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Dr. Buckley
e-mail: gbuckley@cameron.edu
Experiment #8 – Metathesis Reactions
Laboratory Overview
CHEM 1361
August 2010
Gary S. Buckley, Ph.D.
Department of Physical Sciences
Cameron University
Learning Objectives
•Observe evidence of metathesis reactions
•Identify spectator ions in solution
•Write net ionic reactions for metathesis reactions
Table of Contents
(you may click on any of the topics below to go directly to that topic)
•Types of Metathesis Reactions
•Precipitation
•Formation of a gas
•Formation of a weak electrolyte
•Completing and Balancing Metathesis Reactions
•Writing Complete Ionic and Net Ionic Equations
Types of Metathesis Reactions - Precipitation
Metathesis reactions, sometimes called exchange or double displacement reactions, occur
when it appears that cations and anions on the reactant side of the equation have “swapped
partners” to form new products. Evidence of the occurrence of a metathesis reaction can be
the formation of a precipitate, the formation of a weak electrolyte, or the formation of a gas.
As an example of a precipitation reaction, consider:
AgNO3 (aq) + NaBr (aq) →
AgBr(s) + NaNO3 (aq)
More on where the parenthetical expressions come from in a couple of slides, but if you think
of the compounds in the equation as being made up of cations and anions you can envision
the reaction as:
AgNO3(aq) +
NaBr(aq)
→
AgBr(s) +
NaNO3(aq)
Notice the Ag ion and the Na ion have simply swapped partners to form the products.
Types of Metathesis Reactions Precipitation (continued)
A crude visual representation of what happens may look like:
AgNO3 (aq)
+
NaBr(aq)
→
AgBr(s) + NaNO3 (aq)
NO3Ag+
NO3-
Ag+
Ag+
NO3-
+
NO3Ag+
Br Br -
NO3-
Na+
Ag+
Ag+
→
Br -
Na+
Na+
Na+
Br
Br Na+
-,
Na+
Br-
NO3-
Na+
Na
+
NO3-
Notice the
NO3
and
ions are separated from each
other in their respective solutions.
Na +
Na+
Br Ag+
Ag+,
NO3-
Na+
-
Na+
Na+
NO3-
Ag+
Br -
Br Ag+
Ag+
Br -
NO3Ag+
Br -
Ag+
The Ag+ and Br- ions have crystallized
to form a solid at the bottom – a
precipitate. The Na+ and NO3- ions
are still floating unaffected –
spectators.
Types of Metathesis Reactions –
Formation of a Gas
A gas may be formed in a metathesis reaction. An example may
help.
One of the key products formed in the reaction of an acid with a
carbonate (CO32-) or bicarbonate (HCO3-) is carbonic acid (H2CO3).
Carbonic acid readily breaks into water and carbon dioxide.
2 HCl(aq) + Na 2 CO3 (aq)
H 2CO3 (aq) + 2 NaCl(aq)
which leads to water and carbon dioxide formed from carbonic acid
2 HCl(aq) + Na 2 CO3 (aq)
H 2O( ) + CO 2 (g) + 2NaCl(aq)
Types of Metathesis Reactions –
Formation of Weak Electrolytes
A metathesis reaction may involve the formation or destruction of a weak
electrolyte such as a weak acid or base. As an example:
NaF(aq) + HCl(aq)
HF(aq) + NaCl(aq)
The HF formed is a weak electrolyte – it does not break into ions significantly
in solution.
Completing and Balancing Metathesis Reactions
Completing and balancing metathesis reactions is a multistep process that must
be carried out carefully. In order to successfully write these equations, you must:
1. Be familiar with formula writing.
2. Be familiar with the necessary polyatomic ions – know their
formulas and charges.
3. Be able to balance a chemical equation.
The following slide states the steps to be followed in completing and balancing a
metathesis reaction.
Steps to Follow in Completing and Balancing a Metathesis Reaction
Switch the cations to form products. Carry over subscripts that are a part of a
polyatomic ion – sulfates, phosphates, carbonates, etc. DO NOT carry over
subscripts that are for monatomic ions – e.g., chlorides, oxides, bromides,
sodium, barium, etc.
