Materials Included In Kit

Additional Materials Required

Prelab Preparation

Sodium Carbonate/Sodium Bicarbonate Mixture

1. Weigh approximately 15 g of sodium bicarbonate and record the precise mass. Add to a clean sample container.
2. Weigh approximately 5 g of sodium carbonate and record the precise mass. Add to the same sample container.
3. Cap the sample container and thoroughly mix the solids.
4. Label container as sodium carbonate/sodium bicarbonate mixture.

Potassium Carbonate/Potassium Bicarbonate Mixture

1. Weigh approximately 15 g of potassium bicarbonate and record the precise mass. Add to a clean sample container.
2. Weigh approximately 5 g of potassium carbonate and record the precise mass. Add to the same sample container.
3. Cap the sample container and thoroughly mix the solids.
4. Label the container as potassium carbonate/potassium bicarbonate mixture.

Safety Precautions

Potassium carbonate and sodium bicarbonate are slightly toxic by ingestion and are skin irritants. Handle the crucible only with tongs. Do not touch the crucible with fingers or hands. There is a significant burn hazard associated with handling a crucible—remember that a hot crucible looks like a cold one. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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 and potassium bicarbonate, along with sodium and potassium carbonate, may be packaged for landfill disposal according to Flinn Suggested Disposal Method #26a. 

Lab Hints

• The Stoichiometry Laboratory Procedure is included as a Teacher PDF. Each working group will need this procedure to complete the Prelaboratory Assignment.
• Review the recommended Safety Precautions section and demonstrate the proper techniques for handling a crucible with crucible tongs and for heating a crucible in a Bunsen burner flame. The Flinn Scientific Laboratory Techniques Guide, Catalog No. AP6248, provides thumbnail illustrations of these and 14 other common laboratory techniques.
• Students may have a difficult time mathematically relating the combined mass loss of carbon dioxide and water to the mass of sodium or potassium bicarbonate. Make sure the students are clear in this regard.
• All four anhydrous solids absorb moisture from the air. To reduce this source of systematic error, dry the carbonate solids at 115 °C in an oven, then cool and place in a desiccator. Note: Do not place the bicarbonate solids in the oven. They will react at the oven temperature. Place sodium and potassium bicarbonate directly in the desiccator.
• Green chemistry presents a wonderful opportunity to increase safety and impart the values and benefits of science to the next generation. The basic principles of green chemistry as they relate to undergraduate science labs include:
  • Design lab activities to avoid generating hazardous byproducts that require waste disposal.
  • Substitute less hazardous and less toxic chemicals in chemical reactions or lab tests.
  • Perform lab activities on a small or microscale level to reduce the amounts of chemicals used.
  • Use catalysts to avoid by-product formation in chemical reactions.
  • Use safer solvents.
  • Avoid high temperature or high pressure conditions for chemical reactions.
• In reviewing current lab activities, carefully compare the hazards of chemicals versus the learning goals and objectives. Many chemistry departments use lead nitrate, for example, to precipitate lead iodide and demonstrate crystal formation. No doubt, it is a beautiful demonstration! But, does the need for licensed hazardous waste disposal of the heavy metals used in this demonstration justify the learning goals? Could another demonstration accomplish the same objective? Mixing copper chloride and sodium phosphate solutions gives a turquoise solid. It may not be quite as pretty, but it teaches the same thing, and it is “greener” and safer than lead iodide.
• Incorporating application-oriented lab activities into your undergraduate curriculum will help you “color the curriculum green” while at the same time reducing the use of hazardous chemicals and increasing the level of student engagement (e.g., acid–base titrations of fruit juices, redox reactions using vitamin C as the reducing agent, preparation of biodiesel, ion exchange and cation binding properties of soil, specific heat experiments of sand, soil and water).

Further Extensions

Opportunities for Undergraduate Research
As an alternative stoichiometric lab, students could determine the optimum mole ratio of two green reactants using the method of continuous variation.

Answers to Prelab Questions

Carefully read the Procedure for “Determining the Stoichiometry of a Chemical Reaction” and then answer the following questions. Use reference books and the Internet when needed. In the experiment, the silver chromate that is produced forms a dense, colorful precipitate that is easy to see and accurately measure.

1. The products of this lab are silver chromate solid, Ag₂CrO₄(s), and a solution of potassium nitrate, KNO₃(aq).
   1. Is either or both of these products hazardous? If so, in what way?

      Silver chromate is a strong oxidizer and a known carcinogen under long-term exposure. It also causes gastrointestinal and respiratory tract irritation. While potassium nitrate solid has hazards associated with it, a solution of potassium nitrate would pose little hazard to either health or the environment.

