4.3 Yield and Selectivity in Chemical Reactions

Yield calculations are crucial in chemical processes, helping engineers predict and optimize product formation. Understanding theoretical and actual yields allows for assessing reaction efficiency and identifying areas for improvement.

Factors like temperature, pressure, and catalysts significantly impact yield and selectivity. By manipulating these variables, chemists can enhance desired product formation while minimizing waste, a key consideration in industrial applications and sustainable chemical engineering.

Yield Calculations and Concepts

Theoretical vs actual yield

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• Theoretical yield represents maximum product amount based on stoichiometry assuming 100% conversion of limiting reactant
• Actual yield measures experimentally obtained product amount, typically lower due to various factors (incomplete reactions, side products)

Calculating theoretical yield

• Identify limiting reactant determines maximum product amount
• Balance chemical equation ensures proper stoichiometric ratios
• Use molar ratios from balanced equation to convert reactant quantity to product moles
• Convert product moles to mass or volume units (grams, liters) for final theoretical yield

Actual and percent yield

• Measure actual product amount through experimental techniques (weighing, titration)
• Calculate percent yield using formula $\text{Percent Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%$
• Interpret results: <100% indicates incomplete reaction or losses, =100% suggests complete conversion (rare), >100% points to experimental error or impurities

Selectivity in multiple-product reactions

• Measures preference for forming one product over others in reactions with multiple possible outcomes
• Product selectivity calculates ratio of desired product to total products formed
• Reactant selectivity evaluates efficiency of reactant conversion to desired product
• Calculate using formula $\text{Selectivity} = \frac{\text{Moles of desired product}}{\text{Moles of all products}} \times 100\%$
• Critical in industrial processes for maximizing desired product and minimizing waste (petroleum refining, pharmaceutical synthesis)

Factors affecting yield and selectivity

• Temperature influences reaction rate and equilibrium position, impacts competing reactions (exothermic vs endothermic)
• Pressure affects gas-phase reactions, follows Le Chatelier's principle (ammonia synthesis)
• Reactant concentrations alter reaction rates and equilibrium positions (batch vs continuous processes)
• Catalysts lower activation energy, potentially change reaction pathway and product distribution (zeolites in petrochemical industry)
• Reaction time balances conversion and selectivity, determines kinetic vs thermodynamic control of products
• Reactor design impacts mixing and mass transfer, influences yield and selectivity (plug flow vs stirred tank reactors)
• Inhibitors or promoters modify reaction rates and selectivity (poisoning catalysts in catalytic converters)
• Solvents affect reactant solubility and reaction environment (polar vs non-polar solvents in organic synthesis)