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General Chemistry Principles, Patterns, and Applications, 2011

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Exercise<br />

For each equilibrium system, predict the effect that the indicated stress will have on the specified quantity.<br />

a.<br />

H2 g<br />

( ) + CO2( g) H2O( g) + CO( g) : (1) the effect of adding CO on [H2]; (2) the<br />

effect of adding CO 2 on [H 2]<br />

b.<br />

CuO s<br />

( ) + CO( g) Cu( s) + CO2( g) :<br />

c. (1) the effect of adding CO on the amount of Cu; (2) the effect of adding CO 2 on [CO]<br />

Answer:<br />

a. (1) [H 2] increases; (2) [H 2] decreases.<br />

b. (1) the amount of Cu increases; (2) [CO] increases.<br />

Changes in Total Pressure or Volume<br />

Because liquids are relatively incompressible, changing the pressure above a liquid solution has little<br />

effect on the concentrations of dissolved substances. Consequently, changes in external pressure have very<br />

little effect on equilibrium systems that contain only solids or liquids. In contrast, because gases are<br />

highly compressible, their concentrations vary dramatically with pressure. From the ideal gas<br />

law, PV = nRT, described in Chapter 11 "Liquids", the concentration (C) of a gas is related to its pressure<br />

as follows:<br />

Equation 15.37<br />

C = nV = PRT<br />

Hence the concentration of any gaseous reactant or product is directly proportional to the applied<br />

pressure (P) <strong>and</strong> inversely proportional to the total volume (V). Consequently, the equilibrium<br />

compositions of systems that contain gaseous substances are quite sensitive to changes in pressure,<br />

volume, <strong>and</strong> temperature.<br />

These principles can be illustrated using the reversible dissociation of gaseous N2O4 to gaseous<br />

NO2 (Equation 15.1). The syringe shown in Figure 15.12 "The Effect of Changing the Volume (<strong>and</strong> Thus the<br />

Pressure) of an Equilibrium Mixture of N" initially contains an equilibrium mixture of colorless N2O4 <strong>and</strong><br />

red-brown NO2. Decreasing the volume by 50% causes the mixture to become darker because all<br />

concentrations have doubled. Decreasing the volume also constitutes a stress, however, as we can see by<br />

examining the effect of a change in volume on Q. At equilibrium, Q = K = [NO2] 2 /[N2O4] (Equation 15.13).<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

1406

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