Stoichiometry

Introduction

The course description for the College Board AP Chemistry lists the following topics for stoichiometry; net ionic equations, balancing equations, and mass and volume relationships relating to the mole concept, empirical formulas and limiting reactants. Use this set of three acid–base heat of neutralization demonstrations to both engage your students in a hands-on review and test their understanding of this sometimes difficult yet fundamental topic.

The set of three demonstrations includes:

1. Acid–Base Neutralization of Acetic Acid—Start the review with mono-protic acetic acid. Use the method of continuous variations to mix the reactants. Students will then graph the change in temperature versus the volume of acid to determine the mole ratio of two reactants.
2. Di-Protic Acid Neutralization—Once students are comfortable with the continuous variations method, switch to equal molar volumes of sodium hydroxide and the di-protic weak acid, oxalic acid.
3. Neutralization of Poly-protic Citric Acid—All the topics—ionic equations, empirical formulas, balanced equations, limiting reactants, and mass and volume relationships, come together in this final demonstration.

The series of demonstrations may be presented in a variety of ways. Each demonstration may be used to review a specific AP test topic, or all the demonstrations can be performed together to review for student understanding and comprehension of the stoichiometry concepts normally covered in the AP exam. A student worksheet is included as an optional assessment tool for the instructor.

Experiment Overview

Acid–Base Neutralization of Acetic Acid
The demonstration begins with the neutralization of acetic acid with sodium hydroxide. The sodium hydroxide solution along with the acid solutions provided in the kit are approximately 0.8 molar. While this accuracy is sufficient to generate reliable data for stoichiometric calculations, we have included chemicals and a procedure for an accurate standardization of the sodium hydroxide solution. Students plot the demonstration data to determine the mole ratio or fraction of the reactants.

Di-Protic Acid Neutralization
This time we will use the di-protic weak acid, oxalic acid. Students will be asked to convert temperature gains to heat released by the reaction and then plot these heat values versus moles fraction to determine the mole ratio of reactants.

Neutralization of Poly-protic Citric Acid
This final neutralization involves the tri-protic weak acoid, citric acid. In addition to graphing the heat released versus the mole fraction of reactant, students will determine the empirical mass of the citric acid.

Materials

Safety Precautions

Sodium hydroxide solution is corrosive to skin and eyes; skin burns are possible; very dangerous to eyes. Acetic acid solution is toxic and corrosive. Avoid contact with skin and eyes. Wear heat-resistant gloves or use tongs when handling the hot flask. Please review current. Oxalic acid solution is a skin and eye irritant and moderately toxic by ingestion. Sodium hydroxide solution is corrosive to skin and eyes; skin burns are possible; very dangerous to eyes. Citric acid solution may be irritating to tissue, and especially irritating to the eyes. Sodium hydroxide solution is corrosive to skin and eyes; skin burns are possible; very dangerous to eyes. Sodium hydroxide solution is corrosive to skin and eyes; skin burns are possible; very dangerous to eyes. Acetic acid solution is toxic and corrosive. Avoid contact with skin and eyes. Phenolphthalein solution is flammable and is moderately toxic. 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.

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. The waste solutions may be neutralized and flushed down the drain with excess water, according to Flinn Suggested Disposal Method #24a for the acidic solutions, Method #10 for those leftover basic solutions, and the leftover solutions may be neutralize and dispose of according to Flinn Suggested Disposal Method #24b.

