Aktuel tid:0:00Samlet varighed:11:07
0 energipoint
Studying for a test? Prepare with these 6 lessons on Redox-reaktioner og elektrokemi.
See 6 lessons
Video transskription
- [Voiceover] We need to balance the redox reaction for a nickel-cadmium battery, so here's what happens. Solid cadmium plus nickel oxide hydroxide give us cadmium hydroxide and nickel hydroxide. In the battery this takes place in basic solution. So we've already seen how to balance a redox reaction in basic solution, I showed you a video with all of the steps, so this video is a review of what to do. Remember, you pretend like it's an acidic solution and then you can change it to a basic solution. The first step is to assign oxidation states. So we start with solid cadmium, and for cadmium metal this is an element so the oxidation state is zero. Next, for nickel oxide hydroxide we know hydroxide is OH minus and oxygen is O two minus, so therefore nickel must be nickel three plus to balance that out. So this must be nickel three plus ion, which has an oxidation state of plus three. For cadmium hydroxide we know hydroxide is a negative one charge and we have two of them, so this must be cadmium two plus ion, which has an oxidation state of plus two. For nickel hydroxide, same thing. We have two hydroxides, so this must be nickel two plus, a nickel two plus ion, meaning the oxidation state is plus two. First we write our oxidation half reaction, so let's do that. What is oxidized in our battery? Solid cadmium is going from an oxidation state of zero to an oxidation state of plus two in cadmium two plus. That's an increase in the oxidation state, so that's oxidation. So we write down our half reaction. We have solid cadmium going to cadmium hydroxide. Next we look at the atoms that we need to balance. Let's start with cadmium. We have one cadmium on the left and one cadmium on the right, so cadmium is balanced. Next we balance oxygen. How many oxygens are on the right side? Well, there's two, because this two applies to everything in the parentheses. So there's two oxygens on the right and zero oxygens on the left. We balance oxygen by adding water. How many water molecules do we need to add to the left side to balance oxygen? We would need two. So if we add two water molecules, now we have two oxygens on the left and two oxygens on the right. Next we balance hydrogen, and we are going to pretend like this is an acidic solution, so we balance hydrogen by adding protons, by adding H plus. How many hydrogens on the left side? Two times two is four, so four hydrogens on the left. How many on the right? There are two hydrogens on the right. So we need to add two protons to the right side to balance out hydrogen. So we're gonna add two H plus to the right side. So we pretended like this was an acidic solution, but remember, it's actually in basic solution, so we can add hydroxide to both sides to remove the H plus. So if we have two H plusses we need to add two OH minuses, and if we add two hydroxides to the right side, we need to add two hydroxides to the left side. So let's write down what we would have for our oxidation half reaction. We'd have two H two O. So we have two H two O, plus cadmium plus two hydroxide, giving us cadmium hydroxide, so CD(OH) two. And then what is this? What is two H plus, plus two OH minus? H plus and OH minus would give you water, so we get two waters on the right side, so two H two O. Next we need to balance our charge, so let's look at charge for our oxidation half reaction. On the left side, what is the total charge? We have two hydroxides, so that's a charge of two minus. On the right side we have a charge of zero, so an overall charge of zero. We balance charge by adding electrons. So how many electrons do we add, and which side of our half reaction do we add them? If we add two electrons to the right side, that would balance out our charge, so we're gonna add two electrons to the right side. Notice we have two waters on the left and two waters on the right, so we can cancel those out. and write our final half reaction. So cadmium plus two hydroxide give us cadmium hydroxide plus two electrons. This is an oxidation so we should be losing electrons here for our half reaction. We're ready to write our reduction half reaction so next we're gonna do reduction. The oxidation half reaction occurs at the anode of our battery, the reduction half reaction occurs at the cathode of our battery. Let's go back up here so we can see what we write for our reduction half reaction. Well this is going from an oxidation state of plus three to an oxidation state of plus two. That's a decrease or a reduction in the oxidation state. So this would be our half reaction. Nickel oxide hydroxide going to nickel hydroxide. Let's write this down. So we have nickel oxide hydroxide going to nickel hydroxide for our reduction half reaction. We start by balancing our atoms, one nickel on the left, one nickel on the right. Then oxygen. Two oxygens on the left and two oxygens on the right. So we can move on to hydrogen. How many hydrogens on the left side? There's only one. One hydrogen on the left, but there are two hydrogens on the right. So we are going to pretend like this is in an acidic solution, so we balance hydrogens by adding protons. So how many protonsdo we add, and to which side? Well, we have two hydrogens on the right and only one on the left, so we need to add one proton to the left side. So we're gonna add one proton, so H plus. Now hydrogen is balanced. Next we're gonna get rid of the proton we just added because really, this is in basic solution. So since we have one proton we need to add one hydroxide to get rid of that one proton. If we add one hydroxide to the left side we need to add one hydroxide to the right side of our half reaction. So let's write down what we would have now. H plus and OH minus give us H two O. So we have H two O, plus nickel oxide hydroxide. So H two O plus nickel oxide hydroxide. Giving us, on the right side we would have nickel hydroxide plus OH minus, so plus hydroxide. We need to balance charge next, so let's look at the total charge on the left. It would be zero. On the right side we have one hydroxide anion so that's one minus for the charge on the right. So we need to add one electron to the left side of our half reaction. If we add one electron to the left side, now the charge is balanced. It's negative one of the left, negative one on the right. So we add an electron. Gain of electrons is reduction, so that makes sense too. So this is our half reaction. This is our reduction half reaction. And we're ready to think about adding our two half reactions together. So let's go back up here, and remember, this was our final oxidation half reaction, so we lost two electrons. We lost two electrons in our oxidation half reaction. When you're coupling together your half reactions the number of electrons has to be the same, because the electrons that are lost in the oxidation half reaction are the same electrons that are gained in the reduction half reaction. So we need to have two electrons for our reduction half reaction, so we could get two electrons by multiplying everything in our reduction half reaction by two. So we're gonna multiply everything through by two. Two times one gives us two electrons, and then two times H two O, we get two waters. We get two nickel oxide hydroxide, so it's two NiOOH. We would get two nickel hydroxide. Two nickel hydroxide. And finally two hydroxide. Now we have number of electrons, now we have the number of electrons the same so now we can add together everything to get our overall reaction for our nickel-cadmium battery. So we're gonna add the two half reactions that I boxed. This half reaction and this half reaction, we're gonna add them together. We can cancel out some stuff right now to make our lives a little bit easier. We can cancel out these electrons on the product side, on the reactant side. And we can cancel out hydroxide, because if you look-- Let me change colors here-- we have two hydroxides on the reactant side, two hydroxide on the product side. Now we can write our overall reaction. For this half reaction we have cadmium, so let's write that down here. We have solid cadmium, plus two H two O, plus two nickel oxide hydroxide. What would we have for the products? We would have-- Well, let's go back up here to our oxidation half reaction. We would have cadmium hydroxide, so we write that in. Cadmium hydroxide, plus, down here we would have two nickel hydroxide. We have plus two nickel hydroxide. We're finally done. This is the overall reaction for what occurs in a nickel-cadmium battery. And our two products, cadmium hydroxide and nickel hydroxide are both solids that precipitate on the electrodes in the battery, and that makes it easy to reverse the reaction, because if you're reversing your reaction you need to start with cadmium hydroxide and nickel hydroxide, and those are already there, so it's easy to reverse this reaction, which makes it easy to recharge this battery. So the nickel-cadmium battery is like the lead storage battery, it's rechargeable, and therefore it can be very useful.