Limiting Reactant Theoretical and Percentage Yields

EXERCISE 3.16 The British chemist Joseph Priestley prepared oxygen in 1774 by heating mer-cury(ll) oxide, HgO. Mercury metal is the other product. If 6.47 g of oxygen is collected, how many grams of mercury metal are also produced  

The main reaction of a charcoal grill is C(s) + 02(g) the statements below are incorrect Why  

Concentrated hydrochloric add was added to manganese dioxide in the beaker. Note the formation of yellowish-green gas (chlorine), which is depicted by molecular models. 

Often reactants are added to a reaction vessel in amounts different from the molar proportions given by the chemical equation. In such cases, only one of the reactants may be completely consumed at the end of the reaction, whereas some amounts of other reactants will remain unreacted. The limitii reactant (or limiting reagent) is the reactant that is entirely consumed when a reaction goes to completion. A reactant that is not completely consumed is often referred to as an excess reactant. Once one of the reactants is used up, the reaction stops. This means that  

The moles of product are always determined by the starting moles of limiting 

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STRATEGY First, the limiting reactant must be identified (Toolbox M.l). This limiting reactant determines the theoretical yield of the reaction, and so we use it to calculate the theoretical amount of product by Method 2 in Toolbox L.l. The percentage yield is the ratio of the mass produced to the theoretical mass times 100. Molar masses are j calculated using the information in the periodic table inside the front cover of this i book. 

In the problem above, the amount of product calculated based upon the limiting reactant concept is the maximum amount of product that will form from the specified amounts of reactants. This maximum amount of product is 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 one is that most reactions do not go to completion they establish an equilibrium system (see Chapter 14 for a discussion on chemical equilibrium). For whatever reason, not as much product as expected is formed. We can judge the efficiency of the reaction by calculating the percent yield. The percent yield (% yield) is the actual yield divided by the theoretical yield and the resultant multiplied by 100 in order to generate a percentage ... 

In Examples 10.2 and 10.3, we determined the maximum amount of product that could be formed from the given amounts of reactants. This is the amount of product that could be obtained if 100% of the limiting reactant were converted to product and if this product could be isolated from the other components in the product mixture without any loss. This calculated maximum yield is called the theoretical yield. Often, somewhat less than 100% of the limiting reactant is converted to product, and somewhat less than the total amount of product is isolated from the mixture, so the actual yield of the reaction, the amount of product that one actually obtains, is less than the theoretical yield. The actual yield is sometimes called the experimental yield. The efficiency of a reaction can be evaluated by calculating the percent yield, the ratio of the actual yield to the theoretical yield expressed as a percentage. 

The amount of a product formed when the limiting reactant is completely consumed is called the theoretical yield of that product. In Example 3.17, 10.6 grams of nitrogen represents the theoretical yield. This is the maximum amount of nitrogen that can be produced from the quantities of reactants used. Actually, the amount of product predicted by the theoretical yield is seldom obtained because of side reactions (other reactions that involve one or more of the reactants or products) and other complications. The actual yield of product is often given as a percentage of the theoretical yield. This is called the pereent yield ... 

One reactant limits the product of a reaction. Comparing the actual and theoretical yields helps chemists determine the reaction s efficiency. j andP Key Terms limiting reactant Illy ncci ercenti theoretical yield ml wl,.vM lUICIIIId age Yield percentage yield... 

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 ... 

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