Double replacement reactions

Double replacement reactions—also called double displacement, exchange, or metathesis reactions—occur when parts of two ionic compounds are exchanged, making two new compounds. The overall pattern of a double replacement reaction looks like this:

You can think of the reaction as swapping the cations or the anions, but not swapping both since you would end up with the same substances you started with. The solvent for a double replacement reaction is usually water, and the reactants and products are usually ionic compounds—but they can also be acids or bases.

Here is an example of a double replacement reaction:

In this example, the cations are $\greenD{\text{Ba}^{2+}}$start color #1fab54, start text, B, a, end text, start superscript, 2, plus, end superscript, end color #1fab54 and $\maroonD{\text{Na}^+}$start color #ca337c, start text, N, a, end text, start superscript, plus, end superscript, end color #ca337c, and the anions are $\greenD{\text{Cl}^-}$start color #1fab54, start text, C, l, end text, start superscript, minus, end superscript, end color #1fab54 and $\maroonD{\text{SO}_4^{2-}}$start color #ca337c, start text, S, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript, end color #ca337c. If we swap the anions, or cations, we get as our products $\text{BaSO}_4$start text, B, a, S, O, end text, start subscript, 4, end subscript and $\text{NaCl}$start text, N, a, C, l, end text.

Identifying double replacement reactions is usually fairly straightforward once you can recognize the pattern. Predicting whether the reaction will occur can be trickier; it helps to be able to recognize some common types of double replacement reactions. In this article we will be discussing precipitation reactions and neutralization reactions.

A precipitation reaction is when two aqueous ionic compounds form a new ionic compound that is not soluble in water. One example is the reaction between lead (II) nitrate and potassium iodide. Both compounds are white solids that can be dissolved in water to make clear, colorless solutions. When you combine the two clear solutions, you get the following reaction:

We made a beautiful golden solid from two clear solutions! In real life, your reaction flask might look something like the picture below.

The insoluble product compound is called the precipitate. The solvent and soluble components of the reaction are called the supernatant or supernate. We can use solubility rules to predict whether a precipitation reaction will take place. The formation of a solid precipitate is the driving force that makes the reaction proceed in the forward direction.

Neutralization reactions are a type of double replacement reaction that occurs between an acid and a base. The following is an example of a neutralization reaction:

An aqueous neutralization reaction generally produces water and a new ionic compound, also called a salt. The hardest part of identifying a neutralization reaction is recognizing that you have an acid and a base for the reactants. Once you know you have a neutralization reaction, you can generally predict the reaction will occur in the forward direction as long as you have a strong acid and/or a strong base as a reactant.

A fun neutralization reaction that you may have tried is the combination of baking soda—sodium bicarbonate, $\text{NaHCO}_3$start text, N, a, H, C, O, end text, start subscript, 3, end subscript—and vinegar—mostly water with acetic acid, $\text {CH}_3 \text{COOH} (aq)$start text, C, H, end text, start subscript, 3, end subscript, start text, C, O, O, H, end text, left parenthesis, a, q, right parenthesis)—which produces carbonic acid—$\text {H}_2 \text{CO}_3$start text, H, end text, start subscript, 2, end subscript, start text, C, O, end text, start subscript, 3, end subscript—and sodium acetate—$\text{NaCH}_3 \text{COO}$start text, N, a, C, H, end text, start subscript, 3, end subscript, start text, C, O, O, end text. If you have tried this reaction at home, you probably remember a lot of fizzing because the neutralization reaction is accompanied by a gas-producing reaction, where the carbonic acid decomposes into carbon dioxide gas—bubbles!—and water.

Note that double replacement reactions can be written as molecular, complete ionic, or net ionic equations. In this article we are only writing out the molecular equation, but you probably want to be familiar with writing the other forms of the equation as well.

Let’s take a look at an example where we don't know the products:

First, we can identify the cations and anions that will get swapped. The cations are $\text{H}^+$start text, H, end text, start superscript, plus, end superscript and $\text{Ba}^{2+}$start text, B, a, end text, start superscript, 2, plus, end superscript, and the anions are $\text{SO}_4^{2-}$start text, S, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript and $\text{OH}^-$start text, O, H, end text, start superscript, minus, end superscript. Swapping anions gives the products $\text{H}_2 \text O$start text, H, end text, start subscript, 2, end subscript, start text, O, end text and $\text{BaSO}_4$start text, B, a, S, O, end text, start subscript, 4, end subscript:

We can see that our double replacement reaction is also a neutralization reaction since we are reacting sulfuric acid, a strong acid, with barium hydroxide, a strong base. What is the state of the product barium sulfate? If we check our solubility rules, we see that barium sulfate is insoluble and should precipitate out of solution. That means our reaction is a precipitation reaction, too! We can also include that information in our equation by adding the symbol $\blueD{(s)}$start color #11accd, left parenthesis, s, right parenthesis, end color #11accd after the $\text{BaSO}_4$start text, B, a, S, O, end text, start subscript, 4, end subscript.

We aren’t quite done yet, though. Our reaction is not balanced since we have unequal numbers of hydrogens and oxygens on both sides of the arrow. We can fix this by multiplying $\text{H}_2 \text O$start text, H, end text, start subscript, 2, end subscript, start text, O, end text by $\redD{2}$start color #e84d39, 2, end color #e84d39 to give our final, balanced molecular equation:

Double replacement reactions have two ionic compounds that are exchanging anions or cations. Precipitation reactions and neutralization reactions are two common types of double replacement reactions. Precipitation reactions produce an insoluble product from two aqueous reactants, and you can identify a precipitation reaction using solubility rules. Neutralization reactions occur when the reactants are an acid and a base, and neutralization reactions are usually favorable as long as the reaction involves a strong acid and/or a strong base.