The Effect of a Volume Change on Equilibrium

From the ideal gas law (PI/ = nPT), we can see that lowering the number of moles of a gas (n) results in a lower pressure (P) at constant temperature and volume. 

Notice thaf if a chemical reaction has an equal number of moles of gas particles on both sides of the chemical equation, a change in volume has no effect. For example, consider the following reaction  

Bofh fhe leff and fhe righf sides of the equation contain 2 mol of gas particles, so a change in volume has no effect on this reaction. In addition, a change in volume has no effect on a reaction that has no gaseous reactants or products. 

What is the effect of decreasing the volume of the reaction mixture Increasing the volume of the reaction mixture  

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Determining the Effect of a Volume Change on Equilibrium (14.9) Example 14.15 For Practice 14.15 Exercises 67, 68... 

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. 

Consider the effect of a volume change on this reaction at equilibrium. 

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. 

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... 

Equations (XV.5.8) and (XV.5.9) predict a change of rate constant with pressure which will depend logarithmically on the partial molar volume change for the transition-state reaction. An exactly similar equation, the Kelvin equation, can be written for the change of equilibrium constant with pressure. Since AY /RT is of the order of magnitude of 10 atm " for solution reactions, it is evident that the effect of these pressure changes will be of importance only at pressures in excess of 10 atm, and indeed this is verified experimentally. 

EFFECTS OF CHANGING THE VOLUME Le Chatelier s principle also predicts the effect of a change in volume on gas-phase equilibrium. Decreasing the volume of a gaseous system increases its total pressure, and the system responds, if possible, to reduce the total pressure. For example, in the equilibrium... 

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. 

Figure 17.8 The effect of a change in pressure (volume) on a system at equilibrium. The system of...

What is the effect of a change in volume on a chemical reaction (that includes gaseous reactants or products) initially at equilibrium ... 

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