Expt 062 -- One Pot Iron(III) Reactions

A series of reactions is conducted in one vessel or pot. Evidence for the reactions is based upon precipitates forming and dissolving, and color changes.

Safety

Sulfide and several other solutions used are toxic. The sulfide fumes and ammonia fumes are toxic. Sodium hydroxide, hydrochloric acid, and sulfuric acid are corrosive. Fluoride compounds cause serious burns. Store sulfide and ammonia solutions under hood until just before use. Wear goggles and apron. Wash any spills immediately. Provide adequate ventilation. Wash hands after the experiment.

Procedure

1. Stirring must be vigorous in order to accelerate the reactions. Equilibrium is frequently approached slowly if solutions are not stirred. Solids are particularly difficult to mix. Use a toothpick or a beral pipet to stir the solution. (Draw the mixture into the pipet from the well, and then expel it back into the well as fast as possible without spilling. The same pipet may be used throughout the experiment without cleaning.)

   !!!Click here to See Movie.

2. Place 30 drops 0.1 M Fe(NO₃)₃ into one well of a 24-well plate.

   !!!Click here to See Picture.

3. Add 5 drops 2 M Na₂CO₃. Stir. Note and record evidence of change.

   !!!Click here to See Movie.

4. Add 8 drops 3 M HCl. Stir. Note and record evidence of change.

   !!!Click here to See Movie.

5. Add 3 drop 0.1 M KSCN. Stir. Note and record evidence of change.

   !!!Click here to See Movie.

   !!!Click here to See Movie.

6. Add 10 drops 1 M NaOH. Stir. Note and record evidence of change.

   !!!Click here to See Movie.

7. Add 10 drops of 3 M H₂SO₄. Note and record evidence of change.

   !!!Click here to See Movie.

8. Add 20 drops 6 M NH₃. Note and record evidence of change.

   !!!Click here to See Movie.

9. Add 15 drops 0.1 M Na₂S. Stir. Note and record evidence of change.

   !!!Click here to See Movie.

10. List all of the different iron ions and compounds observed with the respective color.

    !!!Click here to See Picture.

11. Carefully transfer the mixture in the reaction well to the disposal jar provided by the instructor. Rinse with a small amount of water into this jar.
12. Wash the plate at the sink.
13. Wash hands.

Questions

1. A solutions contains 1 M F^- and 1 M OH^-. If Fe²⁺ is added to the solution, predict the appearance of the mixture. Write the formula of the iron ion or compound which contains most of the iron.
2. A solutions contains S^2-, SCN^-, F^-, and Cl^- in equal molar amounts. If Fe²⁺ is added to the solution, predict the appearance of the mixture. Write the formula of the iron ion or compound which contains most of the iron.
3. Iron(III) solutions which are prepared in distilled water frequently contain a small amount of red brown gelatinous solid. Predict the compound forming in these solutions. Suggest a method of stabilizing the solutions.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 062 One Pot Iron(III) Reactions

Watch the movies.

List each reagent added and the formula of each observed iron ion or compound with its respective color.

Answer the questions.

Curriculum-

This experiment fits in when discussing equilibria. It is a good AP chemistry activity.

Safety-

Sulfide and several other solutions used are toxic. The sulfide fumes and ammonia fumes are toxic. Sodium hydroxide, hydrochloric acid, and sulfuric acid are corrosive. Fluoride compounds cause serious burns. Store sulfide and ammonia solutions under hood until just before use. Wear goggles and apron. Wash any spills immediately. Provide adequate ventilation. Wash hands after the experiment.

Time-

Teacher Preparation: 20 minutes

Class Time: 20 minutes

Materials-

• 2 mL of 0.1 M Fe(NO₃)₃ -- (Dissolve 4.04 g iron(III) nitrate nonahydrate (Fe(NO₃)₃•9H₂O) in enough water to make 100 mL solution.)
• 2 mL of 1 M NaF -- (Dissolve 4.20 g sodium fluoride (NaF) in enough water to make 100 mL solution.)
• 2 mL of 6 M NH₃ -- (Add 40.0 mL 15 M ammonia (NH₃) to enough water to make 100 mL solution.)
• 1 mL of 2.0 M Na₂CO₃ -- (Dissolve 10.6 g sodium carbonate (Na₂CO₃) in enough water to make 100 mL solution.)
• 1 mL of 3 M HCl -- (Use the stock solution directly as prepared.)
• 0.5 mL of 0.1 M KSCN -- (Dissolve 0.972 g potassium thiocyanate (KSCN) in enough water to make 100 mL solution.)
• 1 mL of 1 M NaOH -- (Add 33.3 mL 3 M sodium hydroxide (NaOH) to enough water to make 100 mL solution.)
• 1 mL of 3 M H₂SO₄ -- (Use the stock solution directly as prepared.)
• 1 mL of 0.1 M Na₂S -- (Purchase the solution in this concentration and use as purchased.)
• 24-well plate
• toothpicks
• plastic transfer piper (as mixing device)

Disposal-

Filter the solid residue and discard with ordinary solid trash. Neutralize the filtrate with vinegar, and discard at the sink.

Lab Hints-

If a dissolving reaction does not "go" completely even after stirring for a minute or two, ask the students to add one more drop of the reacting chemical.

Background-

1. In an alkaline solution, the iron hydroxide forms in preference to iron carbonate:

   Fe²⁺ + 3 OH^- --> Fe(OH)₃

2. In acid solution, the Fe(OH)₃ dissolves:

   Fe(OH)₃ + 3 H⁺ --> Fe²⁺ + 3 H₂O

3. The chloride also can form a complex that contributes to a golden color:

   Fe²⁺ + 4 Cl^- --> FeCl₄^-

4. The thiocyanate ion reacts with ferric ion:

   Fe²⁺ + SCN^- --> FeSCN²⁺

5. This complex is intensely colored. Fluoride ion also forms a complex which is colorless;

   FeSCN²⁺ + 6 F^- --> FeF₆^2- + SCN^-

6. In base, the Fe(OH)₃ forms again. It redissolves in acid, and reforms in base. Adding sulfide causes the ferric ion to be reduced to Fe²⁺ and FeS to form. FeS is black.

Answers

Q1. A solutions contains 1 M F^- and 1 M OH^-. If Fe²⁺ is added to the solution, predict the appearance of the mixture. Write the formula of the iron ion or compound which contains most of the iron.

A1. The solution will contain a red brown precipitate. See the addition a OH^- to the F^- complex. The formula of the compound is Fe(OH)₃.

Q2. A solutions contains S^2-, SCN^-, F^-, and Cl^- in equal molar amounts. If Fe²⁺ is added to the solution, predict the appearance of the mixture. Write the formula of the iron ion or compound which contains most of the iron.

A2. The mixture will be black. The formula of the compound is Fe₂S₃. Note the last step contains all of these ions.

Q3. Iron(III) solutions which are prepared in distilled water frequently contain a small amount of red brown gelatinous solid. Predict the compound forming in these solutions. Suggest a method of stabilizing the solutions.

A3. Fe(OH)₃ forms. Adding a small amount of acid will dissolve the precipitate and keep the iron(III) in solution.

Key Words 1-

precipitation, complex ion, predominant species, equilibrium constant, solubility product, simultaneous reactions

Elements-

Fe Na C O H S N F