Single Displacement reactions

Single displacement reactions (also known as single replacement reactions) occur between an element and an ionic compound or between and element and an acid.

In almost all cases the element is either a metal or hydrogen gas (which acts like a metal since it has a positive charge when it is an ion). The rare exceptions to this rule are treated here.

Recognizing Single Displacement Reactions

A single displacement reaction starts with an element and a compound. The element is usually a metal, but it can also be hydrogen (H₂) or a halogen (F₂, Cl₂, Br₂, I₂)

Completing Single Displacement Reactions

Single displacement reactions involve trading partners just like double displacement reactions, but here one element starts alone and therefore one element will end alone. Here is a simple example:

In this case the sodium joins up with the nitrate ion and the silver ends up alone.

Of course, we need to be careful about charges, just as we did with double displacement reactions. For example:

This reaction is NOT yet balanced, but it is important to note that there are two chlorides on the left to match the charge of Sr (+2) but only one chloride on the right to match the charge (+1) of the sodium.You  MUST write the formulas of compounds correctly before you balance the reaction.

So, we deal with the mismatch of chlorides by balancing:

Here are a few other examples in which I "forgot" to complete the reaction:

In this reaction, the calcium will replace the aluminum and connect to the hydroxide

Once we have determined which elements and ions are paired up, we need to consider charge. The charge on calcium is +2, while the charge on hydroxide is -1, so we will need 2 hydroxides to make that formula work.

Of course we still need to balance the reaction, but it is VERY important to make sure that you have written the formulas correctly BEFORE you balance the reaction.

This entire process becomes a little more complicated when the element on the left can have more than one charge in a compound, as the following example shows:

We've paired up the copper with the nitrate and kicked out the Ag (as we should have), but now we need to figure out the correct formula for the copper nitrate. Copper has two possible charges (+1 and +2). As a general rule in single displacement reactions, the more common charge is the one used. In this case, +2 is the more common charge for copper. That gives the formula Cu(NO₃)₂, and the reaction becomes:

After balancing, we have:

General Rule:

In single displacement reactions, always use the more common charge.

Don't forget that hydrogen is a “HOFBrINCl.” In other words, it always appears as H₂ not as H, so...




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