Question

1A: In 1999, scientists in israel developed a battery based on the followin cell reaction with iron(VI), nicknamed "super iron:" 2K₂FeO₄(aq)+3Zn(s)=Fe₂O₃(s)+ZnO(s)+2K₂ZnO₂(aq). In this reaction, identify the:

-species oxidized

-oxidizing agent

-in order to be a bettery, what must be true of E^o in the forward reaction?

1B: An electrolytic cell is based on the Cu²⁺/Cu and Ag^+/Ag Half cells.

-Write the half reactions and the overall reaction

-Calculate E^o for this cell.

Answer 1

1A

The reaction is

2K2FeO4(aq)+3Zn(s)=Fe2O3(s)+ZnO(s)+2K2ZnO2(aq)

Clearly the oxidation of Fe in K2FeO4 is +6 and in Fe2O3 its +3 so, its getting reduced as oxidation state got decreases so, its the oxidising agent.

The Zn on the other hand has gained +2 oxidation state in ZnO compared to Zn so, Zn got oxidised.

In order to be a battery the forward reaction must be spontaneous which is the case when standard potential of the reaction is positive or E0 > 0

2A

The half cell chemical reactions are -

Ag+ + e -> Ag ---- E1 = 0.8 V (Chemistry Libre text)

Cu+2 + 2e -> Cu ----- E2 = 0.34 V (Chemistry Libre text)

To create a electrolytic cell the net voltage of the cell must be negative .

So, we need to reverse the reaction #1 and then add it to the second one but before that we must make sure that the electrons cancel out when we add them.

So, we multiply reaction #1 by 2 to get

2Ag+ + 2e -> 2Ag ---- G1 = -2FE1 = -2*F*0.8 V = -1.6F V

Cu+2 + 2e -> Cu  ----- G2 = -2FE2 = -2*F(0.34 V) = -0.68F V

G is the change in Gibbs free energy G = -nFE where n is the no. of electrons involved in the hald reaction. F is the Faradays constant, E is the standard electrode potential.

Reversing reaction 1 we get

2Ag -> 2Ag+ + 2e ----- G1' = -2F(-E1) = -2*F(-(0.8 V)) = 1.6F V

Now adding this with reaction 2 we get

Cu+2 + 2e + 2Ag -> Cu + 2Ag+ + 2e

G = -2FEnet =  G1' + G2 = -0.68F V + (1.6F V)

Cancelling out common electrons

Cu+2 + 2Ag -> Cu + 2Ag+

G = -2FEnet = -0.92F V

So, Enet = 0.92F / (-2F) V

Enet = - 0.46 V

So, standard potential for the overall reaction is Enet = -0.46 V

Negative value represents that the reaction needs external voltage to occur and thus its an electrolytic cell