Reaction stoichiometry actual yield

Stoichiometry is the quantitative study of products and reactants in chemical reactions. Stoichiometric calculations are best done by expressing both the known and unknown quantities in terms of moles and then converting to other units if necessary. A limiting reagent is the reactant that is present in the smallest stoichiometric amount. It limits the amount of product that can be formed. The amount of product obtained in a reaction (the actual yield) may be less than the maximum possible amount (the theoretical yield). The ratio of the two is expressed as the percent yield. 

Other Practical Matters in Reaction Stoichiometry— Stoichiometric calculations sometimes involve additional factors, including the reaction s actual yield, the presence of by-products, and how the reaction or reactions proceed. For example, some reactions yield exactly the quantity of product calculated—the theoretical yield. When the actual yield equals the theoretical yield, the percent yield is 100%. In some reactions, the actual yield is less than the theoretical, in which case the percent yield is less than 100%. Lower yields may result from the formation of by-products, substances that replace some of the desired product because of reactions other than the one of interest, called side reactions. Some stoi-... 

The amount of a product obtained from a reaction is often reported as a yield. The amount of product predicted by stoichiometry is the theoretical yield, whereas the amount actually obtained is the actual yield. The percent yield is the percentage of the theoretical amount that is actually obtained ... 

The amount of product actually formed in a reaction divided by the amount theoretically possible and multiplied by 100% is called the reaction s percent yield. For example, if a given reaction could provide 6.9 g of a product according to its stoichiometry, but actually provides only 4.7 g, then its percent yield is 4.7/6.9 X 100% = 68%. 

Chemists use stoichiometry to predict the amount of product that can be expected from a chemical reaction. The amount of product that is predicted by stoichiometry is called the theoretical yield. This predicted yield, however, is not always the same as the amount of product that is actually obtained from a chemical reaction. The amount of product that is obtained in an experiment is called the actual yield. 

In this section, you have learned how the amount of products formed by experiment relates to the theoretical yield predicted by stoichiometry. You have learned about many factors that affect actual yield, including the nature of the reaction, experimental design and execution, and the purity of the reactants. Usually, when you are performing an experiment in a laboratory, you want to maximize your percentage yield. To do this, you need to be careful not to contaminate your reactants or lose any products. Either might affect your actual yield. 

One of the tasks closely related to documentation is simple calculations that have to be performed to prepare an experiment. The number of calculations performed, for instance, in the organic synthesis laboratory is quite small, but those calculations required are very important. The calculations associated with conversion of the starting materials to the product are based on the assumption that the reaction will follow simple ideal stoichiometry. In calculating the theoretical and actual yields, it is assumed that all of the starting material is converted to the product. The first step in calculating yields is to determine the limiting reactant. The limiting reactant in a reaction that involves two or more reactants is usually the one present in lowest molar amount based on the stoichiometry of the reaction. This reactant will be consumed first and will limit any additional conversion to product. These calculations, which are simple rules of proportions, are subject to calculation errors due to their multiple dependencies. 

The amount of product of a chemical reaction predicted by stoichiometry is called the theoretical yield. As shown earlier, if 3.75 g of nitrogen completely react, a theoretical yield of 4.55 g of ammonia would be produced. The actual yield of a chemical reaction is usually less than predicted. The collection techniques and apparatus used, time, and the skills of the chemist may affect the actual yield. 

Hydroxylation of hydrocarbons. Mimoun and De Roch have described several systems that convert oxygen into a polarized peroxidic form that is active in hydroxylation of hydrocarbons at ambient temperatures. The most active system is composed of oxygen, ferrous chloride, hydrazobenzene (or o-phenylenediamine), and a carboxylic acid, usually benzoic acid. The overall stoichiometry of the hydroxylation is represented in equation I, but the reaction is actually more complex. This system converts cyclohexane into cyclohexa-nol in 20-25% yield based on hydrazobenzene or in 35% yield based on the absorbed oxygen. Cyclohexene is oxidized mainly to cyclohexene-3-ol (no epoxide... 

We can rate the efficiency of a reaction by calculating how much product would form under perfect or ideal conditions and then comparing the actual measured result with this ideal. The ideal amount of product is called the theoretical yield, and it is obtained by working a stoichiometry problem. Measuring the amount of product formed gives us the actual yield. From the ratio of the actual yield to the theoretical yield, we can calculate the percentage yield. 

