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Precipitation Reactions: Predicting Precipitates and Net Ionic Equations

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  1. 0:07 Precipitates
  2. 1:10 Solubility Rules
  3. 2:22 Ionic Equations
  4. 4:02 Example
  5. 5:26 Lesson Summary
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Taught by

Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn what a precipitate is and predict when it will form in an aqueous chemical reaction, usually a double-replacement reaction. Learn what an ionic equation is, how it differs from a net ionic equation and how to write a net ionic equation.

Precipitates

When two aqueous solutions react, they sometimes form solids in the solution. The solid is called a precipitate. Precipitation reactions occur when the cations of one reactant and the anions of a second reactant found in aqueous solutions combine to form an insoluble ionic solid, that we call a precipitate.

Most precipitates are formed in a double-replacement reaction. A double-replacement reaction is when the ions in two compounds exchange places with each other in an aqueous solution. AX + BY --> AY + BX, where A, B, X and Y are all ions.

An ionic solution is when the ions of a compound have dissociated in an aqueous solution. A reaction happens when you mix two aqueous solutions. This is when you find out if a precipitate will form or not. A precipitate forms if the product of the reaction of the ions is insoluble in water.

Solubility Rules

Solubility rules for inorganic compounds will help predict whether something will come out of a solution to form a precipitate. There are many different tables in books and on the Internet explaining solubility rules. The problem with making succinct rules is that there are so many exceptions to any rule.

This isn't likely to be something you would memorize, but rather you'd always have a table to refer to. What I am listing here is, to me, the easiest and clearest form of the rules.

Solubility rules:

  1. Common sodium, potassium and ammonium compounds are soluble in water.
  2. Common nitrates, acetates and chlorates are soluble.
  3. Common chlorides are soluble except for silver, mercury and lead.
  4. Common sulfates are soluble except calcium, barium, strontium and lead.
  5. Common carbonates, phosphates and silicates are insoluble except sodium, potassium and ammonium.
  6. Common sulfides are insoluble except calcium, barium, strontium, magnesium, sodium, potassium and ammonium.

Ionic Equations

A net ionic equation is an equation that includes only the substances that are actually participating in the reaction. To write it, you first write the balanced equation. Next you write the separated ions. Last you cancel out the things that appear on both sides of the equation, and what you are left with is the net ionic equation.

Double-replacement reactions and other reactions that involve ions are often represented as net ionic equations. To write a net ionic equation, first write the equation as all ions. When writing these equations, scientists use (aq) to denote something in an aqueous solution and (s) to denote a solid.

So you start with the reaction of:

KCl (aq) + AgNO3 (aq) --> KNO3 (aq) + AgCl (s)

Rewrite it as:

K+ (aq) + Cl- (aq) + Ag+ (aq) + NO3- (aq) --> AgCl (s) + K+ (aq) + NO3- (aq)

This next part is a bit like math. Remember when you can cross out like terms on each side of an equation? You do the same thing here. Get rid of the ions that appear on both sides of the equation.

So the net ionic equation is: Ag+ (aq) + Cl- (aq) --> AgCl (s)

You can see that the final product is a solid. This is the precipitate that is forming. In this reaction, it is silver chloride.

Cancel out the common ions on both sides to get the net ionic equation.
Writing Net Ionic Equation

Example

Here's another example where you not only solve the net ionic equation, but you determine what precipitates.

Write the net ionic equation for:

Zn(NO3)2 (aq) + (NH4)2S (aq) --> ZnS (?) + 2NH4NO3 (?)

The (?) denotes that we don't know what form these products are in, aqueous or solid. When you consult a table which tells you the solubility of compounds, you find out that zinc sulfide is not soluble and therefore precipitates. Now you can fill in those (?).

Zn(NO3)2 (aq) + (NH4)2S (aq) --> ZnS (s) + 2NH4NO3 (aq)

Write the ionic equation:

Zn2+ (aq) + 2NO3- (aq) + 2NH4+ (aq) + S2- (aq) --> ZnS (s) + 2NH4+ (aq) + 2NO3- (aq)

Cross out the things that occur on both sides of the equation to make the net ionic equation:

Zn2+ (aq) + S2- (aq) --> ZnS (s)

Lesson Summary

A precipitate is a solid formed when the cation and anion in an aqueous solution react to form an insoluble compound. An ionic solution is when the ions of a compound have dissociated in an aqueous solution. A net ionic equation is an equation written so that only the substances that are participating in the reaction are shown.

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