   2. What are the proper disposal methods for each of these products?

      For silver chromate: Contact a licensed professional waste disposal service to dispose of this material.
      For potassium nitrate solution: No special pretreatment required prior to normal drain disposal.

2. The balanced equation for the experimental reaction is listed. The desired product of this reaction is the solid silver chromate. Calculate the atom economy of this reaction.

   2AgNO₃(aq) + K₂CrO₄(aq) → Ag₂CrO₄(s) + 2KNO₃(aq)

   {13838_PreLabAnswers_Equation_6}

   = 331.7 g/(339.8 + 194.2) g x 100 = (331.7/534.0) x 100 = 62.1%

3. What are the hazards, if any, of the reactants silver nitrate, AgNO₃, and potassium chromate, K₂CrO₄? What safety practices in the lab should be used to mitigate these hazards?

   Silver nitrate is a corrosive solid, will stain skin and is highly toxic. Potassium chromate is a known carcinogen, highly toxic and harmful to skin, eyes and respiratory tract. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Avoid exposure to dusts created from solid potassium chromate.

Sample Data

Introductory Activity

Calculations and Analysis
From the mass of bicarbonate reactant and the balanced chemical equation, calculate the theoretical mass of carbonate solid that should be produced.

For the sodium bicarbonate sample of 1.987 g:

Grams of Na₂CO₃ = 1.253 g

For the potassium bicarbonate sample of 2.024 g:

Grams of K₂CO₃ = 1.397 g

Calculate the percent yield for the bicarbonate decomposition reaction.

For the sodium bicarbonate sample of 1.987 g:

For the potassium bicarbonate sample of 2.024 g:

Guided-Inquiry Activity

* Theoretical

Calculations and Analysis

For the potassium bicarbonate/potassium carbonate mixture:

Mass of KHCO₃ = (0.00852 moles) x 100.12 g/mol = 0.853 g
% KHCO₃ = (0.853 g/1.244 g) x 100 = 68.5%

For the sodium bicarbonate/sodium carbonate mixture:

Mass of NaHCO₃ = 0.00697 moles x 84.01 g/mol = 0.585 g
% NaHCO₃ = (0.585 g/1.333 g) x 100 = 43.9%

Answers to Questions

Guided-Inquiry Design and Procedure Questions

1. Based on your results in the Introductory Activity, what, if anything, is the product when potassium or sodium bicarbonate is heated at 100 to 200 °C? Does this product undergo further decomposition at this temperature?

   The mass of the product obtained after heating potassium or sodium bicarbonate is close to that expected or calculated if the bicarbonate salt were converted to the corresponding carbonate and the latter did not undergo any further reaction or decomposition.

2. Review the stoichiometry of the bicarbonate decomposition lab. How does mass loss relate to the mass of the starting material? Explain how the mass loss could be used to calculate the percent sodium or potassium bicarbonate in a mixture containing both the bicarbonate and the corresponding carbonate salt.

   From the balanced equation: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g )
   The mass loss is due to mass of H₂O + mass of CO₂.
   Mole ratios are: moles of CO₂ = moles of H₂O

   moles of NaHCO₃ = 2(moles of H₂O)

   Mass loss = (moles of H₂O)(18 g/mole) + (moles of CO₂)(44 g/mole), which reduces to:

   = (moles of H₂O)(18 + 44 g/mole) = (moles of H₂O)(62 g/mole)

   {13838_Answers_Equation_13}
3. Design a laboratory procedure to determine the percent bicarbonate in a mixture, and then answer the following questions. Use reference books and the Internet when needed.
   1. What are the products of this lab?

      Water vapor, H₂O( g ); carbon dioxide, CO₂( g ); and sodium carbonate, Na₂CO₃(s) or potassium carbonate, K₂CO₃(s).

   2. Are any of these products hazardous? If so, in what way?

      Both sodium and potassium carbonate are slightly toxic by ingestion and irritating to body tissues.

   3. What are the proper disposal methods for each of these products?

      Both sodium and potassium carbonate are safe for landfill disposal (normal trash).

4. The balanced equations for the experiment are listed below. The desired products of these reactions are the solid metal carbonates. Calculate the atom economy for each reaction.

   2KHCO₃(s) → K₂CO₃(s) + H₂O(g) + CO₂(g)
   or
   2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)

   {13838_Answers_Equation_14}
5. What are the hazards, if any, of the reactants potassium bicarbonate, KHCO₃, and sodium bicarbonate, NaHCO₃?

