Stoichiometry limiting reactants

Again, we use the standard approach to an equilibrium calculation. In this case the reaction is a precipitation, for which the equilibrium constant is quite large. Thus, taking the reaction to completion by applying limiting reactant stoichiometry is likely to be the appropriate approach to solving the problem. 

Consider the acid-base neutralization reaction and determine its effect on [HA] and [A ]. This step is a limiting reactant stoichiometry calculation. 000 (Section 3.6 and 3.7)... 

A limiting reactant is that reactant which is present in the smallest stoichiometric amount. In industrial reactions, the reactants are not necessarily supplied in the exact proportions demanded by the stoichiometry of the equation. Under these... 

Examine the stoichiometry of the chemical reaction, and identify the limiting reactant and excess reactants. 

A table of amounts is a convenient way to organize the data and summarize the calculations of a stoichiometry problem. Such a table helps to identify the limiting reactant, shows how much product will form during the reaction, and indicates how much of the excess reactant will be left over. A table of amounts has the balanced chemical equation at the top. The table has one column for each substance involved in the reaction and three rows listing amounts. The first row lists the starting amounts for all the substances. The second row shows the changes that occur during the reaction, and the last row lists the amounts present at the end of the reaction. Here is a table of amounts for the ammonia example ... 

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. 

We have data for the amounts of both starting materials, so this is a limiting reactant problem. Given the chemical equation, the first step in a limiting reactant problem is to determine the number of moles of each starting material present at the beginning of the reaction. Next compute ratios of moles to coefficients to identify the limiting reactant. After that, a table of amounts summarizes the stoichiometry. 

The quantitative treatment of a reaction equilibrium usually involves one of two things. Either the equilibrium constant must be computed from a knowledge of concentrations, or equilibrium concentrations must be determined from a knowledge of initial conditions and Kgq. In this section, we describe the basic reasoning and techniques needed to solve equilibrium problems. Stoichiometry plays a major role in equilibrium calculations, so you may want to review the techniques described in Chapter 4, particularly Section 4- on limiting reactants. 

Step 3 Calculate the theoretical yield of product in grams or moles, based on the limiting reactant using simple stoichiometry. 

A second-order reaction may typically involve one reactant (A -> products, ( -rA) = kAc ) or two reactants ( pa A + vb B - products, ( rA) = kAcAcB). For one reactant, the integrated form for constant density, applicable to a BR or a PFR, is contained in equation 3.4-9, with n = 2. In contrast to a first-order reaction, the half-life of a reactant, f1/2 from equation 3.4-16, is proportional to cA (if there are two reactants, both ty2 and fractional conversion refer to the limiting reactant). For two reactants, the integrated form for constant density, applicable to a BR and a PFR, is given by equation 3.4-13 (see Example 3-5). In this case, the reaction stoichiometry must be taken into account in relating concentrations, or in switching rate or rate constant from one reactant to the other. 

We let x represent the number of moles of H2 (the limiting reactant) consumed dining the process. The moles of each species may thus be related to the number of moles of H2 consumed, by stoichiometry. 

Reaction stoichiometry Limiting reactants Percent yield... 

Finally, base the stoichiometry of the reaction on the limiting reactant ... 

In this chapter, you learned how to balance simple chemical equations by inspection. Then you examined the mass/mole/particle relationships. A mole has 6.022 x 1023 particles (Avogadro s number) and the mass of a substance expressed in grams. We can interpret the coefficients in the balanced chemical equation as a mole relationship as well as a particle one. Using these relationships, we can determine how much reactant is needed and how much product can be formed—the stoichiometry of the reaction. The limiting reactant is the one that is consumed completely it determines the amount of product formed. The percent yield gives an indication of the efficiency of the reaction. Mass data allows us to determine the percentage of each element in a compound and the empirical and molecular formulas. 

In the examples above, one reactant was present in excess. One reactant was completely consumed, and some of the other reactant would be left over. The reactant that is used up first is called the limiting reactant (L.R.). This reactant really determines the amount of product being formed. How is the limiting reactant determined You can t assume it is the reactant in the smallest amount, since the reaction stoichiometry must be considered. There are generally two ways to determine which reactant is the limiting reactant ... 

Stoichiometry problems (including limiting-reactant problems) involving solutions can be worked in the same fashion as before, except that the volume and molarity of the solution must first be converted to moles. 

The limiting reactant is what will ran out fust during the reaction, i.e. the reactant whose quantity is less than that defined by the stoichiometry of the reaction. Note that the fluid volume (VL) is generally a variable, i.e. a function of time. If the volume of the reaction mixture is constant, eq.(3.71) becomes... 

