Galvanic (Voltaic Cells) & Electrolytic Cells
Galvanic versus Electrolytic Cells
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hey everyone. So with redox reactions, we will now deal with a new variable. This is a reactions cell potential. And with this self potential, we can have it under standard conditions or under nonstandard conditions remember that little degree sign means under standard conditions, standard conditions here wouldn't mean that you have a concentration of one molar, it would mean that you could have a pressure of one atmosphere. So 1.0 atmospheres Or you could have a ph equal to seven. These are standard conditions. And so we refer to this standard cell potential. Again, if it's not standard, we just say regular self potential. Now we're gonna say the greater this variable then the more likely reduction will occur. So the larger your values, the more reduction wants to happen. And so the smaller this value than the more oxidation wants to happen. Now, we're gonna first talk about our galvanic or voltaic cells. So they go by either name, we're gonna say here that this produces or discharges electricity. So it's basically a battery. So I know you might see that your chemistry professor or ta has a weird sense of humor. One popular joke is that chemists don't die, they go to equilibrium and it's kind of in reference to the galvanic or voltaic cell because once it discharges all of its electricity, we're gonna say that it When it does 100% discharging, that means that it's reached equilibrium and we call that a dead battery. So when a battery dies it's because it's reached equilibrium, it's discharged all the electricity that it has. Now here, we're gonna see an image of a typical galvanic or voltaic cell. And there's gonna be certain periodic trends you still need to remember such as ionization energy and electron affinity. Now here we're going to say that this jar, these two jars here are related to these two half reactions. These half reactions are written as reductions because their electrons are written as reactors. Remember when your electrons are reactant, you are a reduction here, They both have standard cell potentials connected to them. Remember the larger cell potential, the more reduction wants to happen. So this one is positive .40V. So this is where reduction occurs now with all of these electrochemical cells, galvanic voltaic cells, reduction happens at the electrode that we call the cath out. This one is negative 10.25 volts. So oxidation wants to happen here. Oxidation happens at the electrode. That's called the ana. Now taking this piece of information, we have electrodes are just basically for the most part, they're the metal rods that we have here with a galvanic or voltaic cell. We're going to say that the cathode is the positively charged electrode. So here's a positive sign. So this is our cathode and the a note is the negative negative charged electrode. If we go back to these half reactions, the cathode we said was this first reaction here, which is connected. This cadmium solid. So this would be cadmium, you want to make your red. So it's easier to see. And then the anodes was related to this half reaction where nickel is a solid. So this nickel we also should include their ions. So with the cadmium half reaction, we had cadmium two ions floating around. And with the nickel half reaction, we're gonna have nickel two plus ions floating around. Yeah, art remember that when it comes to redox reactions we're losing one side loses electron. The other side gains it. We're going to say here that we said reduction happens at the cathode and we said oxidation happens at the a note. This would explain why electrons are heading in this general direction. They're heading towards the cathode rod. Now we want this to happen as easily as possible. So we want this nickel electrode to be giving up its electrons easily, ionization energy is the energy required to remove an electron. If we want to remove the electron easily, we want to make sure that the ionization energy is low electron affinity is our attraction towards electrons. Our propensity to gain electrons. We want the cattle to gain those electrons because that's where reduction occurs. So you want electron affinity to be high. Now electrons are heading towards the catholic for those who have taken physics at this point, we know something called closing the circuit And if you haven't taken physics yet. Don't worry, I'm going to instruct you right now. Closing the circuit basically we need to have like charges moving in opposite directions to close the circuit. So for this to act as a battery, we have negative electrons traveling this way. So we need negative charges traveling the other way. These negative charges come from this tube here. This two peers called the Salt Bridge. The salt bridge releases neutral ions. Now the name is kind of deceiving neutral ions ions have a charge. So how can they be neutral jewels? Well when I say neutral ions, I mean that the ion, they don't have acidic or basic properties. They're neutral ions. Now typically what we'll have in the salt bridges we could have as our positive ions floating around. We can have N I plus K plus remember group one A ions are neutral ions and then we could have chloride ions or nitrate ions. These are neutral ions because they come from strong acids. If you don't remember that, make sure you go back and take a look at my videos dealing with ionic salts in those videos that teach you how to identify them as acidic, basic or neutral. So the negative ions, we need them to flow from the cathode to the animal to complete the circuit. Remember you need like charges to move in office directions to close the circuit or complete the circuit. These negative ions here traveling towards the an outside and these positive ions are going to gravitate more towards the cat outside. But why are the negative ions going there? Well, they're going there because electrons are leaving this nickel electrode. So what's happening over time nickel electrodes are leaving And as the electrons are leaving, I mean that this side becomes more and more positive because as you lose those electrons you're gonna have more of this, this electrode rod dissolve and they're going to release more nickel two plus ions