   Neither is considered hazardous.

Post-Laboratory Review Questions

1. Calculate the theoretical mass of sodium carbonate solid that should be produced by heating 1.678 g of sodium bicarbonate.
   {13838_Answers_Equation_15}
   Theoretical grams of Na₂CO₃ = 1.059 g
2. If 1.018 g of sodium carbonate were produced from the sodium bicarbonate in Question 1, calculate the percent yield for the bicarbonate decomposition reaction.

   For the sodium bicarbonate sample of 1.678 g:

   {13838_Answers_Equation_16}
3. Calculate the mass of water vapor and carbon dioxide that would be produced by gently heating a mixture of 1.550 g of sodium bicarbonate and 0.463 g of sodium carbonate. What mass of sodium carbonate would remain in the crucible?

   From the stoichiometry of the reaction

   {13838_Answers_Reaction_1}

   Moles of NaHCO₃ = 2 moles of H₂O = 2 moles of CO₂ = 1.550 g NaHCO₃ /84.01 g/mole = 1.845 x 10^–2 moles
   Moles of H₂O = moles of CO₂ = 1.845 x 10^–2 moles/2 = 0.922 x 10^–2 moles
   Mass H₂O = (0.922 x 10^–2 moles) x 18 g/mole = 0.166 g H₂O
   Mass CO₂ = (0.922 x 10^–2 moles) x 44 g/mole = 0.406 g CO₂

4. A classic experiment involves combining a solution of barium nitrate, Ba(NO₃)₂, with a sodium sulfate solution, Na₂SO₄, forming a precipitate of barium sulfate.

   Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)

   1. Identify the hazards associated with the chemicals in this reaction.

      Students should refer to an SDS for each of the reactants and products.
      Barium nitrate solution is moderately toxic by ingestion or inhalation and an irritant. Avoid contact with eyes, skin, and mucous membranes. It is a strong oxidizer and may explode when heated. All soluble barium compounds are poisonous if swallowed and cause nausea, vomiting, stomach pains and diarrhea.
      Barium sulfate is not considered hazardous but avoid prolonged inhalation of dust. Not all health aspects of this substance have been thoroughly investigated.
      Sodium sulfate solution is not considered hazardous.
      Sodium nitrate solution is a body tissue irritant. Avoid all body tissue contact.

   2. The purpose of this lab is to teach the techniques and principles of quantitative gravimetric analysis. Use your knowledge of solubility products to devise a greener set of solutions that would meet the purpose of this lab.

      Students should look for a safer cation than barium that forms an insoluble salt with the sulfate or other greener anions, such as the carbonate anion. The precipitation reaction to form calcium carbonate is a greener alternative to teach the principles of gravimetric analysis.

Supplementary Material Analysis Questions

1. On graph paper, plot the milliliters of reactant 1 versus volume of precipitate for each reaction. Draw the two best-fit straight lines through the data points and determine their point of intersection.
   {13838_Answers_Figure_4}
2. From the point of intersection, determine the stoichiometric mole ratio for the reaction. Write out the correct balanced equation for the reaction.

   Since the optimum reaction ratio is 33 mL AgNO₃ : 17 mL K₂CrO₄, the equation is:
   2AgNO₃ + K₂CrO₄ → 2KNO₃ + Ag₂CrO₄

Teacher Handouts

13838_Teacher1.pdf

Introduction

The Green Chemistry Program was initiated by the Environmental Protection Agency in the 1990s with the goal of applying chemical principles to prevent pollution. The program calls for the design of chemical products and processes that will reduce the use and generation of hazardous substances. The purpose of this lab is to design an experiment for determining the percent composition of a solid by applying the principles of green chemistry.

Concepts

• Stoichiometry
• Green chemistry
• Percent composition
• Decomposition reaction

Background

Much of what makes this world modern is the result of the application of chemistry and chemical reactions. Oil and gasoline, prescription drugs, plastics, solvents, and fertilizers, to name a few, are all products of chemistry.

Over time, many of the processes used to create these products were found to be quite harmful, whether to workers, the consumers or to the environment. In response to these pressing issues, various professional groups have created a different approach to the research and production of chemicals and chemical processes called Green Chemistry.

The Green Chemistry approach uses twelve principles that help evaluate the production and use of chemical products so that the generation of hazardous substances can be reduced or eliminated and, where possible, renewable starting materials can be substituted. These principles are listed.