Prelab Preparation

Sodium Hydroxide Standardization

1. Obtain a sample of potassium hydrogen phthalate (KHP) that has been previously dried in an oven and stored in a desiccator.
2. On an analytical balance, accurately weigh 0.4 to 0.6 g of KHP in a previously tared weighing dish. Record the mass of the KHP in the Standardization Data Table on page 5.
3. Transfer the KHP into an Erlenmeyer flask—pour the solid through a funnel into the flask. Use water from a wash bottle to rinse all of the remaining solid in the weighing dish and in the funnel into the flask.
4. Add about 40 mL of distilled water to the flask and swirl until all the KHP dissolves.
5. Obtain about 75 mL of the sodium hydroxide solution, NaOH, approximately 0.8 molar in a 150-mL beaker.
6. Clean a 50-mL buret, then rinse it with three small portions (about 7 mL each) of the NaOH solution.
7. Fill the buret to above the zero mark with the NaOH solution. Place an empty beaker under the buret.
8. Open the buret stopcock to allow any air bubbles to escape from the tip. Close the stopcock when the liquid level is between the 0- and 10-mL marks.
9. Measure the precise volume of the solution in the buret and record this value in the Standardization Data Table as the “initial volume.” Note: Volumes are read from the top down in a buret. Always read from the bottom of the meniscus, remembering to include the appropriate number of significant figures.
10. Position the buret over the Erlenmeyer flask so that the tip of the buret is within the flask but at least 2 cm above the liquid surface.
11. Add three drops of phenolphthalein solution to the KHP solution in the flask.
12. Begin the titration by adding 1.0 mL of NaOH solution to the Erlenmeyer flask, then closing the buret stopcock and swirling the flask.
13. Repeat step 12 until 15 mL of the NaOH solution have been added to the flask. Be sure to continuously swirl the flask.
14. Reduce the incremental volumes of NaOH solution to 0.5 mL until the pink color starts to persist. Reduce the rate of addition of NaOH solution to drop by drop until the pink color persists for 15 seconds. Note: Remember to constantly swirl the flask and to rinse the walls of the flask with distilled water before the endpoint is reached.
15. Measure the volume of NaOH remaining in the buret, estimating to the nearest 0.01 mL. Record this value as the “ final volume” in the Standardization Data Table.
16. Repeat the standardization titration two more times. Rinse the Erlenmeyer flask thoroughly between trials with deionized water.
    {12146_Preparation_Table_1_Standardization Data Table}

Procedure

Acid–Base Neutralization of Acetic Acid

1. Set up a calorimeter consisting of two nested polystyrene Thermometer cups with a polystyrene or cardboard cover having a hole in it to accept a thermometer (see Figure 1). Place the calorimeter in a 400-mL beaker for stability.
   {12146_Procedure_Figure_1}
2. Obtain approximately 150 mL of the sodium hydroxide solution in a clean 400-mL beaker and 150 mL of acetic 400-mL beaker acid solution as “Solution A” in another clean 400-mL beaker. Label the beakers.
3. Using a digital thermometer, measure the temperature of the NaOH solution and of “Solution A.” Have students record the data in the Data Table. The solutions should be the same temperature. If they are not, determine the weighted average to use as the initial temperature.
4. Using a clean 10-mL graduated cylinder, measure 10.0 mL of NaOH solution and pour the solution into the nested polystyrene cup. Using a clean 50-mL graduated cylinder, me asure 40.0 mL of “Solution A” and add this to the polystyrene cup.
5. Stir with a digital thermometer, and have students record the maximum temperature (°C) of the final solution in the Data Table.
6. Pour the solution into a large plastic cup, rinse the polystyrene cup and thermometer, and repeat steps 4 and 5 using the different volume ratio of the two substances listed in the data table, always keeping the total volume at 50.0 mL.
7. Have students plot the data on their worksheet graphs as shown in the Answer Key on page 10.
8. Students should draw two straight lines that best fit the data, and determine where they intersect. Be sure to have them include the points at the 0:50-mL and 50:0-mL ratios. From this graph, students can find the stoichiometric mole ratio of reactants from the point of intersection on their graphs.

