Stoichiometry and Green Chemistry
Goals:
• Learn about the philosophy of green chemistry
• Determine the composition of a mixture using stoichiometry
• Learn what is important in a good laboratory report
Introduction
Green chemistry seeks to reduce the use and generation of hazardous material through control
of the design and processes of chemical synthesis. Green chemistry, the use of chemistry for
pollution preventions, is distinct from environmental chemistry which focuses on pollution
mitigation. Green chemistry has the power to transform all areas of chemistry into a far safer
enterprise, while simultaneously challenging widely held perceptions that chemistry on the large scale
is a polluting, “dirty” industry and a drain on limited natural resources.
Many experiments used in student laboratory courses are not “green.” Some use toxic
materials, others create excess waste, and still others use unnecessary amounts of natural resources.
Some might do all three or may violate other tenets of green chemistry. A green chemistry approach
to teaching laboratory chemistry meets the same goals as a more traditional approach. Students still
do experiments that teach the same concepts, techniques, and skills, but the experiments use green
materials and processes.
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
below:
1. Create no waste (better to prevent waste than to treat or clean up after it has been created)
2. Nothing should be left over (atom economy principle)
3. No toxicity
4. Green products have to work as well as nongreen products.
5. Get rid of all nonessential additives
6. Reduce energy usage
7. Use renewable materials
8. Get rid of as many steps as possible
9. Make use of reusable method to speed up a reaction
10. Use materials that break down in the environment (biodegradable)
11. Check everything you do against the other principles
12. Safety first
In this lab, you will design a process to determine the mass percent of a metal bicarbonate –
NaHCO3 – in a mixture of itself and its carbonate counterpart. Sodium bicarbonate undergoes
decomposition when heated above 110oC.
2 NaHCO3 (s)  Na2CO3 (s) + H2O (g) + CO2 (g)
At temperatures below 800oC, sodium bicarbonate remains unreacted. Therefore, if a
mixture of bicarbonate and carbonate salts is heated at low temperature, all that remains after
heating should be the carbonate salt. This process is intended as a “greener” experiment for
teaching stoichiometry.
Atom economy is a very important measure used in assessing how “green” a chemical process is.
Although the atom economy calculation is usually applied to organic synthesis processes, it can be calculated
for any chemical reaction. All chemical reactions have the form
Reactants → Products
The products of the reaction can either be desired products or undesirable waste. Green chemistry seeks to
minimize undesirable waste (or not generate it in the first place, if possible). You can think of a reaction instead
as
Reactants → (Desirable Products) + (Undesirable Waste)
The atom economy calculation measures what part of the original input is converted to desirable products. In
essence, atom economy measures how well the process conserves input.
Atom economy =
Mass of all desired products
×100%
Mass of all reactants
The theoretical atom economy for a reaction can be calculated using molar masses instead of actual
masses measured in the laboratory. For example, for the reaction
Fe2O3 + 3 CO  2 Fe + 3CO2
the desired product is iron metal, and the undesirable waste is carbon dioxide. The theoretical sum of the molar
masses
of the reactants is:
Mass of all reactants
= mass of 1 mol of Fe 2 O3 + mass of 3 mol of CO
= (1mol Fe2O3 x 159.7 g/mol) + ( 3 mol CO x 28.01 g/mol)
= 243.8 g
The theoretical mass of the desired product is:
Mass of desired product
= mass of 2 mol of Fe
55.85 g
= 2 mol Fe ×
mol
= 111.7 g
Therefore, the theoretical atom economy is:
Atom economy =
Mass of all desired products
×100%
Mass of all reactants
111.7 g
=
×100% = 45.82%
243.8 g
In principle, a reaction with higher atom economy is preferable to one with lower atom economy, because
higher atom economy means less waste produced for a given amount of product produced (the higher the
atom economy – the greener the reaction). However, it is important to realize that atom economy is only one
measure of a reaction’s greenness. Atom economy tells what fraction of the original materials ends up in the
desired product, but says nothing about what the original materials or desired product are and whether they are
green or not. A reaction that uses highly toxic materials can have high atom economy, and that reaction still
would not be green.
Pre-lab questions
1. The following two reactions are possible methods for refining copper in the final step of a smelting process, i.e., getting pure
copper (Cu) from copper ores found in rocks. Calculate the theoretical atom economy for each reaction.
a. 2CuO (s) + C (s)  2 Cu(s) + CO2 (g)
b. CuO (s) + CO (g)  Cu (s) + CO2 (g)
2. Use your calculations form the previous question (2a, 2b) to answer the following questions:
a. Which one of the methods of refining copper ore is greener according to the atom economy principle of green chemistry?
b. Why is a calculation of atom economy helpful in comparing two chemical reactions to determine which one is greener?
In other words, what does atom economy tell you about “greenness”?
c. What is another possible consideration from the principles of green chemistry that could tell you more about comparing
“greenness” of these two reactions.
After massing an empty crucible and lid, a student placed potassium bicarbonate in
the crucible and maseds the crucible and lid again. She then placed the crucible in a
clay triangle held in a ring on a support stand. She put the crucible cover on loosely
and heated very gently (see figure). It was important she heated the crystals gently
to be sure the escaping vapor does not carry any of the solid along with it. She
heated the crucible and its contents gently for a total of 5 minutes. After removing
the crucible (using tongs), she placed in on a ceramic tile and let cool for 10
minutes, she then massed the crucible and contents. She then repeated the heating
process until a constant mass was obtained.
She constructed the following data table:
Mass of empty crucible + lid
Mass of crucible + lid + KHCO3
Mass of crucible + lid + carbonate
product (after 1st heating)
Mass of crucible + lid + carbonate
product (after 2nd heating)
Mass of crucible + lid + carbonate
product (after 3rd heating)
23.45 g
25.45 g
26.85 g
26.77 g
26.76 g
Using this data, calculate the following:
3. Calculate the percent yield of the carbonate product.
4. Calculate the theoretical atom economy for the carbonate product.
5. How are the 2 ways of measuring efficiency of a reaction (percent yield and atom economy) the same? How do they differ?
Lab Instructions
You will be required to analyze a sample made up of sodium carbonate and sodium bicarbonate.
Design a laboratory procedure to determine the percent bicarbonate in the mixture. Write a detailed step by step procedure for the
experiment. Include all the materials and equipment that will be needed, safety precautions that must be followed, the required
data table and calculations. . (Use about 4-5 grams of your mixture, only do one trial)
Carry out the experiment and record results in an appropriate data table
Analyze the results: Your results should address the following points:
1. The total mass of water and carbon dioxide lost
2. The moles of water lost
3. The mass of sodium bicarbonate originally present in the mixture
4. The mass percent of the sodium bicarbonate in the original mixture.
In addition to the accuracy of the experiment, the discussion should address how the experiment adheres to the principle of
green chemistry (e.g. the atom economy of the experiment).