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Video transcript

- [Instructor] In every chemical reaction, it's the atoms and molecules of the reactants that are getting rearranged to give us new products. And this rearrangement could happen in many ways, like for example, here, the atoms and molecules are combining to give me one single product. Or you can have a big molecule that breaks down into smaller molecules. Or you can have a exchange of atoms between the reactants to give us new products. So depending on the ways of rearrangement, we can classify chemical reactions into various types and in this video, we are going to talk about two of those types, combination and decomposition reactions. Now, before I define them for you, let me give you some examples. So here I have some examples of one of these reactions. Now, can you pause the video and figure out what is the common way in which is the reactants are rearranging to form the products? Pause the video and try it yourself. Okay, now if you have tried, let's see. Over here, in every example I can see, the reactants are combining to give me one single product. Here also, two reactants are combining to give me one single product. Everywhere you can see the same thing is happening. So these are the example of combination reaction. Combination reaction. At a molecular level if you see, the atoms and molecules of the reactants are combining to give me one single molecule, one single product. So I can say that in combination reaction, multiple reactants combine together to form one single product and if I have to write the general structure of these reactions, it looks something like this. A plus B gives me AB whereas it could even be three reactants or four reactants, A plus B plus C plus D, any number of reactants, they all combine to give one single product. That is an example of combination reaction. Now, decomposition reaction, that is totally opposite of combination reaction. See, in combination, multiple reactants combine to give me one single product whereas in decomposition, one single reactant breaks down, decomposes into multiple products. Look at some examples here. Here I have iron sulfate, this is breaking down into iron oxide, sulfur dioxide and sulfur dioxide. One single reactant is breaking down into multiple products. Similarly here, calcium carbonate is breaking down, here lead nitrate is decomposing, right? At a molecular level, something like this is happening. A big molecule is breaking down into multiple products and if I have to write down the general structure, it'll look something like this. AB breaks down into A plus B. Or you know, something like this is also true. ABC is breaking down into A plus BC. The main idea is that there will be only one reactant and that will be breaking down or decomposing into several products, the products could be anything. So now that you know what is combination and decomposition and you understand the basic difference in their definition, let's look at the examples of combination into much more detail. So here I have a magnesium ribbon and I'm going to burn it. Now, you can see that it burns with a dazzling white flame. In fact, there's a lot of light and heat and energy give out and after the reaction, we are left with a white powder. And you know, this whole reaction is an example of combination reaction. Let me write down this equation and you'll be able to understand it better. So here we have magnesium to begin with, Mg and it is reacting with oxygen. See, when anything burns in air, right, it reacts with the oxygen that is present in the air, so it's reacting with oxygen and finally, we get, since this is a combination reaction, these are going to combine and we'll get one single product which is magnesium oxide, MgO is the formula of magnesium oxide. Let me quickly balance this equation. Here we have two oxygen, so here I'm going to put a two here and since this is 2Mg, I'll need to put a two over here also. Yes. So you can see that here magnesium combines with oxygen to give us magnesium oxide. This is the white powder that is being left out and in this process, a lot of heat is being given out also. Now, let's look at another example. Burning of coal. Now, this is also an example of combination reaction. Here the carbon present in the coal, when it burns with the air, right, it combines with the oxygen in the air and finally we get carbon dioxide and this is also giving out a lot of light and heat energy. Let's write down its chemical equation. So here I have carbon that is present in the coal. It reacts with the oxygen of the air and finally we get carbon dioxide, these both combine to give me carbon dioxide and this is a combination reaction. And in this one also, a lot of heat is being generated. Now, let's look at another example. I have a white powder. This is calcium oxide. Now, when I pour some water on top of it, so water is going to combine with the calcium oxide and we will get a white milky mixture which is used to whitewash our walls. We'll talk about that later but let's write down its equation and see what is the combination reaction. So here I have this white powder which is calcium oxide, calcium oxide and this is going to react with the water, I am adding water onto it, H2O and these both are going to combine. And finally, I'm going to get calcium hydroxide. The formula of calcium hydroxide is calcium and hydroxide is OH. OH whole twice and this is balanced, two oxygen, two hydrogen, two oxygen, two hydrogen, one calcium. Yes. And in this reaction also, if you test the container in which this reaction is happening, you'll feel that it is getting warmer, so in this one also, a lot of heat energy is given out. So now we have looked at many examples of combination reaction. Here, multiple reactants combine to give me one single product and we also saw that in all of these reactions, energy was given out and such reactions are called exothermic reaction and in general, combination reactions are exothermic in nature, meaning they give out heat. But not all combination reactions