Double replacement reactions are generally considered to be irreversible. The formation of an insoluble precipitate provides a driving force that makes the reaction proceed in one direction only. The purpose of this demonstration is to find the optimum mole ratio for the formation of a precipitate in a double replacement reaction and use this information to predict the chemical formula of the precipitate.
- Mole ratio
- Double replacement reaction
- Solubility rules
Copper(II) chloride solution, CuCl2, 0.05 M, 210 mL*
Iron(III) nitrate solution, Fe(NO3)3, 0.1 M, 210 mL*
Sodium hydroxide solution, NaOH, 0.1 M, 210 mL*
Trisodium phosphate solution, Na3PO4, 0.05 M, 210 mL*
Graduated cylinders, 50-mL, 4
Graduated cylinders, 100-mL, 14
Marker or labeling pen
Stirring rods, long, 2
*Materials included in kit.
Copper(II) chloride, iron(III) nitrate, sodium hydroxide and trisodium phosphate solutions are skin and eye irritants and are slightly toxic by ingestion. Avoid contact with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Filter or decant the reaction mixtures from Parts A and B to collect the solid products. The solids may be packaged for landfill disposal according to Flinn Suggested Disposal Method 26a. The waste solutions may be rinsed of down the drain with excess water according to Flinn Suggested Disposal Method 26b.
Part A. Reaction of Iron(III) Nitrate with Sodium Hydroxide
Part B. Reaction of Copper(II) Chloride with Sodium Phosphate
- Label seven 100-mL graduated cylinders 1–7.
- Using a clean, 50-mL graduated cylinder, add the appropriate volume of iron(III) nitrate solution to each 100-mL graduated cylinder, as shown in Table 1.
- Use a second 50-mL graduated cylinder to add the appropriate volume of sodium hydroxide solution to each 100-mL graduated cylinder, as shown in Table 1.
- Use a large stirring rod to thoroughly mix the reactants. Observe the signs of chemical reaction in each cylinder. (Mixing the yellow-orange solution of iron(III) nitrate with the colorless sodium hydroxide solution gives a rust-colored precipitate and a pale yellow supernatant.)
- Let the reaction mixtures sit undisturbed for at least 10 minutes to allow the precipitates to settle. During this time, write the reactants on the board and identify the possible products. Ask students to predict which ratio will result in the largest amount of precipitate.
- After the precipitates have settled, record the volume of precipitate in each graduated cylinder.
- What mole ratio gave the maximum amount of precipitate?
- Label seven 100-mL graduated cylinders 1–7.
- Using a clean, 50-mL graduated cylinder, add the appropriate volume of copper(II) chloride solution to each 100-mL graduated cylinder, as shown in Table 2.
- Use a second 50-mL graduated cylinder to add the appropriate volume of sodium phosphate solution to each 100-mL graduated cylinder, as shown in Table 2.
- Use a large stirring rod to thoroughly mix the reactants. Observe the signs of chemical reaction in each cylinder. (Mixing the blue solution of copper(II) chloride with the colorless sodium phosphate solution gives an aqua-colored precipitate and a colorless supernatant.)
- Let the reaction mixtures sit undisturbed for at least 10 minutes to allow the precipitates to settle. During this time, write the reactants on the board and identify the possible products. Ask your students to predict which ratio will result in the largest amount of precipitate.
- After the precipitates have settled, record the volume of precipitate in each graduated cylinder on the board.
- What mole ratio gave the maximum amount of precipitate? Ask your students to explain.
- This kit contains enough chemicals to perform the demonstration as written seven times: 1500 mL each of copper(II) chloride, iron(III) nitrate, sodium hydroxide and sodium phosphate.
- Carrying out the reactions in graduated cylinders makes it easy to measure the volume of precipitate and determine the optimum mole ratio. The reactions may also be carried out in large test tubes in a test tube rack. The quantities may also be scaled down to convenient test tube size.
- This demonstration works best if the precipitates are allowed to settle for at least 15 minutes before recording the volume of solid obtained.
- This demonstration illustrates the method of continuous variation. The best way to use this method is to graph the amount of product obtained in each reaction as a function of the mole ratio. Ideally, the amount of product should increase in a continuous manner, and then begin to decrease. To find the optimum mole ratio, draw two best-fit straight lines through the increasing and decreasing points on the graph. The optimum mole ratio should occur at the intersection of the two lines. Notice that if this graphical method is used, it is not necessary to use the optimum mole ratio in a single run—the ratio is determined by extrapolation. Thus, The 25/35 ratio of CuCl2 and Na3PO4 is conveniently simplified to a 2:3 ratio instead of the actual 5:7 ratio.
- It is not necessary to do all the sample trials for this demonstration. Cylinders 6 and 7 can be eliminated in the iron(III) reaction and cylinders 1 and 7 for the copper(II) reaction—a maximum will still be observed in the graph.
- The continuous variation method may be extended to determine the optimum mole ratio and the chemical formula of the precipitate for any double replacement reaction. Keep a solubility chart and a table of common ion charges handy to help students predict the formulas of the products obtained in double replacement reactions.
- The iron(III) hydroxide demonstration provides somewhat surprising results, in that no precipitate is observed at high concentrations of iron(III) ions. This is due to the formation of soluble, polynuclear complex ions, evidenced by the appearance of a deep red-brown color. (Excess iron(III) ions should appear yellow in solution.) A definite hydroxide Fe(OH)3 probably does not exsist, and the red-brown precipitate commonly called iron(III) hydroxide is best described as hydrous or hydrated iron(III) oxide.
- The color of the liquid above the precipitate may be used to illustrate the excess reagent in each case. In the copper series, the liquid in trials 1–4 are colorless, as Cu(NO3)2 is the limiting reagent. Excess copper nitrate is apparent from the blue liquid in trials 6 and 7.
*Actual ratio = 7 : 5. Optimum ratio obtained from graph.
Answers to Questions
- Draw two graphs, showing the volume of precipitate produced for each cylinder in the iron(III) reaction and in the copper(II) reaction.
- For each reaction, which cylinder and mole ratio produced the most precipitate?
In the iron(III) reaction, cylinder 2 had the most precipitate. Cylinder 2 contained 15 mL of iron(III) nitrate and 45 mL of sodium hydroxide. The mole ratio was 1:3. In the copper(II) reaction, cylinder 5 had the most precipitate. Cylinder 5 contained 35 mL of copper(II) chloride and 25 mL of trisodium phosphate. The ideal mole ratio is 3:2.
- Write two balanced chemical equations, one for each reaction.
Fe(NO3)3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaNO3(aq)
3CuCl2(aq) + 2Na3PO4(aq) → Cu3(PO4)2(s) + 6NaCl(aq)
Equations 1 and 2 summarize the chemical reactions occurring in Parts A and B, respectively. The results of the experiments are illustrated graphically here.
Graduated cylinder 2 should have the most precipitate for the iron(III) reaction and cylinder 5 should have the most precipitate for the copper(II) reaction.
This activity is from Flinn ChemTopic™ Labs, Volume 7, Molar Relationships and Stoichiometry; Cesa, I., Ed; Flinn Scientific: Batavia, IL, 2002.