Chemical quantities Limiting reactant

When two substances react, they react in exact amounts. You can determine what amounts of the two reactants are needed to react completely with each other by means of mole ratios based on the balanced chemical equation for the reaction. In the laboratory, precise amounts of the reactants are rarely used in a reaction. Usually, there is an excess of one of the reactants. As soon as the other reactant is used up, the reaction stops. The reactant that is used up is called the limiting reactant. Based on the quantities of each reactant and the balanced chemical equation, you can predict which substance in a reaction is the limiting reactant. 

The total quantity of reactant is limited to 5.000 g. If either reactant is in excess, the amount in excess will be wasted, because it cannot be used to form product. Thus, we obtain the maximum amount of product when neither reactant is in excess (i.e., when there is a stoichiometric amount of each present). The balanced chemical equation for this reaction, 2 KI + Pb (N03 )2 -> 2 KN03 + Pbl2, shows that... 

In Chapter 3 we covered the principles of chemical stoichiometry the procedures for calculating quantities of reactants and products involved in a chemical reaction. Recall that in performing these calculations, we first convert all quantities to moles and then use the coefficients of the balanced equation to assemble the appropriate molar ratios. In cases in which reactants are mixed, we must determine which reactant is limiting, since the reactant that is consumed first will limit the amounts of products formed. These same principles apply to reactions that take place in solutions. However, there are two points about solution reactions that need special emphasis. The first is that it is sometimes difficult to tell immediately which reaction will occur when two solutions are mixed. Usually we must think about the various possibilities and then decide what will happen. The first step in this process always should be to write down the species that are actually present in the solution, as we did in Section 4.5. 

As indicated by Section 2.3, to determine the content of a batch reactor, or the outlet composition of a flow reactor, the composition of the initial state, or the inlet stream should be specified. So far, the initial contents of a batch reactor Nj 0), or the inlet stream of a flow reactor, Fj., have been specified. However, in some instances it is convenient to characterize the reactor feed in terms of stoichiometric parameters of the chemical reactions that take place in the reactor. Also, as illustrated in Example 2.1, it is useful to identify the limiting reactant. This section covers the common quantities used to characterize the reactor feed. 

A balanced chemical equation contains a large amount of information. What information is given in a balanced equation What is the theoretical yield for a reaction, and how does this quantity depend on the limiting reactant ... 

Notice that the molar density of key-limiting reactant A on the external surface of the catalytic pellet is always used as the characteristic quantity to make the molar density of component i dimensionless in all the species mass balances. effective is the effective intrapellet diffusion coefficient of species i. If there is only one chemical reaction, or one rate-limiting step in a multiple reaction sequence, that is characterized by nth-order irreversible kinetics, then the rate constant in the numerator of the Damkohler numbers is the same for each A -. Hence, kj is written as k , which signifies that has units of (volume/mole)" /time for... 

Notice that the molar density of key-limiting reactant A on the external surface of the catalytic pellet is always used as the characteristic quantity to make the molar density of component i dimensionless in all the component mass balances. This chapter focuses on explicit numerical calculations for the effective diffusion coefficient of species i within the internal pores of a catalytic pellet. This information is required before one can evaluate the intrapellet Damkohler number and calculate a numerical value for the effectiveness factor. Hence, 50, effective is called the effective intrapellet diffusion coefficient for species i. When 50, effective appears in the denominator of Ajj, the dimensionless scaling factor is called the intrapellet Damkohler number for species i in reaction j. When the reactor design focuses on the entire packed catalytic tubular reactor in Chapter 22, it will be necessary to calcnlate interpellet axial dispersion coefficients and interpellet Damkohler nnmbers. When there is only one chemical reaction that is characterized by nth-order irreversible kinetics and subscript j is not required, the rate constant in the nnmerator of equation (21-2) is written as instead of kj, which signifies that k has nnits of (volume/mole)"" per time for pseudo-volumetric kinetics. Recall from equation (19-6) on page 493 that second-order kinetic rate constants for a volnmetric rate law based on molar densities in the gas phase adjacent to the internal catalytic surface can be written as... 

Analyze We are given a chemical equation and the quantity of the limiting reactant (25.0 g of CgHi2). We are asked to calculate the theoretical yield of a product H2CgHg04 and the percent yield if only 33.5 g of product is obtained. 