Write the correct formulas for the products you have made by going
back to the formula writing practice from earlier in the semester.
Balance the equation. This should be easy if you wrote the formulas correctly.
Indicate the correct physical state (s, ℓ, g, or aq) in parentheses after each product.
A summary of solubility rules may be found in your textbook or here.
An example is on the following slide.
Example of Completing and Balancing a Metathesis Reaction
Complete and balance the following reaction:
Na3PO4 (aq) + BaCl2 (aq) →
First step (previous slide): Swap cations:
Na3PO4 (aq) + BaCl2 (aq) → NaCl
+
BaPO4
Second step (previous slide): Write correct formulas
Na3PO4 (aq) + BaCl2 (aq) → NaCl
+
Ba3(PO4)2
Third step (previous slide): Balance equation
2 Na3PO4 (aq) + 3 BaCl2 (aq) → 6 NaCl
+
Ba3(PO4)2
Fourth step (previous slide): Identify the physical states based on solubility
rules, gas formation, etc.
2 Na3PO4 (aq) + 3 BaCl2 (aq) → 6 NaCl (aq) + Ba3(PO4)2 (s)
Writing Complete Ionic and Net Ionic Equations
Writing complete and net ionic equations is helpful in determining
exactly which species take part in the reaction. Each substance is
written as either the complete intact formula or broken into ions. A
compound is broken into ions if it is BOTH soluble and a strong
electrolyte. If it fails either of those tests, it is written as the intact
compound.
The determination of whether a substance is both soluble and a
strong electrolyte flows from categorizing the substance as an acid,
base, or salt. The sheet at this link may be helpful in making that
determination.
The complete ionic equation results from writing each substance in
either its intact form or as separated ions, depending on its
solubility and electrolytic nature. Species that are exactly the same
on both sides of the complete ionic equation are then cancelled,
resulting in the net ionic equation. Those species cancelled are
called spectator ions – they do not take part in the reaction.
Example of Writing Net Ionic Equations
Continuing with our example from a couple of slides back:
2 Na3PO4 (aq) + 3 BaCl2 (aq) → 6 NaCl (aq) + Ba3(PO4)2 (s)
All of the substances in this equation are salts, which makes them strong electrolytes.
The question is their solubility. Following the solubility rules found here, the sodium
salts are both soluble because of the Group 1 sodium cation. The BaCl2 is soluble
because it is a halide and not one of the exceptions. The Ba3(PO4)2 is insoluble since
phosphates are generally insoluble (except for Group 1 and ammonium). Thus, the
complete ionic equation looks like:
6 Na+(aq) + 2 PO43-(aq) + 3 Ba2+(aq) + 6 Cl-(aq) → 6 Na+(aq) + 6 Cl-(aq) + Ba3(PO4)2 (s)
This is the complete ionic equation. Cancelling out the species that are identical on
both sides results in the net ionic equation:
2 PO43-(aq) + 3 Ba2+(aq) → Ba3(PO4)2 (s)
The cancellation of the sodium and chloride ions indicates they were spectators – they
played no significant role in the reaction that occurred.
Troubleshooting Equation Writing
A couple of tips for issues with equation writing:
Completing and balancing metathesis reactions:
- Write formulas for products based on formula writing rules, not on
how many of each atom were on the reactant side.
- If balancing seems difficult, go back and check formulas. These
equations will balance easily if the formula writing is correct.
Writing Complete Ionic and Net Ionic equations:
- Be sure your starting equation is balanced
- Only species that are BOTH soluble and strong electrolytes should be
written as separated ions.
- Include charges on all ions written from separated compounds.
Unless a polyatomic ion, the subscript does not stay with the ion. For
example BaCl2 separates into Ba2+ and 2 Cl- not Ba2+ and Cl22- .
- As a further check on your balancing, the sum of charges on the
reactant side must equal the sum of charges on product side in both
your complete ionic and net ionic equations.