†Adapted with permission from an experiment developed by Professor CJ Scaife, Union College, Schenectady, NY.
1 period; work individually. Complete the Preparation page before laboratory.
Goals
Background
A reaction of two water soluble salts in which cation and anion partners are "traded" is called a metathesis or double displacement reaction. Such a reaction is represented schematically below, where A and B are two different cations, X and Y two different anions.
AX + BY ® AY + BX(1)
For such a reaction to proceed to the right, there must be a thermodynamic driving force. The driving force is provided, according to Le Chatelier's principle, by removing one of the products from solution. This will occur if one of the products precipitates (i.e., leaves solution as a solid), is a weak electrolyte (i.e., is essentially unionized in solution), or decomposes to form a gas, which escapes from solution. Reaction of two salts to form solid, weak electrolyte, or gas is spontaneous. The free energy change, DG, for such a process is negative. This experiment deals with reactions that (potentially) form solids.
The formation of a precipitate is illustrated in equation form below. Three equations, of increasing simplicity, are shown.
|
AgNO3(aq) + |
NaCl(aq) ® |
AgCl(s) + |
NaNO3(aq) (2) |
|
soluble |
soluble |
insoluble |
soluble |
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) ® AgCl(s) + Na+(aq) + NO3-(aq) (3)
Ag+(aq) + Cl-(aq) ---> AgCl(s) (4)
These are called, respectively, the total equation (TE), the ionic equation (IE), and the net ionic equation (NIE). The TE shows what reagents to obtain from the stockroom to carry out a given reaction. An IE is obtained by writing all soluble strong electrolytes from the TE in dissociated form; it bridges the total equation and the net ionic equation. The net ionic equation is obtained by omitting ions that appear on both sides of the ionic equation. Canceled ions are called spectator ions because they do not take part in the chemical reaction. Therefore, the NIE shows in simplest form what species take part in the reaction.
Solubility is the amount of solute (in g or moles) that dissolves in a given amount of solvent (in mL, L, or g). If quite a bit of a solid dissolves in a solvent, the solid is soluble in that solvent. If only a small amount of a solid dissolves in the solvent, the solid is insoluble, or sparingly soluble, in the solvent. A solubility of 1.0 gram solute per 100 mL solvent is considered the border line between soluble and insoluble. For example, 74.5 g of calcium chloride, CaCl2, dissolves in 100 mL H2O; thus CaCl2 is soluble in water. In contrast, only 2.9 x 10-4 g silver chloride, AgCl, dissolves in 100 mL water, so it is insoluble in water.
In this experiment, you investigate many cation-anion combinations to discover which produce insoluble products by carrying out double displacement reactions which form solids. The cations to be investigated are Na+, K+, NH4+, Mg2+, Ca2+, Ba2+, Al3+, Pb2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+. The anions to be investigated are NO3-, Cl-, Br-, I-, CO32-, SO42-, PO43-, PO43-, CrO42-, OH-, S2-. To minimize time, cost, and safety hazard, and to maximize the efficiency of carrying out and cleaning up the experiments, you will work on a very small scale (microscale). Based on observation, you will propose answers to the following questions:
Focus Questions
Equipment and Materials
Safety
All reagents used in this experiment are to be considered harmful. Wear your goggles. Avoid ingestion of solutions or solids.
Experimental
Record all data and observations in your lab notebook or on the Results sheet, whichever is applicable. Obtain a plastic reaction sheet, a paper grid, two paper clips, and three microstirrers (toothpicks). Wipe the sheet with a Kimwipe to remove any residue (contaminants may cause confusing reactions). Attach the grid to the underside of the plastic sheet with paper clips. Practice delivering individual drops of water from a disposable micropipet until you can do it competently.
Several test tube racks, each containing test tubes with cation and anion solutions, and micropipets for dispensing solution, have been placed in the lab. You will share the solutions and pipets with others, so be quick, careful, and courteous. Do not keep a test tube/micropipet set at your bench for longer than you need it. Start with the cation nitrate solutions shown in the left column of the grid. The cation nitrate solutions were prepared by dissolving the nitrate salt of the cation in water (e.g., Ca(NO3)2(s) was used for Ca2+). Obtain a test tube containing a solution of any cation except Na+ (which will be omitted, since it is already present in all the anion solutions). Use the pipet to place one drop of the cation solution in the appropriate cation circle in the leftmost column on the grid. Place nine more drops in the smaller circles horizontally across the grid. Center the drops in the circles. Return the pipet to the test tube, and the test tube to the rack. Continue with other cation solutions until you have a complete matrix of cation drops (160 total).
Proceed similarly with the sodium anion solutions in the top row of the grid. Sodium anion solutions were prepared by dissolving the sodium salt of the anion in water (e.g., NaBr(s) was used for Br-). Obtain a test tube containing solution of any anion except NO3- (omitted, since it is already present in all cation solutions). Use the pipet to place one drop of the anion solution in the appropriate anion circle in the top row on the grid. Place 16 more drops vertically down the sheet. Place each drop directly on the drop of cation solution. Complete the matrix of anion solutions (153 drops in all). Use only one drop of each solution, and be careful not to touch the tip of the dropper to other solutions on the sheet. Finally, place one drop NaNO3(aq) centered in the oval in the upper left corner of the grid.
Record observations on the boxed grid (see following pages). Indicate whether solid is formed, and colors of solids and/or remaining solutions. Cloudiness often suggests solid formation. If in doubt about solid formation, view the drop with a magnifying lens and/or stir the drop carefully with a microstirrer. Use a colored highlighter to indicate on the boxed grid where precipitate formed.
Disposal Methods
Clean the plastic reaction sheet by curling it and emptying the solutions and solids into a 250-mL beaker. Rinse remaining material into the beaker using a distilled water wash bottle. USE NO MORE THAN A TOTAL OF 100 mL OF WATER TO WASH THE SHEET. Finally, rinse the working side of the sheet with tap water and dry carefully with a towel, then with a Kimwipe. When the sheet is completely dry, return it and the paper grid sheet to the instructor.
UNDER A FUME HOOD, add 6 M NaOH solution dropwise to the 250-mL beaker until the mixture is basic to litmus (3-5 drops required). Add 1 drop excess. Add 10 drops 1 M Na2S solution to insure complete precipitation of aqueous cations as insoluble sulfide salts. Finally, heat the mixture using a Bunsen burner just to boiling for 3 minutes to coagulate the precipitate. Pour the mixture into the waste bottle in the hood labelled HEAVY METAL MIXTURE. This will later be filtered to remove heavy metal waste prior to disposal. Wash your hands well.
References
1. C. Scaife, Union College, communication.