are exothermic. There are some exceptions such as when nitrogen combines with oxygen and forms nitrogen oxide. In this reaction, a lot of heat needs to be given in. So this is an endothermic reaction. But we'll talk a lot about energy given out and energy required in a separate video called exothermic and endothermic reactions. Over here, let's now talk about some common examples of decomposition reaction. So first, I have some light green-colored crystals of iron sulfate and I'm going to heat them. Now, as I heat them, I see the color changing from light green to brownish color and also, there is a bad smell coming from here. So this tells me that there is some chemical reaction that has taken place. In fact, iron sulfate has decomposed into multiple products. Let's write this equation to see this. So I started with having iron sulfate, FeSO4, that is the chemical formula for iron sulfate. Now, when I heat it, I am going to heat it. Let me write over here heat. Heat is providing energy which is making it break or decompose. So it is decomposing into a brown powder, right? And you can recall that when iron rusts, iron oxide is formed and that is also brown in color so over here also we are getting iron oxide. Fe2O3 and plus the foul smell, the bad smell that was coming, right? That is because of sulfur oxide, sulfur dioxide and sulfur trioxide. So here we get sulfur dioxide plus sulfur trioxide. In chemistry in fact, whenever you have some bad smell coming out, generally it'll be something to do with sulfur. So sulfur is the smelly one in chemistry. Okay, let me just quickly balance this. Over here I have two iron, so let me just put two iron over here, so now I get oxygen is four twos are eight, three, two, five, five, three, eight, sulfur is two, sulfur is two. Yes, so this is the balanced reaction of the decomposition of iron sulfate. And over here, heat and energy was required and generally, heat energy's required because whenever you're trying to pull away the atoms of a molecule, then we need to provide some energy because there's an attraction between these atoms, so that's why to pull them away, we need to provide some energy and that's what is happening over here also. Wee are trying to break it down, we are trying to decompose this big molecule, that's why we need to provide energy over here. But there are many more things that are also happening and we'll talk about that in detail in exo and endothermic reaction video, okay? Now, let's look at another example. Here I have a spoonful of silver chloride. It is a white powder and I'm going to leave it under the sunlight. Now, after a while, I see that the color has changed, it has become grayish in color. Now, this color change tells me that there is some chemical reaction that is going on here. Let's write down this chemical equation to see. So silver chloride is AgCl and if I leave it under the sun, it is using the energy from sunlight and it is decomposing into silver and chlorine gas, Cl2, since Cl is two, here I'll have to put two and since two Ag, so here two Ag. Now the reaction is balanced. So this is also a decomposition reaction. Here, one big molecule is breaking down into two smaller molecules. And here it is utilizing the energy from the sunlight. Now let's look at one more example. I have a beaker full of water and I'm going to insert some conducting rods into it which are connected to a battery like this. So over here, electricity will start flowing into the circuit and you will notice that there are some bubbles erupting at the point where these conductors touch the water. There'll be some gases being involved, so what's happening here? So actually here, the electrical energy's breaking down the water and hydrogen and oxygen gases are being evolved. Let's write down the chemical reaction. So here we will see that H2O, water is decomposing with the help of electrical energy, electricity. Electricity and it's decomposing into H2 gas and oxygen gas. Let's balance this equation. Over here there is two oxygen, so I'll write two over here. Now, there is four hydrogen. So I'll write two over here. So yes, this is the balanced equation of water dissociating into hydrogen and oxygen because of electrical energy. So okay, now we have looked at some examples of decomposition reaction. In decomposition reaction, one big reactant breaks down or decomposes into multiple products and generally, decomposition requires more energy and such reactions that require more energy are called endothermic reactions. We'll talk more about that later. Sometimes we give energy in terms of heat and these decomposition reactions are given a special name. They are called thermal, thermal decomposition reaction because heat is involved over here. And in some reactions, decomposition happens using the energy of sunlight and these are given a special name. They are called photochemical, photochemical decomposition reaction. And you might be wondering, why photo? Because see, light is said to be made of small packets of energy called photons and that's where this name photo is coming from. So okay, this is photochemical decomposition reaction. And in some decompositions, energy from electricity, electrical energy is required to initiate this decomposition reaction. And now, such reactions are called electrolytic. Electrolytic decomposition reaction. So now, we have studied a lot about decomposition reaction. Let's now summarize this video. We saw that combination reactions are those in which multiple reactants combine to give me one single product, whereas decomposition is totally the opposite. Here, one reactant decomposes into multiple products. And we also saw that generally this requires energy because we are to separate atoms which will require some energy, right? And if this energy's given in form of heat, then it's called thermal decomposition reaction. Whereas if this energy's given in form of light, it will be called as photochemical decomposition reaction. And if the energy's given in the form of electricity, then this will be called as electrolytic decomposition reaction.