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Double displacement reaction

In double displacement reaction, the ions of the reactants exchange position. This could be used to make volcanos for science experiments. Created by Ram Prakash.

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Video transcript

- You might have seen these erupting volcanoes in a science fair. Or you might have made one for your science project. In this video, we are going to talk about the type of chemical reaction that makes this happen. Double Displacement reactions. And towards the end of the video I will also show you by making one volcano. So let's begin. So if I have to explain you in short, in double displacement reactions, the ions of the reactant, they exchange position. Something like this: Now let's see how to write this in a chemical equation. So here I have two random reactants. AB and CD. Now if double displacement reaction is happening here, then the ions are going to exchange position. So here in this reaction, A will go and displace C. And form AD, whereas C will go and displace A, in turn and form CB. And finally our products are going to be AD and CB. Now you can see that why this is called a double displacement reaction. Because in a way we can see that A is displacing C and C is in turn displacing A. And since there are two displacements, so we can call it a double displacement reaction. Okay. Now let's look at an actual reaction. So here I have hydrogen chloride and sodium hydroxide and when they react it is going to be a double displacement reaction. Meaning the ions are going to exchange position here. So here hydrogen ion is going to exchange position with sodium ion, that's it. So here hydrogen will displace, or take the position of sodium, and form HOH or I can say H2O right? Which is water. And sodium will in turn take the position displace hydrogen and form sodium chloride Na Cl and this is going to be our product. HOH or H2O and sodium chloride. Now I've got two reactions and both of these are double displacement reactions. Now I want you to pause the video and try to find out the product of these by yourself first. Now if you have tried it, let's see. So here in this reaction since this is double displacement ions are going to exchange position. Here sodium exchanges position with barium, so okay Sodium displaces barium in one sense and forms sodium chloride and in turn barium takes the place of sodium and forms barium sulfate. So the product is going to be sodium chloride and barium sulfate. Now let's look at this reaction, so here also double displacement so ions exchange so sodium takes the place of hydrogen and forms sodium chloride and hydrogen in turn takes the place of sodium and forms hydrogen sulfide. So the product is going to be sodium chloride and hydrogen sulfide. So okay with this you understand the basic structure of double displacement Reactions. But you might be having many questions like over here why is sodium only exchanging position with hydrogen? Why can't it exchange position with let's say chlorine? Or can sulfur exchange places with chlorine? Or what is happening at the code? Like how are these reactions even taking place? Now all of these questions are amazing questions, but to understand this let's look at a little bit in detail. So here I have sodium sulfide Na2S now if I put this in water meaning I'm making an aqueous solution of sodium sulfide then in that case this molecule is going to dissociate or break down into it's ions. I will get sodium ion and sulfur ion. Something like this, here I'll get sodium ion and and sulfur minus 2 ion. Now this is nothing to be worried about, many molecules when put in water they will dissociate into it's ions. Similarly over here Cl when put in water this will dissociate into hydrogen ion and chlorine ion. Now see why certain molecules dissociate in water is something we will talk about in a separate video. Let's focus over here now so if I only have a solution of sodium sulfide in that case I will only have sodium plus ions and sulfur minus 2 ions and they will be attraction each other because we know that opposite charges attract each other. But if to that solution I also add a solution of Hcl, meaning now I also have H plus ions and chlorine minus ions. Now see sodium has no reason just to be attracted towards sulfur now it also see's that there is chlorine minus ions so some of sodium will also get attracted to chlorine minus ion and we will have sodium chloride being formed which will look something like this sodium chloride and similarly over here hydrogen that was only earlier attracted to chlorine now see's that okay there is sulfur minus 2 ions also present in solution. So some of the hydrogen ions they will get attracted to sulfur minus 2 ions and they will form hydrogen sulfide, something like this. Now this is something that is happening in every double displacement reaction. First of all the reaction will break down into their ions and then the ions will exchange position to form new products. Now this model can help us answer all our questions. So the first question is, can sodium exchange position with chlorine instead of hydrogen? I want you to pause the video and think about this by yourself first. Now if you have tried let's see. If sodium exchanges position with chlorine, that means that a molecule of sodium and hydrogen should be formed, right? But see sodium and hydrogen both are positively charged and we know that like charges they do not attract but they repel each other, and if they are repelling there is no way they can form a molecule, right? So that's why sodium cannot exchange position with chlorine. In fact no positive ion can exchange position with a negative ion. Positive ion can only exchange position with another positive ion. Now