Materials Included In Kit

Additional Materials Required

Prelab Preparation

The sodium sulfite and sodium thiosulfate stock solutions must be diluted to 0.3 M (with 0.3 M NaOH included), for use as “Solution B” in this lab activity. Dilute each solution as described.

1. Prepare 0.3 M sodium sulfite in 0.3 M sodium hydroxide by diluting 450 mL of the 1 M Na₂SO₃ stock solution provided in this kit to a final volume of 1.5 L with distilled or deionized water. Label this solution “Solution B1” for student use.
2. Prepare 0.3 M sodium thiosulfate in 0.3 M sodium hydroxide by diluting 500 mL of the 0.6 M Na₂S₂O₃ stock solution provided in this kit to a final volume of 1.0 L with distilled or deionized water. Label this solution “Solution B2” for student use.

Safety Precautions

Sodium hypochlorite solution is a corrosive liquid; it causes skin burns. The solution reacts with acid to evolve chlorine gas; when heated it evolves chlorine gas. The solution is moderately toxic by ingestion and inhalation. Keep away from skin and clothing. Have students work in a fume hood or well ventilated lab. Remind students to wash their hands thoroughly 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 regulations that may apply, before proceeding. Sodium thiosulfate and sodium sulfite solutions may be treated according to Flinn Suggested Disposal Method #12b. Sodium hypochlorite solution may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The product mixtures may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• Students must decide how many additional values to measure and graph. There should be a minimum of three points on both sides of the value that gives the highest temperature change. The values of ΔT = 0 °C for the ratios of 0 mL NaClO: 50 mL of solution “B” and for 50 mL NaClO: 0 mL of solution “B” are valid points to include in the graph. Ideally, the data should be gathered in a table and then graphed while students are still in the lab. This will allow students to see if they need additional data points, or if they need to repeat a measurement because a data point was too far off the line.
• The best thermometers for small-scale calorimetry are digital electronic thermometers. Digital thermometers are reasonably inexpensive, update every second, and are precise to the nearest 0.1 °C. Temperature measurements may be a significant source of error in calorimetry experiments. Digital thermometers may also be used to stir the solution after mixing. We do not recommend using glass thermometers in this experiment.
• There is a twofold purpose for including NaOH in the solutions. First, the bleach itself is basic. By mixing it with a second solution of approximately the same basicity, the heat evolved due to the dilution of a base does not contribute to the final temperature change due to the redox reaction. Secondly, keeping the solution basic helps to prevent the evolution of sulfur dioxide gas.
• Students can use the data from the experiment to calculate the enthalpy change of the reaction.
• The data may be plotted using a spreadsheet program or a graphing program.

Answers to Prelab Questions

Review the Procedure section before answering the following questions.

1. The following data were obtained in a continuous variation experiment designed to find the coefficients in the equation for the double-replacement reaction between CuCl₂ and Na₃PO₄. Both reactants were used in the form of 0.5 M solutions. One of the products is a precipitate.
   {12051_PreLab_Table_1}
   Plot the volume of CuCl₂ versus the volume of precipitate obtained. 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 the two lines.

   ___3___ CuCl₂ + ___2___ Na₃PO₄ → Products

   {12051_PreLabAnswers_Figure_2}
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 a third gives assurance that the values fall on a straight line.

3. What is the formula of the precipitate?

   Cu₃(PO₄)₂

Sample Data

Initial Temperature ___21.8 ºC___
Solution “B2” = Sodium Thiosulfate

Answers to Questions

1. Fill in the stoichiomentric mole ratio of reactants for the chemical equation.

   For Sodium Thiosulfate as B2; ___4___ NaClO(aq) + ___1___ B(aq) → Products
   For Sodium Sulfite as B1; ___1___ NaClO(aq) + ___1___ B(aq) → Products

2. Why was the total volume of solutions used kept constant in all trials?

   The maximum amount of heat is given off when the optimum mole ratio of reactants is combined. If the solution volume is constant, the change in temperature will be proportional to the amount of heat evolved. If the volumes were not constant, a calculation would have to be made relating temperature change to heat evolved for each measurement.

3. Is it necessary that the concentration of the two solutions be the same?

   Is it not necessary that the concentration of the two solutions be the same. However, if they are not the same, a calculation must be made for each measurement to relate the temperature change to the heat evolved and the moles reactant present.

4. 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 temperature change?

   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.

5. Three unbalanced oxidation–reduction reactions involving sodium hypochlorite are listed. Balance the reactions, then based on your data, select the reaction that corresponds to your experiment.

