9.2: Electrochemistry
This section will focus on electrochemistry, which is the exploration of electricity found in chemistry. We study how chemical energy can be converted into electrical energy and vice versa, to this, we study 2 different kinds of batteries and how they are able to work.
Electrochemical Cells
> There are 2 types of electrochemical cells.
> Voltaic (or galvanic) cell converts chemical energy from an exothermic process to electrical energy.
> Electrolytic cell converts electrical energy into chemical energy.
> Each cell contains an electrode.
> Electrodes are the metal part of the cell that make contact with the solution to conduct electricity.
> The solution is known as an electrolyte, which donates the electrons to the electrode.
> There are 2 electrodes in both electrochemical cells. The cathode is the one that reduces while the anode is the one that oxidizes.
> In a Voltaic cell, the cathode is positive and the anode is negative but in the electrolytic cell, it's the other way around.
Voltaic(galvanic) Cell
> The electrode in a voltaic cell is a metal/metal-ion electrode.
> The electrode is metal and then the solution is the ion of that metal.
> The most common are iron(Fe)(s)/ iron ion(Fe2+)(aq), Zinc(Zn)(s)/ Zinc Ion(Zn2+)( aq), Copper(Cu)(s)/ Copper ion, (Cu2+)(aq) .
> Each cell has a salt bridge which has multiple purposes.
> First, it allows the reactions to act separately. Secondly, it provides a path of migration for the cations and anions. Lastly, it reduces liquid junction potential, the voltage generated when the solutions come in contact.
> A typical salt bridge must not react with the ions and is usually either sodium sulfate(Na2SO4) or potassium chloride(KCl). Figure 2 shows a picture of a Voltaic Cell and here's a youtube video to help. Times 3:20-5:25 are most helpful.
How the Voltaic Cells Works
> The element that is higher on the activity series(figure 3) will be the one that is oxidized and is, therefore, the anode.
> In this case, zinc is higher so it will be the anode and copper will be the cathode. The electrons from the zinc bar will flow to the copper bar which will get bigger and the zinc bar will get smaller.
> The more electrons that are added, the more of the copper solution turn to copper.
>. The more electrons being lost from the zinc bar, the smaller it would be as it's losing zinc atoms and producing zinc ions.
> In the salt bridge, it's safe to assume it is a sodium sulfate bridge, sulfate ions would head towards both solutions while sodium would maintain balance.
> Overall, the current is created when the electrons move from the zinc to the copper. Zinc is oxidized and then copper is reduced. Its also important to consider the half-reactions which can be used to calculate the cell potential, discussing in Topic 19.
Electrolytic Cells
> Electrolytic cells use electrolysis, the process of splitting a molecule into its separate atoms, to separate the chemical solution into its original components and also convert electrical energy into chemical energy.
> It is much smaller than a Voltaic cell and is a simpler structure. Electrolysis is the process of splitting a molecule into its separate atoms.
> It all takes place in a single container with 2 electrodes, an electrolyte, and a battery which is what pumps the electrons.
> In the most common example, which is the of molten salts like lead(II)bromide, the anode which is positive will attract the bromide ions and the negative cathode will attract the lead ions.
> Bromine ions will lose electrons and become Br(g) and the lead ions will gain electrons to become Pb(l). Bromine is oxidized and the lead is reduced. This is shown in figure 4.
Additional Resources
ChemLibre Texts offers a detailed explanation for all you chemistry needs and has published documents not only on electrochemistry, by everything. This link is specifically for electrochemistry.
This website offers a simple and easy to understand comparison of both electrochemical cells.
This pdf is has a few questions regarding both electrochemical cells and offers a historical context about electrochemistry.
This document I made has some Voltaic cell questions, similar to the ones we covered in class.
Here's another youtube video. The first 20 minutes are quite helpful.