Redox Reactions - SS1 Chemistry Past Questions and Answers - page 6

In a chemical reaction, the limiting reactant is the reactant that is completely consumed and limits the amount of product that can be formed. The concept of limiting reactants is crucial in understanding reaction efficiency and predicting the maximum yield of products.

 

The significance of identifying the limiting reactant can be highlighted through the following points:

 

1. Reaction efficiency: The limiting reactant determines the maximum amount of product that can be formed in a reaction. If one reactant is present in excess, it remains unreacted and does not contribute to the formation of additional products. By identifying the limiting reactant, we can optimise the reaction conditions to ensure the complete consumption of the limiting reactant and maximise the efficiency of the reaction.
2. Predicting product yield: The concept of the limiting reactant allows us to predict the maximum yield of products that can be obtained from a given amount of reactants. The stoichiometric ratios in the balanced chemical equation provide the mole-to-mole relationship between reactants and products. By comparing the amounts of reactants, we can determine which reactant is in excess and calculate the theoretical yield of the product based on the limiting reactant.

 

1. Determining excess reactants: By identifying the limiting reactant, we can also determine the amount of excess reactant remaining after the reaction. The excess reactant is the reactant that is not completely consumed. This information is valuable for cost considerations, as it helps in determining the efficiency of reactant utilisation and minimising waste.

Percent yield is a measure of the efficiency of a chemical reaction and represents the ratio of the actual yield of a product to the theoretical yield, expressed as a percentage. It quantifies how much of the desired product is obtained compared to the maximum possible yield.

 

The percent yield can be calculated using the formula:

 

Percent yield = (Actual yield / Theoretical yield) x 100

 

Several factors can affect the percent yield in a chemical reaction:

 

1. Limiting reactant: The identification of the limiting reactant is crucial in determining the maximum theoretical yield. If the reaction is not carried out under ideal conditions or if the reactants are not in the exact stoichiometric ratio, the actual yield may be lower than the theoretical yield, leading to a lower percent yield.
2. Side reactions: Side reactions can occur alongside the desired reaction and result in the formation of unwanted products. These side reactions reduce the amount of the desired product and lower the percent yield.
3. Incomplete reaction: In some cases, the reaction may not go to completion due to various reasons such as reversible reactions, equilibrium conditions, or competing reactions. Incomplete reaction results in a lower actual yield and consequently a lower percent yield.
4. Purity of reactants and products: Impurities in the reactants or products can affect the yield. Impure reactants may react differently, leading to a lower yield of the desired product. Impurities in the product can also affect the accuracy of measuring the actual yield, thus affecting the percent yield.
5. Experimental errors: Errors in measurements, transfers, and calculations can introduce inaccuracies in determining the actual yield, leading to a deviation from the theoretical yield and affecting the percent yield.