Monitoring and controlling chemical reactions Study Guide

In chemistry, when we carry out a reaction, we don't always get back the theoretical maximum amount of product. The theoretical yield is the amount of product we expect to get based on the stoichiometry of the balanced chemical equation, assuming perfect conditions and complete reaction. However, in practice, several factors can prevent us from achieving this ideal amount. The actual yield is the measured amount of product obtained from a chemical reaction in the laboratory.

Percentage yield is a crucial measure that tells us how efficient a chemical reaction is. It compares the actual yield to the theoretical yield and expresses this comparison as a percentage. The formula for percentage yield is:

Percentage Yield = (Actual Yield / Theoretical Yield) × 100%

For example, if you predict you should get 10 grams of product (theoretical yield) but you only isolate 8 grams (actual yield), the percentage yield would be (8 / 10) × 100% = 80%. A higher percentage yield indicates a more efficient reaction, meaning less waste of reactants and resources. Factors that can reduce the percentage yield include incomplete reactions, side reactions forming unwanted products, loss of product during separation and purification processes (e.g., filtration, decantation, or transfer), and impurities in the reactants. Understanding percentage yield helps chemists evaluate and improve their experimental procedures, making industrial processes more economically viable and environmentally friendly.

Atom economy is a measure of how efficiently a chemical reaction converts the atoms in the starting materials (reactants) into the desired product. It is expressed as a percentage. A high atom economy means that most of the atoms from the reactants end up in the useful product, with very few 'waste' products. This is desirable for several reasons: it reduces the consumption of raw materials, minimises the disposal of unwanted by-products, and makes industrial processes more sustainable and environmentally friendly. For example, if a reaction has an atom economy of 100%, it means that all the atoms in the reactants have been incorporated into the desired product, and no atoms are wasted. If the atom economy is low, a significant proportion of the atoms from the reactants form unwanted by-products, which may need to be separated and disposed of, increasing costs and environmental impact. Calculating atom economy involves comparing the total relative formula mass (Mr) of the desired product(s) to the total relative formula mass of all reactants. The formula for atom economy is: (Relative formula mass of desired product / Total relative formula mass of reactants) x 100%. In practical terms, reactions with high atom economy are preferred in industrial chemistry as they are more economically viable and have less impact on the environment.