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Practical Application: Determining Precipitates

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Understanding if a precipitate forms can be confusing, but this lesson helps guide you through the process with rules and practice problems. We will also explore how precipitates impact equilibrium in reactions.

Precipitate Defined

Imagine you went on a double date to prom, but by the end of the evening you had switched partners. Sounds like a lot of drama. But what if I told you that you had just undergone a double displacement reaction where the ions on the reactant side swap places?

Okay, you aren't an ion, or a charged atom, but you did swap partners. You can think of a double displacement reaction as AB + CD --> AD + BC, where the letters represent ions. Also, we should note that the left side of the arrow, or AB + CD, is referred to as the reactants and the right side of the arrow, or AD + BC, is referred to as the products.

Prom date double displacement
DD

Or worse yet, maybe you went to prom and lost your partner completely, in what is termed a single displacement reaction, where one element takes the place of another, or A + CD --> AD + C.

Why do we care about these types of reactions? Well, both of them can result in the formation of a precipitate, which is an insoluble solid that can form during a reaction. Chemists like to figure out the types of precipitates that can form from various reactions, so let's go over some steps, so you can too! And just a quick note, soluble means the substance can be dissolved and insoluble means the substance cannot be dissolved.

A precipitate can look like solid chunks that form
Prec

Solubility Rules

The first thing you will need to familiarize yourself with is the solubility rules, which will govern whether a precipitate will form in a reaction. When looking at a reaction, you need to start at the top of the list and work your way down. This is a simplified version, so don't be alarmed if you find a more comprehensive list as your progress in your chemistry studies.

  1. The following are soluble: ammonium compounds, acetates, nitrates, alkali metal compounds (group 1 on the periodic table), iodides, chlorides, and bromides.
  2. Sulfates are soluble unless they are paired up with Ba2+, Ca2+, or Sr2+
  3. Salts that have mercury (I), lead, or silver are insoluble.
  4. Hydroxides, oxides, phosphates, sulfides and carbonates are insoluble, unless the sulfide is formed with an alkali earth metal (group 2 on the periodic table) OR a hydroxide is formed with barium, calcium, or strontium.

Practice

That was a bit to take in, so let's do a practice problem or two so you can practice using the rules.

Problem #1

For the following double displacement reaction, predict what precipitate forms:

2NaOH + MgCl2 --> 2NaCl + Mg(OH)2

  • Step 1: Write down the products. Here, they are NaCl and Mg(OH)2
  • Step 2: Check out the solubility rules, starting with NaCl. According to the periodic table, Na is an alkali metal, so NaCl will be soluble and no precipitate will form.
  • Step 3: Repeat for the other product, Mg(OH)2. According to the rules, hydroxides are insoluble, so Mg(OH)2 will form a precipitate.

Problem #2:

For the following double displacement reaction, predict what precipitate is formed:

Al(NO3)3 + Na3 PO4 --> AlPO4 + 3NaNO3

  • Step 1: Write down the products: AlPO4 and 3NaNO3
  • Step 2: Check out the solubility rules for AlPO4. PO4 is a phosphate, so it is insoluble and thus AlPO4 is a solid precipitate.
  • Step 3: Repeat for the other product, 3NaNO3. Na is an alkali metal, so 3NaNO3 is soluble and no precipitate forms.

Equilibrium

Before we finish up, we need to take a moment to discuss equilibrium, which is the term used to describe a reaction that can move in both the forward and reverse direction at the same time.

The double arrows are used to represent reactions in equilibrium
Equilibrium

There is a guiding rule for figuring out equilibrium problems called Le Chatelier's Principle, which states that if a reaction experiences a change, the equilibrium will shift to deal with that change.

We are going to keep things really simple here, so realize this can become more complicated. When a precipitate forms, it can no longer be involved in the reaction. While it's still there, just think of it as being 'lost' and therefore no longer participating.

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