Redox Lab question...

A few people have asked about the chemical reactions for the Redox lab hand-in {Redox Hand-in}, and this is often a source of confusion for students, so let me answer it here. First of all, I think a number of people get confused because you're trying to over-think the question and make it more complex than it really is.

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.

Voltaic Cells

Not a lot of information right now, but I just finished drawing a voltaic cell diagram.  Not perfect, but I'm pretty satisfied with it.

Redox - Definitions

If you want to understand chemistry, you have to follow the electrons. If electrons are transferred during a chemical reaction (as opposed to just being rearranged...), then a reduction-oxidation process is taking place. To help keeping track of the electron transfer in redox processes, we can use a couple acronyms/mnemonics related to the definitions of reduction and oxidation.

OIL / RIG - “Oxidation Is Losing electrons” / “Reduction Is Gaining electrons”

LEO / GER - “Losing Electrons is Oxidation” / “Gaining Electrons is Reduction”

Hmm, why is “reduction” associated with gainingelectrons? Remember, electrons are negatively charged, so gaining electrons increases the number of negatively charged particles associated with an atom which reduces its net charge. But are we really looking at “charge” to determine redox chemistry? Sometimes it seems like it, but other times the charge doesn't seem to line up with the processes. We really have to look at oxidation number, which is related to charge, but a with some subtle differences. One way to distinguish charge and oxidation number is that “charge” can be used to describe the net overall balance between electrons and protons in a system that might contain multiple atoms, but “oxidation number” describes the balance between electrons and protons for each individual atom in a structure regardless of its size. Oxidation numbers sound important enough for their own post, look for it soon.

Another thing that can cause some mix-ups is the term “oxidation” or {in verb form} “oxidize”. {Or for the British English spellers in the crowd, “oxidise”.} The element oxygen is very often involved in redox reactions. Is oxygen usually undergoing oxidation or reduction? You know you want to say oxidation, the words look so similar... But if we start with molecular oxygen, O₂(g), it's almost always going to gainelectrons. Gaining Electrons is Reduction. GER, indeed! The important thing to remember here is that reduction and oxidation are ALWAYS coupled processes. You can't have one without the other. This leads to some other terminology...

The process of one substance undergoing oxidation causes something else in the system to be reduced. The substance that is being oxidized is the reducing agent or reductant because it is causing reduction to take place.
The process of one substance undergoing reduction causes something else in the system to be oxidized. The substance that is being reduced is the oxidizing agentor oxidant because it is causing oxidation to take place.

So if oxygen is usually being reduced, it is a good oxidizing agent.