Equilibrium A state of dynamic balance

A reversible reaction is a reaction that can take place in both the forward and reverse directions. An example is the reaction of hydrogen and bromine at elevated temperature to form hydrogen bromide, and the reverse reaction, the decomposition of hydrogen bromide into its elements. 

These two equations may be combined into a single equation with a double arrow to show that both reactions occur. 

When forward and reverse reactions occur at equal rates, a state of chemical equilibrium results in which the concentrations of reactants and products remain constant. 

The law of chemical equilibrium states that at a given temperature, a chemical system may achieve a state in which a certain ratio of reactant and product concentrations has a constant value. The general equation for a reaction at equilibrium is as follows. 

The law of chemical equilibrium may be used to write the equilibrium constant expression for the reaction. 

When you get off a whirling amusement park ride, you probably pause a minute to get your equilibrium. If so, you are talking about getting your balance back after the ride exerted rapidly changing forces on you. But soon you are balanced steadily on your feet once more. Often, chemical reactions also reach a point of balance or equilibrium. The DISCOVERY LAB is an analogy for chemical equilibrium. You found that a point of balance was reached in the transfer of water from the beaker to the graduated cylinder and from the graduated cylinder to the beaker. 

Consider the reaction for the formation of ammonia from nitrogen and hydrogen that you learned about in Chapter 16. 

This reaction is important to agriculture because ammonia is used widely as a source of nitrogen for fertilizing corn and other farm crops. The photo on the opposite page shows ammonia being knifed into the soil. 

Note that the equation for the production of ammonia has a negative standard free energy, AG°. Recall that a negative sign for AG° indicates that the 

The concentrations of the reactants (H2 and N2) decrease at first while the concentration of the product (NH3) increases. Then, before the reactants are used up, all concentrations become constant. 

---

Equilibrium A state of dynamic balance in which the rates of forward and reverse processes (reactions) are equal the state of a system when neither the forward nor the reverse process is thermodynamically favored. 

We have learned much about equilibrium. It is characterized by constancy of macroscopic properties but with molecular processes continuing in a state of dynamic balance. At equilibrium we can conclude that every reaction that takes place does so at the same reaction rate as its reverse reaction. 

The rate and extent of these processes can change over time. A mass balance usually reflects one of two assumptions or their converse the system is at equilibrium or the system is at steady state. Equilibrium is a state of dynamic balance—such as that which occurs when forward and reverse reactions are equal— where there is no impetus for change within the system. Steady state simply means that the condition being evaluated (which might be concentration in a certain phase or the flow rate, for example) is essentially unchanging over a specified time period. 

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 ... 

Metabolic studies on cestodes present a number of challenging problems. A basic difficulty is to relate results from studies carried out in vitro to the processes which actually occur in vivo. Cestodes, with their complex tegumental transport mechanisms (Chapters 5 and 6), are in a state of dynamic equilibrium with their hosts, and removal from the host environment tends to destroy this balance. The most favourable in vitro culture conditions (Chapter 10) can partially mimic some of the complicated interactions which occur between host and parasite, but, nevertheless, it is clear that most metabolic experiments with cestodes in vitro take place under suboptimal, unphysiological conditions. Many (but not all) parasite biochemists are aware of these limitations and appreciate that these artificial systems produce data which must ultimately be tested in vivo, although the technical problems involved in such studies may prove difficult, if not insurmountable. 

Let us return to the equilibrium situation of an n-type semiconductor in contact with a redox electrolyte and reconsider the situation in Figure 9a. This is shown again in Figure 12a to underline the fact that the interface is in a state of dynamic equilibrium. That is, the forward and reverse (partial) currents exactly balance each other and there is no net current flow across the interface. In fact, the situation here is much like that at a metal redox electrolyte interface at the rest potential. We can... 

The rate of evaporation is constant at any given temperature, and the rate of condensation increases with the increasing concentration of molecules in the vapor phase. A state of dynamic equilibrium, in which the rate of a forward process is exactly balanced by the rate of the reverse process, is reached when the rates of condensation and evaporation become equal (Figure 11.34). The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation. We often use the simpler term vapor pressure when we talk about the equilibrium vapor pressure of a liquid. This practice is acceptable as long as we know the meaning of the abbreviated term. 

Balancing To and Fro As you ll learn in this chapter, the continuai back and forth flow of students going to class mimics the forward and reverse steps of a chemical reaction in a state of dynamic equilibrium. 

From the total balance of the flows in the atmosphere, it follows that the main contribution to the increasing CO2 concentration in the air comes from the fossil fuel combustion. At a rate of consumption of 5 miUiard t annually, the CO2 concentration in the atmosphere would increase by 0.7%. The actually measured annual increase is, however, only 1/3 of this value. This means that the remaining 2/3 are rapidly removed from the atmosphere, partly by dissolution in the oceans and partly by consumption for the production of the biomass on the earth s surface. The flow of carbon dioxide is partially maintained in a state of dynamic equilibrium through these autoregulation processes. 

In a closed container pardy filled with water the air over the fiquid soon becomes mixed with water vapor. As the air starts to become saturated with water vapor, some of the water vapor starts to condense. Eventually, the evaporation and condensation of the water estabUsh a dynamic equilibrium—a. state of balance between exactly opposite changes occurring at the same rate. To indicate this equilibrium, a double arrow is placed between the symbols for water in the liquid and vapor states (Figure 8.7) ... 

Dynamic equilibrium A state of balance between opposite changes occurring at the same rate... 

A system at dynamic equilibrium is in a state of balance. When the concentrations of species in the reaction are altered, the equilibrium shifts until a new state of balance is attained. What does shift mean It means that reactant and product concentrations change over time to accommodate the new situation. Shift does not mean that the equilibrium constant itself is altered the equilibrium constant remains the same. Le Chatelier s principle states that the shift is in the direction that minimizes or reduces the effect of the change. Therefore, if a chemical system is already at equilibrium and the concentration of any substance in the mixture is increased (either reactant or product), the system reacts to consume some of that substance. Conversely, if the concentration of a substance is decreased, the system reacts to produce some of that substance. 

To be in equilibrium is to be in a state of balance. A tug of war in which the two sides pull with equal force so that the rope does not move is an example of a sfaf/c equilibrium, one in which an object is at rest. Equilibria can also be dynamic, whereby a forward process and the reverse process take place at the same rate so that no net change occurs. 

Equilibrium C -kw3- li-bre-3m [L aequil-hrium, fr. aequilibris being in equilibrium, fr. aequi- + libra, weight, balance] (1608) n. A state of balance between opposing forces or actions that is either static (as in a body acted on by forces whose resultant is zero) or dynamic (as in a reversible chemical reaction when the velocities in both directions are equal) Chemical a state of affairs in which a chemical reaction and its reverse reaction are taking place at equal velocities, so that the concentrations of reacting substances remain constant. 

---