Double Displacement reactions

Double displacement reactions (also known as double replacement reactions or metathesis reactions) involve the swapping of ions between two ionic compounds or between one ionic compound and one acid.

You need to be able to recognize a double displacement reactions, complete the reaction (if you are just given the left side) and balance the reaction

Recognizing Double Displacement Reactions 

A double displacement reaction is a reaction in which the left side has two ionic compounds or the left side has one ionic compound and one acid. All of the examples on this page are double displacement reactions.

Completing Double Displacement Reactions 

The products of a DD reaction are the result of the ions trading partners. In simpler terms, each positive ion starts with a negative ion as a “partner.” At the end of the reaction, the positive ions are paired with the “other” negative ion.

Here's an example:

In this reaction, the sodium started the reaction with the chloride and ended with the nitrate. At the same time the silver started with the nitrate and ended with the chloride. If it helps, you can follow the color pairs in the same reaction below.

Here is another example:

In this (currently unbalanced) reaction, the barium started with the chloride and ended the reaction with the sulfate, while the lithium started with the sulfate and ended with the chloride.

Take a close look at this reaction. You will notice that there are 2 lithium ions in the formula on the left, but only one on the right. The reason is very straight forward. The charge on lithium ions is +1, while the charge on sulfate ions is -2. Therefore when lithium ions are paired with sulfate, there must be two of them to balance the charge. However, the charge on chloride is -1, so when lithium and chloride are paired, only one of each is needed. This is the MOST LIKELY place to make a MISTAKE!.

Formulas MUST be written according to the charges of the ions, not based on the number of the ions present on the other side of the reaction.

In simple English, do NOT bring subscripts from the left side of a reaction to the right. (Polyatomic ions are the exception, as they stay together and do not change in the reaction.)

Of course, we must deal with this inequality, which is why we balance the reaction.

Information on balancing reactions can be found here.

Completing the reaction

Here is another example. However, this time, I've only given you the left side of the reaction.

To determine the products, we remember that the ions are re-paired on the right side. That means that the potassium will go with the nitrate and the aluminum will be paired with the hydroxide.

Now that we've figured out which ions go with which, we need to write the formulas on the product side of the reaction (the right) correctly (taking into account the charges on the ions). That gives us:

If you don't remember how to do this, you can find some help here.

Then, balancing the equation gives us:

Here is another example:

Switching the ions, gives us:

Taking into account the ion charges on the right side of the equation:

Note: in the work above, there were two possible charges for lead. The key to getting this reaction right is that charge NEVER changes in double displacement reactions. So, since the Pb is a +2 ion on the left, it must still be a +2 ion on the right.

Balancing the reaction gives us:

The last thing you need to know about double displacement reactions is that NOT ALL double displacement reactions occur. That means that you need to be able to determine whether or not they do occur.