 We are going to be looking at redox reactions in this video and this name may sound a little bit unfamiliar to you right now but you have actually seen or you know of some reactions that happen around you that are redox reactions. So one very common example is that of rusting and you would have seen rusting in cars or on your bicycles that look something like this and rusting is a good example of a redox reaction. Another example that you may be aware of is photosynthesis which is also a redox reaction. Then there is also combustion of fuels like methane which again is an example of a redox reaction. So you have seen many of these redox reactions around you but then the question is what are these redox reactions and there is a clue in the name itself. Redox reactions are essentially reduction and oxidation reactions. So the word redox tells us that in these reactions there is oxidation and reduction taking place and we know from before what oxidation and reduction are. Oxidation is the loss of electrons and reduction is the gain of electrons. So redox reactions are those reactions in which some species undergo oxidation whereas others undergo reduction or in other words we can say that some species lose electrons while others gain electrons. So let's take an example and understand this better. So as we saw let's say in a redox reaction some species undergo oxidation and some species undergo reduction but then how do we keep track of which species have lost electrons and which species have gained electrons. So for that we use oxidation numbers which is an idea that we use to keep track of electrons and reactions. So there are some rules used to determine the oxidation numbers of chemical species in a reaction. Let's discuss some of them. The first one is that the oxidation number of a tree species is going to be zero. So if you have pure iron taking part in a reaction in that case its oxidation number will be zero. Similarly if you have chlorine gas or hydrogen gas that is taking part in a reaction in each of these molecules each chlorine will have an oxidation number of zero. The same goes for the hydrogen here. So the idea is if there is a free species in the reaction its oxidation number is going to be zero. Next if we have mono atomic species like let's say this sodium or this bromine. The oxidation numbers in both of these cases will be equal to the charge on these atoms. So we know that in the case of sodium it loses an electron to get to a charge of plus one. So the oxidation state of sodium is plus one. Similarly in the case of bromine in reactions usually it will gain electrons. So the charge on bromine in those cases is negative one and so the oxidation number of bromine is negative one. Now it's not necessary that a given species has only one oxidation number. It could have different oxidation numbers depending on what compound it forms. So for example if we consider hydrogen in most of the compounds that it forms it will have an oxidation number of plus one and similarly in the case of oxygen the most common oxidation number it has is negative two. Sometimes if the oxygen that we are considering is forming a peroxide let's say hydrogen peroxide it can have an oxidation number of minus one as well. But the most common oxidation state that we see is minus two. Another one is fluorine which has an oxidation number of minus one and assigning oxidation numbers of hydrogen, oxygen and fluorine is usually the starting point in calculating oxidation numbers of the other species. Another rule that we have in case of oxidation numbers is that for a neutral compound the sum of oxidation numbers of the atoms will be equal to zero. So like let's take the case of this HNO3 here. Now from what we saw before let's say that we know that the oxidation number of hydrogen is plus one and for each atom of oxygen the oxidation number is negative two. So let's say we want to find the oxidation number of this nitrogen. So we know that the sum of oxidation numbers of all the species in this neutral compound is going to be zero. So we know that the oxidation number of hydrogen plus the oxidation number for nitrogen plus the oxidation number for three atoms of oxygen is going to be zero. So we know that for hydrogen the oxidation number is plus one. Let's say the oxidation number of nitrogen is x and we know that for oxygen the oxidation number is negative two and we know that this sum will be equal to zero. So if we solve for x here we get x to be five. So in that way we can calculate that the oxidation number of nitrogen in this case will be five. So these were some of the rules and some context for oxidation numbers. Now let's see how we can use this idea of oxidation numbers to find out whether a reaction is a redox reaction or not. So now to see how we can identify whether a reaction is a redox reaction or not let's take the example of rusting that we saw before. So we know that rusting is a process in which iron undergoes oxidation forming the reddish colored rust which is an oxide of iron. So if we were to write a reaction to describe the process of rusting it would look something like this. Now usually rust is denoted as Fe2O3.NH2O where N is a number denoting the number of molecules of hydration. So this could be 2H2O or 3H2O and that is because for rusting as a process to happen there is also water involved but for simplicity for now let's just assume that this reaction represents rusting and we are going to use Fe2O3 to represent rust. So we have iron reacting with oxygen to give this oxide of iron which is chemical representation of rust. Now if we look at the oxidation states of iron and oxygen as we saw before since both of them are free species their oxidation numbers are zero and this is before the reaction takes place. Now when the product is formed we know that for oxygen the common oxidation number is negative 2. So as we saw here we know that the sum of the oxidation numbers of iron and oxygen will be zero. So if we take the oxidation state of iron to be x so we have 2x plus 3 times minus 2 that is minus 6 will be equal to zero. So if we solve for x we get x to be 3. So that means for this iron the oxidation state is plus 3. So now if we look at the change in oxidation numbers of both of these as the reaction takes place we notice that iron goes from an oxidation state of zero to plus 3. So that means iron is losing electrons and similarly in the case of oxygen it goes from zero to minus 2 which means that oxygen is gaining electrons or in other words we can say that because iron is losing electrons iron is getting oxidized and because oxygen is gaining electrons oxygen is getting reduced. So based on how we define redox reactions we have oxidation and reduction taking place in this reaction which is why rusting of iron is a redox reaction.