015 Combining Cations and Anions

Expt 015 -- Combining Cations and Anions

The sodium salts of five common anions (C₂H₃O₂^-, SO₄^2-, Cl^-, S^2-, C₂O₄^2-) are mixed with the nitrate salts of five common cations (K⁺, Ba²⁺, Ag⁺, Cu²⁺, Mn²⁺) to determine patterns of solubility. An optional sixth cation may be used to see more complex chemistries.

Safety

Silver, copper(II), barium, and manganese compounds are toxic. Do not ingest the chemicals. Wear goggles and apron. Silver solutions cause stains. Wash spills immediately with large amounts of water. Wash hands after the experiment.

Procedure

Anions:

Add 2 drops of sodium acetate solution to each of the "1" wells in the first five rows: A1, B1, C1, D1, and E1. Add 2 drops of sodium sulfate to each of the "2" wells in the first five rows: A2, B2, C2, D2, and E2. Repeat for all of the remaining sodium solutions, assigning each anion ion to a column of its own.

0.1 M sodium acetate (NaC₂H₃O₂)
0.1 M sodium sulfate (Na₂SO₄)
0.1 M sodium chloride (NaCl)
0.1 M sodium sulfide (Na₂S)
0.1 M sodium oxalate (Na₂C₂O₄)

!!!Click here to See Picture.

Cations:

Add 2 drops of potassium nitrate solution to the first 5 wells in the "A" row of a 96-well plate (A1, A2, A3, A4, and A5). (If a plastic sheet is used instead of the 96-well plate, make puddles of 2 drops each on the sheet.) Add 2 drops of barium nitrate solution to the first 5 wells in the "B" row of a 96-well plate (B1, B2, B3, B4, and B5). Repeat for all of the remaining nitrate solutions, assigning each metal ion to a row of its own.

!!!Click here to See Movie. This movie is sped up 5 times faster than the actual experiment.
1. 0.1 M potassium nitrate (KNO₃)
2. 0.1 M barium nitrate (Ba(NO₃)₂)
3. 0.1 M silver nitrate (AgNO₃)
4. 0.1 M copper (II) nitrate (Cu(NO₃)₂)
5. 0.1 M manganese nitrate (Mn(NO₃)₂)
(optional)
1. 0.1 M Ferrous ammonium sulfate (Fe(NH₄)₂(SO₄)₂)
Note and record observations for the 25 wells. If the well contains a precipitate after adding the two solutions, fill in the corresponding square with the color of the precipitate and note any unusual observations. If there is no evidence of a precipitate, do not write in the square. A clear, colorless solution, indicates that no precipitate is present. A dark background will enhance observation of white precipitates.
If a precipitate is present in small amounts, note that also.

!!!Click here to See Picture.
!!!Click here to See Picture.

Questions

Write balanced chemical equations for all combinations that produced a precipitate.

Handout

Name ___________________________ Class _______

Teacher __________________________

SmallScale 015 Combining Cations and Anions

	Fe²⁺	Mn²⁺	Cu²⁺	Ag⁺	Ba²⁺	K⁺
C₂O₄^2-
S^2-
Cl^-
SO₄^2-
C₂H₃O₂^-

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 015 Combining Cations and Anions

Record observations from the pictures above. Record the formula of each substance added both across and down.

Formulas
_________________________________________ Anions
Formulas
5
4
3
2
1
Answer the questions using your observations.

Curriculum-

Use this activity when discussing solutions, salts, solubility, net ionic equations, or descriptive chemistry. Use this experiment early in the course. Use Experiment 025 for mor complex solution chemistry later in the course. The logical puzzle presented in this problem has value almost any time after the introductory phase of a course is over -- after just a few weeks.

Safety-

Silver, copper(II), barium, iron(II), and manganese compounds are toxic. Do not ingest the chemicals. Wear goggles and apron. Silver solutions cause stains. Wash spills immediately with large amounts of water. Wash hands after the experiment.

Time-

Teacher Preparation: 30 minutes

Class Time: 30 minutes

Materials-

Prepare 1 mL of each reagent.
- 0.1 M KNO₃ (Dissolve 1.011 g of KNO₃ in enough distilled water to make 100 mL of solution)
- 0.1 M Cu(NO₃)₂ (dissolve 2.4 g Cu(NO₃)₂•3H₂O in enough distilled water to make 100 mL of solution)
- 0.1 M AgNO₃ (dissolve 1.7 g AgNO₃ in enough distilled water to make 100 mL of solution)
- 0.1 M Ba(NO₃)₂ (Dissolve 2.613 g of Ba(NO₃)₂ in enough distilled water to make 100 mL of solution)
- 0.1 M Mn(NO₃)₂ (Dissolve 2.870 g of Mn(NO₃)₂•6H₂O in enough distilled water to make 100 mL of solution)
  
