A colorless solution becomes dark orange upon addition of a solution and then a solid. The dark orange color disappears after the addition of another solid but reappears again when more solution and the original solid are added.
- Complex ions
- Le Chatelier’s principle
Iron(III) nitrate solution, Fe(NO3)3, 0.2 M, 6 drops*
Potassium thiocyanate, KSCN, 1 g*
Potassium thiocyanate solution, KSCN, 0.002 M, 20 mL*
Sodium phosphate, monobasic, NaH2PO4•H2O, 1 g*
Graduated cylinder, 50-mL
*Materials included in kit.
Potassium thiocyanate is toxic by ingestion and emits toxic fumes if strongly heated. Iron(III) nitrate solution may be skin/tissue irritant. Sodium phosphate monobasic (monohydrate) is moderately toxic by ingestion. Wear chemical-resistant gloves and chemical splash goggles. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The resulting solution may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. Clean up spills of iron(III) nitrate solution immediately because iron(III) nitrate solution easily stains.
- Using a 50-mL graduated cylinder, measure out 20 mL of potassium thiocyanate solution. Transfer the potassium thiocyanate solution to a Petri dish, and place the Petri dish on an overhead projector. Turn on the overhead projector.
- Add 5 drops of iron(III) nitrate solution in different spots in the Petri dish. Note that the orange spots produced are a little darker than the iron(III) nitrate solution.
- Swirl the solution until the orange color is uniform throughout.
- Add ½ pea size amount of the potassium thiocyanate crystals in one spot. A dark orange spot results. Wait about 30 seconds so the students can observe the movement of the dark orange color through the solution.
- Swirl the solution to dissolve the crystals and the dark orange color will become uniform throughout.
- Add ¼ pea size amount of the sodium phosphate monobasic crystals in one spot. Wait about 60 seconds as the color in the vicinity of the crystals becomes lighter than the rest of the solution. This is a great example of ion diffusion in a solution.
- Swirl the solution to dissolve the crystals and until the solution is colorless throughout.
- Add one drop of the iron(III) nitrate solution in one spot off to the side. Note the blood red color. Don’t stir.
- Add a pea size amount of the potassium thiocyanate crystals in a different spot. Wait about 30 seconds so the students can observe that the area around the crystals becomes orange.
- Enough materials are included in this kit to perform this demonstration seven times; one practice, five classes and one for “Murphy’s Law.”
- Steps 8 and 9 were incorporated into the demo based on the question: “Are the Fe3+ and SCN– ions still present even though there is no color?”
- Put the chemical equations on the overhead while you are doing the demonstration. You can explain or, better yet, have the students explain, the equilibrium shifts.
- Beral pipets or wood splints are ideal to stir the solution.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Planning and carrying out investigations
Engaging in argument from evidence
Analyzing and interpreting data
Obtaining, evaluation, and communicating information
Disciplinary Core Ideas
MS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions
Cause and effect
Stability and change
MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
HS-PS1-6. Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
Answers to Questions
- Write chemical equations for the following reactions.
- The reaction between Fe3+ and SCN– ions
Fe3+(aq) + SCN–(aq) → FeSCN2+(aq)
- The reactions between Fe3+ and H2PO4– ions
Fe3+(aq) + H2PO4–(aq) → [FeH2PO4]2+
- What direction does equilibrium for the first reaction above shift when ferric nitrate or potassium thiocyanate is added?
Equilibrium in the reaction shifts to the right.
- Why did the solution turn colorless when the sodium phosphate monobasic was added?
The addition of the sodium phosphate monobasic shifts equilibrium to the left because of the competing reaction between the Fe3+ and H2PO4– ions. The product of that reaction is colorless.
Fe3+ and SCN– ions form the complex ion FeSCN2+, which is dark red in color (Equation 1). Addition of Fe(NO3)3 or KSCN increases the concentration of these reactants and causes the equilibrium in Equation 1 to shift in the forward reaction, or to the right. This observation illustrates the effect of LeChâtelier’s Principle—a change in any reaction condition causes the equilibrium to shift in a direction that counteracts the effect of the change.
Addition of NaH2PO4•H2O results in the equilibrium shifting in the reverse reaction (to the left). This is due to a competing reaction of Fe3+ and H2PO4– ions to form a colorless [FeH2PO4]2+ complex. This effect can be reversed again by adding more Fe3+ or SCN–.
Special thanks to Jim and Julie Ealy, The Peddie School, Hightstown, NJ, for providing us with the instructions for this activity.