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Current time:0:00Total duration:9:06

Video transcript

here's the reaction for a lead storage battery solid lead plus lead dioxide gives lead sulfate our goal is to balance this redox reaction in acidic solution for a lead storage battery you're dealing with sulfuric acid in a previous video I've already gone over how to balance a redox reaction in acidic solution so this video is just a review of that one using the lead storage battery as an example the first thing we need to do is assign oxidation states and we'll start with solid lead the oxidation state of an element is zero so it's zero for solid lead move on to lead dioxide for oxygen the oxidation state is negative two and we have two of them giving us negative four for the total the sum of the oxidation states in a neutral compound is zero so it must be plus four for lead move on to lead sulfate we know that sulfate is so4 to minus therefore we're dealing with lead two plus the lead two plus cations so the oxidation state is plus two next we write our oxidation half-reaction so what is being oxidized here solid lead is going from an oxidation state of zero to an oxidation state of plus two that's an increase in the oxidation state so that's oxidation so we write down here solid lead is going to lead to plus ion for our oxidation we have sulfate present so we need to have sulfate on both sides so if we have solid lead + s0 for two - that goes to lead sulfate pbso4 next we look at our atoms and we have one lead on the left and one lead on the right one salt from the left one sulfur on the right four oxygens on the left and four oxygens on the right so all of our atoms are balanced we move on to balancing the charge you balance charge by adding electrons so let's figure out how many electrons we need to add and where do we add them on the left side we have two negative charges on the right side our char arge overall charge is zero so we need to make those charges equal we can do that by adding two electrons to the right side so now the charges are balanced remember oxidation is loss of electrons so this is the half reaction that occurs at the anode of our battery next we want to do the reduction half-reaction so we need to write down our reduction half-reaction we can see that we're going from plus 4 to plus 2 that's a decrease in the oxidation state so for our reduction half-reaction it would be PB o 2 going to PB 2 + once again we need to add in our sulfate since that is present so on the left side we would have PB o 2 plus s 0 for 2 minus and then going to pbso4 on the right next we need to balance our atoms so we'll start with lead 1 LED on the Left 1 LED on the right and once that's done alright and the sulfur is balanced to 2 once all for the left one sulfur on the right we look at oxygen so let's look at the left side here so we have 2 oxygens here and 4 here for a total of 6 oxygen so 6 oxygens on the left and on the right we have 4 oxygens so 4 oxygens on the right you balance oxygen by adding water so we need 2 oxygens on the right side so we can get that by adding 2 water so if we add 2 waters now we have the proper number of oxygens we just added 2 more auctions to the right side next we balance hydrogen's by adding H+ so let's look at the right side we have 4 hydrogen's on the right and on the left we have 0 we have 0 hydrogen's so we need we need to add 4 protons to the left side of our half reactions so if we add 4 protons to the left side now hydrogen balances we have 4 on both sides next we balance charge what's the overall charge on the left side we have four positive charges here and two negative charges so they have a total of two plus on the left side on the right side it would be zero so we need to add we need to add two electrons to the left side if we add two electrons to the left side we get our charges balanced so now we have our reduction half-reaction our reduction half reaction occurs at the cathode of our battery so let's add our two half reactions together and we can do this because we have the same number of electrons so let me let me box this half reaction so for our oxidation half reaction we lost two electrons and for our reduction half-reaction we gained two electrons so those electrons cancel out so we're going to add our two half reactions together to get the overall reaction for a lead storage battery so let's add all of our reactants let's start with let's start with these up here so we're going to want to have solid lead and sulfate so let's start by writing that so we have PB plus so4 2 minus and then for our reactants down here we have 4 h+ + PB o 2 + so4 2 - so we have 4 h+ + PB o 2 plus s o 4 - - for the products we have pbso4 alright so we have pbso4 and then down here we have pbso4 + 2 h2o so pbso4 + 2 h2o we've already cancelled out the electrons so we don't need to worry about that let's simplify this alright so how can we simplify this on the left side we have leads so PB + PB o - so PB + PB o 2 now we have four h+ so four rh+ now we have two sulfates we have two sulfates here so we'll write two so4 - - and then for the products we have two pbso4 so that would be two pbso4 + that's a 4 here + 2 h2o alright so we've finally done it this is our overall reaction for a lead storage battery and sometimes you might see this written a little bit different in a textbook you might see you might see two of these protons added on to these 2 sulfates to give you two hso4 minus leaving 2 protons left over so - h+ so that's just another way to write what's happening here so this is this is a voltaic cell right you get a you get a voltage out of this spontaneous redox reaction and one of these cells has about positive 2 volts so the cell potential is about positive 2 volts so we take 6 of these cells and add them together you get a car battery and a car battery has a total of 12 volts so 6 cells each one with each one with 2 volts when the car battery discharges right the spontaneous redox reaction delivers a current and your car engine starts once your car is running the reaction is reversed and that allows you to recharge your lead storage battery the reverse reaction is ready to go because your lead sulfate here is a solid so if we go back and look at our half reactions for our half reactions here we have lead sulfate for both of them and it's a solid that precipitates on your electrodes and so therefore the reverse reaction is ready to go and you can recharge your battery so first you could think about this as a voltaic cell right which produces a current using a spontaneous redox reaction and then you could think about an electrolytic cell a current is used to recharge your battery a current is used to drive a non-spontaneous reaction and so a lead storage battery is rechargeable and that makes it that makes it very useful in car batteries they last an extremely long time
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