Predicting the Direction of Net Chemical Change

In Chapter 13, we used the laws of thermodynamics, specifically fhe second law, to establish criteria (Tables 13.4 and 13.6) for predicting the direction of net chemical change in a system from a given set of initial conditions. We summarize some key results in Table 15.4. 

TABLE 15.4 Using Q and K to Predict the Direction of Net Chemical Change 

We can justify the relationships in Table 15.4 by considering the three experiments we discussed in Section 15-2. These three experiments involve the following reaction 

In Section 15-2, we found that Q must have the value K = 1.48 when this reaction reaches equilibrium at 298 K. To be able to predict the direction of net change when Q K, we must first establish how the value of Q changes when there is net change in the forward direction to the right) or in the reverse direction (to the left). 

Conversely, when there is net change to the left, [Cu ] and [Sn ] decrease whereas [Cu ] and [Sn ] increase. The numerator in the expression for Q decreases, the denominator increases, and thus, the value of Q decreases. The value of Q decreases when there is net change to the left. We conclude that when Q K, the equilibrium condition Q = K can be achieved only if there is net change in the reverse direction or to the left. 

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Predicting the Direction of Net Chemical Change—A comparison of the reaction quotient with the equilibrium constant makes it possible to predict the direction of net change leading to equilibrium (Fig. 15-4). If Q < K, the forward reaction is favored, meaning that when equilibrium is established the amounts of products will have increased and the amounts of reactants will... 

A chemical reaction proceeding toward equilibrium is a spontaneous change. Recall that we can predict the net direction of the reaction—its spontaneous direction— by comparing the reaction quotient (0 with the equilibrium constant (K). But why is there a drive to attain equilibrium And what determines the value of the equilibrium constant And, most importantly, can we predict the direction of a spontaneous change in cases that are not as obvious as burning gasoline or falling books ... 

Predicting the Direction of a Net Chemical Change in Establishing Equilibrium... 

For example, suppose that water vapor is removed from the reaction vessel containing the equilibrium mixture for methanation. Le Chatelier s principle predicts that net chemical change will occur to partially reinstate the original concentration of water vapor. This means that the methanation reaction momentarily goes in the forward direction,... 

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