Precipitation Reaction

Precipitation reaction definition

There are two key points of a precipitation reaction:

1. The reactants have been dissolved in water, so they exist as ions in solution
2. The reactants combine to form a species insoluble in water, so a precipitate is formed

Both of these conditions need to be met for a reaction to be called a precipitation reaction.

Precipitation reaction equation

Now that we know the basics, let's look at some equations.

First, here's the basic formula:

$$AB_{(aq)} + CD_{(aq)} \rightarrow AD_{(aq)} + CB_{(s)}$$

Basically, A and C are "swapping" places. While AD is still soluble, CB is not, so it forms a precipitate while AD remain as ions in solution.

A key thing to remember is that A and C have the same charge: either they are both cations or anions. In our example, A and C are both cations, since cations are typically the first species written in a compound. Basically, they can only swap places since they are similar in nature.

When we talk about precipitation reactions, we often want the net ionic equation, instead of the entire chemical equation.

Here's an example precipitation reaction:

$$NaCl_{(aq)} + AgNO_{3\,(aq)} \rightarrow NaNO_{3\,(aq)} + AgCl_{(s)}$$

Where the net ionic equation looks like this:

$$Ag^+ + Cl^- \rightarrow AgCl_{(s)}$$

So what does this mean? Let's talk about it in this next section

Precipitation reaction method

Firstly, here's what that example looks like in solution:

Fig.1 Precipitation reaction in solution

When a species is "aqueous" (aq), it has been dissolved in water. This means it is present as its ions in solution, as you see on the left. When these species react, the silver cation (Ag⁺) and chlorine ion (Cl^-) form a bond and become a precipitate.

On the other hand, when the sodium cation (Na⁺) and nitrate ion (NO₃^-) react, they form a soluble solid, so they "redissolve" and go back into their ionic state. Essentially, nothing happens, since they go from being an ion to returning to an ion.

This is where the net ionic equation comes in. This equation only shows us the direct precipitate reaction. The ions that are ignored are called spectator ions, since they don't directly participate.

Precipitation reaction examples

Now that we've covered what is happening in solution, let's see how I got that net ionic equation.

I swapped the order of Ag and Cl in the equation, since we normally write the cation first. However, you don't necessarily need to write them in that order.

Precipitation reaction experiment

Precipitation reactions can be helpful for determining what ions are in a solution. To show what I mean, let's do an experiment.

We have a test tube containing 50 mL of an unknown nitrate solution. One way to determine its identity is by reacting the solution with another aqueous solution to produce a precipitate.

We believe that the cation in the nitrate solution is either sodium (Na), lead (Pb), or silver (Ag).

Because of this, we decide to add 10 mL of potassium iodide (KI) to the test tube.

Iodide is a very soluble ion, and is only insoluble when combined with silver (Ag) or lead (Pb). Silver iodide (AgI) is a bright yellow or sometimes grayish solid (due to impurities), while lead iodide (PbI₂) is a bright yellow solid.

When silver iodide is heated, no color change occurs. However, if lead iodide is heated, it will turn an orange or reddish color.

This means that we are looking out for one of three outcomes:

1. No precipitate is formed
   • The cation is sodium
2. A yellow precipitate is formed, color is unchanged when heated
   • The cation is silver
3. A yellow precipitate is formed, color changes to orange when heated
   • The cation is lead

Once the KI solution was added, a bright yellow precipitate started to form. After the precipitate settled, the test tube was heated, causing the precipitate to turn orange as shown below:

Fig.2-Steps of precipitate experiment

Based on this, here is what we know about our reaction:

$$ANO_{3\,(aq)} + KI_{(aq)} \rightarrow KNO_3 + AI_{(s)}\,$$

Where A is our unknown cation.

Since we not only formed a yellow precipitate, and the precipitate changed to orange when heated, our original solution was likely lead nitrate (Pb(NO₃)₂)

Below is what the actual chemical reaction is:

$$Pb(NO_3)_{2\,(aq)} + 2KI_{(aq)} \rightarrow PbI_{2\,(s)} + KNO_{3\,(aq)}$$

Where the net ionic equation is:

$$Pb^{2+}_{(aq)} + 2I^-_{(aq)} \rightarrow PbI_{2\,(s)}$$