Di-Protic Acid Neutralization

1. Again, set up a calorimeter consisting of two nested polystyrene cups with a cover having a hole in it to accept a thermometer. Place the calorimeter in a 400-mL beaker for stability.
2. Obtain approximately 150 mL of the sodium hydroxide solution in a clean 400-mL beaker and 150 mL of oxalic acid solution as “Solution B” in another clean 400-mL beaker. Label the beakers.
3. Repeat steps 3 to 5 from the acetic acid neutralization procedure.
4. Pour the solution out into a large plastic cup, rinse the polystyrene cup and thermometer, and repeat steps 4 and 5 using the different volume ratio of the two substances listed in the data table for oxalic acid, always keeping the total volume at 50.0 mL.
5. Have students record the data on their worksheet data table as shown in the Answer Key on page 9. From these data have students calculate the heat released by each neutralization reaction.
6. Students now plot heat released versus volume of 0.8 M sodium hydroxide, then draw two straight lines that best fit the data, and determine where they intersect. Be sure to have them include the points at the 0:50-mL and 50:0-mL ratios. From this graph, students can find the stoichiometric mole ratio of reactants from the point of intersection on their graph.

Neutralization of Poly-protic Citric Acid

1. Again, set up a calorimeter consisting of two nested polystyrene cups with a cover having a hole in it to accept a thermometer. Place the calorimeter in a 400-mL beaker for stability.
2. Obtain approximately 150 mL of the sodium hydroxide solution in a clean 400-mL beaker and 150 mL of citric acid solution as “Solution C” in another clean 400-mL beaker. Label the beakers.
3. Repeat steps 3 to 5 from the acetic acid neutralization procedure.
4. Pour the solution out into a large plastic cup, rinse the cup and thermometer, and repeat steps 4 and 5 using the different volume ratio of the two substances listed in the data table for citric acid, always keeping the total volume at 50.0 mL.
5. Have students record the data on their worksheet data table. From these data have students calculate the heat released by each neutralization reaction.
6. Students now plot heat released versus volume of 0.8 M sodium hydroxide, then draw two straight lines that best fit the data, and determine where they intersect. Be sure to have them include the points at the 0:50-mL and 50:0-mL ratios. From this graph, students can find the stoichiometric mole ratio of reactants from the point of intersection on their graph.

Student Worksheet PDF

12146_Student1.pdf

Teacher Tips

• In the Acid–Base Neutralization of Acetic Acid activity, typically, the specific heat (J/°C) of the calorimeter is determined experimentally. This value is then multiplied by the change in temperature of the solution to calculate qcal for the reaction. qcal = •T (°C) × heat capacity (J/°C). This value can be neglect for the demonstration.

• The best thermometers to use are digital electronic thermometers (Flinn Scientific Catalog No. AP8716) or temperature sensors connected to a computer- or calculator-based interface system such as LabPro or CBL. 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.
• The use of computer- or calculator-based technology for data collection and analysis is tailor-made for thermochemistry determinations. The graph showing temperature change can easily be drawn using a graphing calculator or a graphical analysis program on a computer.
• If the sodium hydroxide concentration is within the range of 0.75M to 0.85M, then no changes to the volumes of the acidbase combinations or the values of the mole fractions of sodium hydroxide are necessary. If the molarity lies outside that range, recalculate the mole fraction of sodium hydroxide for each volume combination.
• In the Di-Protic Acid Neutralization, ehen students are making their heat released calculations, assume the combined solution has a specific heat capacity of 4.18 Jg/C and the density of the solution is 1.00 g/m L.

Sample Data

Reaction of 0.8 M Solution “A” with 0.8 M Sodium Hydroxide

Reaction of 0.8 M Solution “B” with 0.8 M Sodium Hydroxide

Reaction of 0.8 M Solution “C” with 0.8 M Sodium Hydroxide

Answers to Questions

"Reaction of 0.8 M Solution “A” with 0.8 M Sodium Hydroxide

1. Calculate the change in temperature and the mole fraction of sodium hydroxide for the reaction with solution “A,” then plot on the graph the change in temperature values versus the mole fraction of sodium hydroxide. 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.
   {12146_Answers_Figure_4}
   {12146_Answers_Equation_1}

Reaction of 0.8 M Solution “B” with 0.8 M Sodium Hydroxide

1. Calculate the heat released (q) for each determination. Assume the specific heat capacity of the solution is 4.18 J/g °C and the density of the solution is 1.00 g/mL.

