AP Chemistry Lab — Precipitation Reactions and Solubility Rules

Prelab:

1. To be done on a separate piece of paper and turned in before lab starts!

Write balanced molecular equations for each of the reactions you will be performingin Parts I and II. (There should be a total of 42 equations.) You will be mixing eachcompound with each other compound, but not with itself. It does not matter whatorder in which you react the compounds. (For example. silver nitrate ± bariumchloride will give the same results as barium chloride + silver nitrate!)

2. Prepare two data tables in your lab notebook — one for Part I and one for Part II. Use afull page for each table! For Part I. label 7 columns (1-7 to correspond to the top ofthe 48-well reaction plate you will use) with the correct formulas of each of the sevensolutions for Part I, starting with silver nitrate. Label 6 rows (A-F to correspond to theside of the 48-well reaction plate you will use) with the correct formulas of sixsolutions for Part I, starting with barium chloride since you will not mix silver nitratewith itself. For the second data table, repeat with the seven solutions for Part II. Eachsquare in each data table must be large enough to record the observations andalso to write the formulas of the two possible products formed!

Introduction:

Discover some general rules of solubility for ionic substances. Conduct 42chemical reactions, determine the solubility of the products. analyze the patterns, andformulate the rules.

Background:

An ionic salt is a compound composed of two parts — caflons (positively-chargedions) and anions (negatively-charged ions). When an ionic salt is dissolved in water, thesalt crystal dissociates or separates into its corresponding cations and anions. Forexample, potassium iodide (KI) dissociates into potassium cations (K) and iodide anions(F) according to equation 1:

KI (s) K (aq) + F (aq) Equation 1

Similarly, the ionic salt lead nitrate [Pb(N03)2jdissociates into lead cations (Pb2)andnitrate anions (N03)according to equation 2:

Pb(N03)2(s) Pbt2 (aq) ± 2 N03 (aq) Equation 2

When two ionic salts are mixed together in water. two new combinations of cations andanions are possible. In some cases, the cation from one salt and the anion from the othersalt may combine to form an insoluble solid product. which is called a precipitate. For

example. if the solutions of potassium iodide and lead nitrate are mixed together. a solidprecipitate of lead iodide (Pb12) forms, as shown in equation 3:2Kb(aq) + 21 (aq)+Pb2(iq) + 2N0 (aq) Pb12(s)2K (aq) +2N03 (aq)

Equation 3

Notice that the potassium cations (K) and the nitrate anions (N03)remain dissolved insolution. These ions are referred to as spectator ions. Spectator ions do not participatedin the overall reaction (hence the term spectators) and are often omitted from the net ionicequation. A net ionic equation is one that includes only the ions participating in thereaction. Thus, equation 3 can be reduced to equation 4:

Pb2 (aq) + 2F (aq) -‘ Pb12 (s) Equation 4

The example described above shows that a precipitate of Pb12 forms when a solution ofKI is mixed with a solution of Pb(N03)2.However, when two salt solutions are mixedtogether and no visible precipitate forms (indicating that no reaction occurs), it can beconcluded that all combinations of the cations and anions from the two salt solutions aresoluble in water.

It must be noted that every salt has some degree of solubility. Some salts dissolvereadily in water while others will not. Even if a salt is considered insoluble (forming asolid precipitate). there is still some small amount of that salt dissolved in the water.(Hence the double arrow” in the equations above. A more correct representation wouldhave a top arrow to the right which is longer than a lower arrow to the left.) Tables ofsolubilities are available to predict whether or not a precipitate will form when two saltsare mixed together. However, observing precipitation reactions in a laboratory settingand becoming familiar with the general rules of solubility can be extremely valuable.

In this laboratory experiment, 14 salt solutions will be combined, two at a time —

seven in Part I and seven in part II — totaling 42 chemical reactions. Observations will berecorded as to which combinations form a precipitate and which do not undergo areaction. From these results, a list of soluble substances and a list of insoluble substanceswill be generated. This list will be analyzed and some general rules of solubility willthen be developed.

