General Chemistry Principles, Patterns, and Applications, 2011

to a liquid that can be drawn off. Continuing to remove water from the system forces the reaction to the
right as the system attempts to equilibrate, thus enriching the reaction mixture in methane. This
technique, referred to as driving a reaction to completion, can be used to force a reaction to completion
even if K is relatively small. For example, esters are usually synthesized by removing water. The products
of the condensation reaction are shown here. In Chapter 19 "Electrochemistry", we will describe the
thermodynamic basis for the change in the equilibrium position caused by changes in the concentrations
of reaction components.
E X A M P L E 1 6
For each equilibrium system, predict the effect of the indicated stress on the specified quantity.
a.
2SO2 g
( ) +O2( g) 2SO3( g) : (1) the effect of removing O2 on PSO3 (2) the
effect of removing O 2 on PSO3
CaCO3 s
( ) CaO ( s) + CO2( g) : (1) the effect of removing CO2 on the amount of
CaCO 3; (2) the effect of adding CaCO 3 on PCO2
Given: balanced chemical equations and changes
Asked for: effects of indicated stresses
Strategy:
Use Q and K to predict the effect of the stress on each reaction.
Solution:
a. (1) Removing O 2 will decrease PCO2 thereby decreasing the denominator in the
reaction quotient and making Q p > K p. The reaction will proceed to the left as written,
increasing the partial pressures of SO 2 and O 2 until Q p once again equalsK p. (2) Removing
O 2 will decrease PCO2 and thus increase Q p, so the reaction will proceed to the left. The
partial pressure of SO 3 will decrease.
b. K p and Q p are both equal to PCO2 . (1) Removing CO 2 from the system causes more
CaCO 3 to react to produce CO 2, which increases PCO2 to the partial pressure required
by K p. (2) Adding (or removing) solid CaCO 3 has no effect on PCO2 because it does not
appear in the expression for K p (or Q p).
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