   Example: For mole fraction equal to 0.5; q = m x°C x∆T = 50 g x4.18 (J/g °C) x5.4 °C = 1100 J

2. Plot on the graph the heat released values versus the mole fraction of sodium hydroxide. 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.
   {12146_Answers_Figure_5}
   {12146_Answers_Equation_2}
3. What is the limiting reactant when the mole fraction value of sodium hydroxide is somewhere on the ascending line? On the descending line?

   On the ascending line the sodium hydroxide is the limiting reactant, since the mole fraction ratio of sodium hydroxide to oxalic acid is always less than 2. On the descending line, oxalic acid is the limiting reactant. Here the mole fraction ratio of sodium hydroxide to oxalic acid is always greater than 2.

Reaction of 0.8 M Solution “C” with 0.8 M Sodium Hydroxide

1. Calculate the heat released (q) for each determination. Assume the specific heat capacity of the solution is 4.18 Jg/°C and the density of the solution is 1.00 g/mL.

   Example: For mole fraction equal to 0.5; q = m x°C x∆T = 50 g x4.18 J/g °C x4.7 °C = 980 J

2. Plot on the graph the heat released values versus the mole fraction of sodium hydroxide. 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.
   {12146_Answers_Figure_6}
   {12146_Answers_Equation_3}
3. It is found that 42.00 mL of 0.800 M NaOH is needed to titrate 2.153 g of citric acid to its final end point. Calculate the equivalent mass of the acid. Based on this value and the answer to question 2, calculate the molar mass of citric acid.

   MolesOH^– = moles H⁺ = 0.042 L × 0.800 moles/L = 0.0336 moles H⁺
   E. M. of acid = 2.153 g/0.0336 moles = ___64.1 g/mole___
   Since it take 3 moles of base to neutralize 1 mole of citric acid, 0.0336 moles of base only neutralized one-third this amount of citric acid, or 0.0112 moles.
   M. M. of acid = 2.153g /0.0112 moles = ___192 g/mole___

4. The citric acid molecule can be written as

   H₂C—COOH
   HO—C—COOH or C₃H₄OH(COO)₃H₃
   H₂C—COOH
   Write the net ionic equation for the neutralization reaction of citric acid and sodium hydroxide.
   C₃H₄OH(COO)₃H₃(aq) + 3OH^–(aq) → C₃H₄OH(COO)₃^3–(aq) + 3H₂O(l)

Discussion

Acid–Base Neutralization of Acetic Acid
This demonstration uses the method of continuous variations to determine the mole ratio of two reactants in a chemical reaction. 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 volume 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 due to 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 for the reactants in the balanced chemical equation, should consume the greatest amount of reactants, form the greatest amount of product, or generate the most heat and produce the maximum temperature change.

In this demonstration, sodium hydroxide is reacted with a series of weak organic acids in acid–base neutralization reactions. The change of temperature is the property to be measured. The reactions are all exothermic, so the heat produced will be directly proportional to the amount of reaction that occurs. Students will be given a demonstration handout. The three unbalanced equations for the reactions are listed individually, along with separate data tables for the combinations of reactants used in the demonstration. Students will balance the equations, and then examine the data to answer various questions on stoichiometry.

Di-Protic Acid Neutralization
The diprotic acid, oxalic acid, is now reacted with sodium hydroxide in equal molar portions. This time, it takes two moles of hydroxide ions to neutralize one mole of the weak acid.

For this part, students will take the temperature data and calculate the heat released in joules. They will then plot the heat released versus the mole fraction of sodium hydroxide.

Neutralization of Poly-protic Citric Acid
The polyprotic acid, citric acid, is now reacted with sodium hydroxide in equal molar portions. This time, it takes three moles of hydroxide ions to neutralize one mole of the weak acid.

The mole ratio of sodium hydroxide to acid for the maximum heat release is now 0.75.