into the solution. This is not a good thing because if we have too much of these positive ions being deposited in the solution, the negative electrons will be attracted to them and they won't want to go forward towards the cathode to basically counter this effect. We have these negative ions traveling this way. They themselves will connect with these positive ions helping to bring down their concentration and therefore the electrons won't be as attracted to the AD outside and they'll keep going towards the cat outside. So to produce voltage, we want to make sure that the anodes side with the positive ions is kept low. Now here we want the catholic side concentration of positive ions to get high because you want electrons to again be moving towards that way. So these positive ions are moving more towards this side and we have positive ion c already two. So this side is becoming more concentrated in terms of positive ions, those are attracting electrons towards it. Now what starts to happen as the catholic gains more and more electrons, its surface will become partially negative because the surface becomes more negative. It's gonna track these dissolved calcium two plus ions and they're gonna start to adhere themselves to this electrode and the positive ions and negative surface cancel out each other. And you're going to get an n crusting of the cadmium ions on the electrode. So over time you're catholic actually gonna get fatter and bigger. So the catholic going to what we say plate out plate out means that the dissolved positive ions are being neutralized and solidifying into uh an extra layer on top of the cathode. We said that the an outside it's losing electron so it's dissolving away more and more. It's releasing more of these nickel two plus ions in the solution. Yeah, no, typically dissolves away. Now these are the biggest ideas that you need to remember when it comes to the galvanic slash bill takes up. Besides that, there are certain variables you should come remember and some that you don't know quite yet until now. So we're gonna say for all redox reactions under standard conditions, it is possible to determine if a reaction is spontaneous or not. Remember. Standard conditions means we're using our standard cell potential. Standard conditions are one moller one atmosphere ph equal to seven. So here remember entropy of our universe deals with the second law of thermodynamics that the entropy of the universe is ever increasing Gibbs Free Energy is Delta G0 here remember K. Here represents our equilibrium constant. Our self potential is um is standard cell potential is zero. With these set variables. We can determine if a reaction is spontaneous or not. And we can talk about different types of cells that exist for Well, if our entropy of universe is positive, meaning it's greater than zero and our delta G is negative, meaning it's less than zero and K is greater than one, then our standard cell potential will be positive. And this will represent a spontaneous electrochemical cell. And and that would represent a galvanic or ball takes. Now if we flip everything, we should expect the opposite result. So if you flip everything, you're now non spontaneous. So your non spontaneous electrochemical cell, We haven't talked about it yet, but that's called an electrolytic cell. And then here, If entropy of universe is zero. Standard gibbs free energy zero. Your equilibrium constant is one and your standard cell potential zero that you are at equilibrium. Remember what we said earlier when you're at equilibrium, That represents what type of battery it represents? A dead battery. Alright, so keep in mind a galvanic or voltaic cell represents a spontaneous cell. That's basically a battery. It produces and discharges electricity. Remember the different periodic trends associated with it. Remember that the anodes electrode is negative and the cathode electrode is positive. Remember that reduction always happens at the cathode and at the anote is always oxidation
Electrolytic Cell Concept
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Hey everyone before we talk about the other type of electrochemical sound, let's revisit certain variables. So here we're going to say, in terms of spontaneity, the following correlations between the following variables can be made. So here we have gibbs free energy. Under 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. R 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 of all takes. Now, here, if we were to reverse everything, reverse the sign would expect the opposite result. So this would be a non spontaneous reaction and that 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 going to say that we are at equilibrium and you'd represent a dead battery. Now, here, when we talk about electrochemical cell, it doesn't function spontaneously. So it requires an outside energy source. So it requires a battery. Now, here we're gonna say, our electrochemical cell or electrolytic electrolytic cell is a non spontaneous electrochemical cell and it consumes electricity. And so requires a battery. Remember a galvanic or voltaic cell is different. It is literally a battery it produces and discharges electricity here. This one needs a power source. So it needs a battery. But here it doesn't matter if it's an electrolytic cell or galvanic cell. Oxidation always occurs at the 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 adam. What you should realize here though is that with an electrolytic cell things are not spontaneous or they don't happen naturally. The catholic here is negatively charged negative electrons don't want to go to something that's already negative. Remember life charges repel, we need that outside energy source to force the electrons to go that way and then electrons don't want to leave something that's positive here, the analyst 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 anodes. We don't want the electrons to stay near the anodes 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. 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 and it is still the site of oxidation. Right? So here, just remember these few key things about electronic cells. Remember the variables up above to help us determine if the reaction is spontaneous, non spontaneous or a dead battery.