  Optional (This solution must be made fresh because the ferrous ion air oxidizes.)
- 0.1 M Fe(NH₄)₂(SO₄)₂•6H₂O (Dissolve 3.921 g ferrous ammonium sulfate hexahydrate ((NH₄)₂Fe(SO₄)₂•6H₂O) in enough water to make 100 mL solution.)
Anions:
- 0.1 M sodium acetate (NaC₂H₃O₂) (Dissolve 1.360 g of NaC₂H₃O₂•3H₂O, sodium acetate trihydratein enough water to make 100 mL solution.)
- 0.1 M sodium oxalate (Dissolve 1.340 g of Na₂C₂O₄, sodium oxalate in enough water to make 100 mL solution.)
- 0.1 M sodium chloride (Dissolve 0.584 g sodium chloride (NaCl) in enough water to make 100 mL solution.)
- 0.1 M sodium sulfide (Dissolve 2.401 g of Na₂S•9H₂O, sodium sulfide in enough water to make 100 mL solution.)
- 0.1 M sodium sulfate (Na₂SO₄) (Dissolve 1.420 g of Na₂SO₄, sodium sulfate in water. Dilute to 100 mL with water.)
96-well plate or acetate sheet
toothpicks
cotton swabs

Disposal-

Work at a hood. Add household ammonia to the contents of the disposal jar until they are just basic. Add 0.1 M Na₂S in 10 mL portions until no further precipitation is observed. Filter the precipitate. Discard the precipitate with ordinary solid trash. Acidify the filtrate with vinegar. Add 0.1 M FeCl₃ until no further precipitation is observed. Filter. Discard the solid with ordinary solid trash. Discard the filtrate at the sink with 100 volumes of water.

Lab Hints-

Some common chemicals such as OH^- have complicated chemistry which you may not wish to introduce early in the semester. Phosphates have the complications of a polybasic acid. Carbonate solutions frequently precipitate hydroxides or mixtures of carbonates and hydroxides. Additions and substitutions on the basis of what is available may result in complex chemistry beyond the beginning student. Be sure you know the chemistry before including another cation or anion for beginning students.
Barium is still a problem in this experiment because the precipitate with the sulfide is some contamination product--possibly sulfate or carbonate. The sulfide and hydroxide are both soluble at the concentrations used in this experiment.
This experiment can also be performed using puddles on acetate sheets. Laminating sheets of paper with matrices already printed on them, and with large, dark regions that permit easy observation of light-colored precipitates, works well.

Background-

Reactions:
Most alkali metal salts are soluble
K⁺ displays no reaction with any of the anions.
Most acetates are soluble. Acetate does not react with any of the cations.
Ba²⁺ + SO₄^2- --> BaSO₄ white
Ba²⁺ + C₂O₄^2- --> BaC₂O₄ white
Barium is forming a small amount of white precipitate with the sulfide solution due to an impurity which may be sulfate or carbonate.
Ag⁺ + Cl^- --> AgCl white
2 Ag⁺ + S^2- --> Ag₂S black
2 Ag⁺ + C₂O₄^2- --> Ag₂C₂O₄ white
Mn²⁺ + S^2- --> MnS (mixture of pink and green forms)
Optional
Fe² + S² --> FeS black
Fe² + C₂O₄² --> FeC₂O₄ (yellow slow to form)

Answers-

Q1. Write balanced chemical equations for all combinations that produced a precipitate. A1. Reactions: Ba²⁺ + SO₄^2- --> BaSO₄ Ag⁺ + Cl^- --> AgCl 2 Ag⁺ + S^2- --> Ag₂S 2 Ag⁺ + C₂O₄^2- --> Ag₂C₂O₄ Cu²⁺ + S^2- --> CuS Cu²⁺ + C₂O₄^2- --> CuC₂O₄ Mn²⁺ + S^- --> MnS Mn²⁺ + C₂O₄^2- --> MnC₂O₄ Optional Fe²⁺ + S^2- --> FeS Fe²⁺ + C₂O₄^2- --> FeC₂O₄

Reference-

An early version of this experiment was developed by Bob Trowell, Chester, SC at the 1987 Dreyfus Woodrow Wilson summer program. Many changes have been made in the particular cations and anions to reduce the number of alternative reactions thereby making explanations easier for beginning students.

Key Words 1-

solutions, salts, solubility, net ionic equations, descriptive chemistry

Elements-

Ag Ba Cu Cl C S K Mn Fe