CATHODE VS ANODE HALF REACTION HELP!!!!!

Shelbyhamilton 3H
Posts: 37
Joined: Fri Sep 26, 2014 2:02 pm

CATHODE VS ANODE HALF REACTION HELP!!!!!

Hi so I have a problem where there are two positive cell potentials and the larger cell potential is actually subtracted from the smaller one creating a negative cell potential. I thought we could either decide which one was the cathode/anode by the oxidation numbers or by flippping the smaller half reaction.
So this is question 8 in the 2014 midterm.
using the standard cell potentials
f2 (g) + 2H+ (aq) + 2e- ---> 2HF (aq) Eo = +3.03 V
f2 (g) + 2e- ---> 2F- Eo= +2.87 V
calculate Ka for HF.

so then they write
HF--> H+ (aq) + F- (aq)
what confuses me is that they flip the first equation even though its the bigger one..... this results in e(Cathode)- e (anode) = 2.87-3.03. now our cell potential is -.16 v!!!
why is this?? how come we made the reaction with the bigger cell potential the anode?!?!?!?!

Kayla Denton 1A
Posts: 106
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

You want Ka to be EDIT: [H+][F-]/[HI] because the hydrogen concentration should be at the top for the Ka expression. Therefore, the H+ and F- must be the products so you have to write the reaction this way!
Last edited by Kayla Denton 1A on Wed Feb 11, 2015 10:46 am, edited 1 time in total.

Shelbyhamilton 3H
Posts: 37
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

so we don't follow the "flip the smaller one" rule when finding ka? and does ka only mean aqueous stuff?

Shelbyhamilton 3H
Posts: 37
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

also, how come you want HF in the top anyway? why was the equation HF---> H+ + F- written and not H++ F- ---> HF written?

Kayla Denton 1A
Posts: 106
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

No you don't. And yes, omit liquids and solids! Do this for ALL Qs and Ks, not just Ka.

Edit: you want HI in the bottom because it's a reactant!

All you need to remember is that [H+] must be on the top. Then go from there.

Shelbyhamilton 3H
Posts: 37
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

but you admit gases too?? i thought we could use gases

Chem_Mod
Posts: 17810
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 405 times

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

Given standard reduction half reactions, if you were asked to determine the redox reaction then make it the largest possible voltage when deciding what the redox reaction is.

Given standard reduction half reactions, if you were asked to design a Galvanic cell or battery then make it the largest possible voltage when deciding what the redox reaction is.

But if you are given the redox reaction then that is what you use (and one can have Eo a negative value and E a positive value).

Kayla Denton 1A
Posts: 106
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

Don't omit gases. But F2 isn't present in the OVERALL reaction, that's why it's not in Ka.

Justin Le 2I
Posts: 142
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

You can have the Ecell be negative. It just depends on what the question is asking.

Shelbyhamilton 3H
Posts: 37
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

SO just to clarify.....H+ is always on top when finding kA?

Justin Le 2I
Posts: 142
Joined: Fri Sep 26, 2014 2:02 pm

Re: CATHODE VS ANODE HALF REACTION HELP!!!!!

Ka means a substance is dissolving in solution, so the equation will look like HF---> H+ + F-