Peter Jeschofnig, Ph.D. Version 09.2.02
Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.
Study the nature of ionic reactions, and write net ionic equations for precipitation reactions. Identify spectator ions, perception reactions and solubility of different compounds.
© 2010 Hands On Labs, Inc. LabPaq CK-1 100
To study the nature of ionic reactions To write balanced equations To write net ionic equations for precipitation reactions
© 2010 Hands On Labs, Inc. LabPaq CK-1 101
Label or Box/Bag:
1 1 1
Cotton Swabs Sheet each of white and black paper Distilled water
1 1 1
Goggles-Safety Well-Plate-24 Well-Plate-96
Auxiliary Supplies Bag
Auxiliary Supplies Bag-CK1
Pipet, Empty Short Stem
1 1 1 1 1 1 1 1 1 1
Barium Nitrate, 0.1 M - 2 mL in Pipet Cobalt (II) Nitrate, 0.1 M - 2 mL in Pipet Copper (II) Nitrate, 0.1 M - 2 mL in Pipet
Iron (III) Nitrate, 0.1 M - 2.5 mL in Pipet Nickel (II) Nitrate, 0.1 M - 2 mL in Pipet
Sodium Bicarbonate, 0.1 M - 2 mL in Pipet Sodium Carbonate, 0.1 M - 2 mL in Pipet
Sodium Chloride, 0.1 M - 2.5 mL in Pipet Sodium Hydroxide, 0.1 M - 2 mL in Pipet
Sodium Iodide, 0.1 M - 2.5 mL in Pipet Sodium Phosphate, 0.1 M - 2 mL in Pipet
Sodium Sulfate, 0.1 M - 2.5 mL in Pipet
*Note: The packaging and/or materials in this LabPaq may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List form included in the LabPaq.
© 2010 Hands On Labs, Inc. LabPaq CK-1 102
Discussion and Review
In this exercise you will work with aqueous (aq.) solutions of ionic substances. Aqueous solutions are those solutions in which water is the solvent. Water, because of its high polarity, is an especially good solvent for ionic substances. When ionic substances are dissolved in water, the ions separate and become surrounded by water molecules. This separation of ions is called dissociation. Thus, when sodium chloride dissolves in water, the resulting solution contains sodium ions and chloride ions.
Let us assume that we have two aqueous (aq) solutions, one containing silver nitrate, AgNO3 (aq.), and the other containing sodium chloride, NaCl (aq). The ions present in the two solutions are:
Solution 1: Ag+ and NO3 ̄ Solution 2: Na+ and Cl ̄
If the two solutions are mixed, the combined solution will appear milky. This is caused by the formation of an insoluble solid. Since the two solutions were clear initially, this newly formed substance must be the result of a chemical reaction. An insoluble product such as this is known as a precipitate (ppt), and we call this type of reaction a precipitate reaction.
Precipitates are electrically uncharged. In this experiment all of the precipitates result from the exchange of positive and negative ions between reacting solutions of two ionic compounds. Thus, in this example the precipitate must be either silver chloride, AgCl or sodium nitrate, NaNO3. Sodium nitrate dissolves readily in water and is therefore soluble. Thus we can conclude that silver chloride is the insoluble precipitate. We can represent the formation of this precipitate by a net ionic equation:
Ag+ +NO3 ̄ +Na++Cl ̄→AgCl+NO3 ̄+Na+ Net: Ag+ (aq)+Cl ̄ (aq)→AgCl(s)
Ions present in the solution but not involved in forming the precipitate are called spectator ions; in our example these would be Na+ and NO3-. Spectator ions are not recorded in net ionic equations. To help identify (or predict) which compounds are soluble or insoluble most chemistry texts contain solubility rules and tables.
Basic Solubility Rules:
A. Nitrates: All nitrate salts are soluble.
B. Alkali metals: The salts of lithium, sodium, potassium, rubidium, and cesium are generally very soluble.
C. Ammonium salts: Almost all ammonium salts are soluble.
D. Sulfates: The sulfates of most common elements are soluble, except those of calcium, strontium, barium, and lead (II) ions.
