Writing cell-diagrams with other products such as H+...

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Regina Chi 2K
Posts: 51
Joined: Fri Sep 26, 2014 2:02 pm

Writing cell-diagrams with other products such as H+...

Postby Regina Chi 2K » Sat Feb 07, 2015 12:59 pm

There were many homework problems that illustrate my question; however, I came across an example in the 2014 practice midterm exam. This is problem 7C. When writing the cell diagram, how would we know when to include the H+ (aq) and H20 (l)? Do we always include the H+ if it is present in our equation? And why did they not include H20 (l)? And what would be the order that we put them in the cell-diagram? Thanks for all the help! :)

Sarah H Brown 1L
Posts: 56
Joined: Fri Sep 26, 2014 2:02 pm

Re: Writing cell-diagrams with other products such as H+...

Postby Sarah H Brown 1L » Sat Feb 07, 2015 10:30 pm

Okay, I did some serious Google searches and couldn't find anything explicit about H2O inclusion. After thinking about it (and chem mod/students, feel free to correct me on this if I am wrong), it seems that the elements/molecules are included that contribute to an oxidation number change or the overall charge on a side of a reaction.

When you balance the charge of the reaction
NO3-(aq) + 2H+(aq) + e- --> NO2(g) + H2O(l)

H+ and the other ions contribute to charge difference and therefore, electron balance.
The neutral NO2(g) molecule is important because it has a different oxidation number for N than NO3-(aq).

H2O doesn't really do much because it is neutral and doesn't contribute to electron balance and the oxidation number of either element doesn't change from one side of the reaction to another.

I think that is why it is not included in cell diagrams.

Regina Chi 2K
Posts: 51
Joined: Fri Sep 26, 2014 2:02 pm

Re: Writing cell-diagrams with other products such as H+...

Postby Regina Chi 2K » Sun Feb 08, 2015 8:26 am

Thanks! I'll keep that in mind, but where exactly do we know how to position it? Will it be in the front or back of our main species?

Sarah H Brown 1L
Posts: 56
Joined: Fri Sep 26, 2014 2:02 pm

Re: Writing cell-diagrams with other products such as H+...

Postby Sarah H Brown 1L » Sun Feb 08, 2015 11:15 am

I don't think it matters (as long as it is placed in the proper anode or cathode slot in the proper reactant or product slot). Just remember the anode is on the left and the cathode is on the right and each side is written in order of reactants and then products. Just make sure that the half cell reactions are displayed properly on either side.

oxidation reactant | oxidation product || reduction reactant | reduction product

For example, you have:

2I- --> I2 + 2e- (Oxidation)

and

Cr2O72- + 14H+ + 6e- --> 2Cr3+ + 7H2O (Reduction)

The balanced reaction is:

Cr2O72- + 14H+ + 6I- --> 2Cr3+ + 3I2 + 7H2O

The cell diagram is:

(Pt) | I-, I2 || Cr2O72-, H+, Cr3+ | (Pt)

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

Re: Writing cell-diagrams with other products such as H+...

Postby Niharika Reddy 1D » Sun Feb 08, 2015 11:25 am

As a side note, make sure you include states in the cell diagram!


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