Answer :
Answer:
Only [tex]Fe^{3+}+Mg[/tex] gives spontaneous reaction.
Explanation:
A redox reaction will be spontaneous if standard reduction potential ([tex]E^{0}[/tex]) of the reaction is positive. Because it leads to negative standard gibbs free energy change ([tex]\Delta G^{0}[/tex]), which is a thermodynamic condition for spontaneity of a reaction.
[tex]E^{0}=E^{0}(reduction)-E^{0}(oxidation)[/tex]
Where [tex]E^{0}(reduction)[/tex] and [tex]E^{0}(oxidation)[/tex] represents standard reduction potential of reduction half cell and standard reduction potential of oxidation half cell.
(1) Oxidation: [tex]Mg-2e^{-}\rightarrow Mg^{2+}[/tex] ; [tex]E_{Mg^{2+}\mid Mg}^{0}=-2.38V[/tex]
Reduction: [tex]Fe^{3+}+3e^{-}\rightarrow Fe[/tex] ; [tex]E_{Fe^{3+}\mid Fe}^{0}=-0.04V[/tex]
So, [tex]E^{0}=E_{Fe^{3+}\mid Fe}^{0}-E_{Mg^{2+}\mid Mg}^{0}=(-0.04+2.38)V=2.34V[/tex]
Hence this pair will give spontaneous reaction.
(2) Similarly as above, [tex]E^{0}=E_{Pb^{2+}\mid Pb}^{0}-E_{Au^{+}\mid Au}^{0}=(-0.13-1.69)V=-1.82 V[/tex]
Hence this pair will give non-spontaneous reaction.
(3) Similarly as above, [tex]E^{0}=E_{Ag^{+}\mid Ag}^{0}-E_{Br_{2}\mid Br^{-}}^{0}=(0.80-1.07)V=-0.27 V[/tex]
Hence this pair will give non-spontaneous reaction.
(4) Similarly as above, [tex]E^{0}=E_{Li^{+}\mid Li}^{0}-E_{Cr^{3+}\mid Cr}^{0}=(-3.04+0.74)V=-2.30 V[/tex]
Hence this pair will give non-spontaneous reaction.
The Fe-Mg pair of reactants will react spontaneously without any extra activation energy (ie. thermal or light).
What are Spontaneous Reactions?
- It is the reaction in which reactants react with each other without any extra activation energy (ie. thermal or light).
- The standard reduction potential is positive for spontaneous reaction.
[tex]E^0 = E^0_{rd} + E^0_{ox}[/tex]
Where,
[tex]E^0[/tex] - standard reduction potential
E^0_rd - standard reduction potential of reducing reactant
E^0_ox - standard reduction potential of oxidizing reactant
For, Fe - Mg pair-
E^0_rd of Iron = -0.04 V
E^0_ox of Mg = -2.38 V
Put the values in the formula,
[tex]E^0 = -0.04 + (-2.38)\\\\E^0 = 2.34 \rm \ V[/tex]
Therefore, the Fe-Mg pair of reactants will react spontaneously.
Learn more about Spontaneous Reactions:
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