7.1 Redox Reactions
7.5 Stranger Things / Electrodes
7.10 Battery Facts
Ah, pop culture, who doesn't love it. There is a lot to think about when you are floating through the real world or IRL as certain gamers like to call it. A little history here: There is this thing called Netflix - a streaming service that started as a DVD by mail thing. Yeah, I remember when Netflix was only that. You'd order up a movie and a day or two later you'd get the DVD in your mailbox. You'd watch it and then send it back and get another movie. Worked amazingly well. And so it goes.
Then, one day, Netflix decided to let you just order the movie and watch it online streamed to your TV or computer or whatever. There were many of us who thought they were crazy to try and start a streaming video service - that seemed like a losing proposition. But here we are today, Netflix is a powerhouse of entertainment and shows no sign of weakening. They single handedly brought down Blockbuster and the like. This all just adds to the list of things that we do when we aren't learning all we can about chemistry... right?
I'd say I digress, but that ship sailed several sentences ago. Nonetheless... on Netflix in the summer of 2016 a show called Stranger Things made its debut/premiere. It was a huge hit. Did you see it? Season 4 is coming later this year. And... oh wait, this is the section on electrodes in my chemistry book. I'm having cognitive decline... sorry, yet another cultural reference with the whole political song and dance going on now (2020). I'll get to electrodes but I need to talk a bit more on Stranger Things. It is no big spoiler to say that there are two worlds in Stranger Things – the regular world and this thing called the upside down world. I've included a book cover as reference of this very thing.
You can clearly see the two worlds in the picture and the show wandered in and out of the two worlds. Of course the show is science fiction and creepy as all get out. I highly recommend it! The thing is/was those two worlds are clearly different and to get from one world to the other, you had to find a portal that joined the two. It is the idea of two worlds and a portal that I'm going for here. You see, this whole Stranger Things story line of mine is just a metaphor for the two worlds of electrochemistry and the portal there as well. Lets describe the two worlds first.
Such an old world where wiring and circuits reign supreme. The word itself goes all the way back to the 15th century. But we Americans like to hail good ol' Benjamin Franklin as Mr. Electricity with his key, kite string, and kite which was late 18th century. You see, electricity is OLD and has been around a long long time. Much sooner than anybody anywhere knew anything about atoms and protons and electrons. Electric current (I) was measured in amps and the charge passed was known as coulombs. They did not know the charge was negative electrons at all - it was charged particles or something and the charge was thought to be positive, not negative. And to this day, the direction of electric current is always the opposite direction as electron flow in a circuit. Why? Because the old-timers thought the charge carrier was positive. Electricity world also has potential measured in volts, power in watts, and even resistance, capacitance, and more. All that fun stuff that electricians do.
Well you are beyond knee-deep in a chemistry book so you should have an idea about chemistry world. There are reactants and products that do their thing in very specific ratios known as stoichiometry. Lots of chemistry is solution chemistry where we dissolve the reactants into a solvent and let them react there. We measure concentrations and temperature and we calculate amounts in moles of this and moles of that. We also have potential energy and we can measure it in joules or volts because we can do things in different ways and can convert units. Chemical reactions will proceed and go forward if the conditions and reactants are just right. We call that type of reaction spontaneous. Spontaneous reactions just tend to go forward all on their own. This is chemistry world.
The world of electricity and chemistry DO have an intersection and that intersection is electrochemical cells. The regular world is electricity - electrons moving in metal wires and carrying electric current. The upside down world is chemistry world with its own brand of electron pushing/pulling reactions known as redox reactions. How can those two worlds meet and interact?
Electrodes serve a very specific purpose and that is to channel all electron loses and gains from the reducing agent to the oxidizing agent out into and through electricity world. The electrode surface is the portal between electrons jumping on and off chemical species in solution to electrons zipping through the conduction band (energy level) of the metal wiring. Let's cover the two basic types of electrodes.
In chemistry world we count moles of species - including those electrons we show in our half-reactions. However, when a mole of electrons goes through the electrode portal to electricity world we then switch over to old school thinking in coulombs of charge zipping around a circuit. So what is the conversion factor for moles of electrons to coulombs of charge? The faraday! The faraday is shown as \(F\) and has a value of 96485 coulombs per mole of electons or C/mol e-. We'll be using it quite a bit in sections 7.7 and 7.8.
An active electrode will be made out of a substance that is participating in the redox reaction itself. An active electrode is reacting. Most of the metals on the periodic table can be active electrodes as long as the metal itself can lose electrons and form cations.
active electrodes participate in the redox reaction
A great example of an active metal is zinc, Zn. Zn is used in numerous battery types and is a great anode - meaning it reacts and gets oxidized. This actually has a positive potential:
Zn(s) → Zn2+(aq) + 2e– E° = +0.76 VThe best active metal electrodes tend to be the ones with the most negative E° values on a standard potential table. Why negative? Because all the reactions on a standard potential table are reductions and remember, we want a good oxidation for an anode reaction. So pick very negative and use the metal that is the product on the table. The very best metal like this is lithium which has an oxidizing potential (the opposite of the table) of E° = +3.05 V.
Don't discount metals as cathodes though. You just need to pick a positive standard potential reaction and you'll have a nice cathode reaction that is driven by the desire of the active metal cations to gain electrons and make the metal. More stable metals are used for this type of thing. Copper metal is fairly stable which means it is readily made from a solution of copper(II) ions. Silver is even better from a solution of silver ions. Here are the two reductions and their standard potentials.
Cu2+(aq) + 2e– → Cu(s) E° = +0.34 V
Ag+(aq) + e– → Ag(s) E° = +0.80 V
If you look closely, you'll see that there are lots of half reactions on our table of standard potentials that do NOT have any neutral metals in the reaction. Maybe the reaction is all non-metals, or maybe the metals that are in the reaction are all cations in aqueous solution. So if you think about it, none of those species can conduct electricity. So how do we make an electrical connection? We use an inert electrode. An inert electrode does not participate in the redox reaction, it is there purely to channel electrons in and out of the redox reaction... a true portal between electricity world and chemistry world.
inert electrodes do not participate in the redox reaction / they make the electrical connection only
Our inert electrode of choice is platinum, Pt. Platinum is fairly inert and doesn't really oxidize easily. Platinum also holds up well in strong acids, so it can be used in harsh conditions as well. Another good one is gold, Au. Gold is also fairly inert and doesn't oxidize easily, so it too is a good choice as an inert electrode. The last one is graphite, which is the conducting solid phase of carbon, C(s, graphite). Unlike platinum and gold which are very expensive, graphite is fairly cheap - so if your application will allow it, graphite is the poor man's platinum in the electrochemistry electrode world.
In a world of classrooms and lectures... (say that with a big deep voice over like in a movie trailer) there IS no paying for electrodes, everything is free... just write Pt and you've got it. Write Au and you've got it. It's much like playing online games - you can get away with murder on those things. Just remember that the rules are different IRL. Platinum and gold cost real money and you probably work for a real boss. Controlling costs is a real thing. Certain metal alloys can be made to act as really good and cheap inert electrodes. That is what is in a lot of mainstream batteries, alloys that are cheap.