Exchange reactions, also called double replacement reactions, occur when one of the products of the reaction is insoluble, or is a small molecular compound like H₂O (which is formed in an acid/base reaction) or CO₂ (formed by reaction of a carbonate or bicarbonate salt with an acid). The purpose of this week's experiment is to study a series of possible exchange reactions and identify the products of the reaction (if any) based on your observations. By the end of the experiment, you should be more familiar with the three classes of exchange reactions: precipitation, acid/base, and gas forming.

Precipitation reactions occur when one of the products of the exchange reaction is insoluble (this is called a precipitate). One may use solubility rules to determine whether or not a particular ionic compound will be soluble in water:

(1) Compounds containing the NO₃^- ion are soluble

(2) Compounds containing Cl^-, Br^-, and I^- are soluble, except when the cation is Ag⁺, Pb²⁺, or Hg⁺.

(3) Compounds containing SO₄^2- are soluble, except when the cation is Ba²⁺, Hg²⁺, or Ca²⁺.

(4) Compounds containing a first row metal ion (Na⁺, Li⁺, K⁺, Cs⁺) or an ammonium ion (NH₄⁺) are soluble.

(5) Compounds containing OH^- are insoluble, except when the cation is a first row metal or ammonium ion (see #4, above).

(6) Compounds containing CO₃^2-, S^2-, and PO₄^3- are insoluble, except when the cation is a first row metal or ammonium ion (see #4, above).

You should memorize these solubility rules, as they are very useful in determining whether or not a reaction will occur, and what the products of that reaction might be.

Neutralization reactions occur when an acid (a compound that dissociates to produce H+ ions) reacts with a base (a compound that dissociates to produce OH^- ions) to produce an ionic compound and water. Typical acids and bases we will encounter include:

Note that while acetic acid has a total of 4 hydrogens, only one of these dissociates in solution to produce an H+ ion [the dissociation of CH₃CO₂H produces CH₃CO₂^- (acetate ion) and H⁺]. Also, ammonium hydroxide is produced when NH₃, a gaseous molecular compound, dissolves in (and reacts with) water.

All of the reactants for this week's experiment are available as aqueous solutions. Read labels carefully, and use only the required reagents for the experiments. Work in pairs for this experiment. The reactants for this experiment are listed on the next page.

Obtain approximately 2 ml of each reagent listed above (2 ml = about 40 drops). Mix two drops of one reagent with two drops of another reagent in a small test tube, and record your observations on the table below. Your observations may be one of the following: color changed (C), precipitate formed (P), gas evolved (G), or heat produced (H). You will use your observations to help you write balanced chemical equations for the reactions. Complete each entry in the table by mixing the appropriate reagents together and recording your observations.

Example: Mix 2 drops of CaCl₂ with 2 drops of NaOH. Record your observations in the box found at the intersection of the CaCl₂ column and the NaOH row. Now, repeat the experiment using CaCl₂ and each of the other reactants.

Chemical Disposal: Combine all of your reaction mixtures and any excess reagents in your 400 ml beaker. Rinse your test tubes out with tap water. At the end of the experiment, add 100 ml of water, then add 2 drops of bromothymol blue indicator. Finally, add 6 M NaOH in small amounts until the indicator just turns blue. The contents of the beaker may then be disposed of down the sink.

Using your observations, write balanced total chemical equations as well as net ionic equations for each of the reactant combinations above which produced a chemical change. Be sure to note if a particular reaction was a precipitation reaction, an acid/base reaction, or a gas-forming reaction.

Example: the reaction of CaCl₂ and NaOH produced a white precipitate, indicating that a chemical change occurred. We can now write a balanced exchange (double replacement) reaction to describe this process, noting which of the products is the insoluble material:

CaCl₂ (aq) + 2 NaOH (aq) à Ca(OH)₂ (s) + 2 NaCl (aq) (total reaction)

Ca²⁺ (aq) + 2 OH^- (aq) à Ca(OH)₂ (s) (net ionic equation)

Since a solid was formed, this must be a precipitation reaction.

Gas forming reactions are obvious. Acid/base reactions often do not produce a noticeable change other than the evolution of heat. For reactions which do not change in color, do not produce a precipitate, do not evolve a gas, and do not produce any heat: no chemical change occurred, so don't write any equations to describe the process.

On Wednesday, October 28 at 11:00 AM, a set of completed, balanced chemical equations and net ionic equations for each of the reactions that produced a chemical change should be handed in. Also be sure to indicate the type of reaction that occurred (precipitation, acid/base, gas-forming). One set of reactions should be submitted for each lab group. The reactions may be hand-written, but please, no pages torn out of notebooks.

(Adapted in part from "Reaction Types and Chemical Logic", found in Discovery and Analysis in the Laboratory by J. March and D. Shaw, Published by Harcourt Brace, 1997)