HW prob 13.11d:Writing the half reactions for galvanic cells

[Mia-Kyla Coronel 2N]

13.11(d)

Pt(s) | O2(g) | H+ (aq) || OH-(aq) | O2 (g) | Pt (s)

In the solutions manual the end answer is
4H20(l)-->4H+(aq) + 4OH- (aq)

what happened to the O2?

Thank you in advance!

[Chem_Mod]

To solve this problem, balance the following half-reactions from the cell diagram:

O₂ —> H⁺
O₂ —> OH^-

If you balance the first reaction as a standard reduction reaction in acidic solution, you get:
O₂ + 4H⁺ + 4e- ---> 2H₂O.

If you balance the second reaction as a standard reduction reaction in basic solution you get:
O₂ + 2H₂O + 4e- ---> 4 OH^-
In a cell diagram, the anode is always on the left of the double lines and the cathode is on the right. This means the species on the left are involved in the oxidation half-reaction and the species on the right are involved in the reduction half-reaction.

Therefore:
Anode (oxidation): 2H₂O ---> O₂ + 4H⁺ + 4e-
Cathode (reduction): O₂+ 2H₂O + 4e- ---> 4 OH^-

Adding the above two half reactions gives the balanced redox reaction: 4H₂O(l) ---> 4H⁺(aq) + 4 OH^-(aq)

[Joyce 3H]

One question on the answer: should we eliminate the coefficient 4 of all the reactants and products of the balanced reaction? Or it doesn't matter? Thank you!

[Chem_Mod]

Yes, you can eliminate the coefficients and write the balanced equation as: H20(l) --> H+(aq) + OH-(aq)

[Shelbyhamilton 3H]

In the standard reduction reaction in the acidic solution, how did you get the H+ onto the reactants side with the O2???

[Niharika Reddy 1D]

We initially have O₂ →H⁺

We first balance the 2 oxygens by adding 2H₂O:
O₂ →H⁺ + 2H₂O

Now, balance the hydrogens by adding H⁺. There are 5 hydrogens on the right and none on the left, so add 5H⁺ to the left.
5H⁺ + O₂ →H⁺ +2H₂O

Since H⁺ appears on both sides, we can simplify by canceling the one on the right out and lowering the number on the left by 1:
4H⁺ + O₂ →2H₂O

Add 4 electrons to the left to balance the charges:
4e^- + 4H⁺ + O₂ →2H₂O

[Shelbyhamilton 3H]

Thank you that is so helpful :) I forgot about that last step!

[Victoria Ford 4D]

How do you know whether to balance the equation in acidic or basic solution?