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Double Replacement Reactions - 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 double replacement reaction, two compounds swap atoms forming two different compounds. Like single replacement reactions, metals always replace metals and nonmetals always replace nonmetals. Double replacement reactions often produce precipitates when non-soluble compounds are produced.

Alright so we're going to talk about double replacement reactions also known as double replacement reactions. This is an exchange of ions between 2 compounds typically what's produced is what we call a precipitate. That's a solid within aqueous solution or a liquid solution. We'll actually get a picture of what that looks like soon. alright so here's a skeletal equation of what a double replacement reaction looks like, we have ax and this happens in ionic compounds. So we have ax and we know that a is a metal and x is a non-metal plus by again b is a metal y is a non-metal don't forget metals always comes first in ionic equations so they're going to actually change out partners okay so metals don't want to be as metals and non-metals don't want to be as non-metals so we're actually going to, they're actually going to switch partners and switch them completely. So the a is now not going to be with x anymore, it's now going to be with y and the b is not going to be with y anymore it's now going to be with x okay. an easy way to remember this is the outside ions are going to come together and the inside ions are not going to come together.

But don't forget metals will always, always, always come first. Okay do not ever write the non-metals first. So in this case what's going to happen is ay is going to be a products a is a metal don't forget and y is our non-metal plus bx. b is our metal x is our non-metal. Don't get that mixed up, that can actually get you into trouble. Okay so this is a good skeletal example of a displacement reaction or double replacement reaction. So let's look at actual double displacement reaction, we have 2 ionic compounds that come together aluminum carbonate and potassium oxide and what they're going to do is the metal is now going to bond with this non-metals and this meta is going to replace, is going to go with that non-metal polyatomic ions are going to act as non-metals.

Okay, so we have these subscripts at the bottom 2, 3 and this 2 we're actually going to ignore them for now. Do not use those as, when you're actually making a product. Just ignore them we'll deal with them when we balance the equation okay. So we're just going to deal with the metal, the aluminum itself it's going to bond with the oxygen okay. So how do they come together? Well aluminum makes +3 ions, oxygen makes a -2 cross those to make it even. We come up with a new product al2O3 okay and now potassium is going to bond with carbonate. Carbonate is CO3 that is carbonate, so we're actually going to keep that within compound. So we have K which is going to make a plus ion and sorry aabout that, carbonate which is a minus 2 charge. We're going to cross those and we're going to get K2CO3 okay and these are our products.

And so now we have to balance it to make sure that the elements are the same on both sides. So we're going to just do that and I'm going to just change that just over - okay so let's start with aluminum we have 2 aluminum for the reactant side, 2 on the product side so that's okay. Let's start to talk about carbonate, we have 1 on the, actually we have 3 I didn't copy that correctly, we have 3 on the reactant side, we're going to put 3 on the product side. That means we have 6 potasiums, we're going to make 6 potassiums so we're going to put a 3 here and that means we have 3 oxygens, we have 3 oxygens this is completely balanced. This is an overall double replacement reaction.

Now we actually can take this a step further and do net ionic equations, those are actually the next step in dealing with double replacement reactions but that's going to be shown in another video okay. Let's actually see this inaction; we said that a precipitate occurs so we're actually going to see what a precipitate looks like let's do that.

We're going to do a double replacement reaction with potassium chromate and silver nitrate. Here's my potassium chromate and I see that when I add silver nitrate I'm forming a nice red precipitate and the precipitate is silver chromate. It's a double replacement reaction. We're going to mix potassium iodide and lead nitrate, lead 2 nitrate so I've got my lead nitrate in here and I'm going to add some potassium iodide to it and we're going to see right away that we get a precipitate of lead iodide I'm not sure if you see this but what a pretty precipitate.

Alright so when the 2 reactions that you saw are written up here on the board notice the silver chromate is the precipitate, notice I noticed I wrote that as a solid as a state of matter and down here we have the second reaction that has the yellow precipitate, the lead notice again I wrote that as a solid PPT you might see it frequently that just indicates that it's a precipitate and those are very, very common examples of what a double replacement reaction is.