Chemical reactions going to completion

We may obtain from Equation 46 reasonable bounds on mF for interesting limiting cases in which the chemical reactions go to completion (i.e., Ti + T2 is fixed). For example, if the first step in the two-step reaction is very fast and the second step is very slow, then YN2>n. — 0 and Tn2h2o, ri < 46/64 — 0.72 similarly, if both the first and the second steps are exceedingly fast, then YN2H2o,n 0 and YN2,rz < 28/64 — 0.44 finally, if the first step is very slow, then YN2H2o,rj — TN2,ri -— 0. 

In the foregoing discussion, we have used the basic assumption that chemical reactions go to completion, or until at least one reactant is completely used up — that they are not reversible. Many reactions do go to completion, or so nearly so as to make no difference. But a huge number of reactions are reversible, and to such an extent that the products form and accumulate and then react with each other to re-form the reactants. The reaction ultimately goes to a position of dynamic equilibrium far from completion where the rate of the forward reaction is the same as the rate of the reverse reaction, and the reaction appears to have ceased. Under these conditions the experimenter observes the net rate of reaction, which is simply the difference between the rates of the forward and reverse reactions ... 

There is a difference, however, between cookbook recipes and chemical reactions not all chemical reactions go to completion. For many chemical reactions, a certain amount of the reactants remain in their unreacted state, like pudding that is imperfectly set. A pudding that is not properly set will have some portion that is pudding and some portion that is liquid. Most chemical reactions are like imperfect puddings some portion of the reactants turn into products, but some other portion does not. The conservation of mass holds just as true for these not-perfect-pudding reactions, but for the sake of illustration, we will initially assume that all reactions go to completion that is, we will assume all of the reactants turn into product, a pudding perfectly set. When a reaction goes to completion the amount of product is wholly predictable from the conservation of mass if two pounds of reactants go into the recipe, two pounds of product come out. 

You may get the idea that all chemical reactions go to completion when you watch a hydrogen-oxygen explosion or watch a piece of wood burn in the fireplace. Chemicals do not always react to form products with the complete conversion of reactants, however. Whenever the point is reached at which the forward reaction is proceeding at the same rate as the reverse reaction, equilibrium is established and the amounts of reactants and products remain unchanged. But because equilibrium is a dynamic condition, the forward and reverse reactions are still happening so that each reactant or product is replaced as soon as it is consumed. 

Many chemical reactions go to completion. This means that the reaction continues until all component particles (whether ions, molecules or atoms) of one reagent have reacted, then stops. The reaction between hydrochloric acid and marble chips (calcium carbonate) is an example ... 

Pressure. The chemical reactions go to completion at low pressures. High pressures reverse the desired chemical reactions. The H2-nuclear interface should be at low pressure to (1) minimize the risk of pressurization of the chemical plant and (2) minimize high-temperature materials strength requirements. 

Many chemical reactions go to completion. For example, when magnesium reacts with excess hydrochloric acid, the reaction stops when all the magnesium has been used up. The products cannot be converted back to the reactants. This reaction is irreversible. 

This method is primarily based on measurement of the electrical conductance of a solution from which, by previous calibration, the analyte concentration can be derived. The technique can be used if desired to follow a chemical reaction, e.g., for kinetic analysis or a reaction going to completion (e.g., a titration), as in the latter instance, which is a conductometric titration, the stoichiometry of the reaction forms the basis of the analysis and the conductometry, as a mere sensor, does not need calibration but is only required to be sufficiently selective. 

The point where the heat production rate reaches its maximum value is of critical importance for a chemical process. This maximum value needs to be compared with the total given maximum heat removal capacity. A reaction going to completion can be considered safe, for normal operation, if the maximum heat removal capacity is greater than the maximum heat production rate. For more precise analysis see the literature 19, 10, 11/. 

Stoichiometry. The measurement of reactants and products of a chemical reaction. Fundamentals, rule that the combined weights of reactants will equal combined weights of products in reactions going to completion. 

Many conditions are required for a chemical reaction to proceed. Conditions such as heat, light, and pressure must be just right for a reaction to take place. Furthermore, the reaction may proceed very slowly. Some reactions occur in a fraction of a second, while others occur very slowly. Consider the difference in the reaction times of gasoline igniting in a car s cylinder versus the oxidation of iron to form rust. The area of chemistry that deals with how fast reactions occur is known as kinetics (Chapter 12). Finally, not all reactions go to completion. The amount of product produced based on the chemical equation is known as the theoretical yield. The amount actually obtained expressed as a percent of the theoretical is the actual yield. In summary, it s best to think of a chemical equation as an ideal representation of a reaction. The equation provides a general picture of the reaction and enables us to do theoretical calculations, but in reality reactions deviate in many ways from that predicted by the equation. 

