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Predicting Products - 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.

So here we are and now we understand the different types of reactions there are. We know there's five main types. We can identify them pretty easily but predicting the products, if you know only the reactants can be tough. So the first thing we're going to do is identify the reaction type. We're going to predict whether the reaction is synthesis, decomposition, Single-Replacement, Double-Replacement or combustion. Then we're going to use that information to predict the products. And last we're then going to balance the reaction, because no reaction is really complete unless it's balanced.

So here, I'm given our first reaction, number one. I wrote out in words and then I also wrote it out in symbols. We have silver nitrate plus barium chloride. We might say something like, an aqueous solution of silver nitrate is mixed with an aqueous solution of barium chloride, or something similar to that. But this is what they are chemically; AgNO3 + BaCO2, two compounds.

When we have tow compounds and they're reacting, most likely it's going to be a double replacement reaction. So I know, I'm going to treat this as a double replacement reaction. I'm going to note that as DR.

There are two ways to go about double replacement reactions. I'm not asking for net ionic equation, we'll handle that another time. We're just going to simply predict the products of this reaction. So silver nitrate, let's say silver nitrate is a couple, dancing in a dance. Silver is dancing with nitrate. Then another couple is barium chloride; barium is dancing with chloride. Silver is the boy because he's leading, nitrate is the girl because she's following. Barium is the boy because he's leading, and chloride is the girl because she's following. So what they're going to do is, they come together and they're going to want to switch partners. So barium is now going to dance with nitrate and silver is now going to dance with chloride.

If you don't like my dance analogy, another way to think about this is creating a double rainbow. A lot of my students actually like to create the double rainbow because it tells us very straight forward, what is going to bond with what in the reactants. So I'm actually going to do this underneath here as well; my double rainbow. The two inside things are going to bond and the two outside on this are going to bond.

The problem a lot of students have now is we subscript. So instead of actually looking at the chemical equations, I'm going to actually write out the product. So silver is going to end up with chloride. I'm going to make silver chloride. I've decided that silver goes first because remember silver was a boy. Another way to think about it is, all metals, always go first. Cations always lead and anions always follow.

So silver goes first and then we have barium nitrate. When we figure out our products, we do not want to carry over the subscripts, unless it's a polyatomic ion then we have to, because those are subscripts that stay with the actual atoms.

So silver chloride, the best thing to do is cover up the reactants and just figure out what the products are. Silver chloride we know Ag is +1, chloride is -1. So it's going to be AgCl. Barium is +2, nitrate is a -1. So it's going to be Ba, you need 1Barium and 2Nitrates; Ba(NO3)2. So we then need to balance it. We have 1 nitrate on the reactant side and 2 on the product side. So I'm going to put a 2 here. That means I have 2 solubles, I need to have 2 solubles on this side. That means I had 2 chloride and I also had 2 chlorides because of the subscript here, now it's balanced. So using the rainbow actually really helps in determining the products for a double replacement reaction. Also, the dance scenario helps a lot too.

The next one we're going to see, number two, we have magnesium plus copper 1 nitrate. So I wrote it down again in words and then I also wrote it in symbols. I also have this element and a compound. When I see element and compound something should be going off in my head saying, single replacement. If you do the dance scenario again, we have copper 1 dancing with nitrate. And magnesium is going to come and ask to interrupt the dance.

Metals replace metals and non-metals replace non-metals. Magnesium is a metal, copper is a metal. So my product is going to be magnesium nitrate and copper. In order to know if this reaction actually occurs, I'm going to have to look at my activity series. In my activity series, the most active metals go on top and the least active metals are on the bottom. So I have to check to see if magnesium is more active than copper. It actually is.

Magnesium is above copper on the activity series, that means it will react. If magnesium were below copper on the activity series, it means it's not as active and copper 1 nitrate would not react with magnesium. They would actually be a no reaction.

If you see something like that happen, you can actually write 'NR', saying 'No Reaction'. But in this case, there's a reaction and it actually continues to go through. So again, I'm not going to worry about our subscripts. I know magnesium nitrate. Magnesium is a +2, nitrate is -1, so we're going to need 1 magnesium and 2 nitrates; Mg(NO3)2. Copper stands alone, it is not one of my diatomics. What I mean by diatomic is that there are the 7 elements that need to have a subscript of 2 next to it when it's standing alone. So the way I remember those, is the term called Br-I-N-Cl-H-O-F.

All these atoms must have a 2 next to them, if they're ever by themselves; Br2 I2 N2 Cl2 H2 O2 and F2. You will never see these elements singly by themselves. But copper isn't part of the BrIClHOF so it can stand alone. So now I have to balance this; 2 nitrates there, 2 nitrates there, 2 coppers, 2 coppers and this is my final reaction.