Yield (chemistry)
In chemistry, yield refers to the amount of product produced at the end of a chemical reaction, which involves the combination of elements and compounds. There are three main types of yield that chemists focus on: actual yield, theoretical yield, and percentage yield. Actual yield is the definitive amount of product obtained from an experiment, while theoretical yield is the predicted amount based on ideal conditions, assuming no loss of materials occurs. Typically, theoretical yields are higher than actual yields due to various factors that can cause discrepancies, such as the loss of reactants or unexpected reactions.
To analyze yield effectively, chemists must balance chemical equations, ensuring that the number of atoms of each element remains consistent before and after the reaction, in accordance with the law of conservation of mass. Once a balanced equation is established, scientists can calculate theoretical yield based on the masses of the reactants. After conducting the experiment, they compare the actual yield to the theoretical yield to determine percentage yield, which quantifies how efficiently the reaction occurred. This information is valuable to chemists as it helps refine processes and improve the accuracy of future experiments.
Subject Terms
Yield (chemistry)
In chemistry, yield is what is produced at the end of a chemical reaction, the process by which elements and compounds combine to form new materials. Elements are pure chemical substances that make up everything in the universe. Compounds are formed when two or more elements join together chemically. In calculating and understanding the outcomes of chemical reactions, scientists rely on three types of yield: actual yield, theoretical yield, and percentage yield.
Chemical Equations
Yield is the final part of a chemical equation, a formula that chemists use to visualize what happens during chemical reactions. Yield, also called product, is what is created from the fusion of all the reactants, the substances that are set to combine. The products possess the added masses of the reactants. Mass is the amount of material present in an object; it is generally measured in grams or kilograms. Knowledge of these basic parts of a chemical equation are important to understanding yield.
Types of Yield
No matter how complex a chemical equation becomes, scientists always use the same basic process of adding the reactants' masses to determine a reaction's eventual yield. The types and amounts of products created at the end of a chemical equation are referred to as actual yield. As the term's name suggests, this is the definite, irrefutable result of a chemical reaction.
Usually, an equation's actual yield differs substantially from its theoretical yield, also called predicted yield. Scientists propose theoretical yields before actually conducting chemical experiments. Theoretical yields are not random guesses about what kinds of substances chemical formulas will produce. Rather, they are predictions of how much product will be created if all the reactants in a chemical equation change as they should, without any substances getting "lost" along the way.
The concept of materials being lost in a chemical reaction does not mean that some substances are destroyed or simply disappear. This would oppose the law of conservation of mass, which is a fundamental scientific principle stating that mass cannot be created or destroyed when a substance changes in a chemical process. A material could be considered lost in a chemical reaction if it remained unchanged in the transition from reactant to product when it should have been altered in a specific way. Chemicals could also be "lost" if they unexpectedly combined to form new compound substances. In both of these processes, the atoms, or most basic units, of these chemicals remain intact but shift form on their own. Thus, though atoms in a chemical reaction remain constantly present, they can still alter in ways that change the eventual mass of certain products.
Scientists do not factor in these types of surprises when determining theoretical yields because these predictions assume that chemical reactions will take place under optimal conditions. However, since these conditions rarely exist, theoretical yields are usually higher than actual yields. Once scientists have proposed their theoretical yields and executed the chemical reactions to determine the actual yields, they can use a mathematical formula to calculate percentage yield. This is a measure of the difference between the theoretical and actual yields. Percentage yields are useful because they tell scientists how close their experiments came to achieving their chemical potential.
Calculating Yield
The first step in determining all three types of yield for a chemical reaction is to create a balanced chemical equation. Equations are balanced when the same number of elemental atoms in the reactants is also present in the products. Chemical equations must always be balanced to preserve the integrity of the law of conservation of mass.
For example, when balanced, the chemical formula that combines the compounds sulfuric acid and sodium hydroxide to form sodium sulfate and water looks like this: H2SO4 + 2NaOH → Na2SO4 + 2H2O. This equation is balanced because each individual element exists in the same quantity on both sides of the arrow, or yield sign. For example, there are two atoms of sodium on the left (represented in the two NaOH molecules), and two atoms of sodium on the right (in the molecule Na2SO4).
It is from balanced equations that scientists determine a reaction's theoretical yield. For instance, a chemist could use these substances' masses to predict that 6.9 grams (g) of sulfuric acid could potentially help create 10 g of sodium sulfate. This 10 g is the theoretical yield for this specific product, as each product has its own yield (these are totaled for the complete product yield). When the experiment is performed, the chemist might find that only 7.2 g of sodium sulfate has actually been created. This lower figure, the result of various lost substances, is the actual yield.
The percentage yield can now be used to measure the difference between the theoretical yield and the actual yield. The percentage yield is found by first dividing the actual yield by the larger percentage yield and then multiplying the result by 100. For the reaction that produced 7.2 g of sodium sulfate, the formula for finding the percentage yield looks like this: 7.2 ÷ 10 × 100. The division of 7.2 by 10 produces 0.72. Multiplied by 100, 0.72 becomes 72. This number is the percentage yield of this particular chemical change, meaning that the actual yield is 72 percent of the theoretical yield.
Percentage yields are most useful inside the scientific community. By documenting a chemical reaction's theoretical and actual yields, as well as the percentage of difference between them, chemists supply their colleagues with valuable information about what to expect when performing certain experiments. Other scientists can also try to find ways of producing actual yields that are closer to their theoretical yields.
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