Electrochemical Cells

Hey everyone. So, galvanic or voltaic cells are spontaneous cells that produce or discharge electricity. Now remember when they discharge all their electricity, then they're classified as a dead battery. Now, here we have a basic model of a galvanic cell volta cell. And we're going to say that in a galvanic will take cell that the negative electrode is the a node and the positive electrode is the cathode. Here we have two half reactions that are designated for the cathode and the anote for the cathode, we have three moles of copper, two ion absorbing six moles of electrons to produce three moles of copper solid. So here that means we'd have cu two plus particles floating around within this solution. The metal electrode itself is copper solid. For the anote half reaction, we have two moles of chromium solid, basically losing six electrons to produce two moles of chromium, three ions. So in this compartment here we have our chromium solid here and we have cr three positive ions floating around. Now with the Galvan voltaic cell, we're gonna say that these two jars are connected by this, which is called a salt bridge. Now, in a salt bridge. We have neutral ions, ions that are neither acidic or basic in nature. Typically, we have chloride ions or nitrate ions, those negative ions they flow toward the anodes side. And then we may have sodium ions and potassium ions which move this way towards the cathode side. We say that oxidation always occurs at the A node, reduction always occurs at the cathode. That's why the A node is losing electrons. Electrons are moving away from the A node towards the cathode. As they move, we have the traveling of electrons from one side to another. And in order to close the circuit, we need same charges moving in the opposite direction. So we have our anote here, we have a cathode here, we have electrons moving towards the cathode. And within the salt bridge, we have our negative ions moving towards the anodes side. This completes the circuit. And so through the movement of these two electrons on these two charge like charges in opposite directions, we form a current. So this volt meter here reads how much voltage is being produced. Now, we want our adults to lose electrons easily. So we want their ionization energy, the energy required to move an electron to be low. We want the cathode to attract the electrons. So we want its electron affinity to be high. What starts to happen is you're losing electrons over time. So this a node electrons weigh very little but enough of them over time, you start losing some mass. So we're gonna say here your Anno dissolves away as the cathode gains more and more electrons, its surface starts to become negatively charged. This attracts the submerge uh dissolved copper two ions within the solution. So you have positive ions adhering to a circuit that's slightly negative in charge, they're gonna neutralize each other. And so you're gonna have an encrusting of copper on top of this electrode. So over time, the cathode is gonna get bigger. And we say the cathode plates out now to produce this electricity, you wanna make sure that the cathode side is attracting these negative electrons. So you wanna make sure that this solution here has a good amount of positive charges in it that's gonna attract the negative electrons towards that side. So you wanna make sure the cathode solution concentration is high. At the same time, you wanna make sure that the Anno concentration of positive ions is low because if this solution becomes too saturated with positive ions, those negative electrons will not want to move towards the cathode because remember light charges are tracked each other. So this gives us the break basic breakdown of our galvanic or voltaic cell. Remember with the anade we have oxidation and with the cathode, we have reduction, we have two half reactions involved within this particular example. But we also have other examples of uh half half reactions. All of them are written as reductions because for all of them, the electrons are reactants with each one, we have a standard cell potential associated with it. Remember that the higher your standard cell potential then the more likely reduction will occur. And remember if the more likely reduction occurs, that means you have a stronger oxidizing agent. Conversely, the lower your standard cell potential is and the more likely oxidation will occur. So more likely is oxidation and the more likely oxidation happens, the stronger you're reducing agent. OK. So just remember for this particular example, we're looking at two specific half reactions, but we could easily come up with another galvanic overtake cell example, which incorporates some of these other half reduction reactions.

Hey, everyone before we talk about the other type of electrochemical cell, let's revisit certain variables. So here we're gonna say in terms of spontaneity, the following correlations between the following variables can be made. So here we have gibbs free and our gender standard conditions, we have our equilibrium constant K, our standard cell potential. Here we have our standard entropy and this is entropy of our universe. Now we have reaction quotient versus K our equilibrium constant. And this will tell us the reaction classification as well as cell type. So here in the first row, when we have the configuration, this way, these are all conditions that lead to a spontaneous reaction. And if we're talking about spontaneous reaction, then the electrochemical cell is a galvanic cell or by its other name, a volta. Now here, if we were to reverse everything, reverse the sign we'd expect the opposite result. So this would be a non spontaneous reaction and that'd be connected to the electrochemical cell. We'll talk about right after this chart which is your electrolytic cell. And then finally, if everything is equal to certain variables, we're gonna say that we are at equilibrium and you'd represent a dead battery. Now, here, when we talk about an electrochemical cell, it doesn't function spontan on spontaneously. So it requires an outside energy source. So it requires a battery. Now, here we're gonna say our electrochemical cell or electrolytic, sorry, electrolytic cell is a non electrochemical cell and it consumes electricity and so it requires a battery. Remember, a galvanic or voltage cell is different. It is literally a battery. It produces and discharges electricity. Here, this one needs a a power source. So it needs a battery. But here it doesn't matter if it's uh an electrolytic cell or galvanic cell oxidation always occurs at the A node, reduction always occurs at the cathode. So here we see our electrons moving in this general direction. So they'd be moving towards a cathode that does not change and the electrons are leaving this electrode. So this would have to be our anote. What you should realize here though is that with an electrolytic cell, things are not spontaneously. They don't happen naturally. The cathode here is negatively charged negative electrons don't want to go to something that's already negative. Remember, light charges repel we need that outside energy source to force the electrons to go that way. And then electrons don't wanna leave something that's positive here. The A node is positive. But again, we're using that battery to force the electrons away from our positive A node electrode. Now here electron affinity would have to be low. For the A node, we don't want the electrons to stay near the A node ionization here would have to be high for the cathode. We don't want those electrons once they go there to come off. So basically, when it comes to an electrolytic cell, it's non spontaneous. A lot of the process. Well, a lot of the way of labeling things are the opposite of a galvanic cell. Really. The place that things hold true is in terms of reduction and oxidation, the cathode is still the site of reduction and the a node is still the site of oxidation. All right. So here just remember these few key things about electronic cells and remember the variables up above to help us determine if a reaction is spontaneous, non spontaneous for a dead battery.

