electrochemistry

Question

Which of the following reacts spontaneously with Mn 2+ according to the redox table?

a. Na b. Ag c. Mg 2+ d. Ni

Answer 1

If the Ecell is positive, G is negative, and it's spontaneous (and a galvanic/voltaic cell, which is spontaneous)

From the standard reduction potential table:

MnO4–(aq) + 8H3O+(aq) + 5e– → Mn2+(aq) + 12H2O(l) E=+1.507 V

Other reactions:

Na+(aq) + e– → Na(s) E=+2.71 V

Ag+(aq) + e– → Ag(s) E=+0.80 V

Mg2+(aq) + 2e– → Mg(s) E= -2.38 V

Ni2+(aq) + 2e– → Ni(s) E= -0.26 V

If Mn2+ is reacting it must be a reactant, so it's on the left side of the arrow:

MnO4–(aq) + 8H3O+(aq) + 5e– → Mn2+(aq) + 12H2O(l) E=+1.507 V

So the Mn-containing reaction must be flipped, which changes the sign of E from (+) to (-)

Mn2+(aq) + 12H2O(l) → MnO4–(aq) + 8H3O+(aq) + 5e– E_ox= -1.507 V

Now, since the Mn reaction is oxidation, the other reaction is reduction. Ecell is the Eox added to the Ered, so add the values, and see which is positive:

Ecell=(-1.507)+(2.71)=+1.203 V

Ecell=(-1.507)+(0.8)=-0.707 V

Ecell=(-1.507)+(-2.38)= -3.887 V

Ecell=(-1.507)+(-0.26)= -1.767 V

Only the first Ecell from Na has a positive Ecell value, so that's the only spontaneous reaction.

The answer is choice A