Limiting Reagents

Overview: This tutorial discusses limiting reagents, demonstrates how theoretical and percent yields are determined, and presents possible factors that may preclude the achievement of the full theoretical yield in practice.

Skills:

• Identifying the limiting reagent
• Calculation of theoretical and percent yields

New terms:

• Limiting Reagent
• Theoretical Yield
• Percent Yield

Typically in chemical reactions between two reagents, both are not used completely. In general, one may be used completely while some amount of the other reagent(s) may remain after the reaction has occurred. Those that remain are said to react in excess. The reactant that is used completely is the limiting reagent. The limiting reagent determines the amount of product(s) at the end of the reaction as well as how much remains of the reactant(s) that occurred in excess. When the limiting reagent is completely used the reaction stops. Why would chemists be interested in this information? Answer

In order to determine which reactant is the limiting reagent, take each reactant separately and assume that it is the limiting reagent. The reactant that produces the least amount of product must be the limiting reagent.

Example.

Tin (Sn) and iodine (I2) react to produce tin iodide (SnI4). If there are 10.00 g of tin and 40.00 g of I2 at the beginning of the reaction, what is the maximum amount of SnI4 that may be produced?

What is happening in this reaction? We have iodine, which is a black crystalline solid reacting with tin (a grey-white solid metal solid) to form SnI4. So the skeleton equation is Sn + I2 SnI4.
• Determine the Limiting Reagent: The first step is to balance the reaction: Sn + 2 I2 SnI4. In this case, we do not know which reactant is the limiting reagent. In order to find how much product is formed, however, we must determine this. There are many ways to determine the limiting reagent. We will show one method that involves assuming one reactant is the limiting reagent and determine whether or not there is a sufficient amount of the other reagent(s) for the assumed limiting reagent to react completely. So to start let's look at Sn and assume for now that it is the limiting reagent. How much iodine is needed to completely consume 10.00 g of Sn?

This means that in order for 10.00 g of Sn to be used completely, 42.77 g of iodine is needed. But we only have 40.00 g of I2 (i.e., not enough I2 to completely react with all of the tin). This means iodine is the limiting reagent.

• Determine the amount of SnI4 that may be produced: We must use iodine for this calculation since it is the limiting reagent.

Prove to yourself that iodine is the limiting reagent. How much product could be produced from 10.00 g of Sn (assume that tin is the limiting reagent)? Answer

• Determine the amount of compounds remaining when the reaction is complete: What remains in the reaction container after the reaction has been completed? There is no iodine, only Sn and SnI4. We have already calculated how much SnI4 is produced, but how much tin is left?

So after the reaction has taken place, we would expect 49.35 g of SnI4 and 0.65 g of Sn in the reaction container. An alternative approach to answer this question.

• Determine the Yield: However, when the reaction actually occurred, only 25.8 g of SnI4 was obtained. What happened? Why isn't there 49.35 g of SnI4? What was obtained from the experiment was the actual yield and the mass we calculated (49.35 g SnI4) is the theoretical yield. From this we can calculate the percent yield.

What is the percent yield in our experiment?

Reasons for not achieving the theoretical yield

What are some possible reasons why the theoretical yield is not achieved? Answer

Advanced Applications: An organic chemist at Washington University develops methods for synthesizing pharmacologically active compounds.

Summary

After completing this review, you should be able to determine the limiting reagent and be able to calculate the amount of product formed from the limiting reagent. You should also understand the percent yield of the reaction.

Practice Quizzes: Stoichiometry/Limiting Reagents/Solutions These two quizzes cover the three tutorial modules Stoichiometry, Limiting Reagents, and Solutions. You will probably want to review all three of these modules before trying the quiz.

Note: You will need a pencil, scratch paper, calculator, periodic table and equation sheet to work the practice quiz. Quiz questions are timed (4 minutes per question).

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