WHS AP Chemistry

Lab: Analysis of a Mixture

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. We will focus on the three principles below.

  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 designated 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 Synthesis—synthetic methods should be designated to use and generate substances that possess little or no toxicity to human health and the environment.

Purpose

In this lab, you will design and carry out a green chemistry experiment that can quantitatively measure the weight percent of a metal bicarbonate (sodium bicarbonate) in a mixture of itself and its carbonate counterpart. Sodium bicarbonate undergoes decomposition when heated above 110°C.

2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g)

At temperatures below 800°C, sodium carbonate should remain unreacted. Therefore, if a mixture of bicarbonate and carbonate salts is heated at low temperature, all that remains should be the carbonate solid.

You will use the principles of green chemistry to evaluate the “greenness” of a traditional lab procedure. These three principles will help guide lab design. As part of prevention, you will want to evaluate alternative reactions wherever possible, and identify the reaction that produces the least waste. A typical reaction can be represented by the following equation.

Reactants Desired product + By-product waste

The greater the ration 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.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.

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 your design of an experiment than can quantitatively measure the weight percent of a sodium carbonate/bicarbonate solid 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 Pre-Lab Questions.

Flinn Scientific Advanced Inquiry Labs for AP* Chemistry

WHS AP Chemistry

Pre-Lab Questions

  1. The products of this lab are silver chromate solid, Ag2CrO4(s), and a solution of potassium nitrate KNO3(aq). Silver chromate forms a dense, colorful precipitate that is easy to see and accurately measure. (a) Is either or both of these products hazardous? If so, in what way? (b) 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)

  1. 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

Introductory Activity

Decomposition of Group 1 Bicarbonates

  1. Set up a Bunsen burner on a support stand beneath a ring clap holding a pipe-stem triangle (see figure 1). Do NOT light the Bunsen burner.
  2. Adjust the height of the ring clamp so that the bottom of the 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 (1-2 min).
  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).
  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 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).
  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 bench top. 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 in the designated waste container. Carefully clean the crucible and crucible cover for use in next part of the lab.
  18. Calculate the percent yield for the carbonate product.

Flinn Scientific Advanced Inquiry Labs for AP* Chemistry

WHS AP Chemistry

Inquiry Plan

Form a group with 2 other students and discuss the following questions.

  1. Based on your results in the Introductory Activity, what, if anything is the product when sodium bicarbonate is heated at 100C to 200C? 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 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.
  4. What are the products of this lab?
  5. Are any or all of these products hazardous? If so, in what way?
  6. What are the proper disposal methods for each of these products?
  7. The balanced chemical equation for the experiment is listed below. The desired product of this reaction is the solid metal carbonate. Calculate the atom economy for each reaction.
  8. What are the hazards, if any, of the reactant sodium bicarbonate, NaHCO3?
  9. 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.
  10. Review additional variables that may affect the reproducibility or accuracy of the experiment and how these variables will be controlled.
  11. Carry out the experiment and record results in an appropriate data table.

Analyze the Results

Calculate the mass percent of the bicarbonate composed in the solid mixture. Determine you percent recovery.

Flinn Scientific Advanced Inquiry Labs for AP* Chemistry