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Video transcript

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 solutions so 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 H minus an oxygen is o 2 minus so therefore nickel must be nickel 3 plus to balance that out so this must be nickel 3 plus ion which has an oxidation state of +3 for cadmium hydroxide we know hydroxide is a negative 1 charge and we have two of them so this must be cadmium 2 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 the nickel 2 plus ion meaning the oxidation state is plus 2 so first we write our oxidation half reactions let's do that so what is oxidized in our battery solid cadmium is going from an oxidation state of zero to an ox's to an oxidation state of +2 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 so 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 so how many oxygens are on the right side well there's two because this 2 applies to everything in the parenthesis so there's two oxygens on the right eight and zero oxygens on the left we balance oxygen by adding water so 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 in acidic solution so we balance hydrogen by adding protons by adding H+ how many hydrogen's on the left side two times two is four so four hydrogen's on the left how many on the right there are two hydrogen's on the right so we need to add two protons to the right side to balance out hydrogen so we're going to add two H+ to the right side so we pretended like this was in acidic solution but remember it's actually in basic solution so we can add hydroxides to both sides to remove the H+ so if we have two H plusses we need to add 208 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 2 h2o so we have 2 h2o + cadmium so + cadmium + 2 hydroxide so plus 2 hydroxide giving us cadmium hydroxide so C D o h2 and then what is this what is 2 H + + 2 o H - H + + no H - would give you water so we get two waters on the right side so 2 h2o 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 - on the right side we haven't we have a charge of zero so an overall charge of zero we balanced 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 right that would that would balance out our charge so we're going to 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 so this is an oxidation so we should be losing electrons here for our half reaction we're ready to write our reduction half-reactions and next we're going to do reduction so the oxidation half reaction occurs at the anode of our battery the reduction half reaction occurs at the cathode of our battery so let's go back up here so you'll be right 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 so let's write this down so we have nickel oxide hydroxides 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 hydrogen's on the left side there's only one one hydrogen on the left but there are two hydrogen's on the right so we are going to pretend like this is in an acidic solution so we balance hydrogen's by adding protons so how many protons do we add and to which side well we have two hydrogen's on the right only one on the left so we need to add one proton to the left side so we're going to add one proton so H+ so now hydrogen is balanced next we're going to get rid of the proton that 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 need to add one hydroxide to the right side of our half reaction so let's write down what we would have now alright so h plus and o h minus give us h2o so we have h2o plus nickel oxide hydroxide so h2o plus nickel oxide hydroxide giving us on the right side we would have nickel hydroxide so nickel hydroxide plus o h 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 on the Left it's negative one on the right and so we add an electron gain of electrons is reduction so that makes sense - so this is our half reaction so 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 so 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 right 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 going to multiply everything through by 2 so 2 times 1 gives us 2 electrons and then 2 times h2o we get two waters we get to nickel oxide hydroxide so 2 n io o H we would get 2 nickel hydroxide so two nickel hydroxide and finally two hydroxide so now we have the 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 going to add we're going to add the two half reactions that I've boxed this half reaction in this half reaction we're going to add them together we can cancel out some stuff right now to make our lives a little bit easier all right 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 so now we can write our overall reaction so for this half reaction we would have cadmium all right so let's write that down here we have solid cadmium + 2 h2o so + 2 h2o + 2 nickel oxide hydroxide so plus 2 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 so cadmium hydroxide + down here we would have two nickel hydroxide so we have plus two nickel hydroxide so 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 right because if you're reversing your reaction you need to start 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
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