Galvanic vs. Electrolytic Cells
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Hey guys, in this new video, we're going to continue with our discussion off our galvanic cell with one that spontaneous and see the connection between our K R Delta G and our new value self potential, the E value. So let's take a look at this question. It says, Ah, certain electrochemical cell involves a five electron change. So here I'm telling you, how many electrons are going to be canceled out. So and here equals five and has a value of K Q equal to 3.0 times 10 to the 16 at 2 98 Kelvin. The value of Delta H for the reaction is negative. 68.3 kg per mole. Calculate the values off Delta G, Delta E and for standard electrochemical cell constructed. Based on this reaction and also Delta s for the reaction. So we have a lot of variables we have to look at. Well, here I give us our K value. Remember, if we know okay, we conform a connection between K and with our e value, which we're gonna just refer to it as our SSL. Remember, the equation that connects them together is SL under Standard State conditions. That's what the zero means equals. Rt over. Number of electrons transferred Her changed times. Faraday's constant times. Allen of K. So let's plug in what we know Our is our constant, which is 8.314 Jules over most times K. Our temperature is already in Calvin. I gave it to his 2 98 Kelvin so we can plug that in the number of electrons changing or being transferred US five and then Faraday's constant remember, that's a constant. It doesn't change here. What's 96,500 is a rough approximation to be more accurate. It's 96,000, 485 cool ums over moles of electrons. And this five is five moles of electrons. Mhm, remember, see up Top is cool ums and columns is just charge And then we're gonna do the l n off 3.0 times. 10 to the 16. That's r K value. We plug all that in and we're gonna say the units for S L R volts volts. So he will be 0.195 volts. Okay. And remember, when we're talking about volts, one vault is equivalent toe one. Jewell over one. Cool. Um, so just remember that for later on, that's what one volts equals. So we just found our e value. If we know our e value, we confined our delta G value because they're connected by this equation. Delta G zero equals negative end times. Fair days, constant times R s. L remember. And is the number of electrons transferred, so five moles of electrons transferred. Faraday's constant is 96,485 school ums over moles of electrons. Yeah. Remember, we just said that vaults are jewels over columns. Right. So this is 0.195 jewels oversee cool apps. And in that way, we can see all the units getting canceled out, and we can see why air units are what they are. So moles of electrons cancel out cool apps. Cancel out. Our units will be left in jewels. Okay, so you plug all that in. That gives us negative 94,000 73 jewels. So so far, we found our SL. We found out Delta G. Finally, we have to find our Delta s. Remember, we know Delta G. We know Delta h. We confined Delta s because the equation that connects them all together is Delta G zero equals Delta H zero minus t Delta s zero. So now, technically, our Delta H is in killing jewels. So we should change our delta G also to kill jewels. So this will be negative. 94.73 killer tools equals negative 68 0.3 Killah Joel's over moles times temperatures to 98 Kelvin multiplied by s and actually subtracting 2 90 Calvin. And that's multiplying with Delta s. So we need to isolate Delta s. So we're gonna add 68.3 to both sides. And we're gonna say that answer we're gonna divide by the negative to 98 Kelvin. So when you add those two together, that gives us yeah, 25.77 29 killer jewels and will be negative and he calls negative to times Delta s divide both sides by negative to Kelvin, the negatives council out. So our answer at the end is gonna be a positive. Okay, it's gonna be 0. killer jewels over Kelvin's and this small shouldn't be there. And that right that will represent your Delta s. So remember, we have to remember what formulas connect the different variables together. So this one connects our s a with R K. This connects our delta G with R s. L remember we could also used We could have also said Delta G equals negative rt times L a N k that would have connected us between Delta G and with our K value. So we could have used this first, actually, if we wanted Thio in order to go from the K that I gave you the question directly to Delta G. So remember, these are different equations. All of them connect them to one another as long as you could remember those formulas. You remember the connections that you have between one variable and another variable. Now that we've seen this, I want you guys to attempt to answer this question here, I give you the e values for two half reactions and I want you to figure out what chaos now remember. All we have to remember here is the larger e value means that reduction is occurring. The larger e value means reduction reduction happens at the cathode the smaller e value means oxidation is occurring. And if oxidation is occurring, that must be your an ode. And remember, how do you find your overall e? You're East cell value. It's what Minus what? Once you find that variable, that answer for S l remember. What's the connection between SL and K? What equation do we use? Once you know the equation to use, then it's your job toe. Isolate K. Don't worry. If you get lost, just come back and watch the next video and I'll show you how to approach this problem. Good luck, guys.