We know the actual yield from the experiment. To calculate the percentage yield, first we need to find the theoretical yield. We can do that by calculating the maximum quantity of product that could form, based on the stoichiometry of the reaction. Once we have both the theoretical yield and the actual yield, finding the... 

The theoretical yield of product is the maximum amount of product that can be obtained by a reaction from given amounts of reactants. It is the amount that you calculate from the stoichiometry based on the limiting reactant. In Example 3.16, the theoretical yield of acetic acid is 27.3 g. In practice, the actual yield of a product may be much less for several possible reasons. First, some product may be lost during the process of separating it from the final reaction mixture. Second, there may be other,... 

In Example 10.5 you found that burning 66.0 grams of CyHig will produce 203 grams of CO2. This is the theoretical yield (theo), the amount of product formed from the complete conversion of the given amount of reactant to product. Theoretical yield is always a calculated quantity, calculated by the principles of stoichiometry. In actual practice, factors such as impure reactants, incomplete reactions, and side reactions cause the actual yield (act) to be lower than the theoretical yield. The actual yield is a measured quantity, determined by experiment or experience. 

The amounts of products calculated in the ideal stoichiometry problems in this chapter so far represent theoretical yields. The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant. In most chemical reactions, the amount of product obtained is less than the theoretical yield. There are many reasons for this result. Reactants may contain impurities or may form by-products in competing side reactions. Also, in many reactions, all reactants are not converted to products. As a result, less product is produced than ideal stoichiometric calculations predict. The measured amount of a product obtained from a reaction is called the actual yield of that product... 

When you use stoichiometry to calculate the amount of product formed in a reaction, you are calculating the theoretical yield of the reaction. The theoretical yield is the amount of product that forms when all the limiting reactant reacts to form the desired product It is the maximum obtainable yield, predicted by the balanced equation. In practice, the actual yield— the amount of product actually obtained from a reaction—is almost always less than the theoretical yield. Th e are many reasons for the difference between the actual and theoretical yields. For instance, some of the reactants may not react to form the desired product. They may react to form different products, in something known as side reactions, or they may simply remain unreacted. In addition, it may be difficult to isolate and recover all the product at the end of the reaction. Chemists often determine the efficiency of a chemical reaction by calculating its percent yield, which tells what percentage the actual yield is of the theoretical yield. It is calculated as follows ... 

In the stoichiometry examples worked out in the preceding section, we made the unstated assumption that all reactions "go to completion." That is, we assumed that all reactant molecules are converted to products. In fact, few reactions behave so nicely. Most of the time, a large majority of molecules react in the specified way, but other processes, called side reactions, also occur. Thus, the amount of product actually formed—the reaction s yield—is usually less than the amount that calculations predict. 

Since the stoichiometry of epoxy to amine is not less than 1.0 1.0 and the cure proceeds at a low temperature, one would predict that only reactions (1) and (2) would take place. To test this assumption for the DAB- DGEBA system, several identical runs were analyzed by the method of Bell (16). Utilizing the material balances found above (which assume no side reactions), the various reactive groups were determined and averaged with 95% confidence limits. A similar run using the spectro-photometric method gave the amounts of primary and secondary amines separately. This yields a calculated and actual concentration of primary and secondary amines. Provided there are no side reactions, the two values should be identical. The results were plotted in Figure 3 and found to fall on the same curve, within experimental error. This verifies the assumption that only reactions (1) and... 

As discussed in the previous chapter, there is an important distinction between the stoichiometric equations that describe the overall arithmetic of a chemical reaction—the relative numbers of molecules that are consumed and produced— and the set of equations that constitute a mechanism for the reaction. A mechanism is composed of a set of reactions called elementary steps, each of which is taken to represent an actual molecular event that leads to the overall reaction. The complete set of elementary steps must yield the correct stoichiometry and the experimentally observed rate law of the reaction. 

Proof that a carboxylated en2yme intermediate (enzyme-COf) actually participates in biotin-dependent carboxylations was provided by Yoshito Kaziro in Severn Ochoa s laboratory at New York University. They were able to isolate enzyme-C02 and show an exact stoichiometry between bound biotin and the active carboxy group. Importantly, the isolated enzyme-COr transferred its labile carboxy group to propionyl-CoA yielding methylmalonyl-CoA [reaction (3)] or underwent quantitative decarboxylation upon exposure to ADP, P and Mg + [reverse of reaction (2)]. 

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