4.4 Reaction Yields

Chemical reactions are like cooking recipes. The limiting reactant is like the ingredient that runs out first, determining how much you can make. Theoretical yield is the max amount possible, while actual yield is what you really get.

Factors like incomplete reactions, side reactions, and impurities can reduce actual yield. Green chemistry aims to make reactions more efficient and eco-friendly. Understanding these concepts helps optimize chemical processes and reduce waste.

Reaction Yields and Limiting Reactants

Limiting reactants and theoretical yield

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• Limiting reactant is the reactant that is completely consumed first in a chemical reaction and determines the maximum amount of product that can be formed, limiting the extent of the reaction (baking soda in a recipe)
• Excess reactant remains after the limiting reactant is completely consumed and does not affect the amount of product formed (extra flour in a recipe)
• Stoichiometry used to determine the limiting reactant based on molar ratios of reactants by comparing the mole ratios of reactants to the balanced chemical equation (2:1 ratio of hydrogen to oxygen in water formation)

Calculation of chemical yields

• Theoretical yield is the maximum amount of product that can be formed based on the limiting reactant, calculated using stoichiometry and mole ratios from the balanced chemical equation with the formula: $\text{Theoretical Yield} = \text{Moles of Limiting Reactant} \times \frac{\text{Molar Ratio of Product}}{\text{Molar Ratio of Limiting Reactant}}$ (48g of NaOH from 36g of Na)
• Actual yield is the amount of product actually obtained from a chemical reaction, often less than the theoretical yield due to various factors (42g of NaOH obtained instead of 48g)
• Percent yield compares the actual yield to the theoretical yield and indicates the efficiency of the reaction using the formula: \text{[Percent Yield](https://www.fiveableKeyTerm:Percent_Yield)} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\% (87.5% yield for 42g actual vs 48g theoretical)
  • Reaction efficiency can be improved by optimizing conditions to increase percent yield

Factors affecting actual yield

• Incomplete reactions occur when reactants do not completely convert to products due to equilibrium limitations or kinetic factors, causing a lower actual yield (reverse reaction in Haber process)
• Side reactions consume reactants or products to form undesired compounds, lowering the actual yield of the desired product (formation of CO in methanol synthesis)
• Impurities in reactants can reduce the actual yield by reacting with the desired reactants or products (sulfur impurities in iron ore)
• Loss of product during separation and purification contributes to a lower actual yield as some product may be lost during the isolation and purification process (recrystallization losses)
• Experimental errors from human mistakes in measurement or technique and instrumental limitations or inaccuracies can impact the actual yield (incorrect temperature settings)

Green Chemistry and Reaction Efficiency

• Atom economy measures the efficiency of a chemical reaction by calculating the percentage of atoms from the reactants that end up in the desired product
• Mass balance principles ensure that the total mass of reactants equals the total mass of products, accounting for all atoms in a chemical reaction
• Green chemistry aims to design chemical processes that minimize waste and environmental impact while maximizing reaction efficiency