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.
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 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.
- Common sodium, potassium and ammonium compounds are soluble in water.
- Common nitrates, acetates and chlorates are soluble.
- Common chlorides are soluble except for silver, mercury and lead.
- Common sulfates are soluble except calcium, barium, strontium and lead.
- Common carbonates, phosphates and silicates are insoluble except sodium, potassium and ammonium.
- Common sulfides are insoluble except calcium, barium, strontium, magnesium, sodium, potassium and ammonium.
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.
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) 2 S (aq) --> ZnS (?) + 2NH4 NO3 (?)
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) 2 S (aq) --> ZnS (s) + 2NH4 NO3
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)
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.
When you reach the end of this lesson, you could be able to:
- Define precipitate, double-replacement reaction, ionic solution and net ionic equation
- Consult and understand solubility rules
- Write a net ionic equation and determine whether the products precipitate
Precipitation Reactions: Predicting Precipitates
Precipitates are solids that are formed in chemical reactions. The solids generally look cloudy in the liquid they are in. The color of the precipitate can be a variety of colors. In this activity we are going to practice predicting what precipitates will form.
This is an abbreviated list of solubility rules for our practice problems:
- Group 1 ions are soluble.
- Group 7 ions are soluble except when combined with lead.
- Sulfate, nitrate, hydroxide, and ammonium are soluble. Exceptions are calcium, strontium, and barium with these anions.
Take the outer ions and combine them and take the inner (sandwiched ions) and combine them. All of the compounds have to be electrically neutral. Let's do an example:
Potassium iodide + lead(II) nitrate react.
2KI(aq) + Pb(NO3) 2 (aq) ⟶ 2KNO3 (aq) + PbI2 (s)
Notice we combined the potassium and the nitrate into a soluble compound. Lead and iodide combine to form a precipitate. We always balance the equation too.
Now you try a few!
1. Potassium sulfate + barium nitrate
2. Lead nitrate + ammonium carbonate
3. Sodium hydroxide + barium chloride.
1. K2 SO4 (aq) + Ba (NO3) ⟶ 2KNO3 (aq) + BaSO4 (s)
2. Pb(NO3) 2 (aq) + (NH4) 2 CO3 (aq) ⟶ PbCO3 (s) + 2NH4 NO3 (aq)
3. 2NaOH(aq) + BaCl2 (aq) ⟶ 2NaCl (aq) + Ba(OH)2 (s)