Chemical reactions products, 195 reversible

Theoretically, all chemical reactions are reversible. There are, however, many reactions in which the extent of the reverse reaction (i.e., combination of the products to produce the reactants) is very small as to be considered negligible. Such reactions which are ordinarily found to proceed to completion in one direction are said to be irreversible reactions. The decomposition of potassium chlorate... 

Theoretically, all chemical reactions are reversible and, in the presence of enzymes, a dynamic equihbrium state will be reached rather than a total transformation of substrate molecules to product molecules. The dynamic equilibrium could be characterized by the Haldane equation ... 

The preceding method uses initial rates rather than rates at a later stage of the reaction because chemical reactions are reversible and we want to avoid complications from the reverse reaction reactants <— products. As the product concentrations build up, the rate of the reverse reaction increases. If the reverse rate becomes comparable to the forward rate, the measured rate will depend on the concentrations of both reactants and products. At the beginning of the reaction, however, the product concentrations are zero, and therefore the products can t affect the measured rate. When we measure an initial rate, we are measuring the rate of only the forward reaction, so only reactants (and catalysts see Section 12.12) can appear in the rate law. 

Strictly speaking, all chemical reactions are reversible. What we sometimes call irreversible reactions are simply those that proceed nearly to completion, so that the equilibrium mixture contains almost all products and almost no reactants. For such reactions, the reverse reaction is often too slow to be detected. 

In principle, every chemical reaction is reversible, and so are all of its steps. This is because the decrease in standard free energy accompanying a totally irreversible reaction or step would have to be infinite. In practice, however, a reaction or step is said to be irreversible if, at equilibrium, its reactants are almost completely converted to products. It is left to the practitioner to decide on the merits of the case how strict he wants to be in interpreting this "almost."... 

And equilibrium always carries the day. Equilibrium may be held at bay for a time, and systems may even enter a metastable state, but ultimately, equilibrium rules. Chemical reactions are able to respond to and adjust to lower energy and maximize entropy because chemical reactions are reversible and dynamic. Being reversible and dynamic means that chemical reactions can be quite pliable and can be coaxed into producing more products or reverting to reactants. Such manipulations are obligatory in chemical industry and the chemical lab, but they are more important than that. Equilibrium concerns us all, every day. As we live and breathe. 

Chemical reactions are reversible as the products are formed, products begin to react to form the reactants. This reverse reaction complicates the study of kinetics. For the time being, we will consider only the forward reactions. If we consider only the forward reaction, we can write a rale law for the reaction above as ... 

Chemical reactions are reversible. As reactants are converted to products, the concentration of the reactants decreases, and the concentration of the products increases. Since rates are related to concentrations, the rate of the forward reaction begins to slow, and the rate of the reverse reaction quickens as a reaction proceeds. Eventually, the two rates become equal. This condition, where the forward reaction rate equals the reverse reaction rate, is called chemical equilibrium. At chemical equilibrium, there is no change in the concentration of the products or reactants. Equilibrium will be reached from either direction, beginning with predominantly reactants or predominantly products. Equilibrium is the point of greatest entropy. 

The fuel cell was discussed in Chapter 6. The automobile fuel cell will likely make a profound impact on the private automobile a repeat of the discussion of how fuel cells function is warranted. A fuel cell is a special type of battery. In commonplace batteries, the chemicals and hardware used to produce the electric current are placed within the battery package. As the battery is used, the reaction products remain within the confines of the package. Because of the fixed mass of reactive chemicals, a battery has a fixed total power output. When it is depleted, it must be replaced or recharged. Recharging is the slow process of driving the chemical reaction in reverse with an external source of electric power. 

You know that changes constantly occur, but some changes are not permanent. For example, liquid water freezes into ice, but then ice melts and becomes liquid water again. In other words, the freezing process is reversed. Can chemical reactions be reversed Can the product of a reaction become a reactant ... 

In principle, all chemical reactions are reversible, but this reversibility may not be observable if the fraction of products in the equilibrium mixture is very small, or if the reverse reaction is kinetically inhibited. 

Note that in equation (2) atoms are conserved because the temperatures at which chemical reactions are normally carried out are insufficiently high to cause nuclear reactions to occur. The arrows in equations (1) and (2) denote that the chemical species on the left hand side are the reactants, and that they are being converted into the products given on the right hand side. It is conventional to place the reactants on the left hand side of the stoichiometric expression, and products on the right, and the reaction will proceed by depletion of reactants and formation of products, which is called the forward direction. If the products are mixed initially, in the absence of reactants, the reaction will proceed in the reverse direction. It will be shown below that all chemical reactions are reversible. 

Theoretically, in closed systems all chemical reactions are reversible and can be characteri d with equilibrium constants. You may wonder if the equilibrium reactions described are somehow different from those used in previous chapters where only a single reaction arrow was used. Many of those reactions have large K values and proceed to make mainly products. Equilibrium concentrations (and double reaction arrows) are most meaningful for reactions that have K values that are neither extremely large nor extremely small and have observable quantities of both reactants and products at equilibrium. 

Let us look at equation (4.1.2) from the viewpoint of entropy flow d S and entropy production diS, that was introduced in the previous chapter. To make a distinction between irreversible chemical reactions and reversible exchange with the exterior, we express the change in the mole numbers dN as a sum of two parts ... 