   4NaClO(aq) + 1Na₂S₂O₃(aq) + 2NaOH(aq) → 2Na₂SO₄(aq) + 4NaCl(aq) + H₂O(l)
   3NaClO(aq) + 1KI(aq) → 3NaCl(aq) + 1KIO₃(aq)
   1NaClO(aq) + 1Na₂SO₃(aq) → 1Na₂SO₄(aq) + 1NaCl(aq)

Introduction

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

Concepts

• Stoichiometry
• Mole ratio
• Oxidation–reduction reaction

Background

This experiment uses the method of continuous variation to determine the mole ratio for two reactants in a chemical reaction. Several steps are involved. First, equimolar solutions of the two reactants are prepared in which the concentrations are known. Second, the solutions are mixed a number of times using different volume ratios of the two 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 due to a reactant or product, the mass of precipitate that forms, or the volume of a gas evolved.

In the method of continuous variation, the total number of moles of reactants is kept constant across a series of measurements. Each measurement is made with a different mole ratio of reactants. The optimum ratio, which is the stoichiometric ratio for the reactants in the balanced chemical equation, should consume the greatest amount of reactants, form the greatest amount of products, or generate the most heat and produce the maximum temperature change.

Sodium hypochlorite is an oxidizing agent and both reactions tested in this experiment are oxidation–reduction reactions. The hypochlorite ion will be reduced as another substance is oxidized

Experiment Overview

The purpose of this experiment is to use the method of continuous variation to determine the mole ratio for two reactants. The change in temperature is the property to be measured. Both reactions are exothermic, so the heat produced will be directly proportional to the amount of reaction that occurs. Since the experiment is designed so that the total volume of solutions is constant for all measurements, the temperature change will also be proportional to the quantity of heat evolved and total extent that the reaction occurs.

Materials

Prelab Questions

Review the Procedure section before answering the following questions.

1. The following data were obtained in a continuous variation experiment designed to find the coefficients in the equation for the double-replacement reaction between CuCl₂ and Na₃PO₄. Both reactants were used in the form of 0.5 M solutions. One of the products is a precipitate.
   {12051_PreLab_Table_1}
   Plot the volume of CuCl₂ versus the volume of precipitate obtained. 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 the two lines.

   ______ CuCl₂ + ______ Na₃PO₄ → Products

   {12051_PreLab_Figure_1}
2. Are there enough data points to make a valid conclusion about the optimum mole ratio? Why or why not?
3. What is the formula of the precipitate?

Safety Precautions

Sodium hypochlorite solution is a corrosive liquid; it causes skin burns. The solution reacts with acid to evolve chlorine gas when heated. Sodium hypochlorite solution is also moderately toxic by ingestion and inhalation. Keep away from skin and clothing. Work in a fume hood or well-ventilated lab only. The reaction of sodium hypochlorite and solution B generates an irritating gas. Do not breathe this vapor. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Obtain approximately 160 mL of the sodium hypochlorite solution in a clean 250-mL beaker and 160 mL of “Solution B” in another clean 250-mL beaker. Label the beakers.
2. Measure the temperature of the sodium hypochlorite solution and of “Solution B.” Record the temperatures in the Data Table. The solutions should be at the same temperature; if not, a correction must be made for the average initial temperature of the two solutions.
3. Nest one insulated foam cup inside another. Set the nested cups inside a 400-mL beaker for support. This assembly, together with the thermometer, will serve as your calorimeter.
4. Using a clean 10-mL graduated cylinder, measure 5.0 mL of the sodium hypochlorite solution and pour the solution into the nested insulated foam cup calorimeter.
5. Place a digital thermometer in the liquid. Using a clean 50-mL graduated cylinder, measure 45.0 mL of “Solution B” and add this to the insulated foam cup.
6. Monitor the temperature change. Record the maximum final temperature of the reaction mixture. Hint: The maximum temperature is usually recorded within one minute after mixing.
7. Pour the solution out of the Styrofoam cup, rinse the cup and thermometer, and repeat steps 2–5 using the different amounts of each solution that are called for in the data table. Notice that the total volume of both solutions is kept consistant at 50.0 mL.
8. Determine the temperature change ΔT (T_final – T_initial) and plot the data on the graph.
9. There should be at least three data points on each side of the one that gave the greatest temperature difference. If not, test new ratios until you have at least three on each side. Remember, the solution ratios of 0 mL NaClO:50 mL solution B and 50 mL NaClO:0 mL solution B are valid data points (ΔT = 0 in both cases).
10. Draw two straight lines that best fit the data and determine where they intersect. Be sure to include the points at 0:50 mL and 50:0 mL ratios. If any points do not fall close to the lines, repeat these measurements. Find the stoichiometric mole ratio of reactants from the point of intersection on your graph.
11. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12051_Student1.pdf