Reaction at equilibrium

We need to be able to identify the equilibrium composition of a reaction so that we can discuss the approach to equilibrium systematically. 

At equilibrium, the reaction quotient has a certain (dimensionless) value called the equilibrium constant, K, of the reaction  

We shall not normally write equilibrium-, the context will always make it clear that Q refers to an arbitrary stage of the reaction, whereas K, the value of Q at 

This is one of the most important equations in the whole of chemical thermodynamics. Its principal use is to predict the value of the equihbrium constant of any reaction from tables of thermodynamic data, hke those in the Resource section. Alternatively, we can use it to determine Afi by measuring the equilibrium constant of a reaction. 

The first step in the metabolic breakdown of glucose is its phosphorylation to G6P  

Chemical reactions can not only go one way or the other (our main problem), but they can stop going for two reasons. Either one of the reactants is used up, or the reaction can reach an equilibrium state, with all products and reactants present in a baianced condition. The second possibility is the subject of this chapter-how much can we predict about this balanced state of equilibrium  

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Although a system at equilibrium appears static on a macroscopic level, it is important to remember that the forward and reverse reactions still occur. A reaction at equilibrium exists in a steady state, in which the rate at which any species forms equals the rate at which it is consumed. 

For a reaction at equilibrium, the equilibrium constant determines the relative concentrations of products and reactants. 

The standard-state electrochemical potential, E°, provides an alternative way of expressing the equilibrium constant for a redox reaction. Since a reaction at equilibrium has a AG of zero, the electrochemical potential, E, also must be zero. Substituting into equation 6.24 and rearranging shows that... 

The observation that a system at equilibrium responds to a stress by reequilibrating in a manner that diminishes the stress, is formalized as Le Chatelier s principle. One of the most common stresses that we can apply to a reaction at equilibrium is to change the concentration of a reactant or product. We already have seen, in the case of sodium acetate and acetic acid, that adding a product to a reaction mixture at equilibrium converts a portion of the products to reactants. In this instance, we disturb the equilibrium by adding a product, and the stress is diminished by partially reacting the excess product. Adding acetic acid has the opposite effect, partially converting the excess acetic acid to acetate. 

Le Chatelier s principle (Section 6.10) A reaction at equilibrium responds to any stress imposed on it by shifting the equilibrium in the direction that minimizes the stress. 

In a system of connected reversible reactions at equilibrium, each reversible reaction is individually at equilibrium. 

Let us examine the equilibrium assumption of transition state theory. Consider a reversible elementary reaction at equilibrium. Because the initial and final states are at equilibrium, assuredly the transition state is in equilibrium with each of these. (It follows that for a reaction at equilibrium, transition state theory is exact insofar as the equilibrium assumption is concerned.)... 

The figures below represent the following reaction at equilibrium at different temperatures. 

Figure 5.9 Graph of Gibbs free energy against , the extent of the reaction. The minimum in the curve gives the extent of the reaction at equilibrium.

FIGURE 9.13 Le Chatelier s principle predicts that, when a reaction at equilibrium is compressed, the number of molecules in the gas phase will tend to decrease. This diagram illustrates the effect of compression and expansion on the dissociation equilibrium ot a diatomic molecule. Note the increase in the relative concentration of diatomic molecules as the system is compressed and the decrease when the system expands. 

Le Chatelier s principle When a stress is applied to a system in dynamic equilibrium, the equilibrium adjusts to minimize the effect of the stress. Example a reaction at equilibrium tends to proceed in the endothermic direction when the temperature is raised, leveling The observation that strong acids all have the same strength in water, and all behave as though they were solutions of H,Of ions. 

By definition, the free energy change for a reaction at equilibrium is zero, hence ... 

In a reaction at equilibrium, the forward and reverse reactions occur at equal rates, and there is therefore no net flux in either direction ... 

For a reaction at equilibrium, the rate of the forward reaction is balanced exactly by the rate of the reverse reaction. For this reason, any equilibrium reaction can be written in either direction. The equilibrium constant for the Flaber synthesis of ammonia, for example, can be expressed in two ways ... 

Equation (6.13), in fact, reflects the physical nature of the electrode process, consisting of the anode (the first term) and cathode (the second term) reactions. At equilibrium potential, E = Eq, the rates of both reactions are equal and the net current is zero, although both anode and cathode currents are nonzero and are equal to the exchange current f. With the variation of the electrode potential, the rate of one of these reactions increases, whereas that of the other decreases. At sufficiently large electrode polarization (i.e., deviation of the electrode potential from Eg), one of these processes dominates (depending on the sign of E - Eg) and the dependence of the net current on the potential is approximately exponential (Tafel equation). 

How can you know whether reactants or products are favored in a reaction at equilibrium The answer depends upon the reaction. For the club soda reaction, a measurement of pH indicates the amount of acid present in a solution. The lower the pH, the more acid is present. 

What happens to a reaction at equilibrium if one of the products is removed The reaction goes to completion because a product is not available to react in the reverse direction. 

Ans. In a reaction at equilibrium, the ratio can have only one value at any given temperature. In the Nernst equation, the value can change, since the reaction can be stopped short of equilibrium simply by disconnecting a wire or the salt bridge. 

The first two terms on the right represent the net rate of reaction at equilibrium which must be zero. Hence... 

Fig. 3 The effect of adding in the back reaction at equilibrium is to double the concentration of X.

In a chemical context, therefore, the system A + B —C + D reaches a dynamic equilibrium when the rate of the forward reaction equals the rate of the backward reaction. At equilibrium, the concentrations of A, B, C and D remain constant. 

Surface reaction at equilibrium, only A not in adsorptive equilibrium. 

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