SECTION 2 Balancing Chemical Equations

Throughout Section 11.3, balanced chemical equations are written in such a way that the coefficient of die reactant is 1. In general, if the coefficient of the reactant is a, where a may be 2 or 3 or..., then k in each integrated rate equation must be replaced by the product ak. (See Problem 101.)... 

Sometimes we need to know how much product to expect from a reaction, or how much reactant we need to make a desired amount of product. The quantitative aspect of chemical reactions is the part of chemistry called reaction stoichiometry. The key to reaction stoichiometry is the balanced chemical equation. Recall from Section H that a stoichiometric coefficient in a chemical equation tells us the relative amount (number of moles) of a substance that reacts or is produced. Thus, the stoichiometric coefficients in... 

We need to be able to write balanced chemical equations to describe redox reactions. It might seem that this task ought to he simple. However, some redox reactions can be tricky to balance, and special techniques, which we describe in Sections 12.1 and 12.2, have been developed to simplify the procedure. 

The initial everyday situation (use of fuel eells) was shown to be a valuable starting point for relating observations of ehemieal reaetions to their explanation with models, which is as a foundation for students better understanding of the notation of a balanced chemical equation. Students are more interested in learning what is meaningful to them. [Teacher from School N° 3, Section 2]... 

Sections 2- and 3- describe how to use the relationships among atoms, moles, and masses to answer how much questions about individual substances. Combining these ideas with the concept of a balanced chemical equation lets us answer how much questions about chemical reactions. The study of the amounts of materials consumed and produced in chemical reactions is called stoichiometry. 

Ans. Section 2.4, factor-label method Sec. 4.4, calculation of formula weights Sec. 4.5. changing moles to grams and vice versa Sec. 4.5, Avogadro s number and/or Sec. 7.2, balancing chemical equations. 

We have seen how analytical calculations in titrimetric analysis involve stoichiometry (Sections 4.5 and 4.6). We know that a balanced chemical equation is needed for basic stoichiometry. With redox reactions, balancing equations by inspection can be quite challenging, if not impossible. Thus, several special schemes have been derived for balancing redox equations. The ion-electron method for balancing redox equations takes into account the electrons that are transferred, since these must also be balanced. That is, the electrons given up must be equal to the electrons taken on. A review of the ion-electron method of balancing equations will therefore present a simple means of balancing redox equations. 

Beginning students often find it useful to write a complete description of an experiment, with sections dealing with purpose, methods, results, and conclusions. Arranging a notebook to accept numerical data prior to coming to the lab is an excellent way to prepare for an experiment. It is good practice to write a balanced chemical equation for every reaction you use. This practice helps you understand what you are doing and may point out what you do not understand about what you are doing. 

S balanced chemical equation helps us determine the amount of products that might be formed from given amounts of reactants. The equation, however, tells us little about what is taking place on the submicroscopic level during the reaction. In this and the following section, we explore that level to show how the rate of a reaction can be changed either by changing... 

In Section 5.4 we develop the condition of equilibrium for a chemical reaction for any chemical reaction written as Y.k VA = 0, the condition of equilibrium is vkfik = 0. In these equations each vk represents the stoichiometric coefficient of the substance Bk in the balanced chemical equation the sign is positive for products and negative for reagents. Expressions for the equilibrium constants are readily obtained from this condition. For a general relation we express the chemical potentials in terms of the activities, so... 

The integration of Equation (11.22) to determine the equilibrium constant as a function of the temperature or to determine its value at one temperature with the knowledge of its value at another temperature is very similar to the integration of the Clausius-Clapeyron equation as discussed in Section 10.2. The quantity AHB must be known as a function of the temperature. This in turn may be determined from the change in the heat capacity for the change of state represented by the balanced chemical equation with the condition that all substances involved are in their standard states. 

Write the balanced chemical equation for the reactions in Section 6 of the Procedure section. 

Balancing Chemical Equations Use with Chapter 10, Section 10.1... 

In this section, you learned how to represent chemical reactions using balanced chemical equations. Because there are so many different chemical reactions, chemists have devised different classifications for these reactions. In section 4.2, you will learn about five different types of chemical reactions. 

You can get the same kind of information from a balanced chemical equation. In Chapter 4, you learned how to classify chemical reactions and balance the chemical equations that describe them. In Chapters 5 and 6, you learned how chemists relate the number of particles in a substance to the amount of the substance in moles and grams. In this section, you will use your knowledge to interpret the information in a chemical equation, in terms of particles, moles, and mass. Try the following Express Lab to explore the molar relationships between products and reactants. 

