Chemical Connections stoichiometry, importance

It is difficult to measure reaction rates directly, because we do not directly sense molecular transformation events. We can measure concentrations, however. It is important to connect the reaction rate to the rate of change of the concentrations of the various species in the reactor, which are the quantities we usually care about in a commercial reactor. We define production rate, Rj, as the rate at which the jth species is produced (moles/(time-volume)) due to the chemical reactions taking place. It is clear looking at the stoichiometry in Equation 2.27 that each time the forward reaction event occurs, an Si2He molecule is produced. Each time the reverse reaction occurs, an SizHe molecule is consumed. The production rate of Si2He, Rshtk Is therefore directly related to the reaction rate,... 

When we discussed quantitative aspects of chemical reactions in Chapter 4, we emphasized the importance of ratios of moles. The ideal gas law provides a relationship between the number of moles of a gas and some easily measurable properties pressure, volume, and temperature. So when gases are involved in a chemical reaction, the ideal gas law often provides the best way to determine the number of moles. Using the ideal gas law in a stoichiometry problem really doesn t involve any new ideas. It just combines two kinds of calculations that you ve already been doing. We ll still do the stoichiometric calculation in terms of mole ratios, as always, and we ll use the gas law to connect the number of moles of a gas with its temperature, pressure, and volume. 

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