Galvanic versus Electrolytic Cells

by Jules Bruno
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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