Materials Needed:

Chemicals — Part I Chemicals — Part IISilver nitrate solution, 0.1 M Iron (III) chloride solution. 0.1 MBarium chloride solution, 0.1 M Sodium hydroxide solution, 0.1 MSodium carbonate solution. 0.1 M Magnesium bromide solution, 0.1 MAmmonium sulfate solution, 0.1 M Potassium carbonate solution, 0.1 MLead nitrate solution. 0.1 M Lead nitrate solution. 0.1 MCalcium nitrate solution, 0.1M Sodium sulfate solution, 0.1M

Sodium chromate solution. 0.1MPotassium phosphate solution. 0.1 M

EquipmentReaction plate. 48-wellPipet (cassette) holder, with labeled beral pipets. for Part IPipet (cassette) holder, with labeled beral pipets, for Part IIDistilled water bottle and Q-tips for cleaning well plate

Safety Precautions:Barium chloride, lead nitrate, and sodium chromate solutions are toxic byingestion. Silver nitrate solution is toxic, corrosive, light-sensitive, and stains skin andclothing. Sodium hydroxide solution is corrosive and a body tissue irritant. Avoid alleve and body tissue contact with all solutions, Wear chemical splash goggles. chemical-resistant gloves, and a chemical-resistant apron.

Procedure:

Part I — Mixing Pairs of Solutions in Data Table 1

1. Take the pipet holder with the 7 labeled pipets for Part I to the stock solutionstable. Fill (about half-way) the 7bpipets with the appropriate solutions and takethem back to your lab bench.

2. Place a 48-well reaction plate on top of a black table or black piece of paper.

3. Using your Part I Data Table as a guide. place 4 drops of silver nitrate solutioninto well Al. Be sure to hold the pipet vertically for uniformly-sized drops.Continue adding 4 drops of silver nitrate solution to each well down the verticalcolumn 1. wells Bi to Fl.

4. Fill each vertical column in the well plate as follows:Place 4 drops of barium chloride solution into wells B2-F2.Place 4 drops of sodium carbonate solution into wells C3-F3.Place 4 drops of ammonium sulfate solution into wells D4-F4.Place 4 drops of lead nitrate solution into wells E5-F5.Place 4 drops of calcium nitrate solution into well F6.(Remember. it doesn’t matter which order you mix the chemicals so only half ofthe reactions need to be performed!)

5. Now add 4 drops of barium chloride solution to the solution already in well Al.Record your observations immediately. (Note: If a precipitate appears, write“PPT” in the correct box of Data Table I. Record the color of the precipitate. Ifno precipitate appears to have formed, write NR” for No Reaction in the correct

box of Data Table I. Make sure to leave space in each box — whether or not aprecipitate was formed!- to write the two possible products of the reaction.)

6. Continue the process of mixing solutions by adding 4 drops of each of thesolutions listed along the left side of your Data Table I to the correct well. Recordyour observations immediately, as described in Step #5.

7. Once Data Table I is complete. turn your well plate upside down on severalthicknesses of paper towel and tap it on the towel to empty the wells.Immediately flush the reaction plate with tap water. The remaining solutions maybe flushed down the drain with plenty of water. Rinse the well plate with distilledwater (use a Q-tip if this doesn’t completely clean out a well!) and tap it on apaper towel to dry it out — your plate is now ready for Part II.

Part II — Mixing Pairs of Solutions in Data Table H

8. Take the pipet holder with the 7 labeled pipets for Part II to the stock solutionstable. Fill (about half-way) the 7bpipets with the appropriate solutions and takethem back to your lab bench.

9. Place a 48-well reaction plate on top of a black table or black piece of paper.

10. Using your Part II Data Table as a guide, place 4 drops of iron (III) chloridesolution into well Al. Be sure to hold the pipet vertically for uniformly-sizeddrops. Continue adding 4 drops of iron (III) chloride solution to each well downthe vertical column 1, wells Bi to Fl.

11. Fill each vertical column in the well plate as follows:Place 4 drops of sodium hydroxide solution into wells B2-F2.Place 4 drops of magnesium bromide solution into wells C3-F3.Place 4 drops of potassium carbonate solution into wells D4-F4.Place 4 drops of lead nitrate solution into wells E5-F5.Place 4 drops of sodium sulfate solution into well F6.(Remember, it doesn’t matter which order you mix the chemicals so only half ofthe reactions need to be performed!)

12. Now add 4 drops of sodium hydroxide solution to the solution already in well Al.Record your observations immediately. (Note: If a precipitate appears, write“PPT” in the correct box of Data Table I. Record the color of the precipitate. Ifno precipitate appears to have formed, write ‘NR” for No Reaction in the correctbox of Data Table 1. Make sure to leave space in each box — whether or not aprecipitate was formed!- to write the two possible products of the reaction.)

