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Single Replacement Reaction - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

In a single replacement reaction, a single element replaces an atom in a compound producing a new compound and a pure element. Like double replacement reactions, metals always replace metals and nonmetals always replace nonmetals in a compound.

Alright so we're going to talk about single replacement reactions, you might also know them as single displacement reactions, there're exactly the same thing just different ways of saying them. This when an element replaces another element in a compound and know that metals replace metals, non-metals replace non-metals. Non-metals will never replace metals or vice versa, they'll only replace itself or the only type of reaction that it is or element that it is. So we have, let's look at this just a simple reaction we have a+bx yields ax+b okay let's analyze this. So we have a+bx we know that b is the metal, how do we know b is the metal? Because in ionic compounds metals always come before non-metals.

Okay and know a, a must be a metal also because in this case a is replacing b, metals replace metals okay. So an easy way to remember something like that is to actually like and think about them dancing okay, so we have BX are dancing together b is the metal we're going to say the Boy, when boys and girls dance, boys are typically the leaders so b is going to lead and x is going to follow. The non-metals are girls are going to follow and a will replace b in this case. Alright so let's look at something that's actually a real reaction. We have copper which we know is a metal and we have silver nitrate notice that silver is first, it's a metal. So in this case silver is going to replace copper, the metals are going to replace each other. So then our products are going to be silver plus copper 2 nitrate.

Okay, so how do we know that silver is, copper is going to replace silver in a reaction. Well sometimes metals replace metals but sometimes nothing will happen so how do we know? Well we have to go over to our activity series, okay you might have gotten this sheet in class there's no need to memorize it you'll always pretty much have it in front of you if you need it. We have the most active metals, the metals that are extremely reactive up at top and the least active metals at the bottom okay. Let's say, let's go back to our dancing analogy and say that we, that magnesium and chloride are dancing together. Remember magnesium is the metal, the boy chloride is a non-metal of the girl. Okay then up comes along potassium, potassium is like hey chlorine would you like to dance and chlorine is like heck yeah I would to dance with you.

It's going to ditch magnesium and now dance with potassium, potassium is a more active metal. Let's go back and say okay, let's go back to the beginning and say magnesium is going to be dancing with chlorine and iron comes up, is chlorine going to dance with iron and ditch magnesium? No, iron is not as desirable or it's not as active okay, so that's not going to happen. So if it's lower down on the activity series the reaction is not going to take place, if it's high up on the activity series that reaction will take place. And this is the way that non-metals replace each other, same idea. So let's go and let's do some examples, okay so we have potassium by itself and we have zinc chloride.

Okay potassium is going to go up and say alright chloride would you like to dance with me? You're going to look at your activity series zinc is pretty low, potassium is a little bit higher up. Chlorine is going to be like heck yeah, I'll dance with you so what's going to happen is potassium and chloride are going to come together. Okay so don't mind the 2 they have to come together properly, so we have the potassium we're just going to make a plus charge and we have a chlorine which is going to make a minus charge. Do what we do and this is a 1 here it's going to exchange the numbers at the bottom and you're going to make potassium chloride KCl don't mind the 2 we'll take of that when we balance the equation.

And then zinc is going to be all alone zinc, okay so now we have to balance now that we have the products. We have 2 chlorine on this side, we're going to put a big 2 here which means we have 2 potassiums and I'll put a 2 here, now we have a balanced equation and this is a single replacement reaction. Let's look at the next one, we have iron and once you replace sodium and dance with phosphate. So can iron do that? Let's look at our activity series, iron is down here, sodium is up here. There's no way phosphate is going to ditch sodium to dance with iron, no way so that reaction is not going to happen. Okay I'm going to say no reaction. They'll just going to hang out together and this iron will not be able to react with sodium phosphate no reaction.

This activity series we want to understand where it comes from, I will go to the electro Chemistry film and talk about it, as we talk about the details of that there. Okay so let's actually see a single replacement reaction in action.

Things are going to happen here and I have bored out this egg and emptied out its contents and allowed it to dry, there are holes in either end of it a few millimeters in diameter and what I'm going to do is take some Vaseline and I'm going to coat the bottom third of the egg, not the whole, the bottom third of the egg with Vaseline like that. And now I'm going to place the egg inside of the glass bottle and then the nice plate seal between the side of the glass bottle and that egg and next after that I'm going to go ahead and take my reagent bottle notice how I turn my hand over grab the stopper, keep my mouth away and the nose away from the contents of the reagent bottle same hand and I pour some hydrochloric acid otherwise know as acid inside of the Erlenmeyer flask.

I have approximately 50 milliliters inside and I cover up that hydrochloric acid remove the reagent bottle from the experimental demonstration table and off to the side it goes, then I have some zinc here and I'm going to pour the zinc inside just make sure I have my barbecue lighter out here which is actually very important and here we go, safety goggles are on, gas is being produced, that coming off there and I cover the top of the egg to trap the hydrogen gas which is being produced. So the egg itself is filling up with hydrogen gas. And I'm going to allow that reaction to run for about another 15 seconds and take this off the bottom here remove source of hydrogen gas, light the barbecue lighter and wait because it's a very small flame but it's a centimeter tall on top of the egg which you may not be able to see and I'm actually going to turn off the lights so you can see what's going on there. And then it explodes, that was exciting.

Alright so let's talk about what we just saw in that video, we have hydrochloric acid in the Erlenmeyer flask he then added zinc chips okay, and then they combined to make hydrogen gas which we saw escaping from the Erlenmeyer flask and going in that tube up to the funnel and inside the egg and left in the Erlenmeyer flask we have zinc chloride okay. Now we know that hydrogen gas we tested it because remember he put it through the egg and lit it on fire one of the main characteristics of hydrogen is that it's extremely flammable. We saw it actually explode, which indicated that yes we did produce some hydrogen gas, we also know that this whole reaction is extremely exothermic as he had on the whiteboard in the back. What does that mean? It means that it produces a lot of energy, so if you actually touch that Erlenmeyer flask it will be very, very hot meaning it produced a lot of heat during this reaction. So we also want to note that yes this is a metal, hydrogen isn't a non-metal, hydrogen is one of our rare exceptions that it will act as a metal because it's in group 1 however we know that it's a non-metal. So in this case hydrogen is going to act as a metal.

Hydrogen has a lot of funny characteristics and it'll act as a metal sometimes and act as a non-metal other times. In this case it acted like a metal letting zinc replace it. So that's pretty much single replacement reaction. Hope that was helpful.