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General Chemistry Principles, Patterns, and Applications, 2011

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Initially the<br />

concentration of CO2(g) increases linearly with the amount of solid PbCO3 added, as<br />

PbCO3 decomposes to CO2(g) <strong>and</strong> solid PbO. Once the CO2concentration reaches the value that<br />

corresponds to the equilibrium concentration, however, adding more solid PbCO3 has no effect on<br />

[CO2], as long as the temperature remains constant.<br />

In contrast, the reduction of cadmium oxide by hydrogen gives metallic cadmium <strong>and</strong> water vapor:<br />

Equation 15.31<br />

CdO s<br />

( ) + H2( g) Cd( s) + H2O( g)<br />

<strong>and</strong> the equilibrium constant K is [H2O]/[H2]. If [H2O] is doubled at equilibrium, then [H2] must also be<br />

doubled for the system to remain at equilibrium. A plot of [H2O] versus [H2] at equilibrium is a straight<br />

line with a slope of K (Figure 15.8 "The Concentration of Water Vapor versus the Concentration of<br />

Hydrogen for the "). Again, only those pairs of concentrations of H2O <strong>and</strong> H2 that lie on the line<br />

correspond to equilibrium states. Any point representing a pair of concentrations that does not lie on the<br />

line corresponds to a nonequilibrium state. In such cases, the reaction in Equation 15.31 will proceed in<br />

whichever direction causes the composition of the system to move toward the equilibrium line. For<br />

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

Saylor.org<br />

1394

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