13. Continue the process of mixing solutions by adding 4 drops of each of thesolutions listed along the left side of your Data Table I to the correct well. Recordour observations immediately, as described in Step #5.

14. Once Data Table I is complete, turn your well plate upside down on severalthicknesses of paper towel and tap it on the towel to empty the wells.Immediately flush the reaction plate with tap water. The remaining solutions mayhe tiushed down the drain with plenty of water. Rinse the well plate with distilledwater (use a Q-tip if this doesn’t completely clean out a well!) and tap it on apaper towel to dry it out.

Analysis:

Part I — Making a List of Soluble, Insoluble, and Unknown Substances

I. In each box where a reaction was performed in Data Tables I and II. write theformulas for the two possible double replacement products. This should be doneeven for those combinations that produced no reaction.

2. Use a full page in your notebook to make the following table:

Categories of Substances

Soluble Substances [ Insoluble Substances ] Unknown Substances

Use this table to make a list of all the substances that you know to be soluble inwater. Generate this list of soluble substances in two ways.a. Write down each solution that you used in this experiment from the materials list.

(Since these are all solutions with no precipitates. you can assume they are allsoluble.)

h. Look at your recorded observations in the data tables. For each reaction that didnot produce a precipitate (NR), four substances are now known to be soluble — thetwo starting substances and the two possible products. For example. when mixingtwo soluble substances such as sodium carbonate and ammonium sulfate, noreaction occurs. This tells you that sodium sulfate and ammonium carbonate arealso soluble.

3. Generate a list of all substances that you know to be insoluble. Look at your datatables. Examine each of the reactions that did produce a precipitate. Each pair ofsolutions mixed together can produce 2 new possible substances, as discussed inthe background section. Make the assumption that if a precipitate is formed whenmixing two solutions, the precipitate is formed by only 1 of the 2 new substances.Compare each of the new substances to the list of soluble substances. If one ofthe new substances appears on the soluble list, the other must be the precipitateand can be add to the insoluble column. If neither one of the new substancesappears on the soluble list, then place both substances in the unknown column.Write the two substances together as a pair. This way. later you will know that ifone substance is found to he soluble, the other must be insoluble.

4. Look at your lists of soluble and insoluble substances. Cross out any duplicateentries, using a different colored pen. Leave the list of unknown substancesunchanged.

5. In ‘our notebook. make a separate table like the one above. This time categorizeand alphabetized column 1 (soluble substances) and then column 2 (insolublesubstances) by the anionic part (second work!) of the salt. For example.potassium chloride would be listed as ‘chloride, potassium” and would begrouped with all the other chlorides in that column. Leave column 3 (unknownsubstances) as is, with the salts written in pairs. Do not mix up the pairs!

6. Examine the pairs of substances in the unknown column of your table.Remember, if one substance is found to be soluble, the other must be insoluble.Look for any substances that can now be definitely determine to be soluble orinsoluble. Move these into the appropriate column of your table, and crossing outthe old entry, using your different colored pen.

7. Look for any common relationships among the substances in the unknowncolumn and that of the soluble or insoluble substances. The key objective at thispoint is to remove all of the substances from the unknown column and placethem in either the soluble or the insoluble column using logic and the process ofelimination. There will be a point where no more substances can be moved from

the unknown column by simply looking at laboratory data. At this point, someinferences and generalizations will need to he made. For example, are most (orall) of the nitrate compounds in the same column? How about the ammonium orpotassium compounds? Use the following categories to help with this step:carbonates: chlorides and bromides: chromates; hydroxides; nitrates; phosphates;sulfates: ammonium. potassium and sodium compounds. As you remove asubstance. cross out the old entry and place the substance in its new column usingyour different colored pen. Under your tinal table. for each substance that youremove from the unknown column by generalization/inference, list thereason that you used to move the substance.

8. After completely eliminating the unknown column, examine the substances in thesoluble and insoluble columns. Each column should be organized by group (i.e..carbonates, chlorides, etc.) if you completed step #5 correctly. In your notebook.make a final table - like the one below - with the solubility of each group,including any exceptions that you observe. Be sure that your final tablematches your experimental data and not what you “think” you should get!

General Solubilitv Rules

Substances Soluble or Insoluble ExceptionsCarbonates

Chlorides and Bromides

Chromates

Hydroxides

Nitrates

Phosphates

Sulfates

Ammonium, Potassium,and Sodium Compounds

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General Solubility Rules3ubstances Soluble or Insoluble

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