QUANTITATIVE INFORMATION FROM BALANCED EQUATIONS AND LIMITING REACTANTS (SECTIONS 3.6 AND 3.7) The mole concept can be used to calculate the relative quantities of reactants and products in chemical reactions. The coefficients in a balanced equation give the relative numbers of moles of the reactants and products. To calculate the number of grams of a product from the number of grams of a reactant, first convert grams of reactant to moles of reactant. Then use the coefficients in the balanced equation to convert the nmnber of moles of reactant to moles of product Finally, convert moles of product to grams of product... 

When we carry out chemical reactions, the available supply of one reactant is often exhausted before the other reactants. As soon as we run out of one of the reactants, the reaction stops. The reactant completely consumed has determined how far the reaction can go, limiting the quantity of product produced. We say that the reactant completely consumed in the reaction is the limiting reactant. 

In many chemical processes, the quantities of the reactants used are such that one reactant is in excess. The amount of the product(s) formed in such a case depends on the reactant that is not in excess. This reactant is called the limiting reactant—it limiting reactant limits the amount of product that can be formed. 

In the laboratory or chemical plant, we work with much larger quantities than the few molecules of the preceding example. Therefore, we must learn to deal with limiting reactants using moles. The ideas are exactly the same, except that we are using moles of molecules instead of individual molecules. For example, suppose 25.0 kg of nitrogen and 5.00 kg of hydrogen are mixed and reacted to form ammonia. How do we calculate the mass of ammonia produced when this reaction is run to completion (until one of the reactants is completely consumed) ... 

The quantity of product that is calculated to be formed when all the limiting reactant reacts is termed the theoretical yield. The mass, volume or amount of a product actually obtained in a chemical reaction is termed the experimental yield. 

Given a chemical equation, or information from which it may be determined, and initial quantities of two or more reactants, (a) identify the limiting reactant, (b) calculate the theoretical yield of a specified product, assuming complete use of the limiting reactant, and (c) calculate the quantity of the reactant initially in excess that remains unreacted. 

In a similar way, the reactants in a chemical reaction do not always combine in quantities that allow each to be used up at exactly the same time. In many reactions, there is a limiting reactant that determines the amount of product that can be formed. When we know the qnantities of the reactants of a chemical reaction, we calculate the amount of product that is possible from each reactant if it were completely consumed. We are looking for the limiting reactant, which is the one that runs out first, producing the smaller amount of prodnct. 

Balancing a Chemical Equation 244 Calculating the Quantities of Reactants and Products in a Chemical Reaction 283 Calculating the Moles of Product from a Limiting Reactant 289... 

Chapter 9, Chemical Quantities in Reactions, describes the mole and mass relationships among the reactants and products and provides calculations of limiting reactants and percent yields. A section on Energy in Chemical Reactions completes the chapter. 

When a chemical reaction actually occurs, the reactants are usually not present in the exact stoichiometric ratios specified by the balanced chemical equation. The limiting reactant is the one that is available in the smallest stoichiometric quantity—it will be completely consumed in the reaction and it limits the amount of product that can be made. 

Chemists are interested in assessing the reaction efficiency and, most of the time, use the percent yield of a reaction calculated as the ratio of the acmal yield of a specific product divided by the theoretical yield (based on the limiting reactant). This tool is used to quantify the efficiency of a chemical reaction and to compare the expected product quantity to the actual one. 

The actual yield is defined as the quantity of product actually obtained from a synthesis in a laboratory or industrial chemical plant the theoretical yield is the maximum possible amount of product that can be formed when the limiting reactant is completely used. 

Five tasks must be performed in this problem (1) Represent the reaction by a chemical equation in which the names of reactants and products are replaced with formulas. (2) Balance the formula equation by inspection. (3) Determine the limiting reactant. (4) Calculate the theoretical yield of sodium nitrite based on the quantity of limiting reactant. (5) Use... 

If the quantities of both reactants are in exactly the correct ratio for the balanced chemical equation, then either reactant may be used to calculate the quantity of product produced. (If on a quiz or examination it is obvious that they are in the correct ratio, you should state that they are so that your instructor will understand that you recognize the problem to be a limiting quantities problem.)... 

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