Standard Reduction Potentials?
Guinness asked:
Consider the following standard reduction potentials of some commonly used metals:
?o (V)
Au3+ (aq) + 3 e- ? Au (s) +1.50
Ag+ (aq) + e- ? Ag (s) +0.80
Cu2+ (aq) + 2e- ? Cu (s) +0.34
Fe3+ (aq) + 3e- ? Fe (s) -0.036V
Al3+ (aq) + 3e- ? Al (s) -1.66
Mg2+ (aq) + 2e- ? Mg (s) -2.37
Using the reduction potentials one can compare the reactivities of metals towards oxidation- reduction processes. Evaluate the following statements and determine if they are True or False.
1. Cu (s) can be oxidized by Fe3+ (aq).
2. A positive reduction potential means that the metal is more stable in its ionic state (aq) than in its reduced state (s).
3. Al3+ (aq) can oxidize Ag (s).
4. Elemental aluminum (Al (s)) is more stable than elemental iron (Fe (s)).
5. Aluminum has a strong tendancy to be reduced and is therefore a precious metal.
In order to solve such questions, you must know these facts:
(a) Comparison of the reduction potentials: The ion having the highest reduction potential can be reduced easily.
RP for Au3+ is +1.50 V
RP for Cu2+ is +0.34 V
Au+3 can be reduced by Cu(s), but Cu2+ can not be reduced by Au(s).
(b) Comparison of the oxidation potentials: The metal having the highest oxidation potential can be oxidized easily.
(Reverse of reduction equations and potentials give the oxidation equations and potentials)
OP for Au(s) : Au —–> 3e- + Au3+ ……… – 1.50 V
OP for Cu(s) : Cu —–> 2e- + Cu2+ ……… – 0.34 V
- 0.34 > – 1.50, therefore Cu is oxidized by Au3+, but Au cannot be oxidized by Cu2+.
(c) If the ?o (V) is positive (+) for a reaction , this reaction is SPONTANEOUS, otherwise (-), the REVERSE reaction is spontaneus.
Let’s apply this to your 1st question.
(1)Cu (s) can be oxidized by Fe3+ (aq).
Without writing the equations to save time,
Cu will be oxidized. What is the OP of Cu? (- 0.34 V)
Fe3+ will be reduced. What is the RP of Fe3+? (-0.036 V)
The sum of these will give – 0.376 V. Since the ?o for the reaction is negative, Cu cannot be oxidized by Fe3+, or Fe3+ cannot be reduced by Cu. However the reverse can happen. Fe can be oxidized by Cu2+, or Cu2+ can be reduced by Fe.
If we apply (a) or (b) we also get the same result.
RP of Cu2+ > RP of Fe3+
Cu2+ has greater tendency to be reduced compared to Fe3+. Therefore Fe3+ cannot be reduced, and hence Cu cannot be oxidized. OR
OP of Cu < OP of Fe
Fe has a greater tendency to be oxidized compared to Cu. Therefore Cu cannot be oxidized, and Fe3+ cannot be reduced.
3rd question (similar type):
RP of Al3+ = – 1.66 V
OP of Ag = – 0.80 V
Since the ?o for the reaction is negative, Al3+ CANNOT oxidize Ag.
2nd question:
If the reduction potential is (+), oxidation potential is (-).
(+) shows the spontaneity. The ion can be easily reduced to metallic state, but it is difficult to oxidize it into ionic form. The statement is NOT TRUE.
4th question:
Elemental aluminum (Al (s)) is more stable than elemental iron (Fe (s)).
OP of Al = + 1.66 V
OP of Fe = + 0.036 V
The electron losing tendency of Al (necessary for oxidation) is greater than that of Fe. Therefore Al can be easily oxidized compared to Fe. Fe is more stable. The statement is NOT TRUE.
5th queation:
Aluminum has a strong tendancy to be reduced and is therefore a precious metal.
The statement should be given as;
Aluminum “ion” has a strong tendancy to be reduced and is therefore a precious metal.
Why?
Because metals are NEVER reduced. Only the metal ions can be reduced. Metals never gain electrons. Any way.
This is partly explained in (4). The reduction potential of Al3+ is – 1.66 V. Therefore it is difficult to reduce Al. Both (-) and high voltage value. It has not strong tendency to be reduced. Before the method invented by Charles Martin Hall, Al was a precius metal. But, now it is not. Both first and the second part of the staterment is NOT TRUE.