Why do we use excess reactants in a chemical reaction?
Why are percent yields usually less than 100%?
Is it because we want to maximize the amount of products produced, and reactions usually end up in a loss of reactants to the air, etc?
generally, excess reactants are used to drive the speed of reaction,or the equilibrium toward the desired product.
Yield is usually less than 100 percent because of temperature is not ideal, pressure is not ideal, reaction medium was a trade off, and a mydrid of other factors.
Everything Bob Pursley said is ttue. It boils down to two ways of saying it.
1. There is no free lunch.
2. Nothing is 100% anything.
Great questions! Let's break them down one by one.
1. Why do we use excess reactants in a chemical reaction?
Using excess reactants is a common practice in chemical reactions for a few reasons:
a. Ensure complete reaction: In some reactions, one reactant may be used up before the other reactant is completely consumed. Adding excess reactant ensures that the limiting reactant is not exhausted, allowing the reaction to proceed to completion.
b. Increase yield: By using excess reactants, we can drive the reaction towards the desired products more effectively. It helps to maximize the amount of products produced by shifting the equilibrium toward the desired side.
c. Account for impurities and losses: During a reaction, impurities might inhibit the reaction, or certain side reactions can occur that reduce the overall yield. Using excess reactants compensates for these factors, helping to obtain a higher yield of the desired product.
2. Why are percent yields usually less than 100%?
Percent yield is a measure of how efficiently a chemical reaction converts reactants into products. It is calculated by dividing the actual yield by the theoretical yield, multiplied by 100%.
A percent yield less than 100% can result from various factors:
a. Incomplete reactions: Even with excess reactants, some reactions may not proceed to completion due to unfavorable conditions or reversible reactions. This means that not all reactants are converted to products, resulting in a lower actual yield.
b. Side reactions: During a chemical reaction, side reactions may occur simultaneously and produce undesired by-products. These side reactions can lower the overall yield of the desired product.
c. Losses during purification or separation: Separating the desired product from the reaction mixture may result in losses, such as through filtration, evaporation, or transfers. These losses can reduce the total amount of product obtained.
d. Impurities: The presence of impurities or contaminants in the reactants, or the reaction conditions themselves, can lower the overall yield by interfering with the reaction or affecting the purity of the final product.
It is important to note that while a percent yield less than 100% is common, it can still be improved through various optimization techniques to reduce side reactions, refine purification processes, and enhance reaction conditions.