So we're going to say here that our line notation sometimes called cell notation is a quick, simple method described an electrochemical cell without having to dry it out in detail. Now a line notation or sell notation utilizes vertical lines here when there is one vertical line and represents a phase boundary where we're talking about the same element, but in different phases a double line represents basically a junction that's the result of a salt bridge. So this is referring to an actual separation of your two jars within your electrochemical cell. Now, here we're gonna say that basic setup of a typical line notation or sell notation is we have the lower oxidation state of the element on the exteriors and in the insides we have the higher oxidation states of the elements. And again, the double line here is representative of the salt bridge or just basically the separation of the two jars that comprise my overall electrochemical cell. Here, in this example, we have our typical galvanic overtake cell. We have our own out here on the left remember an out here would be a negative electrode. Over here we have our cathode which is our positive electrode. We're gonna say that we have two half reactions given to us at the cathode. We have the reduction of copper to ion here, three copper to ion moles of it absorb six electrons in order to create three moles of copper solid. So here, that's our catholic. So here we have our copper solid as our electrode. And in the solution we have swimming around our copper, two ions, then we're gonna say here we have our at the and out we have two moles of chromium solid become oxidized to produce two moles of chromium three plus ion plus six electrons that are given off. So here chromium would represent our electrode. Here we have chromium three plus ion floating around and then remember electrons naturally moved from the anad where oxidation occurs to the cathode where reduction occurs. So your electrons have this general trend here. Remember in this salt bridge we have are inert negative ions which could be chloride ion or nitrate ions which move that way to neutralize any of these positive ions from becoming too concentrated. Um you may also have positive ions here, which would migrate more this way in terms of the salt bridge towards the cathode side. Now here we have the transferring about six electrons. Once we've canceled out those intermediate electrons, our overall equation would be three moles of copper two plus ion plus two moles of chromium solid, produce three moles of copper solid plus two moles of chromium three plus ion here when we're talking about cell notation, remember some notation as easy as A B C. So here A would represent are an ode B would be our physical break between the two jars and then see what represent our cathode, we'd say at the annual, remember oxidation is what's occurring. So here the oxidation would be of the chromium, it goes from chromium solid to chromium three plus the oxidation number of chromium solid is zero and the oxidation number of chromium three plus is equal to its charge. So it's plus three. Remember we said that we're going to put the lower oxidation states on the exteriors on the outside. So chromium would go here as a solid and then here we'd have chromium three plus ion. Then at the catholic we have reduction. So reduction here would be the copper two plus ion being reduced to copper solid. The oxidation number here of copper two plus is plus two and of chromium is I'm a copper solid at zero. So it puts C U two plus here and then we have See you solid here. So this would be basic or generic line notation or sell notation. Now if we're going to include um activities, um we could say here if we're dealing with a standard line notation or sell notation, that means our activity a approaches unity. That means it's equal to one. So here we say that we have chromium solid And then its activity of chromium solid is equal to one and then we have chromium three plus its activity is equal to chromium three plus equals one. We have our physical break. Then we have copper two plus Aquarius times activity equals one and then copper solid times acquis equals one. So that would be looking at line notation or salutation when we include activities within our basic setup. But remember here, line notation or salutation is just a quick way of describing what's going on in terms of this galvanic or will take self image now that we've seen this, attempt to do the example question that's left here on the bottom once you do come back and see if your answer matches up with mine.

