Molecules, Moles, and Chemical Equations

Explosions can be very large, as in the photo on the left, or quite small, as in the laboratory demonstrations in the photo on the right. But all chemical explosions are very rapid reactions that release substantial amounts of energy in a short time. 

Left U.S. Department of Energy right Charles D. Winters 

After mastering this chapter, you should be able to 

The most fundamental requirement for an explosive reaction is that it must release a large amount of energy. This release of energy is responsible for what might be described as the force of the explosion. The actual amount of energy 

VV will study the role ofener in chemical reactions in detail in Chapter 9. 

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The essential information implied by the chemical equation is the stoichiometry at the macroscopic level, ie, if a moles of M react, then b moles of B do also p moles of P are formed, etc. No inference should be made about behavior at the microscopic or atomic level, ie, there is no implication thatp molecules of P appear simultaneously. There may or may not be intermediates that appear and disappear in the course of the reaction. 

Stoichiometry in Reactive Systems. The use of molar units is preferred in chemical process calculations since the stoichiometry of a chemical reaction is always interpreted in terms of the number of molecules or number of moles. A stoichiometric equation is a balanced representation that indicates the relative proportions in which the reactants and products partake in a given reaction. For example, the following stoichiometric equation represents the combustion of propane in oxygen ... 

J.I4 The oxides of nonmetallic elements are called acidic oxides because they form acidic solutions in water. Write the balanced chemical equations for the reaction of one mole of each acidic oxide with one mole of water molecules to form an oxoacid and name the acid formed (a) C02 (b) SO,. 

Solid magnesium has been transformed into Mg ions, and hydronium ions have decomposed to give H2 gas and water molecules. Quantitative measurements reveal that for every mole of Mg consumed, the reaction also consumes two moles of H3O+, and it produces one mole of H2 and two moles of water. The reaction can be summed up in the following balanced chemical equation Mg(,S ) + 2 q) Mg (i2 q) + H2(g) + 2 Hz 0(1)... 

The chemical equation then represents a conservation of atoms, which ensures conservation of mass and an alternative view of the species as molecules or moles. The stoichiometric coefficients correspond to the number of molecules or moles of each species. 

This balanced equation can be read as 4 iron atoms react with 3 oxygen molecules to produce 2 iron(III) oxide units. However, the coefficients can stand not only for the number of atoms or molecules (microscopic level) but they can also stand for the number of moles of reactants or products. So the equation can also be read as 4 mol of iron react with 3 mol of oxygen to produce 2 mol ofiron(III) oxide. In addition, if we know the number of moles, the number of grams or molecules may be calculated. This is stoichiometry, the calculation of the amount (mass, moles, particles) of one substance in the chemical equation from another. The coefficients in the balanced chemical equation define the mathematical relationship between the reactants and products and allow the conversion from moles of one chemical species in the reaction to another. 

A balanced chemical equation provides many types of information. It shows which chemical species are the reactants and which species are the products. It may also indicate in which state of matter the reactants and products exist. Special conditions of temperature, catalysts, etc., may be placed over or under the reaction arrow. And, very importantly, the coefficients (the integers in front of the chemical species) indicate the number of each reactant that is used and the number of each product that is formed. These coefficients may stand for individual atoms/molecules or they may represent large numbers of them called moles (see the Stoichiometry chapter for a discussion of moles). The basic idea behind the balancing of equations is the Law of Conservation of Matter, which says that in ordinary chemical reactions matter is neither created nor destroyed. The number of each type of reactant atom has to equal the number of each type of product atom. This requires adjusting the reactant and product coefficients—balancing the equation. When finished, the coefficients should be in the lowest possible whole-number ratio. 

The relationship above gives a way of converting from grams to moles to particles, and vice versa. If you have any one of the three quantities, you can calculate the other two. This becomes extremely useful in working with chemical equations, as we will see later, because the coefficients in the balanced chemical equation are not only the number of individual atoms or molecules at the microscopic level, but also the number of moles at the macroscopic level. 