another question is can sulfur exchange position with chlorine? So yes that is definitely possible. See if sulfur comes here instead of chlorine it will form a molecule with hydrogen hydrogen sulfide and that is what we are actually getting over here hydrogen sulfide. So now if chlorine exchanges position with sulfur it will have to form a molecule with sodium and we'll get sodium chloride, and that is what we are getting over here, right? So that's what happens in double displacement, either the positively charged ions they exchange position or the negatively charged ions they exchange position. They both basically mean the same thing. As long as you do not exchange position between a positively charged ion and negatively charged ion. Now there is one more question to answer. You might be wondering about the mixed reactants some of the sodium ions got attracted to chlorine ions, and that is how sodium chloride was formed and we note that on the product side. But the rest of the sodium was still attracted to sulfur right? And we could still have some sodium sulfide ions, and we should also write that on the product side. Why are we mentioning that? Similarly some of the hydrogen ions they are attracted to sulfur ions and we got hydrogen sulfide and we wrote that on the product side the rest of the hydrogen should be with chlorine, right? And we should still have some hydrogen chloride left and we should also write that on the product side. Why aren't we writing that? So that's a very good question. See hydrogen sulfide this is a gas, and this cannot be present in a liquid this will escape out of the liquid. What this means is the moment hydrogen ions and sulfur ions become close together and combine they will form hydrogen sulfide which is a gas and this gas will escape out of the solution. Now what this means is as time passes by more and more hydrogen ions and sulfur ions are getting lost out of the solution. So they are no longer having sulfur ions in the solution. That means we can no longer form sodium sulfide. And also we are no longer having hydrogen ions in the solution and that means we can no longer form hydrogen chloride. The hydrogen and the sulfide ions they will combine together to form hydogen sulfide gas which will escape out of the solution. And so earlier we did this example barium chloride plus sodium sulfate, now this is also a double replacement reaction. When we mix this in an aqueous solution or that means in water then barium chloride, this will dissociate into it's ions, and it's ions are going to be barium plus 2 ions and chlorine minus 1 ions. Similarly sodium sulfate this will also dissociate into it's ions sodium and sulfate ions as a plus 1 and as a full minus 2. Now see barium plus 2 ions this can get attracted to even sulfate ion now and this will form barium sulfate also. Similarly sodium ion now this can get attracted to chlorine ions also and this will form sodium chloride. And so the products are going to be barium sulfate and sodium chloride. Now see the moment barium ion and sulfate ion come together they form barium sulfate which is insoluble in water. Meaning it will settle down out of the solution, it will participate out of the solution. Just like when you mix sand and water the sand after some time it settles down right? Something very similar is going to happen with barium sulfate. That means after some time barium and sulfate ions they won't be available in the solution anymore. The only ions available in the solution are sodium and chloride, which will form sodium chloride and barium sulfate will settle out of the solution. And since we don't have any barium ions left we can't have any barium chloride being formed. And similarly since we don't have any sulfate ions left we can't have any sodium sulfide being formed, so just two products. Here I have an experiment to show the same thing Here I've taken barium chloride solution and sodium sulfate solution, both of which are colorless. But the moment I mix them I am getting a white compound being formed, now that is barium sulfate. And after some time this barium sulfate is going to settle down at the bottom of the flask. Now such reactions in which a precipitate is formed, something that settles down, these reactions are also called a precipitation reactions. Now let's make our volcano. For this you will require baking soda and vinegar. Now the moment you add vinegar to baking soda wow you can see the amount of froth being generated. A lot of gases evolving over here, and if you put this reaction in a small volcano you will have a lot of bubbles coming up, and this will look amazing. In fact if I zoom in you can still see the bubbles coming out. Now let's look at the chemical equation of this reaction to understand what's happening. So the chemical formula for baking soda is NaHCO3 and vinegar is CH3COOH. Now when they are both mixed in water the dissociate into their ions. We get Na plus ion and HCO3 minus ions. Similarly here we get CH3COO minus ions and H plus ions. Now the ions will exchange positions to have double displacement reactions, right? So the positive ions will exchange position let's see So here Na plus will exchange with H and we will get CH3COONa, similarly H will exchange with Na and we will get HHCO3 or H2CO3. So the products are this CH3COONa and H2CO3. Now this does not stop here itself, see H2CO3 this further dissociates, this further breaks down into water and carbon dioxide, and this carbon dioxide is the gas that is erupting out and is making the bubbles form. You can see the bubbles, right? Those are carbon dioxide molecules coming out of the solution. Now with this, let's summarize the video. In this video we spoke about double displacement reaction, and in these reactions the ions of the reactants they exchange position.