Stoichiometric calculations products

Roberts (1982) uses a fireball s heat production to calculate its final diameter. Roberts assumes that, at the moment of maximal fireball size, the total increase in enthalpy can be related to the initial mass ratio of fuel to air. If R = mjttif for a stoichiometric mixture, the enthalpy rise (//) can be approximated by... 

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). 

Stoichiometric calculations of the amount of product formed in a reaction are based on an ideal view of the world. They suppose, for instance, that all the reactants react exactly as described in the chemical equation. In practice, that might not be so. Some of the starting materials may be consumed in a competing reaction, a reaction taking place at the same time as the one in which we are interested and using some of the same reactants. Another possibility is that the reaction might not be complete at the time we make our measurements. A third possibility—of major importance in chemistry and covered in several chapters of this text—is that many reactions do not go to completion. They appear to stop once a certain proportion of the reactants has been consumed. 

The heat absorbed or given off by a reaction can be treated like a reactant or product in a stoichiometric calculation. 

You know that a balanced equation represents relationships between the quantities of reactants and products. For a reaction that takes place in a cell, stoichiometric calculations can also include the quantity of electricity produced or consumed. Stoichiometric calculations in electrochemistry make use of a familiar unit—the mole. 

All of the important stoichiometric calculations that relate the weights and volumes of starting materials to the weights and volumes of products typically involve just three simple steps. 

Stoichiometric calculations involving solutions of specified molar concentration are usually quite simple since the number of moles of a reactant or product is simply volume x molar concentration. 

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. 

You have learned how to do stoichiometric calculations, using balanced chemical equations to find amounts of reactants and products. In these calculations, you assumed that the reactants and products occurred in the exact molar ratios shown by the chemical equation. In real life, however, reactants are often not present in these exact ratios. Similarly, the amount of product that is predicted by stoichiometry is not always produced. 

You now know how to identify a limiting reactant. This allows you to predict the amount of product that will be formed in a reaction. Often, however, your prediction will not accurately reflect reality. When a chemical reactions occurs—whether in a laboratory, in nature, or in industry—the amount of product that is formed is often different from the amount that was predicted by stoichiometric calculations. You will learn why this happens, and how chemists deal with it, in section 7.3. 

The percentage yield of a chemical reaction compares the mass of product obtained by experiment (the actual yield) with the mass of product determined by stoichiometric calculations (the theoretical yield). It is calculated as follows ... 

Stoichiometric calculations are used to determine the products of a chemical reaction. 

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. 

The applications of chemistry focus primarily on chemical reactions, and the commercial use of a reaction requires knowledge of several of its characteristics. A reaction is defined by its reactants and products, whose identities must be learned by experiment. Once the reactants and products are known, the equation for the reaction can be written and balanced and stoichiometric calculations can be carried out. Another very important characteristic of a reaction is its spontaneity. Spontaneity refers to the inherent tendency for the process to occur however, it implies nothing about speed. Spontaneous does not mean fast. There are many spontaneous reactions that are so slow that no apparent reaction occurs over a period of weeks or years at normal temperatures. For example, there is a strong inherent tendency for gaseous hydrogen and oxygen to combine to form water,... 

Another consideration when calculating f-ratios is the diurnal variabUity of N versus carbon (C) metaboUsm. When relating new production to C export (sensu Eppley and Peterson, 1979), stoichiometric calculations of C export from N uptake can be in error when C and N uptake are not evaluated over appropriate timescales. Diel variability in uptake rates of various N compounds has been demonstrated (e.g., Cochlan and Harrison, 1991b Cochlan et al., 1991a Tremblay et al., 2000 Wilkerson and Grunseich, 1990) and faUure to integrate daUy uptake rates over an entire diel cycle may seriously bias estimates of f-ratios. 

In addition to complicating our view of the marine N cycle, high rates of inorganic N uptake by bacteria and organic N uptake by phytoplankton may compromise stoichiometric calculation of C assimilation from N uptake (or vice versa) using the Redfield C N ratio in the euphotic zone (Wheeler and Kirchman, 1986). Further, bacterial uptake of NH4+ and NOs can confound computation of the f-ratio leading to over- or underestimates of new production. Consequently, estimating C export from N-based estimates of new production needs to be reevaluated for a variety of reasons. 

Figure 2 shows a few of the tanks used to store the millons of metric tons of ammonia made each year in the United States. Stoichiometric calculations are used to determine how much of the reactants are needed and how much product is expected. However, the calculations do not start and end with moles. Instead, other units, such as liters or grams, are used. Mass, volume, or number of particles can all be used as the starting and ending quantities of stoichiometry problems. Of course, the key to each of these problems is the mole ratio. 

Although equations tell you what should happen in a reaction, they cannot always tell you what will happen. For example, sometimes reactions do not make all of the product predicted by stoichiometric calculations, or the theoretical yield. In most cases, the actual yield, the mass of product actually formed, is less than expected. Imagine that a worker at the flavoring factory mixes 500.0 g isopentyl alcohol with 1.25 x 10 g acetic acid. The actual and theoretical yields are summarized in Table i. Notice that the actual yield is less than the mass that was expected. 

The cost of the things you buy is lower because chemists use stoichiometric calculations to increase efficiency in laboratories, decrease waste in manufacturing, and produce products more quickly. 

Suppose a chemist needs to obtain a certain amount of product from a reaction. How much reactant must be used Or, suppose the chemist wants to know how much product will form if a certain amount of reactant is used. Chemists use stoichiometric calculations to answer these questions. 

Recall that stoichiometry is the study of quantitative relationships between the amounts of reactants used and the amounts of products formed by a chemical reaction. What are the tools needed for stoichiometric calculations All stoichiometric calculations begin with a balanced chemical equation, which indicates relative amounts of the substances that react and the products that form. Mole ratios based on the balanced chemical equation are also needed. You learned to write mole ratios in Section 12.1. Finally, mass-to-mole conversions similar to those you learned about in Chapter 11 are required. 

Mole ratios are central to stoichiometric calculations. They are derived from the coefficients in a balanced chemical equation. To write mole ratios, the number of moles of each reactant and product is placed, in turn, in the numerator of the ratio with the moles of each other reactant and product placed in the denominator. 

Stoichiometric calculations allow a chemist to predict the amount of product that can be obtained from a given amount of reactant or to determine how much of two or more reactants must be used to produce a specified amount of product. 

To do stoichiometric calculations that involve both gas volumes and masses, you must know the balanced equation for the reaction involved, at least one mass or volume value for a reactant or product, and the conditions under which the gas volumes have been measured. Then the ideal gas law can be used along with volume or mole ratios to complete the calculation. 

FIGURE 2.4 The steps in a stoichiometric calculation. In a typical calculation, the mass of one reactant or product is known and the masses of one or more other reactants or products are to be calculated using the balanced chemical equation and a table of relative atomic masses. 

The yield of a reaction is the amount of product obtained. This value is nearly always less than what would be predicted from a stoichiometric calculation because side-reactions may produce different products, the reverse reaction may occur, and some material may be lost during the procedure. The yield from a stoichiometric calculation on the limiting reagent is called the theoretical yield. Percent yield is the actual yield divided by the theoretical yield times 100% ... 

The experiment supports the classroom discussion concerning stoichiometric calculations, in particular, % yield calculations. It also promotes critical thinking skills because the student is asked to assess the adaptability of the reaction to full-scale industrial production of NaCl. 

One of the most important areas of chemical arithmetic is based on balanced chemical equations. Chemists call this area of endeavor stoichiometry (stoy-key-om -ah-tree), which concerns the quantitative relationships between the reactants and products in chemical reactions. Stoichiometric calculations can be used to determine the amount of one reactant needed to completely react with another, or to determine the amount of reactant needed to produce a desired amount of product. The key to understanding how this is done is found in the way balanced chemical equations can be interpreted. So that is the place to begin learning the arithmetic of balanced chemical equations. 

Stoichiometry concerns calculations based on balanced chemical equations, a topic that was presented in Chapter 8. Remember that the coefficients in the balanced equations indicate the number of moles of each reactant and product. Because many reactions take place in solution, and because the molarity of solutions relates to moles of solute and volumes, it is possible to extend stoichiometric calculations to reactions involving solutions of reactants and products. The calculations involving balanced equations are the same as those done in Chapter 8, but with the additional need to do some molarity calculations. Let s get our feet wet by working a couple of problems involving solutions in chemical reactions. 

Step 1 is a prerequisite to any stoichiometric calculation. We must know the identities of the reactants and products, and their mass relationships must not violate the law of conservation of mass (that is, we must have a balanced equation). Step 2 is the critical process of converting grams (or other units) of substances to number of moles. This conversion allows us to analyze the actual reaction in terms of moles only. 

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. 

Stoichiometric calculations involve using a balanced chemical equation to determine the amounts of reactants needed or products formed in a reacfion. 

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