Video Transcript
Which of the following explains why a reaction yield may appear to be above 100 percent? (A) The product of the reaction contains impurities. (B) The reagents are of high purity. (C) All reactions are complete; therefore, all reactants are converted to products. (D) There are two or more reactions that occur simultaneously so that some reactants are converted to products.
The question asks about reaction yield. Yield is the amount of product obtained from a chemical reaction. Yield can be expressed in terms of mass or volume or amount in moles. The question asks for the yield in terms of a percent, specifically that above 100 percent. A yield value in terms of percent is called percentage yield. Percentage yield is equal to the actual yield divided by the theoretical yield multiplied by 100 percent. Actual yield is the experimentally obtained or measured yield at the end of a chemical reaction. Theoretical yield is the maximum amount of product that can be obtained from a chemical reaction if all the reactants were converted to product. And this value is calculated.
If a percentage yield obtained is above 100 percent, this means that the value in the numerator of the percentage yield expression is larger than the value in the denominator. For an actual yield value to be higher than a theoretical yield value, this means that the experimentally obtained yield or the measured yield is larger than the calculated maximum possible yield. But this does not seem possible. How can we get more product than the maximum possible amount?
If actual yield is larger than theoretical yield, this can only mean one thing, a contaminated product. In other words, the product contains something else as well adding to its mass. And this is impurities. It is only possible to get a percentage yield greater than 100 percent if the product is contaminated with impurities or if all the solvent from the reaction mixture has not been dried off.
If reagents are of high purity, this decreases the likelihood of the product containing impurities and decreases the likelihood of the percentage yield being greater than 100 percent.
If all reactions are complete and all reactants are converted to products, then the actual yield will be maximized. And this will maximize the percentage yield but will not make it more than 100 percent.
So far, we’ve eliminated answer options (B) and (C). We have one last answer option to check. If two or more reactions occur simultaneously, for example, if hypothetical substances X and Y2 undergo two different reactions at the same time, they might form XY and X2Y2, two different products from two different reactions. And only some of the reactants are converted to the desired product. Let’s say XY was the desired product. Then the percentage yield of XY would definitely not be above 100 because its actual yield will be much lower than the theoretical yield of XY had X2Y2 not being produced. But it could still not be above 100, unless XY was contaminated.
Why might a reaction yield appear to be above 100 percent? The answer is (A), the product of the reaction contains impurities.
