Relative Reduction Potential Series of the Elements

DISCUSSION. I have spent some time on the results of this week's lab. While there was a rather random array of data with regard to those elements many of you have indicated with "equal signs" (no movement of the needle), you do seem to have a strong concensus that C, Au and Pt held on to their electrons much more strongly than the other elements tested, and that Zn and Mg would rather easily release their electrons.

So it seems that those elements which tend to last uncorroded in the shipwrecks are at the top of the chart, and those that deteriorate rapidly and disappear are at the bottom. (Stop and think: don't paleoanthropologists get all excited when they find charcoal in their excavation sites? There is even Neanderthal cave charcoal that is hundreds of thousands of years old. It must be rather stable! And with regard to gold - even over geological time it is still found in its metallic form rather than as some sort of salt or silicate.

On the other hand, elements such as Na, K, Ca or Mg are never found in their metallic states in nature. They are all to happy to give away an electron or two.

Practice Problems

1. See if you are a good predictor of things! If a piece of metallic copper (such as an old penny) is placed in a solution of AgNO₃, the penny begins to turn white with crystalline needles and the solution slowly turns blue. Do this in lab next time and see what happens. AND do the reverse: a piece of silver is dropped into some cupric nitrate. (FYI: what do "cupric" and "cuprous" mean?)

2. After doing #1, can you predict what would happen here? Zinc metal in copper (II) nitrate; or copper metal in zinc nitrate?

3. Or more usefully: Iron metal in zinc nitrate; or zinc metal in ferric nitrate (with this knowledge, explain why galvanizing iron/steel is so useful) Note: galvanize and galvanic electrical cells (batteries)

4. In lab, it was found that whenever Mg was compared to any other electrode, it was always negative (hence, showed up least in the table, and was at the bottom of the frequency list (lowest reduction potential = highest oxidation potential)

   Suppose you had an electrode made of the metal lithium, and you found that the Li was negative relative to Mg. Would Li be above or below Mg on the relative reduction potential series? Discuss the use of "carbon/lithium" batteries versus the old carbon/zinc batteries of your grandparents' days.

5. Let's say you stuck electrodes Cu° and Mg° into the orange. In the immediate neighborhoods around those electrods some of the metals dissolved forming Cu⁺⁺ and Mg⁺⁺.
   1. If Cu° -> Cu⁺⁺ + 2 ε^- and Mg° -> Mg⁺⁺ + 2 ε^-
      where did the electrons go? They have to go somewhere!

   2. Which (Cu° or Mg°) gave off more electrons? How do you know that?

6. Turning to reacting alcohols with acetic anhydride:
   1. Explain what is chemically meant by an "alcohol".

   2. Is an alcohol acidic or basic?

   3. When you reacted acetic acid with HCl (remember your titrating acetic acid with 0.15M NaOH?), you were forming the salt called sodium acetate, which is ionized when dissolved in water. However, when you make, for example, ethyl acetate, which is dissolved in water, is that ionized? (Hint: can you smell sodium acetate? If not, why not?)

   4. Why, after reacting the excess acetic anhydride with water, will you add phenolphthalein and then add NaOH until the dye turns pink? (Hint: see previous question!)

   5. Biologically, acetates are EXTREMELY important. In your introductory biology class, do you emember the compound called acetyl-Coenzyme A? It was involved with connecting glycolysis with the Krebs Cycle. In essence, most foods are broken down to acetate units, which are then joined together in different configurations to make amino acids (which are the building blocks for proteins), into fatty acids (which make up fats and oils and membranes), and many, many other biochemical pathways. Isn't it interesting that the majority of chemicals that make up your body have even numbers of carbons in them? (Fooled you! This isn't really a practice problem!)

   6. Back to reduction potentials: We looked at about a dozen elements, but in organic chemistry (of which biochemistry is a family member), it is usually carbon that is oxidized and reduced. That same carbon that seemed to be so inert as elemental C as in pencil lead and charcoal and diamonds. Let's look at the various oxidation/reduction states of this simple little molecule, CH₃OH (what's its name?) It is your job to put them in order from the most reduced to the most oxidized!

      

   7. Why, oh, why is carbon so inert when you can so easily see in your previous answer that there are more oxidized and more reduced forms of it so abundant in the world around us? (Hint: go back to one of the early labs in the semester.)