Zinc is attached to a ship's steel propeller to prevent the steel from rusting. Write balanced equations for the corro-sion reactions that occur (a) in the presence of Zn and (b) in the absence of Zn.

Video transcript

Hello. In this problem, we are told the resting of steel can be prevented by coating with aluminum brass to provide the balanced reaction for the corrosion that happens when aluminum is present and when aluminum is not present. So let's begin by looking at the standard oxidation potential for the oxidation of aluminum. So the oxidation of aluminum to form aluminum repos people too. The negative of the standard reduction potential. This is 1.66V. And we look at the standard oxidation potential for Iron Form Iron two To find that it is equal to 0.44V. So when we compare the standard oxidation potential of aluminum to that of iron, we see that aluminum is larger and more positive than that of iron. That means then aluminum has a greater potential to be oxidized. So considering first, then the presence of aluminum. So oxidation will occur in the presence of oxygen oxygen gas and is going to be produced from water and with women present, it will be oxidized instead of the iron. Combine these two reactions. To get overall reaction in doing so we need to ensure that the moles of electrons lost is equal to those that are gained. Some multiply. The first reaction through by three And second reaction through by four. And so we'll have 12 moles of electrons being gained and lost. So those were canceled. And then alright, what remains? So we have four moles of aluminum Reacting with three moles of oxygen gas and we produce formals of aluminum ions And two bottles of water. So this is the reaction that is occurring. Then when aluminum is present, if you consider then next the equation for the erosion. When aluminum is absent an aluminum absence, we still have erosion taking place in the presence of oxygen where the oxygen is being reduced to form water. And now with aluminum absent, it means that the iron will be oxidized from iron to we'll combine these two reactions. We want the number of electrons that are being gained equal those that are being lost. So multiply through 2nd reaction by two. Bank it to most of iron with oxygen under acidic conditions going to form two miles time to tomatoes, water and so are electrons that are being gained and lost are the same. So those cancel out and do not appear in this reaction. The iron to then can be further oxidized from Iron three. So again we have our reduction of oxygen on water and we have now iron two Being oxidized from Iron three. We combine these two reactions. So the number of electrons that are being gained has to do with those that are being lost. So we'll multiply this second reaction through by four. So we'll be gaining and losing for multi electrons. So those were canceled. It's left over then we have formals of iron to point form four moles of iron three water. And so we're gonna take these two reactions for the oxidation of iron first to form iron two and then for my iron three. And we're going to include an equation then for iron three reacting with water. Our third reaction then is iron three reacting with water, perform iron three oxide, this is rust. Now we're going to combine these three reactions and doing so recognize that we have iron two on the product and the reactive side and iron three on the product and the reacting side. So we're going to get these to be the same amount. So to do that, then we're going to take the first equation. I'm gonna gonna multiply this through by two and then we're gonna take our third equation and we're gonna also multiply this through by two so that our molds of iron two and three being um on the product and the react inside are the same and so that they will cancel. So simplifying this further. Then we end up with four moles of iron, those oxygen gas and eight Hi Drinan's goes to form four moles of iron two plus water. That's our first reaction. The 2nd 1. Then we have formals iron too reacting with the oxygen under acidic conditions to produce formals martin three and water. The last one. Then we now have four moles of iron three. It loads of water going to form two moles of iron, three oxide and kernels of hydro nines. So combining these and signifying. So we have four moles of iron to on the product and the reacting side. So those cancel four moles of iron three on the product and reacting side. So those cancel, we have eight and four is 12 Hydro nine's on the reacting side, and 12 on the product side. We have four and two is six waters on the product side and eight on the reactive side. To eliminate all but two of those will combine what's what's left over. So we get formals of iron Plus three moles of oxygen, Plus two moles of water. Most form two moles of iron three oxide, and this then is our balanced equation for the corrosion that occurs in the absence of aluminum. This then corresponds to answer a thanks for watching. Hope. This help.