Reactant in excess

The equilibrium conversion can be increased by employing one reactant in excess (or removing the water formed, or both). b. Inerts concentration. Sometimes, an inert material is present in the reactor. This might be a solvent in a liquid-phase reaction or an inert gas in a gas-phase reaction. Consider the reaction system... 

Take the theoretical yield of the product and determine how much of the reactant in excess is used up in the reaction. Subtract that from the starting amount to find the amount left. 

L of Cl2 How many liters of the reactant in excess are present after reaction is complete Assume 100% yield and that all die gases are measured at die same temperature and pressure. 

To solve a limiting quantities problem in which the reactant in excess is not obvious, you should ... 

Step 2 Since there is 2.0 mol NaOH present but the HCI present would require 3.0 mol NaOH, there is not enough NaOH to react completely with the HCI. The NaOH is present in limiting quantity. Put it another way, after 2.0mol NaOH reacts with 2.0mo HCI, there will be l.Omol of HCI and O.Omol of NaOH left. After the 2.0 mol of each has reacted, the rest of the reaction is like the first example in this section There can be no further reaction. Once the reactant in limiting quantity is used up, the reactant in excess cannot react any more. 

A pseudo-order reaction proceeds with all but one of the reactants in excess. This ensures that the only concentration to change appreciably is that of the minority reactant. 

This little relationship shows that a pseudo-order rate constant k is not a genuine rate constant, because its value changes in proportion to the concentration of the reactant in excess (in this case, with [B]). 

Accordingly, we perform the kinetic experiment with a series of concentrations [B]0, the reactant in excess, and then plot a graph of k (as y ) against [B]0 (as V). The gradient will have a value of k2. 

Figure 8.15 The rate constant of a pseudo-order reaction varies with the concentration of the reactant in excess graph of k (as V) against [alkene]0 (as V). The data refer to the formation of a 1,2-diol by the dihydrolysis of an alkene with osmium tetroxide. The gradient of the graph yields k2, with a value of 3.2 x 10 2 dm3 mol-1 s-1...

In the examples above, we indicated that one reactant was present in excess. The other reactant is consumed and there would be some of the reactant in excess left over. The first reactant to react completely is the limiting reactant (reagent). This reactant really determines the amount of product formed. There are, in general, two ways to determine which reactant is the limiting reactant ... 

The reactions, in which molecularity and order are different due to the presence of one of the reactant in excess, are known as pseudo-order reactions. The word (pseudo) is always followed by order. For example, inversion of cane sugar is pseudo-first order reaction. 

Step 4 The reactant in excess is a strong acid or base. Thus, the excess amount results in the same amount of or OH". 

The Friedel-Crafts alkylation reaction usually does not proceed successfully with aromatic substrates having electron-attracting groups. Another limitation is that each alkyl group that is introduced increases the reactivity of the ring toward further substitution, so polyalkylation can be a problem. Polyalkylation can be minimized by using the aromatic reactant in excess. 

Many chemical and biocatalytic conversions involve reactions with an unfavorable equilibrium such as condensations.8 In both cases the Law of Mass Action will apply such that the removal of one species from the reaction mixture will shift the equilibrium position. This is particularly useful for biocatalytic conversions where the alternative approach of using a reactant in excess may have deleterious effects on the biocatalyst. 

Determine the flow rates for reactants in excess and not recycled, and include them in outlet streams. 

For the designer, understanding the mass balance of the plant is a key requirement that can be fulfilled only when the reactor/separation/recycle structure is analyzed. The main idea is that all chemical species that are introduced in the process (reactants, impurities) or are formed in the reactions (products and byproducts) must find a way to exit the plant or to be transformed into other species [4]. Usually, the separation units take care that the products are removed from the process. This is also valid for byproducts and impurities, although is some cases inclusion of an additional chemical conversion step is necessary [5, 6]. The mass balance of the reactants is more difficult to maintain, because the reactants are not allowed to leave the plant but are recycled to the reaction section. If a certain amount of reactant is fed to the plant but the reactor does not have the capacity of transforming it into products, reactant accumulation occurs and no steady state can be reached. The reaction stoichiometry sets an additional constraint on the mass balance. For example, a reaction of the type A + B —> products requires that the reactants A and B are fed in exactly one-to-one ratio. Any imbalance will result in the accumulation of the reactant in excess, while the other reactant will be depleted. In practice, feeding the reactants in the correct stoichiometric ratio is not trivial, because there are always measurement and control implementation errors. 

You now know how to use a balanced chemical equation to find the limiting reactant. Can you find the limiting reactant by experimenting You know that the limiting reactant is completely consumed in a reaction, while any reactants in excess remain after the reaction is finished. In Investigation 7-A, you will observe a reaction and identify the limiting reactant, based on your observations. 

Since chemical reactions usually occur with one or more of the reactants in excess, you often need to determine the limiting reactant before you carry out stoichiometric calculations. You can incorporate this step into the process you have been using to solve stoichiometric problems, as shown in Figure 7.7. 

Equation (99) relates the concentration gradient of the limiting reactant just upstream from the reaction sheet to the temperature and the concentration of the reactant in excess at the reaction sheet. Therefore, it contains all the essential information associated with the reaction-zone structure. The continuity conditions derived previously enable other gradients to be obtained from this result. For example, since 0 — 1 is of order jS Mownstream, we have (00/0( ) =o- = — (0Fi/0( ) =o-6/(Fi,oLei) to the lowest order. The corresponding one-reactant problem is readily... 

If two reactants participate in a reaction and one is considerably more expensive than the other, the usual practice is to feed the less expensive reactant in excess of the valuable one. This has the effect of increasing the conversion of the valuable reactant at the expense of the cost of the excess reactant and additional pumping costs. 

Separation processes are widely used in industry. Chanical conversions often run incompletely as dictated by the thermodynamic equilibrium or by the wish to obtain high selectivity, which may require relatively low corrveraon or the application of one of the reactants in excess Separation methods include distillation, crystallization, centrifiigation, extraction, adsorption and membrane techniques. 

The structural variations possible in R and R make it possible to vary the toughness and elasticity of the polyurethane adhesive. Polyesters or polyethers are prepared with terminal hydroxyl groups that can then be reacted with difunctional or polyfunctional isocyanates. Polyurethane prepolymers can be formed in Reaction 1, with the desired terminal group produced by using one or other of the diol and diisocyanate reactants in excess. 

A technique called the isolation method is often used to determine the order in a particular reactant by keeping the concentration of other reactants in excess. The method is repeated for all reactants and the complete rate law is obtained. A hidden danger is that occasionally the rate law is not the same in the extreme ranges of the concentrations of reactants. Different reaction mechanisms that yield different rate laws may occur under the extreme conditions. 

Does the reactant in excess affect the actual yield for a reaction If it does, explain how. 

Look for. siBipliflcetians. For example, if one of the reactants in excess, assume its concentration is constant. If the gas phase mole fraction of reactant is small, set e = 0-... 

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