I have an electrochem based question. I'm trying to solve for E* based on a chart in my book. I know the equation for solving this is E= Eox + Ered. The chart in my book gives a list of reactions and a Ered value. My teacher told us that the greater, or more positive, Ered value would remain the same and that half reaction would be the reduction, while the smaller Ered value's sign would be flipped along with it's equation to produce an oxidation. My question is this: if my book gives me a reaction that's backwards (ie: I want the reaction Fe+2 + 2e- --> Fe and the book gives me Fe --> Fe+2 + 2e-), would I switch the whole equation around, including the sign, before using the "more positive" rule to figure out the reduction reaction, or would I leave it as is, backwards, and use the rule based on those numbers?

Question

I have an electrochem based question. I'm trying to solve for E* based on a chart in my book. I know the equation for solving this is E= Eox + Ered. The chart in my book gives a list of reactions and a Ered value. My teacher told us that the greater, or more positive, Ered value would remain the same and that half reaction would be the reduction, while the smaller Ered value's sign would be flipped along with it's equation to produce an oxidation. My question is this: if my book gives me a reaction that's backwards (ie: I want the reaction Fe+2 + 2e- --> Fe and the book gives me Fe --> Fe+2 + 2e-), would I switch the whole equation around, including the sign, before using the "more positive" rule to figure out the reduction reaction, or would I leave it as is, backwards, and use the rule based on those numbers?

Are you saying that the table values in your book are listed as oxidations (Fe==> Fe^+2+ 2e is an oxidation) or is the equation you show in the problem as an oxidation and the table values in the book are reduction values.

I just made up an equation on the spot to show that the equations in the book are the opposite of what I want. The table values are reduction.

OK. If the table values are all reductions then you make the decision by the rule as the reactions occur in the table. That rule is to tell you which equation is to be reversed and the sign changed. Then you make the reversal, change the sign, and add the E*ox + E*red for Ecell.

Answer 1

If the equation provided in your book is the opposite of what you want, you will need to reverse the equation, including the sign, before applying the "more positive" rule to determine the reduction reaction.

Let's take your example:
Fe + 2e- --> Fe+2 (book's equation)

You want:
Fe+2 + 2e- --> Fe (desired equation)

To match your desired equation, the book's equation needs to be reversed and the sign changed:
Fe+2 --> Fe + 2e-

Now you can use the "more positive" rule to determine the reduction and oxidation reactions. The reaction with the greater, or more positive, E*red value remains the same, meaning it will correspond to the reduction half-reaction. The reaction with the smaller E*red value will have its sign flipped and its equation reversed, corresponding to the oxidation half-reaction.

After determining the reduction and oxidation reactions, you can calculate E* using the equation E* = E*ox + E*red. This equation represents the overall cell potential, where E*ox is the potential for the oxidation half-reaction and E*red is the potential for the reduction half-reaction.

Remember to pay attention to the signs and directionality of the reactions when applying the "more positive" rule, as it determines the correct oxidation and reduction reactions.