1. Prevention: It is better to prevent waste than to treat or clean up waste after it has been created.
2. Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product, leaving few or no atoms behind.
3. Less Hazardous Chemical Syntheses: Synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
4. Designing Safer Chemicals: Chemical products should be designed to be fully effective while minimizing or eliminating their toxicity.
5. Safer Solvents and Auxiliaries: Minimize the use of auxiliary substances (e.g., solvents, separation agents) wherever possible and make them innocuous when used.
6. Design for Energy Efficiency: Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
7. Use of Renewable Feedstocks: Renewable raw material or feedstock should be used whenever technically and economically possible.
8. Reduce Derivatives: Unnecessary derivatization (use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate additional waste.
9. Catalysis: Catalytic reagents are superior to stoichiometric reagents.
10. Design for Degradation: Chemical products should be designed so that at the end of their function they break down into innocuous products that do not persist in the environment.
11. Real-Time Analysis for Pollution Prevention: Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
12. Inherently Safer Chemistry for Accident Prevention: Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions and fires.

In this lab, you will design a process to determine the weight percent of a metal bicarbonate, either sodium bicarbonate or potassium bicarbonate, in a mixture of itself and its carbonate counterpart. Sodium and potassium bicarbonate undergo decomposition when heated above 110 °C (Equation 1, M = Na, K). At temperatures below 800 °C, potassium and sodium carbonate should remain unreacted. Therefore, if a mixture of bicarbonate and carbonate salts is heated at low temperature, all that remains after heating should be the carbonate solid. This process is intended as a “greener” experiment for teaching stoichiometry.

You will also use the principles of green chemistry to evaluate the “greenness” of a traditional lab procedure used to teach stoichiometry. Specifically, the three principles you will look at in this evaluation are: prevention, atom economy, and less hazardous chemical syntheses.

Prevention: It is better to prevent waste than to treat or clean up the waste after it has been created. In designing a lab activity, evaluate alternative reactions wherever possible, and identify the reaction that produces the least waste.

Atom Economy: When choosing among various reactions for a lab activity, identify which reaction produces the least amount of by-product waste. A typical reaction can be represented by the following equation.

The greater the ratio of desired product to reactants, the greener the reaction. Atom economy can be calculated as a percentage of the mass of the desired product to the mass of all reactants. The higher this percentage, the greener the process.

Let’s look at an example of a product that can be produced by two different reactions. Aluminum can be oxidized by water to aluminum oxide (Equation 2). If solid aluminum hydroxide is strongly heated, aluminum oxide and water are produced (Equation 3). he atom economy for Equation 2 is equal to: The atom economy for Equation 3 is equal to: Of the two reactions, the first reaction is “greener” in terms of atom economy. Remember, atom economy does not address the toxicity or hazards of either the reactants or the byproducts.

Use and Production of Nontoxic Materials—Less Hazardous Chemical Syntheses 
When possible, choose chemicals that have the least toxic effect on humans and the environment. Check the toxicity of all the chemicals involved in the production of the desired products, including the products themselves.

Experiment Overview

The purpose of this inquiry lab is to design and carry out a green chemistry experiment that can quantitatively measure the weight percent of one compound in a mixture of two compounds. The investigation begins with an introductory activity to verify the decomposition reaction of a solid bicarbonate, either potassium or sodium bicarbonate. These solids undergo the reaction outlined in the Background section.

Stoichiometry is defined as the quantitative relationship among constituents in a chemical reaction. Based on the bicarbonate balanced chemical equation and its stoichiometry, you will calculate your atom economy using the experimental data and compare this to the theoretical value. The results provide a model for the guided-inquiry design of an experiment that can quantitatively measure the weight percent of either a sodium carbonate/bicarbonate solid mixture or a potassium carbonate/bicarbonate mixture. You will assess your procedure in terms of the three green principles and then compare this assessment to that of the procedure examined in the Prelaboratory Questions.

Materials

Prelab Questions

Carefully read the Procedure for “Determining the Stoichiometry of a Chemical Reaction” and then answer the following questions. Use reference books and the Internet when needed. In the experiment, the silver chromate that is produced forms a dense, colorful precipitate that is easy to see and accurately measure.

1. The products of this lab are silver chromate solid, Ag₂CrO₄(s), and a solution of potassium nitrate, KNO₃(aq).
   1. Is either or both of these products hazardous? If so, in what way?
   2. What are the proper disposal methods for each of these products?
2. The balanced equation for the experimental reaction is listed below. The desired product of this reaction is the solid silver chromate. Calculate the atom economy of this reaction.

