Materials Included In Kit

Additional Materials Required

Safety Precautions

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. Remind students to thoroughly wash hands with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. Filter or decant the reaction mixtures from Parts 1 and 2 to collect the solid products. The solids may be disposed of according to Flinn Suggested Disposal Method #26a. The leftover solutions may be disposed of down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The laboratory work for this experiment may be completed in one 50-minute lab period. If settling times are slow, the second reaction can be left to settle overnight, and the data collected during the next lab period.
• Carrying out the reactions in graduated cylinders makes it easy to quickly 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 be scaled down to any convenient test tube size.
• This laboratory works best if the solutions and precipitates are allowed to settle for at least 5–10 minutes.
• It is not necessary to do all the sample trials for this laboratory. Cylinders 6 and 7 can be eliminated in the iron(III) reaction and cylinders 1 and 7 for the copper(II) reaction without compromising the results of this laboratory.

Teacher Tips

• This lab 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 CuCl₂ 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.
• For the iron(III) nitrate data, the precipitate is iron(III) hydroxide, Fe(OH)₃. As long as the hydroxide is in excess, the precipitate will settle out. When the iron in solution is in excess, the solution becomes acidic. When this occurs, the precipitate, Fe(OH)₃, begins to dissolve. The volume of precipitate drops dramatically when the iron-to-hydroxide ratio is greater than one to three.
• 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.

Further Extensions

AP^® Standards
This lab fulfills the requirements for the College Board recommended AP Experiment #9: Determination of Mass and Mole Relationship in a Chemical Reaction.

Answers to Prelab Questions

1. The following values were obtained in a continuous variations experiment designed to find the coefficients in the equation for the reaction between 0.5 M solutions of AgNO₃ and K₂CrO₄. One of the products is a precipitate:
   {13901_PreLabAnswers_Table_3}
   Plot the mL of AgNO₃ versus grams precipitate. Label axes and space the data so that the graph reflects the precision of the values given. Use a ruler to draw two best-fitting straight lines through the increasing and decreasing data points. Determine the stoichiometry of the reaction from the intersection of these lines.
   {13901_PreLabAnswers_Figure_2}
   Since the optimum reaction ratio is 33 mL AgNO₃:17 mL K₂CrO₄, the equation is:

   2AgNO₃ + 1K₂CrO₄ → Products

2. Are there enough values to make a valid conclusion? Why or why not?

   Yes, there are at least three points on each side of the maximum. Two points define a straight line, but the third gives assurance that the values fall on a straight line.

Sample Data

Part 1

Answers to Questions

1. On graph paper, plot the milliliters of reactant 1 versus volume of precipitate for each reaction. For the copper chloride graph, draw the two best-fit straight lines through the data points and determine their point of intersection. For the iron nitrate graph, draw the best-fit line through the ascending data and a smooth curve through the descending data. Determine their intersection point.
   {13901_Answers_Figure_3}
   {13901_Answers_Figure_4}
2. From the point of intersection, determine the stoichiometric mole ratio for each reaction. Write out the correct balanced equation for each reaction.

   For Part 1, the optimum reaction ratio is 15 mL of 0.1 M Fe(NO₃)₃ : 45 mL of 0.1 M NaOH. The equation is therefore

   Fe(NO₃)₃ + 3NaOH → Products

   For Part 2, the optimum reaction ratio is 36 mL of 0.1 M CuCl₂ : 24 mL of 0.1 M Na₃PO₄. The equation is therefore

   3CuCl₂ + 2Na₃PO₄ → Products

3. Explain how this method allows you to find the mole ratio of reactants.

   In the continuous variations method the ratio of moles of reactants is gradually changed while a constant solution volume is maintained. The reaction produces precipitate. The maximum amount of precipitate will be produced when the correct mole ratio is combined. Because the solution volume is constant, the volume of precipitate at the optimum mole ratio will be the greatest.

4. Why must you keep a constant volume of reactants?

   The maximum amount of precipitate is produced when the optimum mole ratio of reactants is combined. If the solution volume is a constant, the ratio of the volumes of reactants reflect the mole ratios.

5. Is it necessary that the concentrations of the two solutions be the same?

   It is not necessary that the concentrations of the reactants be the same. However, if they are not the same, a calculation must be made for each measurement to relate the volume of precipitate and moles of reactants present.

