Electrochemical Cells


Voltaic vs Electrolytic Cells

There are two kinds of electrochemical cells - voltaic and electrolytic. Know the difference. I'll do my best here to get you to understand the difference.

voltaic

  • a spontaneous chemical reaction provides the ENERGY to power an external device
  • all batteries are voltaic cells
  • some folks call voltaic cells galvanic cells (same thing)
  • the reactants of a voltaic cell are high energy vs the products - so the reaction is really "wanting" to go forward, hence a positive potential.
  • if it's a voltaic cell, it should have a positive potential (+volts)
  • cathodes and anodes

electrolytic

  • an external power supply is used to force a non-spontaneous reaction to occur
  • the reactants of an electrolytic cell are low energy vs the products - it does NOT want to go forward at all
  • the potential of an electrolytic cell will be negative with no power applied
  • water likes being water forever and ever... an electrolytic cell will split water into hydrogen gas and oxygen gas - we spend energy to get a chemical reaction to go
  • cathodes and anodes

A Very Simple Voltaic Cell

So the Daniell Cell that was mentioned in section 7.01 is the classic electrochemical cell used as an example of an electrochemical reaction. Here are those reactions again.

reductionCu2+(aq) + 2eCu(s)
oxidationZn(s)Zn2+(aq) + 2e
net rxnCu2+(aq) + Zn(s) Cu(s) + Zn2+(aq)

Now we just need to split those reactions into two separate containers that will each contain a ½-reaction. We usually show this with two beakers - each with the appropriate solutions and electrode. The two containers are then linked via a salt bridge which is there to complete the circuit and maintain electroneutrality. Then you hook up your wiring to the electrodes and you've got an electrochemical cell. The wire can go to a voltmeter and show the voltage or you could hook it up to other things - maybe a light bulb if you have enough voltage and current. I've shown this cell below with the wire short circuiting the thing... the current flows directly unimpeded from the anode to the cathode here. A bit of a waste of energy but it DOES allow you to see how we "intercept" the electron transfer between copper(II) ions and zinc metal to get electric current.

I am also following a convention in that I'm putting the anode on the left side and the cathode on the right. Make sure you know the definitions of the two electrode here:

anode

the electrode where oxidation takes place (an ox)
⊖ if voltaic
⊕ if electrolytic

cathode

the electrode where reduction take place (red cat)
⊕ if voltaic
⊖ if electrolytic

Because oxidation is always at the anode, electrons will always leave the anode through the external circuit (wire). And in a likewise manner... because the cathode is always where reduction occurs, electrons will always enter the cathode from the external circuit (wire).

The Daniell cell happens to be a voltaic cell with a standard potential of +1.10 volts. Let's head to the next section to see how that works.

Shorthand Cell Notation

It gets a bit tedious drawing that picture of a cell up above. Once we have the basics of how an electrochemical cell works all we really need is the two half reactions that we are going to use to build it. This is where shorthand cell notation comes in. We still show dividing lines between the cell parts, but we do so in a nice inline manner so that we can easily depict a full cell on a single line. The Daniell cell above can be shown in shorthand notation like this.

Zn(s)
Zn2+(aq)
  
Cu2+(aq)
Cu(s)

Starting on the left and "reading" the cell notation across you start with the anode itself, Zn(s)... then the vertical line is a boundary for a phase change - here is means you are crossing over into the solution that the anode is in. Then you list all the necessary reactants and products to identify the half reaction. We only have to put zinc(II) down here because the half reaction is very simple - Zn oxidizes into Zn2+. Next up in the reading is double vertical bars which is shorthand for going into and then out of the salt bridge. You do not have to include what is in the salt bridge... just a double vertical bar. Now we are in the cathode solution where copper(II) ions are the "active ingredient" and that is all that is listed. Another vertical bar and we arrive at the cathode which in this case is copper metal. That is it! From start to finish and you can depict ANY cell this way. So to summarize in a very general way you have this.

anode
anodic solution
  
cathodic solution
cathode

Always Remember!

ANODE
on the
←LEFT

  
CATHODE
on the
RIGHT→


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