Separation of Cations & Anions

SEPARATION & QUALITATIVE ANALYSIS OF CATIONS & ANIONS The goal of this experiment is to study some double replacement (metathesis) reactions and develop an understanding of the solubility rules which are applied when predicting the outcome of double replacement reactions. What are double replacement reactions? 1. In double replacement reactions, there is an exchange of ions between the two reacting species. The general form of the reaction is: A+B– + C+D–  A+D– + C+B– Notice the exchange of ions. That is, the positive ion, A+ in compound AB, combines with the negative ion, D–, in compound CD combine to form the product AD. Likewise, the positive ion, C+ in compound CD, combines with the negative ion, B–, in compound AB combine to form the product CB. 2. Double replacement reactions only occur when ions are added or removed from the solution. Such as when . . . . . . • An insoluble solid product, called a “precipitate” is formed (the type studied in this lab). • A molecular species such as a liquid, H2O(l), or gas such as carbon dioxide, CO2(g), is formed. • A liquid(l) or solid(s) reactant is consumed (used up). Another goal of this experiment is to use each of the four methods of writing the equation for a chemical reaction. The four types are: 1. Word equation In word equations, the names of the reactants and products are written out. An example of a word equation is: sodium iodide + lead nitrate → sodium nitrate + lead iodide 2. Formula equations Formula equations consist of formulas substituted for the names in the word equation. Once the formulas are determined, the reaction can be balanced. Substituting formulas for names in the reaction above:

3. Ionic equations Using the solubility guidelines, all water soluble compounds in an aqueous solution are separated into ions in ionic equations. If the substance is a solid, liquid, or gas, the substance is NOT separated into aqueous ions as they do not exist with these substances. The next step in the process is to re-write the balanced equation giving the state (solid, liquid, gas, or aqueous) of each of the reactants and products. Since sodium iodide, NaI, lead nitrate, Pb(NO3)2, and sodium nitrate, NaNO3, are predicted to be soluble in water, they are given the aqueous, (aq), designation. Since lead iodide, PbI2, is predicted to be “insoluble” in water, it is given the solid, (s), designation. 2NaI(aq) + Pb(NO3)2(aq)  2NaNO3(aq) + PbI2(s)

Now that we have a balanced formula equation we can write the ionic equation. That is, separate each water soluble substance (aq) in the formula equation into its ions. In this example, NaI, Pb(NO3)3, and NaNO3 have the aqueous solution designation and therefore must be written as ions. 2Na+(aq) + 2I–(aq) + Pb2+ (aq) + 2NO3–(aq)  2Na+(aq) + 2NO3–(aq) + PbI2(s) Since there are two moles of sodium iodide, 2NaI(aq), in the balanced formula equation, we will separate the two moles sodium iodide into 2 moles of Na+(aq) and two moles of I– (aq). Likewise, the one mole of lead nitrate, Pb(NO3)2(aq), becomes one mole of lead ions, Pb2+(aq) and two moles of nitrate ions, NO3–(aq). In the product, only the two moles of sodium nitrate, 2NaNO3(aq), is separated into ions as it’s the only water soluble product. 4. Net ionic equations The net ionic equation contains all of the particles in the ionic equation less any “spectator” ions such as the Na+(aq) and NO3–(aq) in this example. The net ionic equation represents the actual reaction which is taking place in your experiment. Notice that in the ionic equation, the sodium ions and nitrate ions are identical on each side of the balanced equation. Their presence is not important and they can be eliminated from the overall reaction. They are called spectator ions. Spectator ions do not participate in the chemical reaction. That is, they are identical on both sides of the equation. In the reaction above, the sodium, Na+(aq) and nitrate, NO3–(aq), ions are the same on both sides of the equation and therefore can be eliminated. 2Na+(aq) + 2I–(aq) + Pb2+ (aq) + 2NO3–(aq)  2Na+(aq) + 2NO3–(aq) + PbI2(s) Net ionic equation:

