electrochem more fun facts and concepts

Spontaneity and Electrochemistry For those wondering how to determine spontaneity in redox reactions, this is how to do it: We know that ΔG° determines spontaneity and we can manipulate the equation used to determine it to incorporate E°. The equation we can use is ΔG°=-nFE°, where n is the mols of electrons exchanged, F is Faraday’s constant (96500 C/mol of e-), and E° is the standard potential. In case E is given instead of E°, we can find ΔG instead with ΔG=-nFE.

Spontaneity and Electrochemistry Suppose we want to find E° for a cell, but one of the two reactants is an unknown compound. However, let’s say that these two reactants are at their standard state. If we know their equilibrium concentrations at the standard state we can find E°.

Spontaneity and Electrochemistry To do this, we must manipulate the equation ΔG°= -nFE°. Think back to entropy. ΔG° also equals –RTlnK. So if ΔG°= -RTlnK and ΔG°= -nFE°, than we can say that -RTlnK = -nFE°. From that we can derive that E° = RTlnK/-nF As you can see, K is the only real unknown here and solving for it is easy enough given the concentrations at equilibrium.

Spontaneity and Electrochemistry A useful picture to have when talking about ΔG° and E° is:

More Manipluations! Suppose we have reactants that are not at their standard states. We can’t use standard reduction potentials to find E° of the cell. Instead, we must find E of the cell. In this case, we’ll have to think back to ΔG and relate it to ΔG°. The equation for that is ΔG = ΔG° + RT ln Q. We know that ΔG°= -nFE° and ΔG=-nFE, therefore we can change ΔG = ΔG° + RT ln Q to - nFE = -nFE° + RT ln Q.

More Manipulations! If we divide both sides of that equation by -nF, we can get the equation E = E° - RTlnQ/nF. Since we are using E°, the temperature will be 25°C or 298K. This is a very famous equation called the Nernst Equation, which was made by a guy named Walther Nernst.

Electrolysis What is electrolysis? Electrolysis is the process in which electrical energy is used to cause a nonspontaneous chemical reaction to occur. An example of electrolysis is running a current through water in order to produce hydrogen gas.

Electrolysis Generally you will have to know electrolysis when a question asks how many mols of grams of something is produced when subjected to a current. The current most commonly used is Amperes, which are coulombs per second. Along with the information that 1 mole e - = 96,500 C, we can easily apply the idea of electrolysis.

Electrolysis Let say we have a solution of NaCl and we, for some reason, want Na(s). Suppose we run a current of 15 Amperes through the solution for 2 minutes. How many grams of Na(s) will we get? First of all, 2 minutes is really 120 seconds and 15 Amperes is really 15 Coulombs/sec. Thus, in 120 seconds, we ran a total of 120 * 15 coulombs or 1800 coulombs through the solution.

Electrolysis Given that 1 mole e - = 96,500 C, we can now easily find out how many mols of e - we ran through the solution. 1800C/96500C/mols e - = mols e -. Since you only need one mol of electrons to reduce Na+, there are mols Na(s). To find grams of Na(s), just divide the mols by the molar mass, or /22.99 = 8.11E-4g Na(s)

So that’s that. We blame Apple for our aesthetically pleasing, yet still really boring looking PowerPoint. We also blame G because he made us teach this to you kids.