I have a table of standard potentials in water that gives a number of reactions and their e*reduction. Say I'm looking for the reaction Al(s) --> Al+3 + 3e-, but the book gives me the reaction Al+3 + 3e- --> Al(s) and a E*red value of -1.68. If this were to react with something else, would I first flip the second equation, including the sign, so I have it going in the correct direction, and THEN possibly flip it back if I find that this reaction is actually the oxidation half reaction?
I hope this made sense. Our teacher told us that when you have two half-reactions, the one with the more positive E*red will be the reduction half reaction, and the sign of the other half reaction is switched to make it an oxidation. My question is, do you still do the flipping if you have to flip the sign of the oxidation reaction anyway to make it go the right way?
Yes, the reverse reaction will occur if a more electronegative reduction is the other half reaction.
Yes, your understanding is correct. When you have a set of half-reactions and you need to combine them to form a full redox reaction, you will often need to reverse one of the half-reactions in order to achieve the correct overall reaction.
In the case you mentioned, the half-reaction given in the book is Al+3 + 3e- --> Al(s) with a standard reduction potential (E*red) of -1.68. If you want to use this half-reaction to form the reaction Al(s) --> Al+3 + 3e-, you will first need to reverse the given half-reaction. By reversing the reaction, the sign of the reduction potential also flips. So the reversed half-reaction becomes Al(s) --> Al+3 + 3e- with a new E*red value of +1.68.
Now, this new half-reaction represents the oxidation half-reaction. Since the book gave you the reduction half-reaction and its reduction potential, you can compare the reduction potentials of the two half-reactions to determine which one is the reduction and which one is the oxidation. As you mentioned, the more positive (less negative) reduction potential indicates the reduction half-reaction.
Therefore, the half-reaction Al(s) --> Al+3 + 3e- with E*red = +1.68 is the oxidation half-reaction, while the half-reaction Al+3 + 3e- --> Al(s) with E*red = -1.68 is the reduction half-reaction.
In summary, when combining half-reactions to form a full redox reaction, you may need to reverse one of the half-reactions to ensure the reaction proceeds in the desired direction. Furthermore, you can determine which half-reaction is the reduction and which one is the oxidation by comparing their reduction potentials. The more positive reduction potential represents the reduction half-reaction.