Mass relations in reactions

A chemist who carries out a reaction in the laboratory needs to know how much product can be obtained from a given amount of starting materials (reactants). To do this, he or she starts by writing a balanced chemical equation. 

Chemical reactions are represented by chemical equations, which identify reactants and products. Formulas of reactants appear on the left side of the equation those of products are written on the right. In a balanced chemical equation, there are the same number of atoms of a given element on both sides. The same situation holds for a chemical reaction that you carry out in the laboratory atoms are conserved. For that reason, any calculation involving a reaction must be based on the balanced equation for that reaction. 

Beginning students are sometimes led to believe that writing a chemical equation is a simple, mechanical process. Nothing could be further from the truth. One point that seems obvious is often overlooked. You cannot write an equation unless you know what happens in the reaction that it represents. All the reactants and all the products must be identified. Moreover, you must know their formulas and physical states. 

N2H4 and N2O4, respectively. The products of the reaction are gaseous nitrogen, N2, and water vapor. To write a balanced equation for this reaction, proceed as follows  

Write a skeleton equation in which the formulas of the reactants appear on the left and those of the products on the right In this case, 

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The approach followed in Chapter 3 to calculate mole-mass relations in reactions is readily applied to solution reactions represented by net ionic equations. 

The general topic of this chapter is stoichiometry (stoy-key-OM-e-tree), the study of mass relations in chemistry. Whether dealing with atomic masses (Section 3.1), molar masses (Section 3.2), chemical formulas (Section 3.3), or chemical reactions (Section 3.4), you will be answering some very practical questions that ask how much or how many—." For example—... 

By working parts (a) through (d) of this sample in succession, you can see how many different ways there are to ask a question about mass relations in a reaction. That should cushion the shock should you see only part (d) in an exam. 

EXERCISE 4.15 Using Mass Relations in a Neutralization Reaction... 

It is customary to define the faraday as this quantity of positive electricity. The quantitative treatment of electrochemical reactions is made in the same way as the calculation of mass relations in ordinary chemical reactions, with use of the faraday to represent one mole of electrons. 

In this equation, m. is the effective mass of the reaction coordinate, q(t -1 q ) is the friction kernel calculated with the reaction coordinate clamped at the barrier top, and 5 F(t) is the fluctuating force from all other degrees of freedom with the reaction coordinate so configured. The friction kernel and force fluctuations are related by the fluctuation-dissipation relation... 

A proper resolution of Che status of Che stoichiometric relations in the theory of steady states of catalyst pellets would be very desirable. Stewart s argument and the other fragmentary results presently available suggest they may always be satisfied for a single reaction when the boundary conditions correspond Co a uniform environment with no mass transfer resistance at the surface, regardless of the number of substances in Che mixture, the shape of the pellet, or the particular flux model used. However, this is no more than informed and perhaps wishful speculation. 

The selection of reactor type in the traditionally continuous bulk chemicals industry has always been dominated by considering the number and type of phases present, the relative importance of transport processes (both heat and mass transfer) and reaction kinetics plus the reaction network relating to required and undesired reactions and any aspects of catalyst deactivation. The opportunity for economic... 

Yield and other mass-related metrics such as atom economy, reaction mass efficiency and mass intensity have been examined by Constable et al with regard to their significance concerning greenness and costs. The importance of using a (product) concentration term, which can be mass intensity or mass index, is additionally emphasized by Laird et al This is in compliance with Winterton, who in his twelve more green chemistry principles demands the establishment of full mass balances. 

Only a little effort is necessary to reduce solvent 1 demand used during reaction scale-up. The quantity used in the laboratory stage was reduced to 59% in the operation stage (Table 5.1). However, related to substrate 2, 96% of solvent 1 is still used. Thus, 87% of the original quantity of solvent will be fed to the incinerator for disposal, while the recycle rate is only 9.1% (from 96% to 87%, Table 5.1). Considering that there is a factor of five difference in solvent 1 demand between the operation scale and the literature procedure (see the segments Solvent of the mass index, in Figure 5.10), the potential for optimiz-... 

The mass-related metrics shown in Figure 5.11 indicate that the amount of a substrate (see also byproduct formation), an auxiliary material for reaction, and of a solvent have to be reduced. The detailed view of the mass indices of the pilot scale, for example, the segments Substrates and Aux (R) and the size of segments Substrates (excess) and Aux (R) of the environmental factor E, deliver the information listed in Table 5.2 108% base and 162% auxiliary (R) are used. The measure to increase base addition for recycling purposes was successful at the expense of 193% base, much auxiliary material Aux (R) was saved in operation scale (reduction from 162% to only 13%). This leads to an overall... 

To extend the applicability of the SECM feedback mode for studying ET processes at ITIES, we have formulated a numerical model that fully treats diffusional mass transfer in the two phases [49]. The model relates to the specific case of an irreversible ET process at the ITIES, i.e., the situation where the potentials of the redox couples in the two phases are widely separated. A further model for the case of quasireversible ET kinetics at the ITIES is currently under development. For the case where the oxidized form of a redox species, Oxi, is electrolytically generated at the tip in phase 1 from the reduced species, Red], the reactions at the tip and the ITIES are ... 

The performance of adsorption processes results in general from the combined effects of thermodynamic and rate factors. It is convenient to consider first thermodynamic factors. These determine the process performance in a limit where the system behaves ideally i.e. without mass transfer and kinetic limitations and with the fluid phase in perfect piston flow. Rate factors determine the efficiency of the real process in relation to the ideal process performance. Rate factors include heat-and mass-transfer limitations, reaction kinetic limitations, and hydro-dynamic dispersion resulting from the velocity distribution across the bed and from mixing and diffusion in the interparticle void space. 

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