Yield, percent

Percent yield is important as an indicator of the efficiency of a particular reaction. 

AIM To learn to calculate actual yield as a percentage of theoretical yield. 

In the previous section we learned how to calculate the amount of products formed when specified amounts of reactants are mixed together. In doing these calculations, we used the fact that the amount of product is controlled by the limiting reactant. Products stop forming when one reactant runs out. 

The amoimt of product calculated in this way is called the theoretical yield of that product. It is the amoimt of product predicted from the amounts of reactants used. For instance, in Example 9.8, 10.6 g of nitrogen represents the theoretical yield. This is the maximum amount of nitrogen that can be produced from the quantities of reactants used. Actually, however, the amount of product predicted (the theoretical yield) is seldom obtained. One reason for this is the presence of side reactions (other reactions that consume one or more of the reactants or products). 

The actual yield of product, which is the amount of product actually obtained, is often compared to the theoretical yield. This comparison, usually expressed as a percent, is called the percent yield. 

EniZKES Percent yield is a measure of the efficiency of a chemical reaction. 

Real-World Reading Link Imagine that you are practicing free throws and you take 100 practice shots. Theoretically, you could make all 100 shots. In actuality, however, you know you will not make all of the shots. Chemical reactions also have theoretical and actual outcomes. 

Theoretical and Actual Yields In many of the stoichiometric calculations you have performed, you have calculated the amount of product produced from a given amount of reactant. The answer you obtained is the theoretical yield of the reaction. The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant. 

A chemical reaction rarely produces the theoretical yield of product. A chemist determines the actual yield of a reaction through a careful experiment in which the mass of the product is measured. 

The actual yield is the amount of product produced when the chemical reaction is carried out in an experiment. 

Given the actual quantity of product, determine the percent yield for a reaction. 

In our problems up to now, we assumed that all of the reactants changed completely to product. Thus, we have calculated the amount of product as the maximum quantity possible, or 100%. While this would be an ideal situation, it does not usually happen. As we carry out a reaction and transfer products from one container to another, some product is usually lost. In the lab as well as commercially, the starting materials may not be completely pure, and side reactions may use some of the reactants to give unwanted products. Thus, 100% of the desired product is not actually obtained. 

When we do a chemical reaction in the laboratory, we measure out specific quantities of the reactants. We calculate the theoretical yield for the reaction, which is the amount of product (100%) we would expect if all the reactants were converted to the desired product. When the reaction ends, we collect and measure the mass of the product, which is the actual yield for the product. Because some product is usually lost, the actual yield is less than the theoretical yield. Using the actual yield and the theoretical yield for a product, we can calculate the percent yield. 

For your chemistry class party, you have prepared cookie dough from a recipe that makes 5 dozen cookies. You place dough for 12 cookies on a baking sheet and place it in the oven. But then the phone rings and you run to answer it. While you are talking, the cookies on the baking sheet bum and you have to throw them out. You proceed to prepare four more baking sheets with 12 cookies each. If the rest of the cookies are edible, what is the percent yield of cookies you provide for the chemistry party  

The theoretical yield of cookies is 5 dozen or 60 cookies, which is the maximum or 100% of the possible number of cookies. The actual yield is 48 edible cookies, which is 

If we know the chemical equation and the amounts of reactants, we can calculate the theoretical yield of that reaction. But in reality, the yield depends on many other factors also. Most of the time in synthesis reactions, even in your own lab experiments, you probably noticed that the actual yield is lower than the theoretical yield. The percent yield denotes the amount of actual yield in terms of the theoretical yield. The formula to find the percent yield is given below  

A student conducted the above reaction in a lab as apart of her research assignment. She used 138 g of sodium nitrite, with excess of hydrogen chloride. What is the percent yield of HNO, if the actual yield of HNOj was 61.1 g  

you should find the munber of moles of NaNO. Since she used 138 g, the number of moles of NaNO is 2 (MoLwt of NaNOj is 69 g/mol). Since the ratio of formation of HNO is 1 1 with respect to NaNO, theoretically 2 moles of HNOj should be form. Two moles of HNO correspond to 94 g. But actually, only 61.1 g of HNOj was formed. Now it is just a matter of plug and chug in the percent yield formula. 

In this experiment, the actual yield was not high (i.e., only 65% of the theoretically predicted yield) as expected. 

Match the following reactions with the appropriate type of reaction 

In almost any reaction, you re going to produce less of the product than you expected. You may produce less because most reactions are equilibrium reactions (see Chapter 8), because of sloppy technique or impure reactants, or because some other conditions come into play. Chemists can get an idea of the efficiency of a reaction by calculating the percent yield for the reaction using this equation  

The actual yield is how much of the product you get when you carry out the reaction. The theoretical yield is how much of the product you calculate you ll get. The ratio of these two yields gives you an idea about how efficient the reaction is. 

