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Writing a cell diagram, electrodes

Posted: Mon Feb 09, 2015 1:38 pm
by Madison Davis 3F
A galvanic cell

Zn + 2NO3(-) + 4H(+) --> 2NO2 + 2H20 + Zn(2+)


The cell diagram is:

Zn| Zn2+ || NO3- , H+ | NO2 | Pt (s)

How do you know where to put the electrode Platinum?
Also, why is the H+ included but not the water?


Thanks!

Re: Writing a cell diagram, electrodes

Posted: Mon Feb 09, 2015 2:25 pm
by Sarah H Brown 1L
Platinum is an inert conductor that must be in the cell with the half reaction that has no conducting solids. For example, Zn(s) is a conducting metal so Pt is not needed in the Zn cell. In the other cell, N and O aren't conducting metals so Pt added to conduct electrons from cell to cell. Pt doesn't participate in the reaction. A good trick if you are unsure is to check the states of the participants in the half reactions. If there are no solids and everything is (g), (l), or (aq), you definitely don't have a metal conductor and need to add Pt.

Re: Writing a cell diagram, electrodes

Posted: Mon Feb 09, 2015 2:27 pm
by Sarah H Brown 1L
My response to this previous question https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=140&t=5207&sid=a8c15a447679e63652a57835cb025e98 answers the second part of your question.

Re: Writing a cell diagram, electrodes

Posted: Tue Feb 10, 2015 12:26 am
by Justin Le 2I
Don't forget to write out the states when you're doing the cell diagram. That will make it much easier.

Re: Writing a cell diagram, electrodes

Posted: Tue Feb 10, 2015 5:19 pm
by Amy Luong 1L
For the cell diagrams, why don't we include the numbers of the electrode (for ex: Midterm 2009 we put lH+l when there are 2H+'s?
And is it acceptable to place a comma between two of the ions rather than another line?

Re: Writing a cell diagram, electrodes

Posted: Tue Feb 10, 2015 6:08 pm
by Sarah H Brown 1L
Only place a line if the compounds/anions/cations/elements are in different phases! A comma is used in-between if the pieces are in the same phase. Between ions, there should be commas because ions are aqueous.

Re: Writing a cell diagram, electrodes

Posted: Tue Feb 10, 2015 8:12 pm
by Madison Davis 3F
Thank you so much! Would there be a time when you don't need to place the Pt in the equation? If there is, how do you know?

Re: Writing a cell diagram, electrodes

Posted: Tue Feb 10, 2015 8:23 pm
by Sarah H Brown 1L
You don't need Pt (s) if the electrode is a conducting, solid metal such as Zn (s) and Cu (s). So look for the clues--first you need something in the solid phase. Then, check if it is a metal. In the case where the electrode is a conducting solid metal, the electrode participates in the redox reaction.

Remember, if an electrode has only gas and aqueous phases, you definitely need that inert conductor such as Pt (s). This conductor does not participate in the reaction.