IB Chemistry - Stoichiometry

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 The atom economy is a measure of efficiency in green chemistry. Syllabus reference Reactivity 2.1.5 - The atom economy is a measure of efficiency in green chemistry. Calculate the atom economy from the stoichiometry of a reaction. Guidance Include discussion of the inverse relationship between atom economy and wastage in industrial processes. The equation for calculation of the atom economy is given in the data booklet. Tools and links Structure 2.4, Reactivity 2.2 - The atom economy and the percentage yield both give important information about the “efficiency” of a chemical process. What other factors should be considered in this assessment?

Atom Economy in Green Chemistry

Definition of Atom Economy

Atom economy measures the efficiency of a chemical reaction in utilizing atoms from the starting materials. It is defined as the ratio of the molar mass of the desired product to the total molar mass of all products, multiplied by 100.

Atom Economy in the Industrial Synthesis of Aspirin

In this process, salicylic acid (C7H6O3) reacts with ethanoic anhydride (C4H6O3) to produce aspirin (C9H8O4) and ethanoic acid (CH3COOH). The reaction equation is:

C7H6O3 + C4H6O3 → C9H8O4 + CH3COOH

Molar Mass Calculation

The molar mass of aspirin (C9H8O4) is calculated as the sum of the molar masses of carbon, hydrogen, and oxygen in its formula, resulting in 180.16 g/mol. The total molar mass of the reactants, salicylic acid (138.12 g/mol) and ethanoic anhydride (102.09 g/mol), is 240.21 g/mol.

Atom Economy Calculation

The atom economy of the reaction is calculated by dividing the molar mass of the desired product (aspirin) by the total molar mass of the reactants and multiplying by 100%. This gives us an atom economy of approximately 75%, indicating that 75% of the total mass of the reactants is converted into the desired product.

Conclusion

The atom economy for the synthesis of aspirin is a measure of the reaction's efficiency in terms of material utilization. While the atom economy of approximately 75% is relatively efficient, it also highlights areas for potential improvement in industrial chemical processes to further minimize waste and enhance sustainability.

Importance in Green Chemistry

• Resource Efficiency: Ensures most reactants are converted into the desired product.
• Environmental Impact: Reduces hazardous byproducts and environmental burden.
• Economic Benefits: Leads to cost savings by reducing material and waste disposal needs.

Example 1. Synthesis of Ibuprofen

Improving the synthesis of ibuprofen increased its atom economy from 40% to 77%, reducing waste.

Example: The Diels-Alder Reaction

This reaction, forming a cyclohexene system, is an example of a 100% atom economy, utilizing all reactant atoms in the product.

Conclusion

Atom economy is essential for assessing the sustainability of chemical processes, with high values indicating efficient and environmentally friendly reactions.

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