Limiting Reactant 92 Reaction Yield

Molar Ratios from Balanced Equations Limiting Reactants Reaction Yields... 

Reaction stoichiometry Limiting reactants Percent yield... 

You are given the mass of the reactants, carbon and copper(I) oxide, as well as the mass of copper formed by the reaction. You are asked to find the limiting reactant, theoretical yield, and percent yield. 

Limiting Reactant, Theoretical Yield, and Percent Yield The limiting reactant in a chemical reachon is the reactant that limits the amount of product that can be made. The theoretical yield in a chemical reaction is the amount of product that can be made based on the amount of the limihng reactant. The actual yield in a chemical reachon is the amount of product actually produced. The percent yield in a chemical reaction is the actual yield divided by theorehcal yield times 100%. 

A chemist allows 14.4 g of CaO and 13.8 g of CO2 to react. When the reaction is finished, the chemist collects 19.4 g of CaCOs. Determine the limiting reactant, theoretical yield, and percent yield for the reaction. 

Let s return to our pizza analogy to understand three more important concepts in reaction stoichiometry limiting reactant, theoretical yield, and percent yield. Recall our pizza recipe from Section 4.2 ... 

The combustion of liquid ethanol (C2H5OH) produces carbon dioxide and water. After 4.62 mL of ethanol (density = 0.789 g/mL) is allowed to bum in the presence of 15.55 g of oxygen gas, 3.72 mL of water (density = 1.00 g/mL) is collected. Determine the limiting reactant, theoretical yield of H2O, and percent yield for the reaction. (Hint Write a balanced equation for the combustion of ethanol.)... 

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% ... 

It is concluded [634] that, so far, rate measurements have not been particularly successful in the elucidation of mechanisms of oxide dissociations and that the resolution of apparent outstanding difficulties requires further work. There is evidence that reactions yielding molecular oxygen only involve initial interaction of ions within the lattice of the reactant and kinetic indications are that such reactions are not readily reversed. For those reactions in which the products contain at least some atomic oxygen, magnitudes of E, estimated from the somewhat limited quantity of data available, are generally smaller than the dissociation enthalpies. Decompositions of these oxides are not, therefore, single-step processes and the mechanisms are probably more complicated than has sometimes been supposed. 

The limiting reactant in a reaction is the reactant that governs the maximum yield of product. A limiting reactant is like a part in short supply in a motorcycle factory. Suppose there are eight wheels and seven motorcycle frames. Because each frame requires two wheels, there are enough wheels for only four motorcycles, so the wheels play the role of the limiting reactant. When all the wheels have been used, three frames remain unused, because they were present in excess. 

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. 

J 2 Identify the limiting reactant of a reaction and use the limiting reactant to calculate the yield of a product and the... 

Reactor Performance Measures. There are four common measures of reactor performance fraction unreacted, conversion, yield, and selectivity. The fraction unreacted is the simplest and is usually found directly when solving the component balance equations. It is a t)/oo for a batch reaction and aout/ciin for a flow reactor. The conversion is just 1 minus the fraction unreacted. The terms conversion and fraction unreacted refer to a specific reactant. It is usually the stoichiometrically limiting reactant. See Equation (1.26) for the first-order case. 

Phase transfer catalysts these have been around for about 40 years and were developed as a means of increasing the rates and yields of reactions in which the reactants are in two separate phases. In these cases poor mass transport often limits the reaction. Phase transfer catalysts act by transporting the reactants from one phase into another, thus overcoming mass-transport limitations. 

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. 

C04-0041. Several examples of chemical reasoning are introduced in this chapter. Write out the reasoning steps that you will follow in (a) balancing a chemical equation (b) identifying the limiting reactant (c) determining whether a precipitate forms and (d) computing a reaction yield. 

The simplest way to calculate the energy yield is to multiply the thermodynamic drive by the mass of the limiting reactant, the reactant that will be first exhausted from the fluid as the reaction proceeds. Figure 22.8 shows energy yields calculated in this way, for the various metabolisms considered. An energy yield calculated in this manner is approximate, since the reaction s thermodynamic drive does not... 

Taking sulfide oxidation (Reaction 22.19) as an example, when the fluid mixture reaches 25 °C, there are about 5 mmol of H2S(aq) and 0.6 mmol of 02(aq) in the unreacted fluid, per kg of vent water. The 02(aq) will be consumed first, after about 0.3 mmol of reaction turnover, since its reaction coefficient is two it is the limiting reactant. The thermodynamic drive for this reaction at this temperature is about 770 kJ mol-1. The energy yield, then, is (0.3 x 10-3 mol kg-1) x (770 x 103 J mol-1), or about 230 J kg-1 vent water (Fig. 22.8). In reality, of course, this entire yield would not necessarily be available at this point in the mixing. If some of the 02(aq) had been consumed earlier, or is taken up by reaction with other reduced species, less of it, and hence less energy would be available for sulfide oxidation. 

The limiting reactant is P4, and 111 gPCl3 should be produced. This is the theoretical yield. The actual yield is 104gPCl3. Thus, the percent yield of the reaction is... 

The HTE characteristics that apply for gas-phase reactions (i.e., measurement under nondiffusion-limited conditions, equal distribution of gas flows and temperature, avoidance of crosscontamination, etc.) also apply for catalytic reactions in the liquid-phase. In addition, in liquid phase reactions mass-transport phenomena of the reactants are a vital point, especially if one of the reactants is a gas. It is worth spending some time to reflect on the topic of mass transfer related to liquid-gas-phase reactions. As we discussed before, for gas-phase catalysis, a crucial point is the measurement of catalysts under conditions where mass transport is not limiting the reaction and yields true microkinetic data. As an additional factor for mass transport in liquid-gas-phase reactions, the rate of reaction gas saturation of the liquid can also determine the kinetics of the reaction [81], In order to avoid mass-transport limitations with regard to gas/liquid mass transport, the transfer rate of the gas into the liquid (saturation of the liquid with gas) must be higher than the consumption of the reactant gas by the reaction. Otherwise, it is not possible to obtain true kinetic data of the catalytic reaction, which allow a comparison of the different catalyst candidates on a microkinetic basis, as only the gas uptake of the liquid will govern the result of the experiment (see Figure 11.32a). In three-phase reactions (gas-liquid-solid), the transport of the reactants to the surface of the solid (and the transport from the resulting products from this surface) will also... 

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 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. 

Finally, the molar enthalpy of the reaction can be calculated as described, dividing A0bsH by the amount of substance of the limiting reactant converted to products (n see equation 10.6). Alternatively, the value of the quantum yield and equation 10.13 can be used (Q = AriC s and Q = Ap e). 

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