Redux reactions are a fundamental process in chemistry and involve the transfer of electrons from one substance to another in these reactions oxidation occurs when a substance loses electrons while reduction occurs when a substance gains electrons when two different Metals undergo Redux reaction the more readily oxidized metal will lose electrons and transfer them to the other metal which will be reduced for example the more reactive zinc will transfer electrons to the less reative copper since Redux reactions involve the movement of electrons we can separate our two metals and connect them with a wire forcing these electrons to move through the wire and therefore generate current or we could do the opposite we can use electrical current supplied by a power source such as a battery to drive a Redux reaction that is otherwise nonspontaneous in these two examples we have constructed what are called electrochemical cells Electro referring to the electrical current and chemical referring to the reacting chemical substances in this video we'll investigate the two types of electrochemical cells we'll start with a general overview and then dive into the specifics and usefulness of each cell an electrochemical cell in which a spontaneous Redux reaction generates an electrical current is called a voltaic or galvanic cell and contrast a cell in which current from an external power source such as a DC battery or AC power source drives a non-spontaneous Redux reaction is called an electrolytic cell both voltaic and electrolytic cells consist of two electrodes which we use to conduct current electrodes can be made of metals that may be the same or different depending on intended use or inert non-metals like carbon and graphite we previous established that to allow electrons to flow between the two electrodes we must connect them with a wire which we show in red and black in an electrolytic cell the wire intersects with the power source because this is a non-spontaneous reaction we can place both electrodes in the same electroly whereas in a voltaic Cell the two electrodes are immersed in separate electrolytes we call these half cells in both types of electrochemical cells the electrolytes are composed of ions that act as charge carriers the ions need to be free moving to transfer current in voltaic cells these ions are dissolved in solution for example in the zinc and copper voltaic cell we have electrolyte Solutions containing zinc or copper salts respectively in electrolytic cells ions are either dissolved in solution or are in the molten phase note we'll often see a voltaic cell drawn drawn with a voltmeter across the two electrodes like this a high resistance voltmeter only really measures potential between the two electrodes and does not conduct current through it nevertheless we'll draw in our voltmeter when discussing current flow the voltage that we can now measure depends on multiple factors including electrod type and electrolyte concentration this table shows how different half cell combinations generate different voltages okay with our general comparison of mind let's compare our two types of electrochemical cells in more depth starting with voltaic cells the zinc copper cell which we've already seen is a typical example of a voltaic cell zinc is more readily oxidized compared to Copper so when these Metals undergo a reduxx reaction together zinc will be oxidized and transfer electrons to Copper ions zinc will become aquous zinc 2+ the copper plus ions will be reduced gaining electrons and forming solid copper when we set up our voltaic cell for this reaction we have one half cell containing a zinc electrode immersed in a solution containing a dissolved zinc salt for this example we'll use a solution of zinc nitrate we call the zinc electrode the anode as it's the site where oxidation occurs oxidation will in fact always occur at the anode since since the anode in a voltaic Cell could be considered the source of electrons it's identified as the negative electrode we say that its polarity or sign is negative the half equation for the oxidation of zinc summarizes this process where we see the oxidized zinc will lose to electrons and form the zinc 2+ ion this means that as the reaction takes place the concentration of zinc 2 plus ions in the solution will increase and the mass of the zinc anode will decrease electrons will travel from the zinc anode Through the Wire into the copper electrode this electrode is immersed in a solution containing a copper salt for this example we use a copper 2 nitrate solution summarized by the half equation for the reduction of copper the electrons entering the copper electrode attract positive copper 2+ ions in the solution each copper ion will gain two electrons and be reduced to copper metal over time we see the concentration of the copper 2+ ions in the solution decrease in the overall mass of the copper electrode increase with the addition of the newly plated copper the name of the electrode at which reduction occurs is the cathode reduction will in fact always occur at the cathode since the cathode in a voltaic Cell could be considered the receiving end of the flow of electrons it is identified as the positive El electrode we say that its polarity or sign is positive during the course of this Redux reaction we've described a continual buildup of positive or negative charge in each of our half cells if left unchecked this will ultimately kill our voltaic cell this problem is solved by the addition of the salt bridge easily forgotten by many chemistry students the salt bridge acts as a bit of an unsung hero in the voltaic cell consisting of a piece of filter paper soaked in an inert electrolyte solution or an inverted glass u-shaped tube filled with an inert electrolyte solution the salt bridge connects our two half cells and completes our circuit in doing so we allow for ions or charge to move between our electrolyte Solutions without allowing them to mix as the voltaic cell runs ions within the salt bridge will move to balance our buildup of positive or negative charge in this example negative ions will