So you need to draw a voltaic cell... you can draw this by hand, or you can draw it electronically. DO NOT just find one online and copy-paste it into your hand-in, if that's all I wanted I would have pasted this {voltaic cell} into the hand-in myself before I posted it.
The part that causes some confusion is the "...write a correctly balanced net ionic equation for the spontaneous process..." Don't over-read that! The metal cation solutions you were using in lab were probably nitrate salts, but nitrate (or whatever anion might have been present) was a spectator in all of your reactions. Net ionic equations are actually easier than full-formula equations because they're not cluttered up with with a bunch of extra stuff, and net ionic equations actually just describe the CHEMISTRY that's happening rather than distracting you with a bunch of spectator ions and species.
Now, I'm not going to write out a net ionic equation that's the exact answer to one that you have to write, but here's an example. Let's say I made up the voltaic cell Fe|Fe+3||Cd+2|Cd with the black/negative lead of my meter hooked up to the Fe(s) electrode and the red/positive lead connected to the Cd(s). The potential I measure is -0.32V. Because the measured potential is negative, the cell is running backwards, so the spontaneous cell reaction is Cd|Cd+2||Fe+3|Fe. Translating that into a reaction, we can write the two half-reactions as:
Cd(s) <=> Cd+2(aq) + 2 e-
3e- + Fe+3(aq) <=> Fe(s)
Adding those up gives the overall (or "net") reaction:
3Cd(s) + 2 Fe+3(aq) <=> 2 Fe(s) + 3 Cd+2(aq)
{Remember to multiply each half-reaction by an appropriate integer to make all the electrons cancel...}
That's a "correctly balanced net ionic equation for the spontaneous process" in this case. Now go do that for all the cells you measured in the experiment.
That is helpful. My question is on number 3 and I figured it'd be easiest to post on here. Do we need to post the actual cell potential of every reaction that happened and calculate the predicted potential of every cell and show the percentage difference of every cell?
ReplyDeleteQuestion 3 is related to parts IID and IIE of the experimental procedure. These are NOT the cell potentials calculated from tables of standard reduction potentials, they are the potentials that you calculate for pairs of metals that are not adjacent to one another in your activity ranking based upon the relationships you determine in parts IID and IIE. For example (similar to the example in IIE), you can calculate an expected potential for the M1-M3 cell from the relationship you determine, but you also directly measured the potential of the M1-M3 cell... Compare the expected/predicted/calculated cell potential to your measured potential by calculating a percent difference.
ReplyDeleteAnd yes, I'm being a little vague on purpose because I want you to look at your data and think about the relationships. Good luck.
Lets say we have four metals, much like in the problem. The ranking is A, B, C, D. Obviously, I am going to calculate a percent difference of my expected/predicted/calculated for a cell between A and C. However, do you still want us show a percent difference between observed and predicted voltages for a cell of lets say A and B, since they are adjacent in the ranking? I apologize for all the questions, I'm just confused with the wording of the question. Thanks!
ReplyDeleteYou are going to calculate the expected potentials for non-adjacent cells (A-C, A-D, B-C) using the measured potentials of the adjacent pairs (A-B, B-C, C-D) and the relationships you determined in IID and IIE.
ReplyDeleteYes, the wording of that question is a little awkward, largely because it's a rather difficult question to ask without giving away the answer. The purpose of that question is to have you discover the relationship between the potentials of these different cells, not just test how well the equipment worked, so it can be a little stumbly when I'm trying to ask it...
Is addition all you need for the calculations in this part of the hand in? Or is it supposed to be more complicated than that?
ReplyDeleteIt just might be... What does your data show? Not every problem requires complex formulas or pages of calculus. Remember, reduction potentials are a thermodynamic quantity... How are {delta}G for a stepwise process related to the {delta}G for an overall process?
ReplyDelete