which of these reactions if spontaneous?
(a) Fe^0 + Mg +2 --> Mg^0 +Fe+2
(b) Fe+2 + Mg ^0 --> Fe^0 + Mg +2
write the half cell equations for the spontaneous reaction.
which element is oxidized and which is reduced?
Look up the reduction potentials and calculate Ecell. If Ecell is positive the reaction is spontaneous.
The element oxidized is the element that lost electrons. Reduction is the gain of electrons.
To determine which of these reactions is spontaneous, we need to consider the standard reduction potentials of the species involved.
The standard reduction potential measures the tendency of a species to gain electrons and be reduced. The higher the reduction potential, the stronger the species' tendency to be reduced.
For reaction (a): Fe^0 + Mg+2 → Mg^0 + Fe+2
We can write the half-cell reactions as follows:
1. Fe^0 → Fe+2 + 2e- (oxidation half-cell)
2. Mg+2 + 2e- → Mg^0 (reduction half-cell)
For reaction (b): Fe+2 + Mg^0 → Fe^0 + Mg+2
The half-cell reactions for this reaction are:
1. Fe+2 + 2e- → Fe^0 (reduction half-cell)
2. Mg^0 → Mg+2 + 2e- (oxidation half-cell)
Now we need to compare the standard reduction potentials for the reduction half-reactions listed above.
The standard reduction potential of Fe^0 → Fe+2 is -0.44V.
The standard reduction potential of Mg+2 → Mg^0 is -2.37V.
Looking at reaction (a), the reduction half-reaction (Mg+2 + 2e- → Mg^0) has a more negative reduction potential (-2.37V), which suggests it has a stronger tendency to be reduced. The oxidation half-reaction (Fe^0 → Fe+2 + 2e-) has a less negative reduction potential (-0.44V), indicating it is less likely to be oxidized.
Therefore, reaction (a) is spontaneous because the reduction half-reaction has a more negative reduction potential and the oxidation half-reaction has a less negative reduction potential.
In reaction (b), the situation is reversed. The reduction half-reaction (Fe+2 + 2e- → Fe^0) has a less negative reduction potential (-0.44V), suggesting it has a weaker tendency to be reduced. The oxidation half-reaction (Mg^0 → Mg+2 + 2e-) has a more negative reduction potential (-2.37V), indicating it is more likely to be oxidized.
Therefore, reaction (b) is not spontaneous as the reduction half-reaction has a less negative reduction potential, and the oxidation half-reaction has a more negative reduction potential.
To summarize:
(a) Fe^0 + Mg+2 → Mg^0 + Fe+2
Spontaneous reaction with the reduction half-reaction: Mg+2 + 2e- → Mg^0
(b) Fe+2 + Mg^0 → Fe^0 + Mg+2
Not spontaneous. The reduction half-reaction: Fe+2 + 2e- → Fe^0