Limiting Reactants and Percent Yield - SS1 Chemistry Lesson Note

In chemical reactions, the limiting reactant is the substance that is completely consumed, limiting the amount of product that can be formed. The percent yield, on the other hand, measures the efficiency of a reaction by comparing the actual yield of a product to the theoretical yield. We will delve into the concepts of limiting reactants and percent yield, exploring their significance and how they relate to reaction efficiency.

 

Limiting Reactants:

In a chemical reaction, reactants combine in specific stoichiometric ratios based on the balanced chemical equation. However, the actual amounts of reactants used in a reaction may not match these ratios. The limiting reactant is the one that is completely consumed first, limiting the amount of product that can be formed. The other reactant is called the excess reactant, as it remains after the limiting reactant is consumed.

 

To determine the limiting reactant, you can use stoichiometry and compare the amount of product that can be formed from each reactant. The reactant that produces the smaller amount of product is the limiting reactant.

 

Understanding the limiting reactant is crucial for calculating the theoretical yield of the product and predicting the maximum amount of product that can be obtained from a given amount of reactants.

 

Theoretical Yield:

The theoretical yield is the maximum amount of product that can be obtained from a reaction, assuming complete conversion of the limiting reactant into the desired product. It is calculated using stoichiometry and the mole ratio between the limiting reactant and the product.

 

The theoretical yield provides a benchmark for comparison with the actual yield, allowing us to evaluate the efficiency of the reaction.

 

 

 

 

Actual Yield:

The actual yield is the amount of product obtained from a reaction under real laboratory or industrial conditions. It may be less than the theoretical yield due to various factors such as incomplete reactions, side reactions, product loss during isolation, or experimental errors.

 

Percent Yield:

The percent yield is a measure of the efficiency of a reaction and is calculated by dividing the actual yield by the theoretical yield and multiplying by 100. The formula for percent yield is:

 

Percent Yield = (Actual Yield / Theoretical Yield) × 100

 

The percent yield indicates the proportion of the maximum possible yield that was actually obtained. A percent yield of 100% indicates that the actual yield matches the theoretical yield, suggesting an efficient and complete reaction.

 

Factors Affecting Percent Yield:

Several factors can impact the percent yield of a reaction, including:

 

1. Reaction Conditions: Temperature, pressure, and reaction time can influence the extent of a reaction and affect the percent yield.

 

1. Purity of Reactants: Impurities in reactants can hinder the reaction and reduce the yield. Using pure reactants helps maximise the percent yield.

 

1. Side Reactions: Side reactions can occur concurrently with the desired reaction, reducing the yield of the desired product.

 

1. Reaction Efficiency: The efficiency of the reaction itself, such as the completeness of the reaction and the presence of catalysts, affects the percent yield.

 

1. Product Isolation: Loss of product during isolation and purification steps can reduce the actual yield.

 

Importance of Limiting Reactants and Percent Yield:

Understanding limiting reactants and percent yield is essential in chemical synthesis, process optimization, and quality control. They help predict the maximum achievable yield, optimise reactant amounts, and assess the economic viability of chemical processes. The percent yield provides insights into the efficiency of reactions, allowing chemists to identify and improve reaction conditions, minimise wastage of resources, and maximise product output.