move to our zinc half cell attracted to the addition of positive zinc ions positive ions in the salt bridge will move to the Copper half cell attracted to the negative charge buildup by the loss of copper ions in solution each half cell will be neutralized by this process and the voltaic cell will be able to run for a longer period of time the overall flow of negative charge in this example Le is in the clockwise Direction this negative charge current consists of electrons in the external circuit and negative ions in the two half cells and salt bridge the flow of positive charge in this example is in the anticlockwise or counterclockwise Direction This positive charge current consists of conventional current in the external circuit and positive ions in the two half cells in salt bridge let's talk about cell potential and how we see it impact and common applications of voltaic cells in a conventional voltaic cell the potential between the two half cells is at its highest in the initial stages of the Redux reaction this potential drives the reaction forward over time we say that our cell discharges which means that the concentration of reacting ions increases in the anod half cell and decreases in the cathode half cell this will lower our cell potential until eventually one of the reactants has been completely used and the Redux reaction can no longer proceed at this point current stops flowing trying to recharge a voltaic cell that contains an irreversible Redux reaction will not work these types of cells are called primary cells a disposable alkaline battery is an example of a primary cell in everyday life we may use non-rechargeable or run flat cells like these in TV remotes clocks or flashlights since these cells cannot be recharged they must be replaced when they can no longer Supply Power rechargeable batteries in contrast contain reversible reactions we call these secondary cells a secondary cell is a cell in which electrical current from an external power source such as a wall socket can be supplied to drive the reverse Redux reaction to recharge the cell and increase its voltage again when a secondary cell or battery is recharged the supplied current flows opposite to the current when the cell is in use and being discharged now that we've seen a couple of applications of voltaic cells let's go into more depth with our other electrical chemical cell type we said that in an electrolytic cell electrical energy is used to drive a non-spontaneous Redux reaction we use a power source to set up potential across our two electrodes making the electrode connected to the positive terminal of the battery the positive electrode and the electrode connected to the negative terminal of the battery the negative electrode electrons will flow toward the positive electrode and then away from the negative electrode this makes the negative electrode the cathode as it is the site of reduction in the positive electrode the anode as it's the site of oxidation you may have noticed that the polarity or sign of the anode and cathode in this electrolytic cell is opposite to that found in a voltaic Cell this is an important thing to note as it can be a place of high confusion for IB chemistry students we use the Redux reaction within an electrolytic cell most often for electrolysis electrolysis is the process by which electrical current decomposes or breaks down a substance into simpler substances Electro refers to electrical current and Lis refers to the breakdown cutting loosening or decomposition of a substance the electrolysis of water which we've seen here is a bit of a more complex example let's take a look at a simplified version of electrolisis and identify our cathode anode and flow of charge particles in this example we'll use a bunson burner to melt the salt and use as our molten electrolyte a solid lead to bromide will melt into a molten mixture of lead and bromide ions we'll use inert graphite electrodes to pass current from our external power source into our molten ionic salt this will cause our ions to move toward the electrode of opposite charge positive ions will move toward the negative electrode gain electrons and be reduced therefore at the cathode we see the reduction of lead cat ions negative ions move toward the positive electrode lose electrons and are oxidized at the anode we see the oxidation of bromide ions these ions have been discharged as they've been converted to neutral elements at their respective electrodes from which they can also be collected in this example bromine Vapor would be collected at the anode and Lead would be collected at the cathode the overall equation for this reaction sees lead to bromide turn into liquid lead and bromine vapor in summary we discussed the general definitions and structures of our two types of electrochemical cells within a voltaic cell a spontaneous Redux reaction generates current across a wire while in an electrolytic cell an external source of electricity is used to drive a non-spontaneous Redux reaction while oxidation will always occur at the anode in reduction at the cathode in either type of cell the polarities of their respective electrodes are opposite we saw a few specific examples of voltaic and electrolytic cell in action in primary voltaic cells like an alkaline battery the Redux reaction stops when one of the reactants is used up the reactions in these cells are irreversible making them non-rechargeable in secondary cells like that in your smartphone the reactions are reversible allowing these cells to be recharged for electrolytic cells a molten ionic substance can be discharged to form neutral metal and non-metal elements we can use this as a means of collecting these elements in their natural state understanding these principles is crucial for chemists as electrolytic cells are foundational to numerous industrial processes and energy storage Technologies and as energy storage becomes more important for tackling large issues such as climate change mastering this topic becomes even more essential for IB chemistry students who are looking to change the world through science but