The limiting reactant is S02 and thus, by using die stoichiometry of the reaction, we have... 

The conversion of B can be evaluated by using the stoichiometry of the reaction. It is easy to show that if A is the limiting reactant,... 

Introduction and Orientation, Matter and Energy, Elements and Atoms, Compounds, The Nomenclature of Compounds, Moles and Molar Masses, Determination of Chemical Formulas, Mixtures and Solutions, Chemical Equations, Aqueous Solutions and Precipitation, Acids and Bases, Redox Reactions, Reaction Stoichiometry, Limiting Reactants... 

Because 3.12 mol H20 is required and 5.55 mol H20 is supplied, all the calcium carbide can react so the calcium carbide is the limiting reactant and water is present in excess, (b) The reaction stoichiometry implies that 1 mol CaC2 — 1 mol C2H2. It follows that the mass of ethyne (of molar mass 26.04 g-mol ) that can be produced is... 

Complex stoichiometry problems should be worked slowly and carefully, one step at a time. When solving a problem that deals with limiting reactants, the idea is to find how many moles of all reactants are actually present and then compare the mole ratios of those actual amounts to the mole ratios required by the balanced equation. That comparison will identify the reactant there is too much of (the excess reactant) and the reactant there is too little of (the limiting reactant). 

Using the Limiting Reagents (Stoichiometry) simulation (eChapter 3.6), perform the redox reaction between magnesium metal and oxygen gas by combining equal masses of both reactants. 

In the next section, we will study a two-reactant case in which the adjustment of the fresh feeds to balance the stoichiometry of the reaction is crucial from a plantwide perspective. In addition, the limiting reactant concept can be used to improve dynamic controllability. 

Several topics are suggested here that could be de-emphasized or eliminated, affording instructors time to explore biochemical topics more fully electron configuration, quantum numbers, atomic orbitals, the mole concept, limiting reactant and stoichiometry, organic nomenclature, and organic reactions by functional group. 

Two concepts are often used to describe the behaviour of a process involving reactions conversion and selectivity. Conversion is defined with respect to a particular reactant, and it describes the extent of the reaction that takes place relative to the amount that could take place. If we consider the limiting reactant, the reactant that would be consumed first, based on the stoichiometry of the reaction, the definition of conversion is straight forward ... 

The correct answer is (A). When you see two masses in a stoichiometry problem, you should be alerted that you are dealing with a limiting reactant problem. This problem will have two stages—the first is to determine the limiting reactant, and the second to determine the mass of the hydrogen gas. Before we do anything, we need to see the balanced equation for the reaction ... 

According to the balanced chemical equation, the ratio of lithium nitride to water is 1/3. The ratio of lithium nitride to water, based on the mole amounts calculated, is 0.14 0.32. Divide this ratio by 0.14 to get 1.0 2.3. For each mole of lithium nitride, there are only 2.3 mol water. However, 3 mol are required by stoichiometry. Therefore, water is the limiting reactant. 

You will use the concepts of stoichiometry and limiting reactants in the Chemistry Course Challenge. Ifyou have two reactants and you want to use up all of one reactant, which is the limiting reactant ... 

Assume that your friend has missed several chemistry classes and that she has asked you to help her prepare for a stoichiometry test. Unfortunately, because of other commitments, you do not have time to meet face to face. You agree to email your friend a set of point-form instructions on how to solve stoichiometry problems, including those that involve a limiting reactant. She also needs to understand the concept of percentage yield. Write the text of this email. Assume that your friend has a good understanding of the mole concept. 

If, however, 2.50 X 103 kilograms of methane is mixed with 3.00 X 103 kilograms of water, the methane will be consumed before the water runs out. The water will be in excess. In this case the quantity of products formed will be determined by the quantity of methane present. Once the methane is consumed, no more products can be formed, even though some water still remains. In this situation, because the amount of methane limits the amount of products that can be formed, it is called the limiting reactant, or limiting reagent. In any stoichiometry problem it is essential to determine which reactant is the limiting one to calculate correctly the amounts of products that will be formed. 

In doing stoichiometry calculations, we assume that reactions proceed to completion—that is, until one of the reactants is consumed. Many reactions do proceed essentially to completion. For such reactions it can be assumed that the reactants are quantitatively converted to products and that the amount of limiting reactant that remains is negligible. On the other hand, there are many chemical reactions that stop far short of completion. An example is the dimerization of nitrogen dioxide ... 

Notice that contains two tenns and they involve stoichiometry and the initial mole fraction of the limiting reactant. The parameter becomes important if the density of the reacting system is changing as the reaction proceeds. 

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