Equilibrium arrow

We have now encountered a number of different types of arrow routinely used in chemistry to convey particular meanings. We have met curly arrows used in mechanisms, double-headed resonance arrows, equilibrium arrows, and the simple single arrows used for reactions. This is a convenient point to bring together the different types and provide a checklist for future reference. We are also showing how additional information about a reaction may be presented with the arrow. 

Many types of cirrows cire used in organic chemistry, and each of them conveys information about the particular reaction. These arrows include the resonance arrow, equilibrium arrow, reaction arrow, double-headed arrow, and singleheaded cirrow. 

Electron delocalization is shown by double-headed arrows ( ). Equilibrium is shown by two arrows pointing in opposite directions (... 

Suppose we have two water tanks connected by a pipe. When the water levels in the tanks are equal, water flows in the forward direction from Tank A to Tank B at the same rate as it flows in the reverse direction from Tank B to Tank A. If we add more water to Tank A, there is an increase in the rate at which water flows from Tank A into Tank B, which is shown with a longer arrow. Equilibrium is reestablished when the water levels in both tanks become equal. The water levels are higher than before, but the water flows equally between Tank A and Tank B. 

The equilibrium shape of a liquid lens floating on a liquid surface was considered by Langmuir [59], Miller [60], and Donahue and Bartell [61]. More general cases were treated by Princen and Mason [62] and the thermodynamics of a liquid lens has been treated by Rowlinson [63]. The profile of an oil lens floating on water is shown in Fig. IV-4. The three interfacial tensions may be represented by arrows forming a Newman triangle ... 

By convention, species to the left of the arrows are called reactants, and those on the right side of the arrows are called products. As Berthollet discovered, writing a reaction in this fashion does not guarantee that the reaction of A and B to produce C and D is favorable. Depending on initial conditions, the reaction may move to the left, to the right, or be in a state of equilibrium. Understanding the factors that determine the final position of a reaction is one of the goals of chemical thermodynamics. 

The accompanying sketch qualitatively describes the phase diagram for the system nylon-6,6, water, phenol for T > 70°C.f In this figure the broken lines are the lines whose terminals indicate the concentrations of the three components in the two equilibrium phases. Consult a physical chemistry textbook for the information as to how such concentrations are read. In the two-phase region, both phases contain nylon, but the water-rich phase contains the nylon at a lower concentration. On this phase diagram or a facsimile, draw arrows which trace the following procedure ... 

Fig. 1.20 Cell consisting of two reversible Ag /Ag electrodes (Ag in AgN03 solution). The rate and direction of charge transfer is indicated by the length and arrow-head as follows gain of electrons by Ag -he- Ag—> loss of electrons by Ag - Ag + e- —. (o) Both electrodes at equilibrium and (f>) electrodes polarised by an external source of e.m.f. the position of the electrodes in the vertical direction indicates the potential change. (K, high-impedance voltmeter A, ammeter R, variable resistance)...

In this equation /g, is th equilibrium exchange current, and the arrow convention adopted is that / g represents the rate of cathodic reduction... 

The value of the equilibrium constant tells which side of the reaction arrow is energetically favored. If Keq is much larger than 1, then the product concentration term [C 4 [Dlrf is much larger than the reactant concentration term A " B, and the reaction proceeds as written from left to right. If Keq is near 1, appreciable amounts of both reactant and product are present at equilibrium. And if Koq is much smaller than l, the reaction does not take place as written but instead goes in the reverse direction, from right to left. 

The double arrow implies that the forward and reverse processes are occurring at the same Liquid-vapor equilibrium. Under the... 

The painting, "Equilibrium," shows arrows pointing in opposite directions. The same symbols are used to denote a chemical reaction in equilibrium. 

This open question is symbolized by the unusual type of equilibrium arrows used in Scheme 11-2. 

FIGURE 9.6 The relative sizes of the reaction quotient Q and the equilibrium constant K indicate the direction in which a reaction mixture tends to change. The arrows show that, when Q < K, reactants form products (left and when Q> K, products form reactants (right). There is no tendency to change once the reaction quotient has become equal to the equilibrium constant. 

Another example of an acid is hydrogen cyanide, HCN, which transfers its proton to water when it dissolves to form the solution known as hydrocyanic acid, HCN(aq). However, only a small fraction of the HCN molecules donate their protons, and so we classify HCN as a weak acid in water. We write the proton transfer reaction with equilibrium half-arrows ... 

Like all chemical equilibria, this equilibrium is dynamic and we should think of protons as ceaselessly exchanging between HCN and H20 molecules, with a constant but low concentration of CN and H30+ ions. The proton transfer reaction of a strong acid, such as HCl, in water is also dynamic, but the equilibrium lies so strongly in favor of products that we represent it just by its forward reaction with a single arrow. 

Figure 2. The carbon dynamics of a primary forest prior to and following deforestation and slash burning. Arrows represent the relative magnitude of C flux. In the primary forest (represented by the large box at the top of the figure), the C pool is in a dynamic equilibrium with inputs approximately equalling exports. With deforestation and fire, the balance is altered with exports far exceeding imports.

The double-headed arrow is intended to imply the existence of a resonance hybrid a stmcture with an electronic distribution intermediate between the two shown. Every instmctor knows the hazards of this portrayal. Firstly, the doubleheaded arrow is misinterpreted by some students to mean either (i) that there is an equilibrium condition involving the two different species, or (ii) that flipping occurs between the two species. A second problem is demonstrated by those students who ask Are these not the same If we rotate one of the molecules by 60°, we see that they are identical . We can hypothesise that the latter problem may be exacerbated by the tendency of textbooks (and probably teachers) to talk about these two different resonance stmctures as though we are referring to two different molecules - when, in fact, we are talking about different electron distributions in just one molecule. It seems so important for instractors to refer to just one set of six carbon atoms joined by ct bonds, and then to discuss alternative distributions of the six TT electrons within that system. 

Notice that we are using equilibrium arrows here (=... 

If the concentration of a solute is lower than its solubility, additional solute can dissolve, but once the concentration of solute reaches the solubility of that substance, no further net changes occur. Individual solute molecules still enter the solution, but the solubility process is balanced by precipitation, as Figure 12-6 illustrates. A saturated solution in contact with excess solute is in a state of dynamic equilibrium. For eveiy molecule or ion that enters the solution, another returns to the solid state. We represent d Tiamic equilibria by writing the equations using double arrows, showing that both processes occur simultaneously ... 

Look again at Figure 16-1 If two NO2 molecules can form a bond when they collide, then that bond also can break apart when an N2 O4 molecule distorts. The concept of reversibility is a general principle that applies to all molecular processes. Every elementary reaction that goes in the forward direction can also go In the reverse direction. As a consequence of reversibility, we can write each step in a chemical mechanism using a double arrow to describe what happens at chemical equilibrium. 

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