Many chemical reactions are reversible under ordinary conditions of temperature and concentration. They will reach a state of equilibrium unless at least one of the substances involved escapes or is removed from the reaction system. In some cases, however, the forward reaction is so predominant that essentially all reactants will react to form products. Here, the products of the forward reaction are favored, meaning that at equilibrium there is a much higher concentration of products than of reactants. Hence, we can say that the equUibrium "hes to the right, because products predominate, and products conventionally are written on the right side of a chemical equation. An example of such a system is the formation of sulfur trioxide from sulfur dioxide and oxygen. 

Most chemical reactions are reversible. Whether a reaction is proceeding from reactants to products or from products back to reactants, the transition state is the same in both directions. 

A great many chemical reactions are reversible. That is, under certain conditions it is possible to start with the products and make a measurable amount of the reactants. In these cases an equilibrium state can exist in which the reaction comes to a standstill because the forward and reverse rates are equal. This equilibrium point determines how much of the reactants can be converted into products and also what conditions will favor conversion. 

From a macroscopic perspective, a chemical reaction consists of a vast amount of sequential, individual, chemical steps. Each step takes place when one or more molecules of one or more chemical species (the reactants) interact via collisions and transform their chemical nature to give rise to a different set of molecules of distinct chemical species (the products). Strictly speaking, all chemical reactions are reversible because it is possible that the product molecules collide in such a way that they react and give rise to the reactant molecules. The usual way to represent these processes is as follows ... 

All chemical reactions are reversible, and reactants and products interconvert to varions degrees. When the concentrations of reactants and products no longer change, the reaction is in a state of equilibrium. In many cases, equilibrium Ues extensively (say, more than 99.9%) on the side of the products. When this occurs, the reaction is said to go to completion. (In such cases, the arrow indicating the reverse reaction is usually omitted and, for practical purposes, the reaction is considered to be irreversible.)... 

In principle, all chemical reactions are reversible, and given sufficient time an equilibrium will be established for any chemical reaction. At equihbrium, the reactions in the forward and reverse directions occur at equal rates, and there is no net change in the concentrations of reactants and products. The extent to which reactants are converted to products is expressed by an equihbrium constant. 

In principle, all chemical reactions are reversible if we wait long enough they come to some equilibrium state at which some amount of reactants are in equilibrium with some amount of products. However, as the following example shows, in practice, some reactions seem irreversible, with practically complete consumption of the reactants. 

The MR concept is based on the removal of a reaction product in order to avoid the reaction equilibrium conditions to be achieved and promoting reaction kinetics. Most of industrial chemical reactions are reversible... 

As seen in previous sections, the standard entropy AS of a chemical reaction can be detemiined from the equilibrium constant K and its temperature derivative, or equivalently from the temperature derivative of the standard emf of a reversible electrochemical cell. As in the previous case, calorimetric measurements on the separate reactants and products, plus the usual extrapolation, will... 

The process temperature affects the rate and the extent of hydrogenation as it does any chemical reaction. Practically every hydrogenation reaction can be reversed by increasing temperature. If a second functional group is present, high temperatures often lead to the loss of selectivity and, therefore, loss of desired product yield. As a practical measure, hydrogenation is carried out at as low a temperature as possible which is stiU compatible with a satisfactory reaction rate. 

Complexes 79 show several types of chemical reactions (87CCR229). Nucleophilic addition may proceed at the C2 and S atoms. In excess potassium cyanide, 79 (R = R = R" = R = H) forms mainly the allyl sulfide complex 82 (R = H, Nu = CN) (84JA2901). The reaction of sodium methylate, phenyl-, and 2-thienyllithium with 79 (R = R = r" = R = H) follows the same route. The fragment consisting of three coplanar carbon atoms is described as the allyl system over which the Tr-electron density is delocalized. The sulfur atom may participate in delocalization to some extent. Complex 82 (R = H, Nu = CN) may be proto-nated by hydrochloric acid to yield the product where the 2-cyanothiophene has been converted into 2,3-dihydro-2-cyanothiophene. The initial thiophene complex 79 (R = R = r" = R = H) reacts reversibly with tri-n-butylphosphine followed by the formation of 82 [R = H, Nu = P(n-Bu)3]. Less basic phosphines, such as methyldiphenylphosphine, add with much greater difficulty. The reaction of 79 (r2 = r3 = r4 = r5 = h) with the hydride anion [BH4, HFe(CO)4, HW(CO)J] followed by the formation of 82 (R = Nu, H) has also been studied in detail. When the hydride anion originates from HFe(CO)4, the process is complicated by the formation of side products 83 and 84. The 2-methylthiophene complex 79... 

Every chemical reaction can go in either forward or reverse direction. Reactants can go forward to products, and products can revert to reactants. As you may remember from your general chemistry course, the position of the resulting chemical equilibrium is expressed by an equation in which /Cec], the equilibrium constant, is equal to the product concentrations multiplied together, divided by the reactant concentrations multiplied together, with each concentration raised to the power of its coefficient in the balanced equation. Eor the generalized reaction... 

Chemical reactions involving gases carried out in closed containers resemble in many ways the H20(/)-H20(g) system. The reactions are reversible reactants are not completely consumed. Instead, an equilibrium mixture containing both products and reactants is obtained. At equilibrium, forward and reverse reactions take place at the same rate. As a result, the amounts of all species at equilibrium remain constant with time. 

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