Molar Interpretation of a Chemical Equation

In the Haber process for producing ammonia, NH3, nitrogen (from the atmosphere) reacts with hydrogen at high temperature and pressure. 

Hydrogen is usually obtained from natural gas or petroleum and so is relatively expensive. For this reason, the price of hydrogen partly determines the price of ammonia. Thus, an important question to answer is. How much hydrogen is required to give a particular quantity of ammonia For example, how much hydrogen would be needed to produce one ton (907 kg) of ammonia Similar kinds of questions arise throughout chemical research and industry. 

To answer such quantitative questions, you must first look at the balanced chemical equation one N2 molecule and three H2 molecules react to produce two NH3 molecules (see models above). A similar statement involving multiples of these numbers of molecules is also correct. For example, 6.02 X 10 N2 molecules react with 3 X 6.02 X 10 H2 molecules, giving 2 X 6.02 X 10 NH3 molecules. This last statement can be put in molar terminology one mole of N2 reacts with three moles of H2 to give two moles of NH3. 

You may interpret a chemical equation either in terms of numb of molecules (or ions or formula units) or in terms of numbers of moles, depending on your needs. 

Suppose you ask how many grams of atmospheric nitrogen will react with 6.06 g (3 X 2.02 g) of hydrogen. You see from the last equation that the answo- is 28.0 g N2. We formulated this question for one mole of atmospheric nitrogen. Recalling the question posed earlier, you may ask how much hydrogen (in kg) is needed to yield 907 kg of ammonia in the Haber process. The solution to this problem depends on the fact that the number of moles involved in a reaction is proportional to the coefficients in the balanced chemical equation. In the next section, we will describe a procedure for solving such problems. 

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If you use the molar interpretation of a chemical equation, there is nothing unreasonable about using such coefficients as and . 

The molar interpretation of a chemical equation involves reading the coefficients as the number of moles of the reactants and products. This is still a particulate-level explanation, but we are grouping the particles into counting units that make it easier to translate into a macroscopic-level interpretation. On the molar level, fractional coefficients are acceptable. H2(g) + V2 02(g) H20(g) can be read as one mole of... 

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 ... 

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. 

Knowledge Required (1) The definition of molarity, M. (2) The interpretation of a balanced chemical equation. 

As we discussed in Section 3.4, the coefficients in a chemical equation can be interpreted as molar relationships as well as molecular relationships. So we can say that the equation shows that one mole of methane reacts with two moles of oxygen to produce one mole of carbon dioxide and two moles of water. From the chemical equation, we can write the following set of mole ratios ... 

A chemical equation may be interpreted in terms of moles of reactants and products, as well as in terms of molecules. Using this molar interpretation, you can convert from the mass of one substance in a chemical equation to the mass of another. The maximum amount of product from a reaction is determined by the limiting reactant, the reactant that is completely used up the other reactants are in excess. 

In Section 8.4 we introduced the three levels on which a chemical equation may be interpreted particulate, molar, and macroscopic. Let s briefly review the particulate and molar interpretations of an equation. Consider the equation... 

In this reaction, the number of silver atoms that reacts is twice the number of sulfur atoms. When 200 silver atoms react, 100 sulfur atoms are required. However, in the actual chemical reaction, many more atoms of both silver and sulfur would react. If we are dealing with molar amounts, then the coefficients in the equation can be interpreted in terms of moles. Thus, 2 mol of silver reacts with 1 mol of sulfur to produce 1 mol of Ag2S. Because the molar mass of each can be determined, the moles of Ag, S, and AgjS can also be stated in terms of mass in grams of each. Thus, 215.8 g of Ag and 32.1 g of S react to form 247.9 g of Ag2S. The total mass of the reactants (247.9 g) is equal to the mass of product (247.9 g). The various ways in which a chemical equation can be interpreted are seen in Table 9.1. 

To interpret a titration, we need the stoichiometric relation from the chemical equation for the reaction. This relation is used to write the mole ratio in the usual way. The only new step is to use the molarities of the solutions to convert between the moles of reactants and the volumes of... 

You see from the preceding discussion that a balanced chemical equation relates the amounts of snbstances in a reaction. The coefficients in the equation can be given a molar interpretation, and using this interpretation you can, for example, calculate the moles of product obtained from any given moles of reactant. Also, yon can extend this type of calcnlation to answer questions about masses of reactants and products. 

Thermochemical equation the chemical equation for a reaction (including phase labels) in which the equation is given a molar interpretation, and the enthalpy of reaction for these molar amounts is written directly after the equation. (6.4) Thermochemistry the study of the quantity of heat absorbed or evolved by chemical reactions, (p. 225)... 

The equilibrium constant for the complexation of cholesterol with Eu(fod)3 has been evaluated from measurements on a series of solutions at varying total concentrations but identical molar ratio. The same analysis provides values for the chemical shifts of protons in the uncomplexed steroid, and in the steroid-europium complex the latter value is not accessible by direct measurement, since the complex is always in equilibrium with the free steroid. The mathematical equations presented in this paper should be generally applicable. In another paper the validity of various procedures for interpreting lanthanide-induced shifts is explored comments on cholesterol are included. No one mathematical model at present available is considered to have general validity. 

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