Given the following standard reduction potentials,
Hg22+(aq) + 2 e– 2 Hg (l) E° = +0.789 V
Hg2Cl2(s) + 2 e– 2 Hg (l) + 2 Cl-(aq) E° = +0.271 V
determine Ksp for Hg2Cl2(s) at 25 °C.
Cell Notation & The Nernst Equation
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here in this example in states, the some notation for Redox reaction is given as the following at temperature of 2 98. Kelvin remember, when we're talking about some notation, we say it's easy, like a B C. So this represents are an ode, our break or physical break that separates the two solutions from each other and then our cathode. So here first it says right, the balance, half reactions occurring at the anote and the cathode. So we know what theano do. We have zinc solid going to make zinc two plus ion. Remember, we wanna put electrons on the more positive side. This side is neutral, cause Inc has no charge. Zinc here has a plus to charge. So discharges plus two, which means we would add two electrons to this side so that both sides are zero at the end. Then for the cathode, it's nickel two plus going to nickel solid again. We add electrons with a more positive side, so we'd add two electrons to this side. So this would be at the an ode. This would be at the cathode. If we combine those two together, that helps us to give the complete balance. Redox reaction. So bringing them both down. We have zinc solid. Gives us zinc two plus ion plus two electrons. Then we have two electrons, plus nickel. Two plus gives us nickel solid. So what happens here? Both electrons cancel out, and what's left at the end is our overall equation. So zinc solid plus nickel two plus gives us zinc two plus plus nickel solid. Remember, your ions are acquis in solution, and the neutral metal forms are solids. Now that we know that we can determine our self potential, remember self potential. This means under standard conditions, equals cathode minus an ode. Yeah, here, I'm giving you the values. I didn't give them to you beforehand, but now I am So we said that the cathode had the nickel and here the value for nickel. It's self potentials 0.23 volts and then the ano deals with zinc. Its value is negative. 76 volts. So remember it's cathode minus an Oh, so that's negative. 00.23 volts minus ah minus 0.76 volts, Remember, minus of a minus really means positive. So this negative 0.23 volts plus 0. volts, which comes out to a positive 0.54 volts. For the next question, we have to calculate the maximum electrical work that could be produced by this cell. So just realize when they're asking for electrical work, they're really asking about Gibbs Free Energy and that connects to sell potential in this form. So Gibbs free energy equals negative end times f times SL and is the number of electrons that are transferred. So basically the electrons that cancel out which would be to so n would be two electrons involved in this reaction. So two moles of Electrons F is known as Faraday's constant. So here Faraday's constant is 485 cool ums per mole of electrons. Sometimes your professor will just round it off to 96,500. That's fine, too, but here I just wanna be as accurate as possible. So that would be on negative will be 96, columns per mole of electrons, so most of electrons cancel out. Now we have columns and realize here that when we say volts because this is in volts, volts equals jewels over Colom's. So this is 0.50 for Jules over. Colom's so cool it was canceled. So we're gonna have jewels. So this comes out to negative 1000 and four negative. 104,204 jewels as our answer Now, Finally, our last portion asked us to calculate the reacted quotient for this cell and the cell potential under nonstandard conditions. So we need room for this guys. So let me take myself out of the equation out of the picture. Remember, Q is you're reacting question or reaction questioned. It's just equal toe products. Overreacting. It's it ignores solids and liquids. So what we have to do is we look at the overall equation that we have here again. We ignore solids and liquids. So here this is a quiz. So we're gonna keep this around. This is a quick we're gonna keep it around. The other two are solid, so ignore them. So this equals zinc two plus ion over nickel two plus ion. And what values do we plug in here when we go back up here? And look, I gave you the concentration for each ion. Those are the numbers. They're gonna plug in there to find que So that's 0.37 divided by 0.59 which comes out to 6. 