Pb2+ (aq) + 2I–(aq)  PbI2(s)

What does the net ionic equation tell us? When solutions of sodium iodide, NaI(aq), and lead nitrate, Pb(NO3)2(aq), are mixed, an insoluble product (called a “precipitate”) of lead iodide, PbI2(s), is formed. The sodium and nitrate ions remain is solution as they are soluble in water. Pre-Lab Assignment: 1. For each test to be performed (26 total): (a) Assuming that each reaction is a double replacement reaction, write the names and formulas of the products and record in the data table. (b) Using the solubility guidelines, predict if the reactants and products are soluble or insoluble, and record that information in the space provided in the data table.

MATERIALS: A one foot square section of plastic wrap, paper towels, stirring rod, dropping bottles containing 0.1M solutions of the following: Na2CO3 NaI Sr(NO3)2 CuSO4 Pb(NO3)2 CaCl2 ZnC2H3O2 KBr

PROCEDURE: You and your partner will do half of the reactions (13 tests) and then pool your results with the other group working at your table. 1. Spread a piece of plastic wrap over white paper on the lab bench. Smooth out any wrinkles. 2. For each trial: Obtain the dropper bottle of the “main” soluble compound you will be testing and place one drop of the solution on the plastic wrap for each test to be conducted with that solution. Be sure to leave enough space between the test spots so that they do not mix with other tests. Record the names and formulas of all the compounds to be tested with the main solution in the data table. Holding the dropper tip a few inches above the main drop, add one drop of each of these solutions to the main solution and observe. *DO NOT touch the dropper tips to drops of other solutions as this will contaminate the solutions and give inaccurate results!* If the solution turns cloudy, a precipitate or nearly insoluble product has been produced by that combination of materials. Record your observations by writing "ppt" to denote a precipitate and "NR" if there is no reaction observed (both products are soluble in water) in the "reaction" column on your data sheet. (Even though a color change with no precipitate is considered to be "no reaction or NR" in this lab, it doesn’t mean that there was no reaction…it just means that the products were soluble…) 3. When you are finished with your tests, collaborate with the other group working at your table and obtain the data for the solutions you did not mix together. 4. Clean up: place another piece of paper towel on the plastic wrap to absorb the solutions. (Don’t touch it, just let it soak up.) Then roll everything up in the plastic wrap and place in the trash. Thoroughly wash your hands as heavy metals used in this experiment are toxic. Group 1 Tests:

Group 2 Tests:

Strontium nitrate with Calcium chloride Sodium carbonate Copper sulfate Zinc acetate Sodium iodide Potassium bromide

Calcium chloride with Sodium carbonate Copper sulfate Zinc acetate Sodium iodide Lead nitrate Potassium bromide

Sodium carbonate with Copper sulfate Zinc acetate Lead nitrate Potassium bromide

Copper sulfate with Zinc acetate Sodium iodide Lead nitrate Potassium bromide

Zinc acetate with Sodium iodide Lead nitrate Potassium bromide

Sodium iodide with Lead nitrate Potassium bromide Lead nitrate with Potassium bromide

QUESTIONS: 1. For only the tests in which a reaction occurred (i.e. a precipitate formed): (a) Write a complete word equation for the double replacement reaction studied. Be certain to note whether the material is dissolved in water with the (aq) for aqueous solution or if the product is considered insoluble using (s) for solid notation. (b) Write a complete, balanced formula equation for each of the reactions studied. (c) Give the ionic equation for each of the reactions studied. (d) Eliminate the spectator ions from the ionic equation and write the net ionic equation for each reaction studied. 2. Were there any discrepancies between the solubility guidelines and what you observed in this lab? If so, explain possible reasons for this. 3. Some of the solutions were not tested with one another. For example, sodium carbonate was not tested with sodium iodide, nor was strontium nitrate tested with lead nitrate. Explain why you did not have to test these solutions with one another.