How Double Replacement Products Form

 General Chemistry Index

Where are we going with this? This page will assist in developing the ability to predict products of simple reactions as listed in of reactions: synthesis (i.e., combination), decomposition, single displacement, double displacement, acid/base, and combustion.

How Double Replacement Products Form 

These reactions look like this…

AB + CD --> AD + BC

The double replacement reaction looks more complex than synthesis or single replacement. However (ironically), the possible products are a lot easier to predict. 

Remember that opposites attract thing? Also, that chemistry grammar thing of putting positives first? Well, for double replacement, those two things add up to making this really easy.

So, look at this…

MX + NY --> ???

Grammar says M and N are positive and X and Y are negative. (But you should actually look it up!)

Presuming whoever wrote the question followed the grammar rules, since opposites attract, the only thing that might be possible is for the X and Y to swap places. 

Example? Sure. And how about those polyatomic ions? Okay, that too.

AgNO3 + K2SO4 --> ????

The trick is to realize that SO4 and NO3 are polyatomic ions. How about some coloring?

AgNO3 + K2SO4 --> ????

Blue is positive. Red is negative (don't hate… just a rough life)

Opposites attract. Blue cannot be with blue. Red cannot be with red.

There's only one possible product. Swap the negative parts: 

AgNO3 + K2SO4 = Ag2SO4 + KNO3

The only thing left to do is figure out IF IT ACTUALLY WILL react. That… That's a whole 'nother thing. (The reaction above does, by the way.) 

There's a lot of rules related to this, and then one of the rules has rules. Yeah… I know.