Lesson 4.3 Precipitation Reactions
Suggested Reading

Zumdahl Chapter 4 Section 4.5
Essential Question

What are the characteristics of precipitation reactions?
Learning Objectives

Predict if a precipitate will form using solubility rules.
Among the several million known substances, many millions of chemical
reactions are possible. Although the sheer number of possibilities can be
bewildering, most reactions can be classified by type. The majority if the
reactions we will study in this course belong to one of three types:
1. Precipitation reactions: In these reactions, you mix two ionic
solutions and a solid ionic substance (a precipitate) forms.
2. Acid-base reactions: An acid substance reacts with a substance
called a base. Such reactions involve the transfer of a proton
between reactants.
3. Oxidation-reduction (redox) reactions: These involve the transfer of
electrons between reactants.
We will begin our study with precipitation reactions. In the previous lesson,
we used a precipitation reaction to illustrate how to convert a molecular
equation to an ionic equation. A precipitation reaction occurs in aqueous
solution when one product is insoluble; that is, it does not dissolve readily
in water.
In this lesson, we will look at some simple solubility rules and then we will
see how to use these rules to predict whether mixing two ionic compounds
will result in a precipitate as in the illustration above.
Solubility Rules
Substances vary widely in their solubility, or ability to dissolve in water.
Some compounds, such as sodium chloride, dissolve readily are are said
to be soluble. Others, such as calcium carbonate (which occurs naturally
as limestone and marble), have quite limited solubilities and are said to be
insoluble.
A precipitate is an insoluble solid compound formed during a chemical
reaction in solution.
To predict whether a precipitate will form when you mix together two
solutions of ionic compounds, you need to know whether any of the
potential products are insoluble.
Here is a list of general solubility rules (there is also a list in your Chapter 4
study guide). You must memorize the solubility rules. There are a lot of
videos on YouTube with songs and mnemonic devices to help you
remember the rules.
These rules apply to most of the common ionic compounds that we will
discuss in this course. To see how you would use the rules, consider the
following ionic compounds: lead(II) nitrate, Pb(NO3)2, sodium phosphate,
Na3PO4, and silver chloride, AgCl.
lead(II) nitrate, Pb(NO3)2: Rule 2 states that acetates and nitrates are
soluble, so we predict that lead(II) nitrate is soluble.
sodium phosphate, Na3PO4: Rule 1 states that Group 1A and ammonium
compounds are soluble, so we predict that sodium phosphate is soluble.
Rule five says most phosphates are insoluble, but lists Group 1A
compounds as exceptions.
silver chloride, AgCl: Rule 3 says most binary compounds of Group VIIA
compounds are soluble, but lists Ag as an exception so we predict that
AgCl is insoluble.
Predicting Precipitation Reactions
Now let us see how you would go about predicting whether a precipitation
reaction will occur. Suppose you mix together solutions of nickel(II)
chloride, NiCl2, and sodium phosphate, Na3PO4. You can write the
potential reactants as follows:
NiCl2 + Na3PO4 →
How can you tell if a reaction will occur, and if it does, what products will
form?
When you write a precipitation reaction as a molecular equation, the
reaction has the form of an exchange (or metathesis) reaction. An
exchange reaction is a reaction between compounds that, when written as
a molecular equation, appears to involve the exchange of parts between
the two reactants. In a precipitation reaction, the anions are exchanged
between the two cations (or vice versa).
Let us assume that a reaction does occur between nickel(II) chloride,
NiCl2, and sodium phosphate, Na3PO4. If you exchange the anions, you
get nickel(II) phosphate,Ni3(PO4)2, and sodium chloride, NaCl. The
balanced molecular equation is, assuming there is a reaction, is
3NiCl2 + 2Na3PO4 → Ni3(PO4)2 + 6NaCl
Now let’s determine if the reactants and products are soluble. Rule three
says that chlorides are soluble, with certain exceptions, but NiCl2 is not
one of them. Thus, we predict that nickel(II) chloride is soluble. We know
that sodium phosphate is soluble because of rule 1. The potential products
are nickel(II) phosphate and sodium chloride. According to rule 5,
phosphates, except for those of Group 1A elements, are insoluble.
Therefore, we predict that nickel(II) phosphate is insoluble. Sodium
chloride is soluble (Rules 1 & 3). We can now write the equation above
with phase labels.
3NiCl2(aq) + 2Na3PO4(aq) → Ni3(PO4)2(s) + 6NaCl(aq)
We predict that reaction occurs because nickel(II) phosphate is insoluble
and precipitates from the reaction mixture. The molecular equation
summarized the reactants and products involved.
To see the reaction that occurs on an ionic level, you need to rewrite the
molecular equation as a net ionic equation. To do this, you first write the
strong electrolytes (the soluble compounds) in the form of ions, leaving the
formula of the precipitate unchanged.
3Ni+(aq) + 6Cl-(aq) + 6Na+(aq)+ PO4 3-(aq) → Ni3(PO4)2(s) + 6Na+(aq) + Cl(aq)
After canceling spectator ions, you obtain the following net ionic equation:
3Ni+(aq) + PO4 3-(aq) → Ni3(PO4)2(s)
This equation represents the essential reaction that occurs between nickel
ions and phosphate ions in aqueous solution in order to form solid
nickel(II) phosphate. If nickel(II) phosphate were soluble, then a reaction
would not have occurred. In which case, we would have had to change our
prediction to no reaction.
Recap: Steps for Predicting a Precipitation Reaction
1. Deduce the products of reaction by exchanging the anions between
the two cations to determine the potential products.
2. Write the balanced molecular equation for the proposed reaction.
3. Use the solubility rules to determine if the products and reactants are
soluble.
4. After applying the solubility rules, use labels to indicate the phases of
the reactants and products.
5. Make a prediction about whether or not a reaction will occur.
6. Write the net ionic equation.
HOMEWORK: Book questions pg. 172 questions 35-41
Book page 173 52, 55-57, 59, 61, 63, 64