
have the and
same
amount.
+ multiplying
- the whole
together.
You
by
Aubalance
H
NO3
H2O(l)½ reaction by a
(s)
(aq)
(aq)
number in front.
SRA
RA
RA
Fourth, write out the ½ reactions for the SOA and
exactly as they appear in the Redox table.
2SRA
X
Third, from your choice (SA/RA), choose the
strongest SA and
*****Remember
to OA.
write the SRA Right
To Left
+
2H
O

O
+
4H
+4e
2 (l)
2(g)
(aq)
(Backwards)***
Predicting Redox Reactions
Example
Last, we cancel out anything that shows up on both
sides.
4 NO3-(aq) + 8H+(aq)+ 4e-  2N2O4(g)+ 4H2O(l)
+ 2H2O(l)  O2(g) + 4H+(aq) +4e-
2
+
4 NO3-(aq) +8H+(aq) +2H2O(l) + 4e- 2N+2O4(g) + 4H2O
(l)+O2(g) + 4H (aq) +4e
2 X 2H2O(l)  O2(g) + 4H (aq) +4e
/
/
/
/
+
2H
O

O
+
4H
+4e
2 +8H
(l) +(aq)  2(g)
4 NO3-(aq)
2N2O4(g) + 2H2(aq)
O(l)+O2(g) + 4H+(aq)
Predicting Redox Reactions
Practice
Predicting Redox Reactions
Practice
SOA
SRA
Predicting Redox Reactions
Practice
1X
5X
Predicting Redox Reactions
Practice
Can A Reactant React/Redox With Itself?
If you have looked closely at your Redox table and
practiced assigning RA and OA to species in a redox
question... Which you all have…. Right?
Anyway, there ARE species that can act as either an OA
or an RA, and in fact, react with themselves.
This is called Disproportionation.
***If you run into a question where you find the SRA
AND SOA are the same species, its ok...your not
wrong!***
Just treat it like any other Redox and follow the
same 5 steps.