Percent Yield, Theoretical Yield and Actual Yield
Introduction
In chemical reactions, the actual quantity of product obtained often differs from what is expected theoretically. Understanding theoretical, actual, and percent yields helps chemists analyze efficiency, optimize processes, and troubleshoot errors in laboratory or industrial settings.
Definitions of Yield Terms
- Theoretical Yield: The maximum possible amount of product that can be formed from given reactants, calculated using stoichiometry.
- Actual Yield: The measured amount of product obtained from an experiment.
- Percent Yield: The ratio of actual yield to theoretical yield, expressed as a percentage.
Why Yields Matter in Chemistry
Yields provide insights into reaction efficiency, cost-effectiveness, and feasibility of chemical production. High percent yield indicates minimal loss and high efficiency, while a low percent yield suggests issues such as incomplete reaction, loss of product, or side reactions.
Calculating Theoretical Yield
Theoretical yield is based on stoichiometric calculations. Steps include:
- Write the balanced chemical equation.
- Convert the mass of the limiting reactant to moles.
- Use mole ratios to find moles of desired product.
- Convert moles of product to grams using molar mass.
Understanding Actual Yield
Actual yield is determined by experiment. It may be lower than the theoretical yield due to human error, reaction conditions, or competing reactions. It is typically measured after purification and drying of the final product.
How to Calculate Percent Yield
The formula is:
Percent Yield = (Actual Yield ÷ Theoretical Yield) × 100
This value tells how close the experimental result is to the maximum possible outcome.
Numerical Examples
- If the theoretical yield of a reaction is 10.0 g and the actual yield is 8.5 g, then:
Percent Yield = (8.5 g ÷ 10.0 g) × 100 = 85% - Given 12.0 g of reactant A yields 6.0 g of product B, but theoretical yield is 7.5 g:
Percent Yield = (6.0 ÷ 7.5) × 100 = 80%
Reasons for Low Percent Yield
- Side reactions forming unwanted products
- Loss of product during purification or transfer
- Incomplete reactions or unfavorable equilibrium
- Measurement errors or incorrect reaction conditions
Conclusion
Understanding percent, theoretical, and actual yields is essential for evaluating chemical processes. These calculations allow chemists to refine procedures, minimize waste, and improve efficiency in both academic and industrial laboratories.