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Chapter 9: Problem 57

Why is the actual yield of a reaction often not equal to the theoretical yield?

Short Answer

Expert verified
The actual yield of a reaction often differs from the theoretical yield due to factors such as reversible reactions, side reactions, loss of product during purification, and limiting reagents. These factors can lead to a smaller amount of the desired product being formed as compared to what is predicted by the stoichiometry of the balanced chemical equation.

Step by step solution

01

Introduction to Yield

In chemistry, yield is a measure of the quantity of product formed in a chemical reaction. Theoretical yield is the maximum amount of product that can be formed from a given amount of reactants, as predicted by the stoichiometry of the balanced chemical equation. The actual yield, on the other hand, is the amount of product that is actually obtained from the reaction in a real-life experimental scenario.
02

Factors Affecting Actual Yield

There are several factors that can cause the actual yield of a reaction to be different from the theoretical yield. These factors include: 1. Reversible reactions: Some reactions are reversible, meaning they can proceed in both the forward and reverse directions. In such cases, the reaction may not go to completion, and the equilibrium position might not favor the formation of products to the extent predicted by the balanced chemical equation. 2. Side reactions: In many cases, the reactants can undergo other reactions besides the main reaction of interest, leading to the formation of unwanted side products. This can reduce the amount of reactants available for the main reaction, which results in a smaller actual yield of the desired product. 3. Loss of product during purification or isolation: When the products of a reaction are isolated and purified, some of the product may be lost due to factors such as transfer loss, filtration loss, and evaporation. This can result in a lower actual yield than theoretically predicted. 4. Limiting reagent: Often, one of the reactants is not present in stoichiometric amounts, meaning there is not enough of it to react with all the other reactants. This limiting reagent determines the maximum amount of product that can be formed, which can be less than the theoretical yield based on the initial quantities of all reactants.
03

Conclusion

The actual yield of a reaction is often not equal to the theoretical yield due to factors such as reversible reactions, side reactions, loss of product during purification, and limiting reagents. These factors can result in the formation of a smaller amount of the desired product than what is predicted by the stoichiometry of the balanced chemical equation.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Theoretical Yield
The theoretical yield is a concept in chemistry where we calculate the maximum possible amount of product that can be formed in a chemical reaction. This calculation is based on the stoichiometry of the balanced chemical equation.
  • The process starts with identifying the balanced chemical equation for the reaction.
  • Using stoichiometry, we then determine the maximum amount of product that can be systematically predicted from the reactants.
Theoretical yield serves as a guideline or a prediction. It assumes that every single molecule reacts perfectly, and no side reactions occur. However, in real life, reactions rarely achieve an ideal 100% yield.
Actual Yield
The actual yield refers to the amount of product that is actually obtained from a chemical reaction. Unlike the theoretical yield, the actual yield can vary due to several real-world factors.
  • Some reactions are reversible, limiting the full conversion of reactants to products.
  • Unintended side reactions can occur, producing side products.
  • Losses can happen during purification or isolation steps, such as material sticking to containers.
These factors make the actual yield typically lower than what is theoretically predicted, highlighting the gap between ideal predictions and practical results.
Stoichiometry
Stoichiometry is the heart of calculating yields in chemistry, involving the quantitative relationships between reactants and products in a chemical reaction.
  • It starts with a balanced chemical equation, where every atom that goes into the reaction has a corresponding place in the products.
  • Using this equation, you determine mole ratios, setting the stage for calculating theoretical yields.
Stoichiometry not only helps in finding out how much product can be expected but also in identifying which reactant will limit the reaction's progress, known as the limiting reagent.
Limiting Reagent
The limiting reagent is a critical concept in chemistry, referring to the reactant that is completely consumed first, limiting the amount of product formed in a reaction.
  • Even if other reactants are present in excess, the limiting reagent determines the maximum possible yield of the reaction.
  • To find the limiting reagent, compare the mole ratios of the reactants to the product according to the balanced equation.
Understanding the limiting reagent is essential for predicting the actual workability of the reaction, as the theoretical yield calculations must accommodate this real-world constraint.

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Most popular questions from this chapter

For the reaction \(\mathrm{Ca}_{3} \mathrm{P}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{PH}_{3}\), (a) Balance the equation. (b) How many grams of \(\mathrm{H}_{2} \mathrm{O}\) would you need to react with \(60.0 \mathrm{~g}\) of \(\mathrm{Ca}_{3} \mathrm{P}_{2} ?\) (c) How many grams of \(\mathrm{PH}_{3}\) could theoretically be produced from the reactant amounts calculated in part (b)?

Saccharin, which is used as an artificial sweetener, is made up of \(45.90 \%\) by mass \(C, 2.75 \%\) by mass \(\mathrm{H}, 26.20 \%\) by mass \(\mathrm{O}, 7.65 \%\) by mass \(\mathrm{N}\), and \(17.50 \%\) by mass \(\mathrm{S}\). If the molar mass of saccharin is \(183.19 \mathrm{~g} / \mathrm{mol}\), what is its molecular formula?

The ingredients for making 1 dozen cupcakes are: 5 tablespoons butter 1 cup sugar 2 eggs 2 cups flour 1 cup milk If you have only 5 eggs but plenty of all of the other ingredients: (a) How many cupcakes can you make? (b) How many cups of sugar do you need to make the number of cupcakes calculated in (a)?

\(5.00 \mathrm{~g}\) of solid sodium (Na) and \(30.0 \mathrm{~g}\) of liquid bromine \(\left(\mathrm{Br}_{2}\right)\) react to form solid \(\mathrm{NaBr}\). (a) Write a balanced chemical equation for this reaction. (b) Which reactant is limiting? (c) What is the theoretical yield for this reaction in grams? (d) How many grams of excess reactant are left over at the end of the reaction? (e) When this reaction is actually performed, \(14.7 \mathrm{~g}\) of \(\mathrm{NaBr}\) is recovered. What is the percent yield of the reaction?

Consider the unbalanced chemical equation \(\mathrm{KCl}+\mathrm{O}_{2} \rightarrow \mathrm{KClO}_{3}\) If you react \(42.6 \mathrm{~g}\) of \(\mathrm{KCl}\) and \(36.5 \mathrm{~g}\) of \(\mathrm{O}_{2}\) and the reaction has a percent yield of \(56.0 \%\), how many grams of \(\mathrm{KClO}_{3}\) are produced?
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