Video: Determining the Metal Most Prone to Corrosion Using the Standard Electrode Potentials of Different Metals

Using the standard electrode potentials shown in the table, determine which of the following metals is the most prone to corrosion. [A] Zn [B] Mg [C] Au [D] Hg [E] Fe


Video Transcript

Using the standard electrode potentials shown in the table, determine which of the following metals is the most prone to oxidation. (A) Zn, (B) Mg, (C) Au, (D) Hg, or (E) Fe.

Before we get started, let’s make sure we have the right names for each candidate. (A) is zinc, (B) is magnesium, (C) is gold, (D) is mercury, and (E) is iron. The term “prone to oxidation” is not a technical term, but we can surmise what we’re looking for is the metal which, when exposed to oxygen, will form an oxide the fastest. Just bear in mind that one of the metals we’re dealing with, mercury, is a liquid at room temperature rather than a solid. However, liquid metals can form oxides on their surface just like solid metals can.

We’ve been given lots of standard electrode potentials for various reduction processes. Standard electrode potentials are a measure of the electrical potential of an electrode. Ions are in effective one molar concentration. And the pressure is at one atmosphere. Standard electrode potentials are measured relative to the standard hydrogen electrode, which has a defined electrode potential of 0.00 volts. The electrical potential is just another way of expressing the energy per electron.

The standard units volts are simply joules per coulomb of charge. Standard electrode potentials are defined against the reduction version of the half equation. These can be labeled as 𝐸 red. In this question, what we’re looking for is the metal most prone to oxidation, so we need to look at the equations, find the metals, and then flip them around. We’ll also need to pay attention to the fact that we have two half equations for mercury and two for iron. We’ll come to the details of those in a moment.

To get the electrode potential for the oxidation, the reverse of the reduction, we simply take the negative of the standard electrode potential. For the case of zinc turning into zinc two plus, our standard oxidation potential is positive 0.7618 volts. For magnesium transforming into magnesium two plus, the standard oxidation potential is positive 2.372 volts. For the conversion of gold into its ion, the value is negative 1.498 volts. For the conversion of mercury into Hg2+ or Hg2 2+, the values are minus 0.851 volts or minus 0.7973 volts.

Remember that electrical potentials can be expressed per electron, so we can scale up or scale down an equation without changing the electrical potential. And therefore, we don’t change the standard oxidation potential value.

The values with respect to iron are a little bit more complicated. We start out with solid iron and then move to iron two plus. The oxidation potential for that process is a positive 0.447 volts. However, we then move from Fe2+ to Fe3+ with an oxidation potential of negative 0.771 volts. When we add the two together, we need to find the average energy per electron. The first two electrons were delivered at positive 0.447 volts above standard hydrogen electrode voltage. The third one is delivered at negative 0.771 volts. Averaging these out gives us a standard oxidation potential for the iron-iron three electrode of positive 0.041 volts.

So, after much work, we’ve ended up with the oxidation potentials for all our metals. To solve this question, we need one last principle. The higher the oxidation potential, the more prone to oxidation the metal is. Like many systems, electrical systems are driven towards the lowest electrical potential. Let’s take, for instance, magnesium. Magnesium has a highly positive oxidation potential. This means that when given the opportunity, for instance, if it’s paired with gold, it will give up that energy and form the oxide. Of course, all these oxidation potentials are measured relative to the standard hydrogen electrode.

And, of course, metals can’t be oxidized without a companion to induce the oxidation. However, we can use oxidation potentials to rank metals by how prone to oxidation they are. So, based on its oxidation potential alone, magnesium is the most prone to oxidation, followed by zinc. Followed by iron to iron two plus, followed by iron to iron three plus, followed by mercury to mercury two two plus, and slightly later to mercury two plus. And lastly, gold is least prone to oxidation. Therefore, using standard electrode potentials given in the table, we have determined that, of the five metals given, the one most prone to oxidation is magnesium. If you looked up these elements on the reactivity series, you’d see exactly the same trend.

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