For the reaction of rust to iron (see the preceding section), your theoretical yield is 699.5 grams of iron suppose your actual yield is 525.0 grams. Therefore, the percent yield is 

A percent yield of about 75 percent isn t too bad, but chemists and chemical engineers would rather see 90+ percent. One industrial plant using the Haber reaction has a percent yield of better than 99 percent. Now that s efficiency  

In a chemical reaction, you normally run out of one of the reactants and have some others left over. The reactant you run out of first is called the limiting reactant, and it determines the amount of product formed. (In some of the problems sprinkled throughout this chapter, 1 tell you which reactant is the limiting one by saying you have an excess of the other reactant(s).) 

In the preceding problems, the amount of product calculated based on the limiting-reactant concept is the maximum amount of product that could be formed from the given amount of reactants. This maximum amount of product formed is called the theoretical yield. However, rarely is the amount that is actually formed (the actual yield) the same as the theoretical yield. Normally it is less. There are many reasons for this, but the principal reason is that most reactions do not go to completion they establish an equilibrium system (see the Equilibrium chapter for a discussion of chemical equilibrium). For whatever reason, not as much as expected is formed. The efficiency of the reaction can be judged by calculating the percent yield. The percent yield (% yield) is the actual yield divided by the theoretical yield, and the result is multiplied by 100% to generate percentage  

Consider the problem in which it was calculated that 60.8 g NH3 could be formed. Suppose that reaction was carried out, and only 52.3 g NH3 was formed. What is the percent yield  

Let s consider another percent yield problem in which a 25.0-g sample of calcium oxide is heated with excess hydrogen chloride to produce water and 37.5 g of calcium chloride. What is the percent yield of calcium chloride  

Note All the units except % must cancel. This includes canceling g CaCl2 with g CaCl2, not simply g. 

For example, if the reaction considered in Example 9.8 actually gave 6.63 g of nitrogen instead of the predicted 10.6 g, the percent yield of nitrogen would be 6.63. g  

The maximum amount of a given product that can be formed when the limiting reactant is completely consumed 

Now set up and solve the problem completely. The setup is exactly the same as it would be with grams and moles, except that the unit g becomes mg and the unit mol becomes mmol. 

This reaction is used in the laboratory and in electroplating factories to find the nickel ion concentration of a solution. 

Lean H Now You have reached a critical point in your study of chemistry. Students usually have no difficulty changing grams to moles, moles of one substance to moles of another, and moles to grams when those operations are presented as separate problems. When the operations are combined in a single problem, however, trouble sometimes follows. It doesn t help when, as in the following sections, many other ideas are added. In other words, you will not understand the following concepts unless you have already mastered this one. 

Just studying the foregoing examples will not give you the skill that you need. That comes only with practice. You should solve some end-of-chapter problems now—before you begin the next section. We strongly recommend that, at the very least, you solve Problems 1, 13, 17, 21, 23, and 27 at this time. If you understand and can solve these problems, you are ready to proceed to the next section. 

Co to http //now.brookscole.com/ cracoliceSe and click Chemistry Interactive for the module Weight Relations in Chemical Reactions. 

Most chemical reactions do not produce the predicted amount of product. Although your work so far with stoichiometric problems may have led you to believe that chemical reactions proceed according to the balanced equation and always produce the calculated amount of product, it s not true. Many reactions stop before all the reactants are used up, so less product is formed than expected. Also, products other than those expected sometimes form from competing chemical reactions, thereby reducing the amount of the desired product. 

Percent yield tells you how efficient a chemical reaction is in producing the desired product. 

Aspirin (CC1H8O4) can be made from salicylic acid (C HgOj) and acetic anhydride ( 411 03). Suppose you mix 13.2 g of salicylic acid with an excess of acetic anhydride and obtain 5.9 g of aspirin and some water. Calculate the percent yield of aspirin in this reaction. 

Calculate the percent yield for each chemical reaction based on the data provided. 

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The efficiency of a synthetic transformation is normally expressed as a percent yield or percentage of the theo retical yield Theoretical yield IS the amount of prod uct that could be formed if the reaction proceeded to completion and did not lead to any products other than those given in the equation... 

G-5 Aliphatic Petroleum Resins. Carbocationic polymerization of C-5 feedstreams has been accomptished with various Friedel-Crafts catalyst systems. Table 3 compares the efficiencies of selected Lewis acids ia the polymerization of a typical C-5 stream containing 43 wt % C-5—C-6 diolefias and 47 wt % C-5—C-6 olefins (20). Based on weight percent yield of resia at equimolar coaceatratioas of catalyst (5.62 mmol/100 g), efficieacy follows AICI3 AlBr3 > BF3etherate-H20 > TiCfy > SnCl. The most commonly used catalyst in petroleum resin synthesis is AlCl. ... 

Pig. 4. Synthesis of tigemonam where Boc is the /-butoxycarbonyl group PyrH" is the pyridinium ion and the numbers given are percent yield of product. 

The Wacker process for the oxidation of ethylene to acetaldehyde with PdCb/CuCb at 100°C (212°F) with 95 percent yield and 95 to 99 percent conversion per pass. 

