Chemical quantities stoichiometric calculations

Stoichiometry is the study of the relative quantities of reactants and products in chemical reactions. Stoichiometric calculations are used for many purposes. One purpose is determining how much of a reactant is needed to carry out a reaction. This kind of knowledge is useful for any chemical reaction, and it can even be a matter of life or death. 

Stoichiometry is the quantitative study of products and reactants in chemical reactions. Stoichiometric calculations are best done by expressing both the known and unknown quantities in terms of moles and then converting to other units if necessary. A limiting reagent is the reactant that is present in the smallest stoichiometric amount. It limits the amount of product that can be formed. The amount of product obtained in a reaction (the actual yield) may be less than the maximum possible amount (the theoretical yield). The ratio of the two is expressed as the percent yield. 

Stoichiometry is the quantitative study of products and reactants in chemical reactions. Stoichiometric calculations are best done by expressing both the known and unknown quantities in terms of moles and then converting to other units if necessary. 

Gases in Chemical Reactions Stoichiometric calculations involving gases are similar to those that do not involve gases in that the coefficients in a balanced chemical equation provide conversion factors among moles of reactants and products in the reaction. For gases, the amoimt of a reactant or product is often specified by the volume of reactant or product at a given temperature and pressiue. The ideal gas law is then used to convert from these quantities to moles of reactant or product. Alternatively at standard temperature and pressure, volume can be converted directly to moles with ffie equality ... 

A common feature of both these methods is that the quantity of treatment chemical can be calculated from stoichiometric relationships in the reactions involved. This is not so with conventional inhibitor treatments. With these the concentration of inhibitive chemicals can only be determined on the basis of experimental laboratory studies, service trials and overall practical experience. 

The equation for a chemical reaction speaks in terms of molecules or of moles. It contains the basis for stoichiometric calculations. However, in the laboratory a chemist measures amounts in such units as grams and milliliters. The first step in any quantitative calculation, then, is to convert the measured amounts to moles. In mole units, the balanced reaction connects quantities of reactants and products. Finally, the result is expressed in the desired units (which may not necessarily be the same as the original units). 

Since chemical reactions involve combination of atoms or molecules to form new compounds or decomposition of compounds to form simpler ones, it is most convenient m stoichiometric calculations to employ molecular units rather than weight units, This particular kind of unit is called a mole and represents the quantity of substance numerically equal to its molecular weight This weight quantity may be based on any system of weight units desired, and it is thus necessary to designate this basis by referring co pound moles, giam moles, etc. 

Stoichiometry establishes the quantities of reactants (used) and products (obtained) based on a balanced chemical equation. With a balanced equation, you can compare reactants and products, and determine the amount of products that might be formed or the amount or reactants needed to produce a certain amount of a product. However, when comparing different compounds in a reaction, you must always compare in moles (i.e., the coefficients). The different types of stoichiometric calculations are summarized in Figure 5.1. 

Calculations of volumetric analysis ordinarily consist of transforming the quantity of titrant used (in chemical units) to a chemically equivalent quantity of analyte (also in chemical units) through use of a stoichiometric factor. Use chemical formulas (NO CALCULATIONS REQUIRED) to express this ratio for calculation of the percentage of (a) hydrazine in rocket fuel by titration with standard iodine. Reaction ... 

Including stoichiometric calculations within the purview of student skills as the heart of chemical knowledge rather than resorting to only arithmetic conversions of simpler quantities. 

The utility of this formulation for simple stoichiometric calculations is illustrated by Example 1.2, in which the quantities appropriate to each species are arranged underneath the formula of that species in the chemical equation. Its utility in other applications will be demonstrated in later parts of the book. 

Stoichiometry is the calculation of the quantities of reactants or products involved in a chemical reaction. The importance of stoichiometry can be appreciated by visualizing industrial operations that process hundreds or thousands of tons of chemicals per day. The economics of many chemical manufacturing processes are such that an unnecessary excess of only a percent or so of a reacting chemical can lead to waste that can make the operation unprofitable. Obtaining accurate values in chemical analysis, which may need to be known to within about a part per thousand, often involves highly exacting stoichiometric calculations. 