© 2010 Hands On Labs, Inc. LabPaq CK-1 103
E. Hydroxides: Most of the hydroxides are insoluble, except those of the alkali metals and barium; calcium hydroxide is moderately soluble.
F. Halides: The chloride, bromide, and iodide salts are generally soluble, except those of silver, lead (II), and mercury (I) ions.
G. Sulfides: Most sulfides are insoluble, except from alkali metals and ammonium ion.
H. Acetates: All acetates are soluble, except silver acetate, which is slightly soluble.
I. Silver salts: All silver salts are insoluble, except silver nitrate, silver nitrite, and silver perchlorate. Silver acetate and silver sulfate are slightly soluble.
J. Carbonates: All carbonates are insoluble, except those of ammonium, sodium, potassium, and the other alkali metals.
K.Phosphates: Phosphates are insoluble, except those of ammonium, sodium, potassium, and the other alkali metals.
IMPORTANT: If there is no Auxiliary (Aux) Chemicals Bag in the LabPaq, skip the following steps for chemical dilutions, as your chemicals are already diluted to the molarity needed for this experiment.
If the LabPaq contains an Auxiliary (Aux) Chemicals Bag, follow the procedures below to dilute the chemicals to the molarity needed for this experiment.
Auxiliary Chemical Dilutions:
Before you begin this experiment, prepare the chemicals from the auxiliary chemicals bag as instructed below:
First dilute the 6 M NaOH (sodium hydroxide) to 0.1 M in one well of the 24-well plate: add 1 drop of 6M NaOH directly from the auxiliary bottle, and then using the empty unlabeled pipet add 59 pipet drops of distilled water.
Next, read through the following procedures and then draw a data table in your notebook to record your observations about any precipitates or gases that form when the two solutions are combined. Remember to include color for precipitates. Examine the reactions against both dark and white backgrounds. If there is no reaction for that combination of solutions write NR (no reaction).
© 2010 Hands On Labs, Inc. LabPaq CK-1 104
CAUTION! Sodium hydroxide is caustic and can burn skin and clothes if it touches them. Rinse any spills well with copious amounts of water. Also, most of these chemicals are toxic by ingestion. So remember, no food or drinks in the lab!
1. All reactions will be performed in the 96-well plate. a. Place 2 drops of cobalt (II) nitrate solution into seven of the A row wells. b. Place 2 drops of copper (II) nitrate into seven of the B wells. c. Place 2 drops of iron (III) nitrate into seven of the C wells. d. Place 2 drops of barium nitrate into seven of the D wells. e. Place 2 drops of nickel (II) nitrate into seven of the E wells.
2. When performing the following, NEVER TOUCH THE TIP OF THE PIPET TO THE SOLUTION ALREADY IN THE WELL!
a. Place 2 drops of sodium phosphate solution into five vertical wells under column number 1.
b. Place 2 drops of sodium iodide solution into five vertical wells under 2. c. Place 2 drops of sodium sulfate solution into five vertical wells under 3. d. Place 2 drops of sodium chloride solution into five vertical wells under 4. e. Place 2 drops of sodium bicarbonate solution into five vertical wells under 5. f. Place 2 drops of sodium carbonate solution into five vertical wells under 6. g. Place 2 drops of sodium hydroxide solution into five vertical wells under 7.
Cleanup: When all observations have been recorded rinse the 24-well plate and the 96-well plate under tap water until all remnants of chemicals have disappeared. Use a Q-tip® to remove stubborn precipitates. If the precipitates are allowed to dry they are much harder to remove. Dispose of any unused solutions by flushing them down the drain with lots of water and throw the empty pipets in the trash.
© 2010 Hands On Labs, Inc. LabPaq CK-1 105
A. Compare your results with the solubility rules and/or solubility table in your chemistry text.
B. Do your results agree with your expectations from the solubility rules/table? C. Which anions generally form precipitates? What are exceptions? D. Which anions generally do not form precipitates? What are the exceptions? E. Which cations generally do not form precipitates?
F. Select 10 reactions that produce a precipitate, color change, or gas and write balanced chemical equation and a net ionic equation for each. Remember, a reaction may be indicated by the formation of a precipitate, color change, or the formation of gas. Record the well numbers of the precipitates you chose for your equations.
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