So here we have to write the half reactions as well as the overall net ionic equation. For the following line notation. Remember with line notation we have a B and C. So here we have our an ode our break and our cathode for the unknown portion. We're gonna say we have copper solid And here it gives us copper two plus ion Aquarius. We're gonna say here that this is an oxidation because at the panel we have oxidation on this side, the overall charges zero over here, the overall charge is plus two. We need to make sure that both sides of the equation have the same overall charge. So we're gonna have to put electrons on the more positive side. So this side is plus two and this side here is zero in order to make this side here also equal to zero. We have to add two electrons. So this here represents our reaction at the anodes where we have copper solid being oxidized to produce copper two ion with the release of two electrons. Now at the cathode we have silver ion being reduced to give us silver solid. So the overall charge here is plus one. The overall charge here is zero. We have electrons with a more positive side. So we have to add one electron here so that one electron gives us an overall charge of zero. Just like the product side. Now the number of electrons have to be the same in terms of both half reactions because they're going to cancel one another out here. We have to multiply this entire equation times to have the same number of electrons. So what we're gonna have now are we're gonna have our two equations and we have two electrons plus to a G positive gives me to a G solid. So electrons cancel out. So at the end we have copper solid plus to a G positive Aquarius produces. And here you can have double irreversible arrows. We have copper two ion plus two silver solids. So that that represents our equation, our overall net ionic equation as well as the two half reactions. So your two half reactions would be uh this one here And then this one after we've multiplied by two. And then we have our overall equation at the end. So remember line notation or some notation is just a convenient way for us to quickly describe what's going on in terms of our electrolytic cell, which substances are being oxidized versus which ones are being reduced and the number of electrons that are transferred between the two um cells within our electrochemical cell.

So here we need to sketch the galvanic cell and determine the cell notation for the following redox reaction. Alright, so what we have here is we have our hydrogen ion becoming a church to gas here, its oxidation state for each H plus ion is just plus one because it's equal to the charge of the ion here, it's in its natural neutral form. So it's oxidation number is zero. So we're gonna say that hydrogen H plus goes from being plus one to being zero. We're gonna say its oxidation number decreased, therefore it was reduced because it was reduced. That means it must represent the cathode. Then here we have iron in its natural neutral form. So when it's in its natural or standard state, its oxidation number is equal to zero here, it's in its ion form with a plus to charge, the charge of an ion is equal to its oxidation number. So now it's plus two, its oxidation number increased by going from zero to plus two. Therefore it was oxidized. And because it was oxidized, it represents the anodes. So here we're gonna draw our quick sketch of our galvanic cell. So here we have our iron electrode and we're gonna have our salt bridge here, we have our wire connected to our volta meter, which measures the amount of voltage that's being generated from the electrons moving from the outside to the cath outside. Now here, this is interesting because we're dealing with HPE plus and H. Two. These two species one together means we're talking about the she electrode and that stands for the standard hydrogen electrode. And when we're talking about she, we have to realize that we don't have hydrogen electrode because hydrogen exists in a gaseous state. So it's not a solid like we have iron here. So to show the she electro what we have is we have a platinum wire and that platinum wire is connected to a platinum surface. So we're gonna have this little beaker here and it's gonna have a porous opening here to allow H Plus ions to flow through. So we have H plus ions dissolved within this solution here. We have iron, two ions dissolved here. Electrons are traveling from the outside to the cath outside. Okay, and they're traveling down this wire at the same time. We have another tube that's connected to the outside. And from this tube we have H two gas coming in. So we have electrons that are traveling down this platinum wire here. And at the same time, we also have hydrogen traveling down as well. They will meet here near this opening here. And at the same time, we have hydrogen coming in through these openings in the small little jar. The H plus will come into contact with electrons and create hte gas. So, you have the generation of H two gas Here within this container and the H2 gas will bubble out of the solution. You have hydrogen gas H2 gas also getting pumped in as well. So that also comes out of the solution to over time. That's the basic setup of the she electrode or standard hydrogen electrode. Now, how would we write the sum notation for this? What we have our iron solid. Remember we're gonna put the lower oxidation numbers on the outside here. We're gonna say that the activities of these approach unity. So that means they're equal to one. So we have the activity for iron equals one. We have our phase boundary. We have F. E. Two plus acquis its activity Is equal to one. So here we have our actual physical breakdown. Next we're gonna have a church plus Aquarius with its activity. We have our phase boundary. We have a church to gas, its activity. Mhm. And then we have just platinum solid over here. So remember we need physical solid electrodes involved. That's why the platinum solid is being used. And because the platinum solid is in itself not being involved in the redox reaction, we call it an inert electrode. So this compartment here of my line notation or some notation, this represents she. And realize here that she's also referred to as our reference electrode because it has a potential equal to zero. So when we're saying that a metal or an element has a potential that's positive, it's positive relative to the she electrode. When we say that it's a negative potential for an ion or metal. It's negative in reference to the she electrode. So the electrode just represents our reference electrode where we compare all the other half reactions that we deal with when it comes to redox reactions. So realize here that the some rotational line notation that we wrote is a much quicker way of illustrating what's going on. In terms of our galvanic cell. Instead of having to draw out the whole thing. And remember here, this would be our an ode over here would be our cathode. Remember that oxidation always occurs at the anodes. So electrons leave the an out and go to the cathode where reduction will occur. So just remember the little intricate parts of this idea of the she electrode and how it relates to line notation in this particular example. Now that we've seen this. Move on to the next question. Our practice question this one same basic understanding, we have to determine which one is being oxidized and therefore represents the anodes and which ones being reduced. Therefore representing the cathode. Figure that out in order to sketch the galvanic cell and also write the line or cell notation from the information given. Once you do that, come back and see if your answer matches up with mine