hi everyone Welcome to our next video on redox reactions we're going to be talking about different types of redox reactions and so many of our learning outcomes are the same as what you've seen in a previous video so to remind you we determined that a redox reaction is one in which one or more elements is going to change its oxidation number and so in order to determine if a reaction is a redox reaction we would want to First assign the oxidation numbers to all the species involved and then see if one of them has changed in the process so last time we talked about the mnemonic device Leo the Lion says gur we can use Leo to remember that the loss of electrons is oxidation and gur to remember that the gain of electrons is reduction another way that helps me to remember what oxidation and reduction are is that oxidation is an increase in oxidation number and both oxidation and increase start with vowels and then a reduction is when we see a decrease in oxidation number and both of those start with consonants so my vowels together tell me oxidation is increase and my consonants together tell me reduction is a decrease in oxidation number so as we said earlier we're going to look at different types of redox reactions and um we'll see what makes them redox reaction so first we have combustion reaction a combustion reaction is one and where we have some sort of fuel which is going to be burned in the presence of an oxidant and usually this generates a good amount of heat and often a flame right so often we think about burning a fuel in the presence of oxygen so here's an example of such a reaction the combustion of Ethylene so ethylene is c2h4 and we see that it is burned in the presence of oxygen gas and in this type of combustion reaction and in the ones that you'll mainly be dealing with in general chemistry the products are always going to be carbon dioxide gas and water so this is always true when our fuel that is being burned is organic meaning that it has carbon hydrogen and usually some oxygen present are products when we burn something like that are always going to be CO2 gas and water so we know that this is a redox reaction it's a type of redox reaction so let's look at the oxidation numbers in order to determine what is changing so I'm going to fill those in for you and then I'll restart the video once I have those ready all right so I've added in all of my oxidation numbers and I found these by looking at the individual molecules and assigning the oxidation number for example in c2h4 I assigned the oxidation number of plus one for hydrogen I recognized that there were four hydrogens so the total contribution to charge would be plus four and then recognizing that this is a neutral molecule the sum of my total charges has to equal zero so the carbon would have to have a minus four total contribution to charge since there are two carbons each one would have an oxidation number of minus two and so that's the strategy I used when assigning all of these oxidation numbers for the different elements and compounds involved so now what we want to do is look to see which species are changing their oxidation number so the first one that I see is that oxygen is going from zero right to minus two in the carbon dioxide we also see oxygen as minus two in the water so we can see that oxygen is having its oxidation number decreased it goes from zero to minus two so we see a reduction in our oxidation number which tells me that this is a the species that is being reduced we also can see that the carbon goes from being minus two to plus 4 in the carbon dioxide gas so the C of c2h4 goes from -2 Plus 4. so this is an oxidation right so this the carbon is oxidized so this is a redox reaction a combustion reaction our next example is a single displacement or replacement reaction so these are redox reactions in which an ion in solution is displaced or replaced via oxidation of a metallic element so here's an example we see that the ion in solution when I look at my reactants would be the silver nitrate right the agno3 so the cation that's present in that would be the silver right the silver ions are present in solution but when this is reacted with the copper the copper replaces the silver as the ion in solution so here we have copper two plus ions that now are in solution and we see then as a product the silver is now our metallic element so this is a replacement or single displacement reaction so I'm going to fill in my oxidation numbers so we can look at what is changing in this reaction so I filled in our oxidation numbers for this reaction so let's look to see which numbers have changed first I can see that copper starts off as the element so it has an oxidation number of zero and then that changes it increases to plus two in the copper nitrate so we can see that copper goes from zero to plus two so there is an increase in its oxidation number and an increase means that we're seeing oxidation when I look to see what else has changed I notice that my silver ions start out as plus one and these decrease in oxidation number down to zero so my silver ions go from a plus one oxidation number down to zero so the decrease tells me that this is the reduction so copper is oxidized and the silver ions are being reduced and the pictures at the bottom of this slide show this reaction so inside the test tube is silver nitrate solution the wire is made of copper once these are once you stir the solution with the copper wire then the displacement reaction occurs and what's happening is the copper is going into solution so that's why we see the characteristic blue color and then the silver is actually going to be deposited onto the wire so that's the Gray colored metal that's being deposited so we have a few more examples so in these we want to identify whether these equations represent redox reactions and if they do then we're going to name the oxidant or the oxidizing agent and the reducing agent so let's fill in our oxidation numbers all right with those oxidation numbers filled in let's look to see if our first example is a redox reaction so we're looking to see if any of our oxidation numbers have changed when I look at zinc as a reactant it's plus two and as a product it's still plus two when I look at carbon as a reactant it's plus four and as a product it's also plus four and similarly for oxygen as a reactant it's minus two and its oxidation state is -2 as a product so here I see no change in terms of my oxidation numbers which tells me that this is not a redox reaction let's look at our second example here we have gallium reacting with bromine to make gallium bromide let's look to see if we have a change in oxidation number I do see that gallium starts as zero and increases to plus three and my bromine starts at zero and decreases down to negative one so gallium goes from zero to plus three so this is increasing oxidation whereas my bromine goes from zero down to negative one so this is a decrease or reduction now this is asking us to name the oxidant and reductant it's just another way for calling the oxidizing agent and the reducing agent so remember the species that is oxidized is my reducing agent it essentially provides the electrons that can be given to the bromine for its reduction the species that is reduced is my oxidizing agent so here the bromine is accepting the electrons from the gallium and so it oxidizes the gallium and serves as our oxidizing agent all right let's look at our last two examples in this first reaction we have barium chloride reacting with potassium sulfates to give us barium sulfate and potassium chloride so let's look to see if any of our oxidation numbers have changed we see that barium starts at plus two as a reactant and then the products it's plus two the chloride starts at -1 and is minus 1 as the product potassium is plus one as a reactant plus one is a product sulfur plus six plus six as a product and oxygen stays at -2 as a reactant end product so here I see no change in terms of my oxidation numbers so this means that this is not a redox reaction let's look at our last example here we have hydrogen peroxide that is decomposing to give water plus oxygen gas when we look at the oxidation number for oxygen in the peroxide remember that this is one of our exceptions normally oxygen has a minus two oxidation number but when it's in a peroxide it would have a minus one the reason is that we want to First assign a plus one oxidation number for hydrogen and here we have two hydrogens so that gives us a plus two total contribution to charge to balance that and have this equal to zero for the neutral molecule the oxygens have to have a minus two overall charge and since we have two oxygens each one would carry a minus one charge right for their oxidation number so here we have oxygen at a minus one oxidation number and we look to see oxygen in our products has a minus two oxidation number so we go from -1 down to minus two right so oxygen in the peroxide goes from minus one to minus 2 in water so here we see a reduction but we also see that oxygen goes from -1 to zero when we look at the oxygen gas so the oxygen in the peroxide goes from minus one to zero in the oxygen gas because this is an increase in our oxidation number this is oxidation so let's assign the oxidizing agent and the reducing agent we know the species that is reduced is our oxidizing agent right so the peroxide is our oxidizing agent and we know that the species that is oxidized is our reducing agent so it just happens to be that the peroxide is both the oxidizing agent and the reducing agent so this is a little strange it's actually a specific type of redox reaction it's called a disproportionation reaction so we have a reaction in which the same element undergoes both oxidation and reduction