6. What is meant by the term limiting reagent?

   The limiting reagent is the reagent that is completely consumed in a chemical reaction.

7. Which reactant is the limiting reagent along the upward sloping line of your graph? Which is the limiting reagent along the downward sloping line?

   For Part 1, the Fe(NO₃)₃ is the limiting reactant along the upward sloping line, the NaOH is the limiting reactant on the downward curving line. For Part 2, CuCl₂ is the limiting reactant on the upward sloping line, and Na₃PO₄ is the limiting reactant on the downward sloping line.

8. Why is it more accurate to use the point of intersection of the two lines to find the mole ratio rather than the ratio associated with the greatest volume of precipitate?

   It may be that the exact mole ratio was not chosen as a data value. Also, the graph averages several values to find the optimum ratio rather than relying on only one value.

Introduction

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 laboratory 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.

Concepts

• Stoichiometry
• Mole ratio
• Double replacement reaction

Background

A balanced chemical equation gives the mole ratios of reactants and products for chemical reactions. If the formulas of all reactants and products are known, it is relatively easy to balance an equation to find out what these mole ratios are. When the formulas of the products are not known, experimental measurements must be made to determine the ratios.

This laboratory uses the method of continuous variations to determine the mole ratio of two reactants. Several steps are involved. First, solutions of the reactants are prepared in which the concentrations are known. Second, the solutions are mixed a number of times using different ratios of reactants. Third, some property of the reaction that depends on the amount of product formed or on the amount of reactant that remains is measured. This property may be the color intensity of a reactant or product, the mass of a precipitate that forms, or the volume of a gas evolved.

In the method of continuous variations, the total number of moles of reactants is kept constant for the series of measurements. Each measurement is made with a different mole ratio of reactants. The optimum ratio, which is the stoichiometric ratio in the equation, should consume the greatest amount of reactants, form the greatest amount of product, and, if the reaction is exothermic, generate the most heat and maximum temperature change.

In this laboratory, the amount of precipitate formed in a double replacement reaction is the property that will be measured. Seven different mole ratios of reactants are added to 50-mL graduated cylinders. The volume of precipitate formed for each mole ratio is measured and these volumes are plotted versus the mole ratio. The graph will resemble Figure 1.

Materials

Prelab Questions

1. The following values were obtained in a continuous variations experiment designed to find the coefficients in the equation for the reaction between 0.5 M solutions of AgNO₃ and K₂CrO₄. One of the products is a precipitate:
   {13901_PreLab_Table_1}

   Plot the mL of AgNO₃ versus grams precipitate on graph paper. Label axes and space the data so the graph reflects the precision of the values given. Use a ruler to draw two best-fitting straight lines through the increasing and decreasing data points. Determine the stoichiometry of the reaction from the intersection of these lines.
   
   ______ AgNO₃ + ______ K₂CrO₄ → Products

2. Are there enough values to make a valid conclusion? Why or why not?

Safety Precautions

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. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part 1. Reaction of Iron(III) Nitrate with Sodium Hydroxide

The iron in iron nitrate acts as a Lewis acid in solution. When combined with sodium hydroxide, the precipitate formed remains insoluble as long as iron nitrate is not in excess of the stoichiometric mole ratio. When iron nitrate is in excess, the precipitate will begin to dissolve. The larger the excess, the greater the amount of precipitate that dissolves. Your plot of the data will reflect this.

1. Label seven 100-mL graduated cylinders 1–7.
2. 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.
3. 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.
   {13901_Procedure_Table_1}
4. 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.)
5. Let the reaction mixtures sit undisturbed for at least 10 minutes to allow the precipitates to settle.
6. After the precipitates have settled, record the volume of precipitate in each graduated cylinder in the Part 1 Data Table.

1. Label seven clean 100-mL graduated cylinders 1–7.
2. 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.
3. 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.
   {13901_Procedure_Table_2}
4. 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.)
5. Let the reaction mixtures sit undisturbed for at least 10 minutes to allow the precipitates to settle.
6. After the precipitates have settled, record the volume of precipitate in each graduated cylinder in the Part 2 Data Table.

Student Worksheet PDF

13901_Student1.pdf