|Chemistry 8th Edition / Chang|
|Student Study Guide
FACTORS AFFECTING CHEMICAL EQUILIBRIUM (14.5)
Le Chatelier's Principle. When a reaction reaches a state of chemical equilibrium under a particular set of conditions, no further changes in the concentrations of reactants and products occur. If a change is made in the conditions under which the system is at equilibrium, chemical change will occur in such a way as to establish a new equilibrium. The factors that can influence equilibrium are change in concentration, change in pressure (or volume), and change in temperature.
What effect does a change in one of these factors have on the extent of reaction? This question can be answered qualitatively by using Le Chatelier's principle: When a stress is applied to a system in a state of dynamic equilibrium, the system will, if possible, shift to a new position of equilibrium in which the stress is partially offset. To interpret this statement take, for example, the reaction:
First, the reaction must be at equilibrium. Let's add more N2O4 as an illustration of a change in concentration. The concentration of N2O4 increases, and the equilibrium is disturbed. The system will respond by using up part of the additional N2O4. In this case, a net reverse reaction will partially offset the increased N2O4 concentration. The net reverse reaction brings the system to a new state of equilibrium. When equilibrium is reestablished, more NO2 will be present than there was before the N2O4 was added. Thus, the position of equilibrium has shifted to the left.
Le Chatelier's principle will predict the direction of the net reaction or "shift in equilibrium" that brings the system to a new equilibrium. In this case the stress of adding more N2O4 was partially offset by a net reverse reaction that consumed some of the additional N2O4. The key to the use of Le Chatelier's principle is to recognize which net reaction, forward or reverse, will partially offset the change in conditions.
Changes in Volume and Pressure. The pressure of a system of gases in chemical equilibrium can be increased by decreasing the available volume. This change causes the concentration of all components to increase. The stress will be partially offset by a net reaction that will lower the total concentration of gas molecules. Consider our previous reaction:
When the molecules of both gases are compressed into a smaller volume their total concentration increases (this is the stress). A net forward reaction (shift to the right) will bring the system to a new state of equilibrium, in which the total concentration of molecules will be lowered somewhat. This partially offsets the initial stress on the system. Notice that when 2 moles of NO2 react only 1 mole of N2O4 is formed. When equilibrium is reestablished, more moles of N2O4 and fewer moles of NO2 will be present than before the pressure increase occurred. In this case the equilibrium has shifted to the right. In general, an increase in pressure by decreasing the volume will result in a net reaction that decreases the total concentration of gas molecules. A special case arises when the total number of moles of gaseous products and of gaseous reactants are equal in the balanced equation. In this case no shift in equilibrium will occur.
Changes in Temperature. If the temperature of a system is changed, a change in the value of Kc occurs. An increase in temperature always shifts the equilibrium in the direction of the endothermic reaction, while a temperature decrease shifts the equilibrium in the direction of the exothermic reaction. Therefore, for endothermic reactions the value of Kc increases with increasing temperature, and for exothermic reactions the value of Kc decreases with increasing temperature. In the case of our example reaction, Kc will decrease as the temperature is increased because the equilibrium will shift in the direction of the endothermic reaction, that is, in the reverse direction.
2NO2(g) N2O4(g) Horxn = –58.0 kJ
We can explain this in terms of Le Chatelier's principle. As heat is added to the system, it represents a stress on the equilibrium. The equilibrium will shift in the direction that will consume some of the added heat. This partially offsets the stress. In this reaction, the equilibrium shifts to the left, and Kc decreases.
Remember, of these three types of changes; concentration, pressure, and temperature, only changes in temperature will actually alter the Kc value.
There are two other factors related to chemical reactions that do not affect the position of equilibrium. The first of these is a catalyst. Catalysts speed up the rate at which equilibrium is reached by lowering the activation energy barrier. Of course, this speeds up the reverse reaction as well. The net result is that changes in the concentration of a catalyst will not affect the equilibrium concentrations of reactants and products, nor will they change the value of Kc.
The second of the two factors is the addition of an inert gas to a system at equilibrium. This will cause an increase in the total pressure within the reactor. However, none of the partial pressures of reactants or products are changed, and so the equilibrium is not upset, and no shifting is needed to bring the system back to equilibrium.
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