Section 8.4 Section 6.3 8.2 To balance chemical equations—that is, to get the same number of atoms of each element on each side... 

Gay-Lussac s law of combining volumes relates the volumes of gases involved in a reaction, all measured separately at the same temperature and pressure. The volume ratio under these conditions is equal to the mole ratio and, therefore, to the ratio of coefficients in the balanced chemical equation (Section 12.9). 

The balanced chemical equation governs the concentrations produced and used up by the reaction (the second row of a tabulation such as those used in the examples in Section 18.4). We can treat concentrations in this way because the concentration ratio of the substances in a given solution or gas mixture is the same as the mole ratio. 

Heat is a reactant or product in most chemical reactions. Before we consider including heat in a balanced chemical equation, first we must learn how to measure heat. When heat is added to a system, in the absence of a chemical reaction the system may warm up, or a change of phase may occur. In this section change of phase will not be considered. 

Calculations that involve chemical reactions use the proportions from balanced chemical equations to find the quantity of each reactant and product involved. As you learn how to do these calculations in this section, you will assume that each reaction goes to completion. In other words, all of the given reactant changes into product. For each problem in this section, assume that there is more than enough of all other reactants to completely react with the reactant given. Also assume that every reaction happens perfectly, so that no product is lost during collection. As you will learn in the next section, this usually is not the case. 

Probably the most fundamental concept of chemistry is the law of conservation of mass that you first encountered in Chapter 3. All chemical reactions obey the law that matter is neither created nor destroyed. Therefore, it is also fundamental that the equations that represent chemical reactions include sufficient information to show that the reaction obeys the law of conservation of mass. You have learned how to show this relationship with balanced chemical equations. The flowchart shown in Figure 10-4 summarizes the steps for balancing equations. You will undoubtedly find that some chemical equations can be balanced easily, whereas others are more difficult to balance. All chemical equations, however, can be balanced by the process you learned in this section. 

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. 

To solve the problem, you need to know how the unknown moles of hydrogen are related to the known moles of potassium. In Section 12.1 you learned to use the balanced chemical equation to write mole ratios that describe mole relationships. Mole ratios are used as conversion factors to convert a known number of moles of one substance to moles of another substance in the same chemical reaction. What mole ratio could be used to convert moles of potassium to moles of hydrogen In the correct mole ratio, the moles of unknown (H2) should be the numerator and the moles of known (K) should be the denominator. The correct mole ratio is... 

The calculations you did in Section 12.2 were based on having the reactants present in the ratio described by the balanced chemical equation. How can you calculate the amount of product formed when one reactant limits the amount of product and the other is in excess The first thing you must do is determine which reactant is the limiting reactant. 

Section 12-1 Interpret the following balanced chemical equation in terms of particles, moles, and mass. 

Section 12-4 13. Tin(IV) iodide is prepared by reacting tin with iodine. Write the balanced chemical equation for the reaction. Determine the theoretical yield If a 5.00-g sample of tin reacts in an excess of iodine. Determine the percent yield. If 25.0 g Snl4 was actually recovered. 

Given the mass of a reactant or product in a chemical reaction, use a balanced chemical equation to calculate the masses of other reactants consumed and other products formed (Section 2.4, Problems 33-46). 

Given a set of initial masses of reactants and a balanced chemical equation, determine the limiting reactant and calculate the masses of reactants and products after the reaction has gone to completion (Section 2.6, Problems 47 and 48). 

As we pointed out in Section 1-1, there is no detectable change in the quantity of matter during an ordinary chemical reaction. This guiding principle, the Law of Conservation of Matter, provides the basis for balancing chemical equations and for calculations based on those equations. Because matter is neither created nor destroyed during a chemical reaction. 

To solve many of the problems in this chapter, you will need to apply the limiting reactant concept (Section 3-3). In Example 11-1, we confirm that the two reactants are initially present in the mole ratio required by the balanced chemical equation they both react completely, so there is no excess of either one. In Example 11-2, we need to determine which reactant limits the reaction. Before you proceed, be sure you understand how the ideas of Section 3-3 are used in these examples. 

Interpret information conveyed by a balanced chemical equation (Section 4.1). 

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