119 That's cute. Now that you know Q, we can answer the second portion. We have to find the self potential under nonstandard conditions. Remember, standard conditions really means one atmosphere. Ah, concentration of one Mueller and 25 degrees Celsius. That's what standard conditions are, and the concentrations here are not one Moeller. So that's what we're looking at under nonstandard conditions. And if we're looking for under nonstandard conditions, that means we use what's called the nursed equation. So in the nurse equation, it's self potential. Under nonstandard conditions equals self potential under standard conditions minus 0.5916 over the number of electrons transferred times log of cute. So we found himself. Potential understanding conditions earlier was 54 volts, minus 0.591 6/2 electrons transferred times Log of Q, which we just found A 6. When we punch this into our calculator, gives us point 516 volts as our final answer. So As you can see, just with a simple some notation, There's a lot of information that we can derive from it. We can figure out the overall equation. What's happening at the Catherine the An ode gives free energy equilibrium constants. So there's a lot of information that we went over here. Remember each one of the situations, what it entailed for us to do on our part, yeah.
The Nernst Equation
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So if we take a look at this question, it says, answer each of the following questions based on the following half reactions. So for party asked, which is the strongest oxidizing agent. So let's think of this in terms of a road map. So oxidizing agent means that you've been reduced and if you've been reduced, that would mean that you are the Catholic and if you're the cathode, that must mean you have the largest e value. And if you've been reduced, that means that you are next to the electrons. All right, so to be able to do this, make sure all the electrons are on the same side. For all behalf, reactions and all the half reactions, the electrons are reacting. So we're good now that they're all on the same side. We can just look at the e values largest value would be of the first equation. It's 1.36 and so we know that would represent the cathode because reduction is happening. We're looking for the elements or ion next to the electrons. So here go the electrons that we have. Who's next to them? Chlorine gas is next to them, so the answer here is BCL two gas. Now let's say that they all didn't have the electrons as reacting bowl we do. Let's say that we had another equation. Br to let's say we have br two of them and we had the electrons on the product side on. Let's say here that it's e value waas negative 0. So that's not it's actual number. I'm just making it up here. This won't be the odd one out. Everyone else has their electrons on the react inside, which is the side you really wanted to be on its on the product side. What will we do? We would reverse this reaction. So the products have become reactions and the reactor and become a product. What effect is reversing the reaction? Have reversing The reaction would reverse the sign of e r self potential have become positive point for 10 So again, make sure all the electrons are on the same side. Probably the react inside if they're not reverse the reaction so that they are. When you reverse the reaction, it flips the sign off yourself potential, which is delta, which is delta Eat. All right, So now that we've answered that one, let's take a look at another one. So we're looking for the strongest reducing agent reducing agent here means you've been oxidized, which means you are the an ode normally added would have the smallest e value and realize that you've been oxidized. That would mean that you are losing electrons, so you're gonna be away from electrons. So if you go back up, this equation has the smallest value. And here are the electrons. Since we're being oxidized, we don't wanna be on the same side with them. So the answer here would be vanadium solid. So the V solid. So that would be your answer here. And then finally, we'll I minus reduced call to to see l minus. So if I minus is reducing CEO, that means that I minus is being oxidized, which means that it should have the smaller e value. So if we look, does I minus have a smaller e value than cl? Yes, it does its point 535 versus 1. So we'd say Yes, I minus will be oxidized and in the process, cl two will be reduced. So that's how we compare different half reactions to each other, and it's all based on the given some potential that we have for each half reaction.
Additional resources for Galvanic Cell
PRACTICE PROBLEMS AND ACTIVITIES (29)
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