Reactant excess reactants

What is meant by limiting reactant Excess reactant ... 

In industrial reactors you will rarely find exact stoichiometric amounts of materials used. To make a desired reaction take place or to use up a costly reactant, excess reactants are nearly always used. This excess material comes out together with, or perhaps separately from, the product—and sometimes can be used again. Even if stoichiometric quantities of reactants are used, but if the reaction is not complete or there are side reactions, the products will be accompanied by unused reactants as well as side products. In these circumstances some new definitions" must be understood ... 

Bread Peanut Butter Sandwiches Limiting Reactant Excess Reactant... 

In a single reaction (where selectivity is not a problem), the usual choice of excess reactant is to eliminate the component which is more difficult to separate in the downstream separation system. Alternatively, if one of the components is more hazardous (as is chlorine in this example), again we try to ensure complete conversion. 

The use of excess reactants, diluents, or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes the recycling of unwanted byproduct to the reactor can inhibit its formation at the source. If this can be achieved, it improves the overall use of raw materials and eliminates effluent disposal problems. Of course, the recycling does in itself reuse some of the other costs. The general tradeoffs are discussed in Chap. 8. 

A reartant A diffuses into a stagnant liquid film where the concentration of excess reactant B remains essentially constant at C q. At the inlet face the concentration is Making the material balance over a differential dz of the distance leads to the second-order diffusional equation,... 

To simplify calculations, but also by convention, the amount of excess reactant in a reaction is defined on the basis of the reaction going to completion for the limiting reactant. In the case of methane (CH4) burned with excess air, the volume of air needed to combust the methane is calculated as though there is complete combustion of the methane, converting it entirely to carbon dioxide and water. 

To a solution of vanillin in toluene is added nitroethane, butylamine and glacial acetic acid. The mixture is refluxed and the water of reaction is steadily azeotropically removed by distillation. After the theoretical amount of water is distilled out, distillation Is continued to remove excess reactants. The last trace of excess reactants is then removed at room temperature under a vacuum. The product is then triturated with a hydrocarbon solvent such as Skellysolve B and is thus obtained in a crystalline state. In general, however, it is preferred to dissolve the residue directly In toluene for use in the next step, without isolating the 1-(2-nitropropen-1-y I )-4-hydroxy-3-methoxy benzene. 

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

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

In situations such as this, a distinction is made between the excess reactant (Sb) and the limiting reactant, I2. The amount of product formed is determined (limited) by the amount of limiting reactant With 3.00 mol of 1 only 2.00 mol of Sbl3 is obtained, regardless of how large an excess of Sb is used. 

Often you will be given the amounts of two different reactants and asked to determine which is the limiting reactant, to calculate the theoretical yield of the product and to find how much of the excess reactant is unused. To do so, it helps to follow a systematic, four-step procedure. 

Calcium carbide, CaC2, reacts with water to form calcium hydroxide and the flammable gas ethyne (acetylene). This reaction was once used for lamps on bicycles, because the reactants are easily transported, (a) Which is the limiting reactant when 1.00 X 102 g of water reacts with 1.00 X 102 g of calcium carbide (b) What mass of ethyne can he produced (c) What mass of excess reactant remains after reaction is complete Assume that the calcium carbide is pure and that all the ethyne produced is collected. The chemical equation is... 

Self-Test M.2A (a) Identify the limiting reactant in the reaction 6 Na(l) + Al20 (s) — 2 Al(l) + 3 Na20(s) when 5.52 g of sodium is heated with 5.10 g of Al203. (b) What mass of aluminum can be produced (c) What mass of excess reactant remains at the end of the reaction ... 

Suppose that 2.00 L of propane gas, C3Hg, at 1.00 atm and 298 K is mixed with 5.00 L of oxygen gas at the same pressure and temperature and burned to form carbon dioxide gas and liquid water. Ignore the volume of water formed, and determine the final volume of the reaction mixture (including products and excess reactant) at 1.00 atm and 298 K if reaction goes to completion. 

Reactant Excessively mixture leaner or richer mixture... 

Attainment of abnormal reaction conditions, e.g. overpressure, over-temperature, segregation of reactants, excessive reaction rate, initiation of side reactions. 

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 ... 

In this example, oil is the limiting reactant. Excess hydrogen is easily recovered from a gas-phase reactor, so margarine manufacturers make the oil the limiting reactant to ensure complete conversion of oil into margarine. The excess H2 gas is recovered and used again in a subsequent reaction. 

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