Predicting the Direction of Reaction

To answer this question, we substitute the starting concentrations of N2, H2, and NH3 into the equilibrium-constant expression and compare its value to the equilibrium constant  

To reach equilibrium, the quotient [NH3]2/[N2][H2]2 must decrease from the starting value of 0.500 to the equilibrium value of 0.105. Because the system is closed, this change can happen only if [NH3] decreases and [N2] and [H2] increase. Thus, the reaction proceeds toward equilibrium by forming N2 and H2 from NH3 that is, the reaction as written in Equation 15.6 proceeds from right to left. 

This approach can be formalized by defining a quantity called the reaction quotient. The reaction quotient, Q, is a number obtained by substituting reactant and product concentrations or partial pressures at any point during a reaction into an equilibrium-constant expression. Therefore, for the general reaction 

Although we use what looks like the equilibrium-constant expression to calculate the reaction quotient, the concentrations we use may or may not be the equilibrium concentrations. For example, when we substituted the starting concentrations into the equilibrium-constant expression of Equation 15.22, we obtained = 0.500 whereas Kc = 0.105. The equilibrium constant has only one value at each temperature. The reaction quotient, however, varies as the reaction proceeds. 

Of what use is Q One practical thing we can do with Q is tell whether our reaction really is at equilibrium, which is an especially valuable option when a reaction is very slow. We can take samples of our reaction mixture as the reaction proceeds, separate the components, and measure their concentrations. Then we insert these numbers into Equation 15.23 for our reaction. To determine whether we are at equilibrium, or in which direction the reaction proceeds to achieve equilibrium, we compare the values of Qc and Kc or Qp and Kp. Three possible situations arise  

Suppose a gaseous mixture from an industrial plant has the following composition at 1200 K 0.0200 M CO, 0.0200 M H2, 0.00100 Af CH4, and 0.00100 M H2O. If the mixture is passed over a catalyst at 1200 K, would the reaction 

To answer this question, you substitute the concentrations of substances into the reaction quotient and compare its value to Kc- The reaction quotient, Q, is an expression that has the same form as the equilibrium-constant expression but whose concentration values are not necessarily those at equilibrium. For catalytic methanation, the reaction qnotient is 

Recall that the equilibrium constant for catalytic methanation is 3.92 at 1200 K. For the reaction mixture to go to equilibrium, the value of must decrease from 6.25 to 3.92. This will happen if the reaction goes to the left. In that case, the numerator of Qc ([CH4],[H20],) will decrease, and the denominator ([CO],[H2] ) will increase. Thus, the gaseous mixture wiU give more CO and H2. 

Consider the problem more generally. You are given a reaction mixture that is not yet at equilibrium. You would like to know in what direction the reaction will go as it approaches equilibrium. To answer this, you substitute the concentrations of substances from the mixture into the reaction quotient Q. Then, you compare Qc to the equilibrium constant Kc- 

The figure shows the relative sizes of Q and Reaction proceeds in the direction of the arrows.Thus, on the left,Qc Kc,so the reaction goes to the right,from reactants to products. 

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Strategy First calculate the partial pressures of N204 and N02, using the ideal gas law as applied to mixtures P, = tiiRT/V. Then calculate Q. Finally, compare Q and K to predict the direction of reaction. 

The reaction quotient Qc is useful because it lets us predict the direction of reaction by comparing the values of Qc and Kc. If Qc is less than Kc, movement toward equilibrium increases Qc by converting reactants to products (that is, net reaction proceeds from left to right). If Qc is greater than Kc, movement toward equilibrium decreases Qc by converting products to reactants (that is, net reaction proceeds from right to left). If Qc equals Kc, the reaction mixture is already at equilibrium, and no net reaction occurs. 

The value of the equilibrium constant for a reaction makes it possible to judge the extent of reaction, predict the direction of reaction, and calculate equilibrium concentrations (or partial pressures) from initial concentrations (or partial pressures). The farther the reaction proceeds toward completion, the larger the value of Kc. The direction of a reaction not at equilibrium depends on the relative values of Kc and the reaction quotient Qc, which is defined in the same way as Kc except that the concentrations in the equilibrium constant expression are not necessarily equilibrium concentrations. If Qc Kcr net reaction goes from left to right to attain equilibrium if Qc > Kc/ net reaction goes from right to left if Qc = Kc/ the system is at equilibrium. 

For each of the following equilibria, use Le Chatelier s principle to predict the direction of reaction when the volume is increased. 

To predict the direction of reaction, use the balanced equation to identify the proton donors (acids) and proton acceptors (bases), and then use Table 15.1 to identify the stronger acid and the stronger base. When equal concentrations of reactants and products are present, proton transfer always occurs from the stronger acid to the stronger base. 

The solubility product expression can be used for predicting whether or not precipitation will occur upon mixing two solutions and whether or not a precipitate will dissolve when in contact with a given solution. This represents an application of the general criteria for predicting the direction of reactions and was developed earlier. For the purposes of this discussion, it will be convenient to call the product of the concentrations of the ions each raised to the appropriate power, i.e., the right-hand side of the solubility product expression,... 

Predicting the direction of reaction. Consider a reaction mixture that is not at equi-hbrium. By substituting the concentrations of substances that exist in a reaction mixture into an expression similar to the equihbrium constant and comparing with K, you can predict whether the reaction will proceed toward products or toward reactants (as defined by the way you write the chemical equation). 

Predict the direction of reaction when H2 is removed from a mixture (lowering its concentration) in which the following equilibrium has been established ... 

Using the reaction quotient Given the concentrations of substances in a reaction mixture, predict the direction of reaction. (EXAMPLE 15.5)... 

The question just asked can be stated more generally Given the concentrations of substances in a reaction mixture, will the reaction go in the forward or the reverse direction To answer this, you evaluate the reaction quotient and compare it with the equilibrium constant K. The reaction quotient has the same form as the equilibrium-constant expression, but the concentrations of substances are not necessarily equilibrium values. Rather, they are concentrations at the start of a reaction. To predict the direction of reaction, you compare with K. ... 

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