A major disadvantage of this system is the limitation of the single-pass gas-chlorination phase. Unless increased pressure is used, this equipment is unable to achieve higher concentrations of chlorine as an aid to a more complete and controllable reaction with the chlorite ion. The French have developed a variation of this process using a multiple-pass enrichment loop on the chlorinator to achieve a much higher concentration of chlorine and thereby quickly attain the optimum pH for maximum conversion to chlorine dioxide. By using a multiple-pass recirculation system, the chlorine solution concentrates to a level of 5-6 g/1. At this concentration, the pH of the solution reduces to 3.0 and thereby provides the low pH level necessary for efficient chlorine dioxide production. A single pass results in a chlorine concentration in water of about 1 g/1, which produces a pH of 4 to 5. If sodium chlorite solution is added at this pH, only about 60 percent yield of chlorine dioxide is achieved. The remainder is unreacted chlorine (in solution) and... 

The theoretical yield is the maximum amount of product that can be obtained. In calculating the theoretical yield, it is assumed that the limiting reactant is 100% converted to product. In the real world, that is unlikely to happen. Some of the limiting reactant may be consumed in competing reactions. Some of the product may be lost in separating it from the reaction mixture. For these and other reasons, the experimental yield is ordinarily less than the theoretical yield. Put another way, the percent yield is expected to be less than 100% ... 

Peicent yield. If you started with 20 popcorn kernels, but only 16 of them popped, the percent yield of popcorn from this "reaction would be 16/20 x 100% = 80%. 

Click Coached Problems for a self-study module on percent yield. 

Suppose that in part (a) the percent yield is 78.2%. How many grams of Sbl3 are formed ... 

The thin backweb, typically 0.2 mm thick with a porosity of 60 percent yields excellent electrical resistance values of 50 rafl cm2, permitting further optimization of high-performance battery constructions. These require very thin electrodes due to the overproportionally increasing polarization effects at higher current densities and consequently also low distances most modern versions have separators only 0.6 mm thick. Such narrow spacings enforce microporous separation ... 

TABLE 7.7 Percent Yield of Methylene and Ether Linkages of 2-Hydroxylmethyl-4,6-Dimethylphenol Self-reaction, 1 1 with 2,4-Xylenol, and 1 1 with 2,6-Xylenol... 

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 ... 

When we calculate a percent yield, the amounts can be expressed in either moles or mass, provided both the actual and theoretical amounts are in the same units. Example shows how to use Equation. ... 

According to Example, it is possible to make 443 g of geranyl formate from 375 g of geraniol. A chemist making geranyl formate for the preparation of a perfume uses 375 g of starting material and collects 417 g of purified product. What is the percent yield of this synthesis ... 

To calculate a percent yield, we need to compare the actual amount obtained in the synthesis with the theoretical amount that could be produced, using Equation. ... 

The theoretical yield of geranyl formate is 443 g. This is the amount of product that would result from complete conversion of geraniol into geranyl formate. The actual yield, 417 g, is the quantity of the desired product that the chemist collects. The percent yield is their ratio multiplied by 100% ... 

Yields can never exceed the theoretical amount, so a percent yield always must be less than 100%. A percent yield between 90% and 100% is quite reasonable. 

C04-0010. If the reaction described in Example produces 195 kg of HCN from 175 kg of methane reactant, what is the percent yield ... 

If the percent yield of a reaction is already known, we can calculate how much of a product to expect from a synthesis that uses a known amount of starting material. For example, the Haber synthesis of ammonia stops when 13% of the starting materials have formed products. Knowing this, how much ammonia could an industrial producer expect to make from 2.0 metric tons of molecular hydrogen First, calculate the theoretical yield ... 

This is a two-stage probiem that requires a yieid caicuiation and a conversion among molar amounts of products and reactants. This is typicai of yieid caicuiations. Use the percent yield to determine the theoreticai yieid, and then use the stoichiometric ratios to caicuiate masses of starting materials. A flowchart summarizes the stepwise strategy. 

First, rearrange the equation for percent yield to find the theoretical yield that will give an actual yield of... 

C04-0012. When heated, potassium chlorate decomposes to potassium chloride and gaseous molecular oxygen 2 KCIO3 2KC1 + 3 O2 What is the theoretical yield of oxygen when 5.00 g of potassium chlorate decomposes Calculate the percent yield if a 5.00-g sample gives 1.84 g O2 on decomposition. Give possible reasons why the actual yield is less than the theoretical yield. 

A reaction that is carried out under limiting reactant conditions nevertheless has a yield that generally will be less than 100%. The reasons why reactions yield less than the theoretical amounts, given in Section 4-1. apply to all reactions. When a reaction operates under limiting reactant conditions, we calculate the theoretical yield assuming that the limiting reactant will be completely consumed. We then determine the percent yield as described in Section 4A. Example shows how to do this. 

The problem asks for a yield, so we identify this as a yield problem. In addition, we recognize this as a limiting reactant situation because we are given the masses of both starting materials. First, identify the limiting reactant by working with moles and stoichiometric coefficients then carry out standard stoichiometry calculations to determine the theoretical amount that could form. A table of amounts helps organize these calculations. Calculate the percent yield from the theoretical amount and the actual amount formed. 

The final amount in this table represents the theoretical yield. To determine the percent yield, we need to have actual and theoretical amounts in the same units. In this example, we work with moles ... 

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