   2AgNO₃(aq) + K₂CrO₄(aq) → Ag₂CrO₄(s) + 2KNO₃(aq)

3. What are the hazards, if any, of the reactants silver nitrate, AgNO₃, and potassium chromate, K₂CrO₄? What safety practices in the lab should be used to mitigate these hazards?

Safety Precautions

Potassium carbonate and sodium bicarbonate are slightly toxic by ingestion and are skin irritants. Handle the crucible only with tongs. Do not touch the crucible with fingers or hands. There is a significant burn hazard associated with handling a crucible—remember that a hot crucible looks like a cold one. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron. Thoroughly wash hands with soap and water before leaving the laboratory. Follow all laboratory safety guidelines.

Procedure

Introductory Activity

Decomposition of Group 1 Bicarbonates

1. Set up a Bunsen burner on a support stand beneath a ring clamp holding a pipe-stem triangle (see Figure 1). Do NOT light the Bunsen burner.
   {13838_Procedure_Figure_1}
2. Adjust the height of the ring clamp so that the bottom of a crucible sitting in the clay triangle is 1–2 cm above the burner. This will ensure that the crucible will be in the hottest part of the flame when the Bunsen burner is lit.
3. Place a crucible with a cover in the clay triangle and heat over a burner flame until the crucible is red hot.
4. Turn off the gas source and remove the burner.
5. Using crucible tongs, remove the crucible cover and place it on a wire gauze on the bench top. With the tongs, remove the crucible from the clay triangle and place it on the wire gauze as well (see Figure 2).
   {13838_Procedure_Figure_2}
6. Allow the crucible and its cover to cool completely on the wire gauze for at least 10 minutes.
7. Use an analytical balance to find the mass of the crucible and crucible cover. Handle with tongs to avoid getting fingerprints on the crucible and cover.
8. Record their mass.
9. Now add about 2 g of potassium or sodium bicarbonate to the crucible. Weigh the crucible, cover and sample. Record their combined mass.
10. Set the crucible at an angle in the clay triangle held in the ring on the support stand. Cover the crucible loosely with the crucible cover, and heat very gently. It is important that the escaping vapor does not carry any of the solid along with it, so be sure that the crystals are heated very gently for at least five minutes (see Figure 3).
    {13838_Procedure_Figure_3}
11. Turn off the gas source and remove the burner.
12. Use tongs to remove the crucible cover and place it on wire gauze on the benchtop. With the tongs, remove the crucible from the clay triangle and place it on the wire gauze as well.
13. Allow the crucible and its cover to cool completely on the wire gauze for at least 10 minutes.
14. Measure and record the mass of the crucible, cover and carbonate product.
15. Repeat the procedure until constant mass is obtained.
16. Record the final mass of the crucible, cover and carbonate product.
17. Dispose of the crucible contents according to your instructor’s directions. Carefully clean the crucible and crucible cover for use in the next part of the lab.
18. Calculate the percent yield for the carbonate product.

Guided-Inquiry Design and Procedure
Form a working group with other students and discuss the following questions.

1. Based on your results in the Introductory Activity, what, if anything, is the product when potassium or sodium bicarbonate is heated at 100 °C to 200 °C? Does this product undergo further decomposition at this temperature?
2. Review the stoichiometry of the bicarbonate decomposition lab. How does mass loss relate to the mass of the starting material? Explain how the mass loss could be used to calculate the percent sodium or potassium bicarbonate in a mixture containing both the bicarbonate and the corresponding carbonate salt.
3. Design a laboratory procedure to determine the percent bicarbonate in a mixture, and then answer the following questions. Use reference books and the Internet when needed.
   1. What are the products of this lab?
   2. Are any or all of these products hazardous? If so, in what way?
   3. What are the proper disposal methods for each of these products?
4. The balanced equations for the experiment are listed below. The desired products of these reactions are the solid metal carbonates. Calculate the atom economy for each reaction.

   2KHCO₃(s) → K₂CO₃(s) + H₂O(g) + CO₂(g)
   or
   2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g)

5. What are the hazards, if any, of the reactants potassium bicarbonate, KHCO₃, and sodium bicarbonate, NaHCO₃?
6. Write a detailed step-by-step procedure for the experiment. Include all the materials, glassware and equipment that will be needed, safety precautions that must be followed, the mass of reactants, accuracy of any equipment, the required data table and calculations.
7. Review additional variables that may affect the reproducibility or accuracy of the experiment and how these variables will be controlled.
8. Carry out the experiment and record results in an appropriate data table.

Analyze the Results
Calculate the mass percent of the bicarbonate compound in the solid mixture. Determine your percent recovery.

Student Worksheet PDF

13838_Student1.pdf