In a chemical reaction, there is a definite ratio between the number of moles of a particular reactant or product and the number of moles of any other reactant or product. These ratios are readily seen by simply examining the coefficients in front of the reaction species in the chemical equation. Normally, a stoichiometric calculation is performed to relate the quantities of only two of the reaction participants. The objective may be to determine how much of one reactant will react with a given quantity of another reactant Or, a particular quantity of a product may be desired, so that it is necessary to calculate the quantity of a specific reactant needed to give the amount of product To perform stoichiometric calculations involving only two reaction participants, it is necessary only to know the relative number of moles of each and their molar masses. The most straightforward type of stoichiometric calculation is the mole ratio metiiod defined as follows ... 

In Chapter 2, we described a chemical equation as a representation of what occurs when molecules react. We will now study chemical equations more closely to answer questions about the stoichiometry of reactions. Stoichiometry (pronounced stoy-key-om -e-tree ) is the calculation of the quantities of reactants and products involved in a chemical reaction. It is based on the chemical equation and on the relationship between mass and moles. Such calculations are fundamental to most quantitative work in chemistry. In the next sections, we will use the industrial Haber process for the production of ammonia to illustrate stoichiometric calculations. 

The measure of the change in chemical potential associated with reaction j is defined byAG = A/xO = 2 f= j It is emphasized that this quantity is calculated from the stoichiometric coefficients of the reaction and not from the number of moles of the different species transformed by the reaction. Nevertheless, the AG j is frequently... 

Many of the calculations of analytical chemistry involve quantities of materials that take part in chemical reactions. Therefore, stoichiometric calculations may be very important in analytical chemistry. The reader should refer back to stoichiometry in Chapter 5. 

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. 

Stoichiometry (from the Greek stoikeion—element) is the practical application of the law of multiple proportions. The stoichiometric equation for a chemical reaction states unambiguously the number of molecules of the reactants and products that take part from which the quantities can be calculated. The equation must balance. 

Calculate the stoichiometric quantities of reactants and products given the chemical equation. 

In the preceding sections, the stoichiometric relationships used to quantify the operation of chemical reactors were expressed in terms of extensive quantities (moles, molar flow rates, reaction extents, etc.) whose numerical values depend on the basis selected for the calculation. In most applications, it is convenient to define intensive dimensionless quantities that characterize the operation of chemical reactors and provide quick measures of the reactor performance. In this section, we define and discuss some common stoichiometric quantities used in reactor analysis. 

We have seen already that chemical equations are always written in terms of the numbers of particles involved. Whether we interpret them in terms of individual molecules or moles of molecules, the stoichiometric coefficients that balance a chemical equation refer to numbers of particles and not to masses. Usually, we can t measure the number of particles directly in the laboratory masses and volume of liquids are the quantities that are more likely to be measurable. Thus if we want to make quantitative calculations for a chemical reaction, fi-equently we need to convert between the measured value of a mass or volume and the desired value of a number of moles. Because such calculations are common and important, chemists have developed a standard approach to overcome this variable mismatch. Although you might think of this approach as an algorithm for solving a particular class of chemistry problems, it is instructive to understand its conceptual... 

The total requirements for Na2C03 and NaOH are 333 and 134 kg hr" . Note that the carbonate consumption is more than 3.5 times the stoichiometric quantity with NaOH, the factor approaches 10. This is a bit extreme because of the low level of impurities assumed in our salt. However, it is conunon for the excess quantities to exceed those theoretically required. With the solution strengths we have chosen, the total flow of treating chemical solution is about 3.6 m hr" or 90 tpd. Very precise calculation should recognize the 1 % increase in solution volume. With salts of lower quality, the increase in volume will become more important. The weight of solids precipitated is 97.2 kg hr" together with the insoluble impurities accompanying the salt, we have 167.7 kg hr" , or about 4 tpd of dry solids for disposal. 

The following thermochemical cycle has been proposed as a method to spUt water chemically into its constituent elements. Show that the net result of this process is the production of hydrogen and oxygen and that all reactants needed are regenerated in the necessary stoichiometric quantities. Using standard heat of formation data, calculate the heat of each reaction and the net reaction. 

We have seen how to calculate the theoretical voltage required for electrolysis. Equally important are calculations of the quantities of reactants consumed and products formed in an electrolysis. For these calculations, we will continue to use stoichiometric factors from the chemical equation, but another... 

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