B. N. Taylor, Guide to the Use of the International System of Units (SI), NIST Special Publication 811, Gaithersburg, MD, 1995. http //www.physics.nist.gov/cuu/units/current.html. http //www.bpim.fr. The amount of substance should be expressed in units of moles, with one mole being Avogadio s constant number of designated particles or groups of particles, whether these are elections, atoms, molecules, or the number of molecules of reactants and products specified by a chemical equation. 

Second, you must realize that, when the two substances you are interested in are both gases you can make a much simpler calculation than that involved in the "three simple steps You recall (see p 160) that equal volumes of gases under the same conditions of temperature and pressure contain the same number of moles (or molecules) The chemical equation shows that you need 5 moles of 02 per mole... 

FIGURE 3.2 A summary of conversions between moles and grams for a chemical reaction. The numbers of moles tell how many molecules of each reactant are needed, as given by the coefficients of the balanced equation the numbers of grams tell what mass of each reactant is needed. 

The amount of substance should be expressed in units of moles, 1 mole being Avogadro s constant number of designated particles or group of particles, whether these are electrons, atoms, molecules, or the reactants and products specified by a chemical equation. 

The relative numbers of reactant and product molecules (or the relative numbers of moles) are indicated by the coefficients of a balanced chemical equation. Using molar masses, we can compute the relative masses of reactants and products in a chemical reaction. 

A chemical equation represents the relationship of the reactants and products through a numerical relationship expressed by the coefficients associated with the participants. The coefficients can be interpreted as telling us the number of molecules or moles of materials involved but they also represent the volumes of those participants that are gases, assuming a constant temperature and pressure (T and P). An example of these relationships is as follows ... 

Temperature units/conversions Periodic table Basic atomic structure Quantum mechanical model Atomic number and isotopes Atoms, molecules, and moles Unit conversions Chemical equations Stoichiometric calculations Week 3 Atmospheric chemistry... 

The atomic processes that are occurring (under conditions of equilibrium or non equilibrium) may be described by statistical mechanics. Since we are assuming gaseous- or liquid-phase reactions, collision theory applies. In other words, the molecules must collide for a reaction to occur. Hence, the rate of a reaction is proportional to the number of collisions per second. This number, in turn, is proportional to the concentrations of the species combining. Normally, chemical equations, like the one given above, are stoichiometric statements. The coefficients in the equation give the number of moles of reactants and products. However, if (and only if) the chemical equation is also valid in terms of what the molecules are doing, the reaction is said to be an elementary reaction. In this case we can write the rate laws for the forward and reverse reactions as Vf = kf[A]"[B]6 and vr = kr[C]c, respectively, where kj and kr are rate constants and the exponents are equal to the coefficients in the balanced chemical equation. The net reaction rate, r, for an elementary reaction represented by Eq. 2.32 is thus... 

This is how we know how to balance chemical equations and work out the masses reacting together, because we know that equal molar masses contain the same numbers of particles. In a chemical equation like C + 02 —> C02, we always know that there will be one mole of carbon atoms reacting with one mole of oxygen molecules (02) to give one mole of carbon dioxide. Providing the chemical equation balances, any fraction of moles will also be true. One-tenth of a mole of carbon will require one-tenth of a mole of oxygen gas molecules (02), to make one-tenth of a mole of carbon dioxide gas (C02). 

Balanced chemical equations can be interpreted in terms of representative particles (atoms, molecules, formula units), moles, and mass. 

I hapter 1 explained how chemical and physical methods are used to estab-lish chemical formulas and relative atomic and molecular masses. This chapter begins our study of chemical reactions. We start by developing the concept of the mole, which allows us to count molecules by weighing macroscopic quantities of matter. We examine the balanced chemical equations that summarize these reactions and show how to relate the masses of substances consumed to the masses of substances produced. This is an immensely practical and important subject. The questions how much of a substance will react with a given amount of another substance and how much product will be generated are central to all chemical processes, whether industrial, geological, or biological. 

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