We have considered the change of properties of a system in which a chemical reaction occurs with an extent of reaction, c,. We also need ways to determine just what extent of reaction will occur in a particular system. In many cases, this question will be answered by stoichiometry, namely by assuming that the reaction will proceed until essentially all the limiting reagent is converted into product, a situation that is called a reaction going to completion. 

Example 4.2 applied the method of false transients to a CSTR to find the steady-state output. A set of algebraic equations was converted to a set of ODEs. Chapter 16 shows how the method can be applied to PDEs by converting them to sets of ODEs. The method of false transients can also be used to find the equilibrium concentrations resulting from a set of batch chemical reactions. Formulate the ODEs for a batch reactor and integrate until the concentrations stop changing. Irreversible reactions go to completion. Reversible reactions reach equilibrium concentrations. This is illustrated in Problem 4.6(b). Section 11.1.1 shows how the method of false transients can be used to determine physical or chemical equilibria in multiphase systems. 

Up to now, chemical reactions have been written as though reactants are completely converted to products, or as is usually said, go to completion. However, there are many cases in which, as products are formed, they react with each other to reform the reactants. These situations are characterized by the simultaneous occurrence of two opposed chemical reactions. In these cases, some product is formed, but some reactants are also present at the same time. The reactions proceed to a point where no further change in concentrations can be detected, and at that time, a state called chemical equilibrium has been reached. Even those cases where reactions go to completion are equilibrium systems in which the amounts or reactant left over are too small to be of practical importance. Nitrogen reacts with oxygen to produce nitrogen monoxide, and the reaction can be written... 

Chemistry, the study of chemical reactions, would be a much simpler (and less interesting) science if all chemical reactions went to completion. This is not the case, however chemistry cannot be based on stoichiometry alone. Many reactions are reversible and do not go to completion. In such mixtures, the final composition is quite different from that which could be expected from stoichiometric considerations. A detailed knowledge of the composition, i.e., the concentration of each species present, of such mixtures is essential for the understanding of their chemistry. Short of actually measuring the amount of each and every species present, the only way to obtain this knowledge is from a consideration of the appropriate equilibria. 

Presenting the idea of the chemical equation as a very useful model that can help us summarise what is going on in reactions, and which does a good enough job for most reactions, will both enable students to see the value of the formalism and stop them making unfortunate generalisations about all reactions going to completion (which could then later act as a misconception when they meet reactions that do not fit the model). 

Deduction of the mechanism (5.1) and of the rate equations (5.2) is greatly simplified if the various decay processes occur at vastly different rates. Analysis of the case n = 2 exhibits all chemically significant features of the general mechanism. To introduce the ideas of time-scale separation and of a trace intermediate assume that the reactions go to completion, i.e., k = kJ 0. Then, if initially only A is present, the concentrations of A, B, and C are found by direct integration of the rate equations... 

Cmd Chemicals is producing a copolymer of uniform composition in a semibatch reactor, as illustrated in Figure 11.4a. For their system, r/ = 2.0, V2 = 0.5, and/) = 0.5. They adjust the rate of addition of the more reactive monomer according to the heat evolution in the reactor. Normally, the system works quite well. One day, however, the feed valve sticks full open at 50% conversion, suddenly dumping the remaining monomer into the reactor. Nevertheless, they let the reaction go to completion. Plot FI and Fj) versus conversion (of all monomer) for this screwed-up reaction. 

Another difficulty is that spontaneous chemical reactions do not go to completion. Even if a spontaneous reaction is exothermic, it proceeds only till it reaches equilibrium. But in our golf ball analogy, equilibrium is reached when all of the golf balls are on the lower level. Oui analogy would lead us to expect that an exothermic reaction would proceed until all of the reactants are converted to products, not to a dynamic equilibrium. 

Stoichiometric calculations of the amount of product formed in a reaction are based on an ideal view of the world. They suppose, for instance, that all the reactants react exactly as described in the chemical equation. In practice, that might not be so. Some of the starting materials may be consumed in a competing reaction, a reaction taking place at the same time as the one in which we are interested and using some of the same reactants. Another possibility is that the reaction might not be complete at the time we make our measurements. A third possibility—of major importance in chemistry and covered in several chapters of this text—is that many reactions do not go to completion. They appear to stop once a certain proportion of the reactants has been consumed. 

