Reaction stoichiometry limiting reactant

Reaction stoichiometry Limiting reactants Percent yield... 

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... 

Let s return to our pancake analogy to xmderstand two more concepts important in reaction stoichiometry limiting reactant and percent yield. Recall our pancake... 

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. 

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 ... 

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. 

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. 

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. 

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

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 ... 

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,... 

Detailed information on mechanistic aspects of the ligand oxidation reactions is limited by the fact that well-defined tractable kinetics is only found for systems so very dilute in the metal ion reactants that stoichiometric studies including isolation of reaction products have not yet been practicable. Some selected systems have, however, been studied in some detail, but at significantly higher metal ion concentrations than used for the kinetic studies. It is relevant to recall, however, that under such conditions the rate usually does not follow Eq. (1) and the stoichiometry does not conform to Eq. (2) with a value of n about 6. 

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... 

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 ... 

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. 

Figure 11.16, is key to the selectivity of the desired product R. Here the reaction stoichiometry controls the yield of R, If the reactant >4 is consumed before B (molar feed ratio of B o A Ygf, greater than 1), the only reaction for the remaining B is the undesirable reaction according to Eq. (11-4) which further converts R, The end result is a reduced yield of R. If, on the other hand, is less than 1, B runs out before A and the conversion of A is limited to low values. 

Atom efficiency the percentage yield (molar flow of the desired product divided by the molar flow of the limiting reactant, taking into account the stoichiometry of the reaction) multiplied by the atom economy. It could be used to replace yield and A E. Eor example, AE could be 100% and yield 5%, making this a not very green reaction. 

---