 We're going to look at decomposition reactions in this video and these reactions can be represented like this where a compound AB breaks down into products A and B. So these are reactions where the original compound breaks down or in a way decomposes into these products. So let's look at an example of a decomposition reaction. So here we have mercury oxide which on heating decomposes into mercury and oxygen. So this is an example of a decomposition reaction. But this specific reaction has an interesting story behind it. So this reaction was first reported by Anton Lavoisier who was one of the earliest chemists studying chemical reactions and he used such a setup where this is a furnace and this mercury oxide would be placed inside this and on heating he would separate out the gas and study the properties of this gas which led to the discovery and identification of oxygen. Now let's take one more example which you would be probably familiar with which is calcium carbonate on heating breaking down into a quick line and releasing carbon dioxide. Now one thing when you consider both of these reactions is if we look at the oxidation states in this example because we know that the oxidation number of oxygen is minus 2 the oxidation number of mercury will be plus 2 because on the whole this is neutral and here because it's free species the oxidation numbers are zero for both mercury and oxygen. So now if you see here since there is a change in oxidation numbers and there is a reduction and an oxidation happening this reaction is also a redox reaction. But on the other hand if you look at this reaction we know that the oxidation number of oxygen is minus 2 and for calcium it is usually plus 2 and if you solve for the oxidation number of this carbon so let's say it is x plus the oxidation number of calcium which is 2 plus the oxidation number of the oxygen which is 3 times minus 2 should be equal to zero. So if you solve for x here which is the oxidation state of the carbon we get x equal to plus 4 which is the oxidation state of this carbon. Now on the product side we have plus 2 for the calcium minus 2 for the oxygen and since oxygen here is minus 2 and there are two atoms here for carbon it will be plus 4 here. So now if we look at the change in oxidation numbers we notice that the oxidation number for carbon has not changed after the reaction nor has it changed for calcium or oxygen so that means although both of these are decomposition reactions the first one is also a redox reaction while the second one is not. So this is one thing to keep in mind because in most decomposition reactions we see the oxidation numbers changing but that does not necessarily mean that all decomposition reactions are also redox reactions. Now let's see one more example of a decomposition reaction where we have hydrogen peroxide decomposing to give water and oxygen. So hydrogen peroxide is unstable in nature which is why it readily decomposes into water and oxygen and one more thing to note about hydrogen peroxide is this is one of the exceptions in which oxygen will have an oxidation number of minus 1. So we know that hydrogen has an oxidation number of plus 1 so if we sum up the oxidation numbers we have 2 times 1 plus 2 times let's say the oxidation number of oxygen in this case is x the sum should be equal to 0 because on the whole this is neutral. So now if we solve for x here we find that x is equal to minus 1 and usually oxygen has an oxidation number of minus 2. So this is one of those cases which is an exception where oxygen shows an oxidation number of minus 1 and this reaction happens spontaneously because hydrogen peroxide is very unstable but in some cases like we saw before the decomposition may not be spontaneous and an external factor like heat may be required to actually bring about the decomposition. So for decomposition reactions they can be thermal decomposition reactions like we saw here or they could also be electrolytic decomposition reactions like the electrolysis of water or they can also be photolytic where the reaction happens because of sensitivity to light. So the point is that decomposition reactions can be spontaneous or non-spontaneous and in case they are non-spontaneous they can be initiated by any external factors like these.