The Effect of a Concentration Change on Equilibrium

EquUibiiuin is disturbed Population is added to Middle Earth 

Disturbance is minimized Population flows out of Middle Earth and into Narnia 

When we say that a reaction shifts to the left we mean that it proceeds in the reverse direction, consuming products and forming reactants. 

The reaction shifts to the left because the value of Q changes as follows  

A FIGURE 14.9 Le Chateliet s Principle The Effect of a Concentration Change Adding NO2 causes the reaction to shift left, consuming some of the added NO2 and forming more N2O4. 

A FIGURE 14.10 Le Chatelier s Principle Changing Concentration The graph shows the concentrations of NO2 and N2O4 for the reaction 

In both of these cases, the system shifts in a direction that minimizes the disturbance. Lowering the concentration of a reactant (which makes Q K) causes the system to shift in the direction of the reactants to minimize the disturbance. Lowering the concentration of a product (which makes Q K) causes the systan to shift in the direction of products. 

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Determining the Effect of a Concentration Change on Equilibrium (14.9) Example 14.14 For Practice 14.14 Exercises 63-66... 

Effect of a Volume Change on Equilibrium Like the effect of concentration, the effect of pressure on equilibrium allows a chemist to choose the best conditions under which to carry out a chemical reaction. Some reactions are favored in the forward direction by high pressure (those with fewer moles of gas particles in the products), and others (those with fewer moles of gas particles in the reactants) are favored in the forward direction by low pressure. 

It is seen from the equilibrium expressions that the effectiveness of a buffer depends on the concentrations of the buffering substances a tenfold dilution of the buffer decreases by the factor 10 the amount of acid or base per liter which can be added without causing the pH to change more than the desired amount. 

At constant temperature, a decrease in volume (iucrease in pressure) increases the concentrations of both A and D. In the expression for Q, the concentration of D is squared aud the conceutratiou of A is raised to the first power. As a result, the numerator of Q increases more than the denominator as pressure increases. Thus, Q > K, and this equilibrium shifts to the left. Couversely, an increase in volume (decrease in pressure) shifts this reaction to the right until equilibrium is reestablished, because Q < K. We can summarize the effect of pressure (volume) changes on this gas-phase system at equilibrium. 

Catalysts increase the rate of reactions. It is found experimentally that addition of a catalyst to a system at equilibrium does not alter the equilibrium state. Hence it must be true that any catalyst has the same effect on the rates of the forward and reverse reactions. You will recall that the effect of a catalyst on reaction rates can be discussed in terms of lowering the activation energy. This lowering is effective in increasing the rate in both directions, forward and reverse. Thus, a catalyst produces no net change in the equilibrium concentrations even though the system may reach equilibrium much more rapidly than it did without the catalyst. 

The expression for K involving the concentrations of the species involved is found to be independent of volume. This implies that any change of pressure is not going to change the final state of equilibrium. The same result can be obtained by taking into consideration the alternative expression involving the partial pressures. If the pressure on the system is increased to n times its original value then all the partial pressures will be increased in the same proportion. This obviously implies that the equilibrium is independent of the pressure. The effect of some other factors on this reaction may now be considered. One such factor can be the addition of substances. For example, on addition of more A2, the partial pressure of A2 in the reactor would increase momentarily from pAl to some value, p A/. It has already been seen that... 

As the concentration of EtOH increased from 0 to 10%, the effective steady-state mass transfer coefficient declined from 0.17 1/hr to 0.11 1/hr, which was due in part to the change in Darcy velocity. Using correlations developed for Ke,ss as a function of Darcy velocity and alcohol concentration (Taylor, 1999), the effect of EtOH concentration can be evaluated at a single, representative Darcy velocity. For example, using a Darcy velocity of 4.0 cm/hr, the value of Ke,ss would be 0.14, 0.13, and 0.13 for 4% Tween 80, 4% Tween 80 + 5% EtOH and 4% Tween 80 + 10% EtOH, respectively. Thus, the addition of EtOH to 4% Tween 80 had no discemable influence on the effective steady-state mass transfer coefficient. It should be recognized, however, that although the mass transfer coefficient remained essentially unchanged, the steady-state concentration of PCE in the column effluent (C") and the cumulative PCE mass recovery increased substantially as a result of EtOH addition (Table 2). This behavior can be explained by the fact that the equilibrium solubility of PCE (C" sat) increased by more than 50%, from 26,900 mg/L to 42,300 mg/L, with the addition of 10% EtOH. 

Another example of the effect of a change of concentration upon the cathodic process can be found in electrolysis of a solution of salts of copper and bismuth. As the respective deposition potentials, which practically equal the equilibrium potentials are fairly close (7c( — 0.34 V, 71 it, = 0.23 V) the two metals cannot he separated from each other electrolytically. On the addition of cyanide, however, Cu++ ions are converted into cupricyanide ions from which copper cannot be deposited prior the cathode reaches the potential Ttt u equalling to about — 1.0 V. As bismuth does not form cyanide complexes the resulting difference in potentials, 7ti — 7Cou — 1.23 V is a sufficient guarantee that during electrolysis only bismuth will be, preferentially deposited. 

To see how we can predict the effects of a change in concentration on a system at equilibrium, we will consider the ammonia synthesis reaction. Suppose there is an equilibrium position described by these concentrations ... 

Chemical equilibrium represents a balance between forward and reverse reactions. In most cases, this balance is quite delicate. Changes in experimental conditions may disturb the balance and shift the equilibrium position so that more or less of the desired product is formed. When we say that an equilibrium position shifts to the right, for example, we mean that the net reaction is now from left to right. Variables that can be controlled experimentally are concentration, pressure, volume, and temperature. Here we will examine how each of these variables affects a reacting system at equilibrium. In addition, we will examine the effect of a catalyst on equilibrium. 

Up to this point, the discussion about the effect of a change in free fraction on free and total drug concentrations has been restricted to steady-state concentrations. However, there is the possibility that a drug, immediately after being displaced from a binding protein and before elimination equilibrium has been achieved, will have a transient change in its total or free concentration. 

The effect of a change in concentration on the equilibrium position is shown in Example 14.11. 

The measurement of pH is further complicated by the effect of high concentrations of sucrose (e.g., 60 Brix or 60%w/w) on hydrogen ion activity. Clarke (1970) has discussed the effect of sucrose solution structure on pH and calcium ion electrode processes and shown a decreased response of these electrodes to changes in ionic activity in sucrose solutions at 60 Brix and 24 °C. This reduced electrode response can in part be explained by the structural order of the sucrose-water mixture (molecular association in sucrose-water systems has been reviewed by Allen et at. (1974). In a 60 Brix sucrose solution the ratio of water molecules to sucrose molecules is 12.7 1, with water molecules hydrogen-bonded to sucrose (/.e., in the solvation shell) in dynamic equilibrium with free water. Therefore, the concentration of free water molecules and dissociated ions is much less than in dilute sucrose solutions. The number of water molecules in the sucrose solvation... 

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