One difficulty Haber faced is that the reactions used to produce compounds from nitrogen do not go to completion, but appear to stop after only some of the reactants have been used up. At this point the mixture of reactants and products has reached chemical equilibrium, the stage in a chemical reaction when there is no further tendency for the composition of the reaction mixture—the concentrations or partial pressures of the reactants and products—to change. To achieve the greatest conversion of nitrogen into its compounds, Haber had to understand how a reaction approaches and eventually reaches equilibrium and then use that... 

An excellent overview of the problems that students experience in learning the notions underlying chemical equilibrium is available (van Driel Graber, 2002). Research shows that conceptual problems arose when students, who had been introduced to chemical reactions through examples that evidently go to completion , first met examples of incomplete reactions . In this situation, they... 

Other reactions do not go to completion because they reach dynamic equilibrium. While reactant molecules continue to form product molecules, product molecules also interact to re-form reactant molecules. The Haber reaction and many precipitation reactions, described later in this chapter, are examples of reactions that reach d3Tiamic equilibrium rather than going to completion. We treat chemical equilibria in detail in Chapters 16-18. 

As an indispensable source of fertilizer, the Haber process is one of the most important reactions in industrial chemistry. Nevertheless, even under optimal conditions the yield of the ammonia synthesis in industrial reactors is only about 13%. This Is because the Haber process does not go to completion the net rate of producing ammonia reaches zero when substantial amounts of N2 and H2 are still present. At balance, the concentrations no longer change even though some of each starting material is still present. This balance point represents dynamic chemical equilibrium. 

In this chapter, we present basic features of chemical equilibrium. We explain why reactions such as the Haber process cannot go to completion. We also show why using catalysts and elevated temperatures can accelerate the rate of this reaction but cannot shift Its equilibrium position in favor of ammonia and why elevated temperature shifts the equilibrium In the wrong direction. In Chapters 17 and 18, we turn our attention specifically to applications of equilibria. Including acid-base chemistry. 

As described in Chapter 4, acid-base reactions that go to completion can be exploited in chemical analysis using the method of titration. Titrations can be understood in greater detail from the perspective of acid-base equilibria. Protonation of a weak base by a strong acid is a reaction that goes virtually to completion because of its large... 

This method is primarily concerned with the phenomena that occur at electrode surfaces (electrodics) in a solution from which, as an absolute method, through previous calibration a component concentration can be derived. If desirable the technique can be used to follow the progress of a chemical reaction, e.g., in kinetic analysis. Mostly, however, potentiometry is applied to reactions that go to completion (e.g. a titration) merely in order to indicate the end-point (a potentiometric titration in this instance) and so do not need calibration. The overwhelming importance of potentiometry in general and of potentiometric titration in particular is due to the selectivity of its indication, the simplicity of the technique and the ample choice of electrodes. 

Ans. Only parts (d) and (e) are correct. The balanced equation governs the reacting ratios only. It cannot determine how much of any chemical may be placed in a vessel—(a) and (b)—or if a reaction will go to completion—(c). 

Chemical facilities have to be operated safely during normal operation as well as during deviations from the specified process and equipment parameters. Chemical reactions that go to completion can only become a hazard for humans and the environment when process pressures or temperatures rise beyond the equipment design parameters of a facility e.g., as result of a runaway reaction. For example unacceptable pressure increases can develop as a result of exothermic processes with inadequate heat sinks or reactions that produce gaseous products (e.g., decompositions). 

Only single step homogeneous reactions are performed. Side reactions will not go to completion. The reaction mechanisms remain unchanged for the processes under consideration. Interactions of the chemical reactants and/or reaction mixtures with the material of construction are excluded. 

In the problem above, the amount of product calculated based upon the limiting reactant concept is the maximum amount of product that will form from the specified amounts of reactants. This maximum amount of product is the theoretical yield. However, rarely is the amount that is actually formed (the actual yield) the same as the theoretical yield. Normally it is less. There are many reasons for this, but the principal one is that most reactions do not go to completion they establish an equilibrium system (see Chapter 14 for a discussion on chemical equilibrium). For whatever reason, not as much product as expected is formed. We can judge the efficiency of the reaction by calculating the percent yield. The percent yield (% yield) is the actual yield divided by the theoretical yield and the resultant multiplied by 100 in order to generate a percentage ... 

In the stoichiometry of chemical reactions, it is assumed that the reaction goes to completion, so that one of the reactants is consumed. However, many reactions do not go to completion, but rather establish an equilibrium. 

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