Lecture 10 - Entropy (Part 2), Electrochemistry

Lecture Recording:

The test: We covered 18.2~20.4 since the last test.

Lecture Summary/Notes:

Van't Hoff Equation

The Van't Hoff equation relates two different k values and T values.

Now, if we have a K1, a T1, and a K2, T2, plugged it into the starred equation and put them together, we'd have the following:

Partial pressure is proportional to the # of moles = x <- proportionality constant

Sum = 1 atm. The sum of these numbers (0.85x + 2(0.15x) = 1 atm. The sum of all the individual partial pressures adds up to 1 atm.

Redox Reactions

If we read a voltage meter, where the salt bridge was NOT present, it would read zero, since the charges are unbalanced. The salt bridge conducts the ions to balance the charge, and thus, the voltage would read 1.103V at 25'C. Voltage is caused by a REDOX REACTION. Zinc is oxidized to get Zn2+, and Cu2+ is reduced to Cu. (Reduction in oxidation state). 

For the diagram shown above, these two reactions (half reactions) would take place.

This is a cell diagram. Electrolytic cells are represented by it. The two lines in the middle represent the salt bridge. On the left is the anode (the substance being oxidized, oxidized state), and the right is the cathode (the substance being reduced, reduced state). Oxidation always occurs at the anode!

Note:  To balance a redox reaction

1. Balance every atom except Oxygen and Hydrogen
2. Balance Oxygens by adding water.
3. Balance Hydrogens by adding protons
4. Balance charge by adding electrons

Electrode Potentials (Eo)

What is a potential? In this case, it is a voltage DIFFERENCE. Like with energy, there is no such thing as an absolute voltage. Potential must have a reference point. For this purpose, we have standard electrodes (an electrode potential that we define as 0 volts). We can use said standard electrode as a reference point. The most common standard electrode is the Standard Hydrogen Electrode (SHE). It looks like this:

It consists of a tube, partially filled with a solution containing H+ (the concentration must be 1 molar), a platinum disk (used because of its inert properties - doesn't associate itself in most reactions. Platinum is also used because of hydrogen's great solubility in platinum. You can make 6, 8, 10, molar solutions of Hydrogen gas in solid platinum since platinum actually breaks the H-H bonds.). I'm not really sure what else, but apparently the solution bubbles Hydrogen....


*p. 872 gives us a table of reduction potentials. The chart tells you what the standard is (SHE in this case).


How do you use these numbers?


Basically, if you have an electrochemical cell, you can take any two of the processes, and you can predict the voltage at standard conditions (25'C and 1M). But remember, since all the numbers are REDUCTION potentials, and you have to have one oxidation process and one reduction process to have a redox reaction. So what you do is reverse the sign of one (i.e. multiply by (-1)) to reverse the reaction, getting an oxidation potential...

You don't multiply the potential by 5, as you do with enthalpies, since voltage is POTENTIAL, not energy.  i.e. Voltage = Joules/Coulombs, or energy/charge. If you multiply V by 5, you have 5 times as much energy, but you also have 5 times as much charge! They cancel out, giving you back your original number. Also, the number is negative, since the sign was reversed (oxidation).

E0cell came out to be positive. That means that under standard conditions (the 0 above the e), if I had 1 mole of H+, 1 mole of Chromium II, and 1 atm of Oxygen above the solution, 1 mole of Chromium III, and pure water is the solvent, I would have 1.653V generated by that reaction! Since it is positive, the reaction will go to the right as written - spontaneous as written. If the number was negative, the reaction would go reverse! If the reaction potential was 0, the reaction would be at equilibrium.

Conduction/Work

If you have some kind of material, and you put two electrodes on either side of it. One will be negative, and the other will be positive. Under the influence of the voltage, the electrons will be pushed from the negative side to the positive side:

Conduction is caused by electrons moving... 

How much energy is dissipated by that material under the influence of that voltage?

The energy dissipated, which is equal to the work done by the current passing through it, is equal to nFE. E is the voltage, F is Faraday's constant, and m is the number of moles.

W = nfE (moles x C/moles x J/C = J)

How much electrical work is done on that block of copper, upon passing 10 moles of electrons, and 2.70V?

2.61…. Is the amount of energy dissipated by a current passing through the block, amounting to 10 moles of electrons, under the electrical force of 2.70V.

If the source of those electrons is a REDOX reaction (i.e. electrons came from a battery), we substitute the W for the energy that is sourcing the electron! i.e. W = - delta G. (Not H, cause delta G is the free energy, and delta H is the exothermicity).

What about for Equilibrium conditions? What about the equilibrium constant? There is a relationship between delta G and the equilibrium constant: