Balancing chemical equations stoichiometric calculations

To determine the limiting reactant, first calculate the number of moles of each reactant present. Then determine how these numbers of moles correspond to the stoichiometric ratio indicated by the balanced chemical equation for the reaction. For each reactant, use the stoichiometric ratios from the balanced chemical equation to calculate how much of the other reactants would be required to react completely. 

In Chapter 13, you learned how to use moles and molar mass along with a balanced chemical equation to calculate the masses of reactants and products in a chemical reaction. Now that you know how to relate volumes, masses, and moles for a gas, you can do stoichiometric calculations for reactions involving gases. 

The emphasis is on writing and balancing chemical equations for these reactions. All of these reactions involve ions in solution. The corresponding equations are given a special name net ionic equations. They can be used to do stoichiometric calculations similar to those discussed in Chapter 3. 

Entropy changes are important in every process, but chemists are particularly interested in the effects of entropy on chemical reactions. If a reaction occurs under standard conditions, its entropy change can be calculated from absolute entropies using the same reasoning used to calculate reaction enthalpies from standard enthalpies of formation. The products of the reaction have molar entropies, and so do the reactants. The total entropy of the products is the sum of the molar entropies of the products multiplied by their stoichiometric coefficients in the balanced chemical equation. The total entropy of the reactants is a similar sum for the reactants. Equation... 

The net ionic equation, like all balanced chemical equations, gives the ratio of moles of each substance to moles of each of the others. It does not immediately yield information about the mass of the entire salt, however. (One cannot weigh out only Ba2+ ions.) Therefore, when masses of reactants are required, the specific compound used must be included in the calculation. The use of net ionic equations in stoichiometric calculations will be more important after study of molarity (Chap. 10). 

Each calculation uses the stoichiometric coefficients from the balanced chemical equation and the molar mass of the reactant. 

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. 

O WI Why is a balanced chemical equation needed to solve stoichiometric calculations ... 

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. 

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. 

The four steps in stoichiometric calculations begin with the balanced chemical equation. 

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. 

This equation can be extended to calculate the standard-state enthalpy change for any chemical reaction by adding up the standard-state enthalpy of formation for all the products (each multiplied by its stoichiometric coefficient in the balanced chemical equation) and subtracting off the total for all the reactants (each multiplied by its stoichiometric coefficient in the balanced chemical equation). In mathematical form, this procedure is represented by the equation... 

To perform this calculation the number of moles of phosphorus available has been multiplied by a stoichiometric factor, a mole ratio factor relating moles of the required reactant to moles of the other reactant. The stoichiometric factor comes directly from the coefficients in the balanced chemical equation. This is the reason you must balance chemical equations before proceeding with calculations. Here the calculation shows that 0.0702 mol of Gig is required to react with aU the available phosphorus. 

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. 

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

I Mole ratios derived from the balanced chemical equation are used in stoichiometric calculations. 

The process of using a chemical equation to calculate the relative amounts of reactants and products involved in the reaction is called doing stoichiometric calculations. To convert between moles of reactants and moles of products, we use mole ratios derived from the balanced equation. 

We are given the mass in grams of LiOH and asked to calculate the mass in grams of CXD2-can accomplish this task by using the three conversion steps in Figure 3.16. The conversion of step 1 requires the molar mass of LiOH (6.94 + 16.00 + 1.01 = 23.95 g/mol). The conversion of step 2 is based on a stoichiometric relationship from the balanced chemical equation 2 mol LiOH — 1 mol CO2. For the step 3 conversion, we use the molar mass of COj 12.01 + 2(16.00) = 44.01 g/mol. 

We will need values for the heats of formation of the reactants and products to determine the desired heat of combustion. First, we must write a balanced chemical equation for the process. Then we can use Equation 9.12 to calculate the heat of the reaction (in this case the heat of combustion) by looking up heats of formation in the table in Appendix E. The stoichiometric coefficients needed will be obtained... 

Like balancing chemical equations, making stoichiometric calculations requires practice. Several worked examples follow. Study this material and practice on the problems at the end of this chapter. 

STRATEGIZE Draw the solution map beginning with moles of chlorine and using the stoichiometric conversion factor to calculate moles of sodium chloride. The conversion factor comes from the balanced chemical equation. SOLUTION MAP ... 

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 ... 

The grams of oxygen needed to completely combust a given fuel can be calculated from the balanced chemical equation. Use the Pyro Valence method discussed in Chapter 3 to determine the proper ratio of oxidizer to fuel for the stoichiometric composition. A sample calculation is shown below ... 

A balanced chemical equation is the key step in all stoichiometric calculations, because the mcle ratio is obtained directly frcm it. Solving any reaction stoichiometry problem must begin with a balanced equation. 

You should take time at this point to look at each type of stoichiometric conversion and make note that in every t3q>e, you must begin with a correctly balanced chemical equation. It is important to remember that without a balanced equation, you will not have an accurate molar ratio and will not be able to calculate the correct molar mass to use in your conversions. 

In an ideal stoichiometric calculation, the mass or the amount of any reactant or product can be calculated if the balanced chemical equation and the mass or amount of any other reactant or product is known. 

Since the stoichiometric coefficient for magnesium in the balanced chemical equation is 1, the calculated value represents A// n for the reaction as written. 

Stoichiometric mass-to-mass conversion If you were preparing to carry out a chemical reaction in the laboratory, you would need to know how much of each reactant to use in order to produce a certain mass of product. This is one instance when you would use a mass-to-mass conversion. In this calculation, you can find the mass of an unknown substance in a chemical equation if you have the balanced chemical equation and know the mass of one substance in the equation. 

The first step in a stoichiometric calculation is to write a balanced equation for the reaction. The balanced chemical equation for the reaction is given below. 

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). 

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,... 

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